LARVAL ECOLOGY AND MORPHOLOGY IN FOSSIL

GASTROPODS

by ALEXANDER N €UTZELSNSB-Bayerische Staatssammlung f€ur Pal€aontologie und Geologie, Department of Earth and Environmental Sciences, Palaeontology and Geobiology,

GeoBio-Center LMU, Richard-Wagner-Str. 10, 80333, M€unchen, Germany; e-mail: a.nuetzel@lrz.uni-muenchen.de

Typescript received 2 November 2013; accepted in revised form 30 January 2014

Abstract: The shell of marine gastropods conserves and

reflects early ontogeny, including embryonic and larval stages,

to a high degree when compared with other marine inverte-

brates. Planktotrophic larval development is indicated by a

small embryonic shell (size is also related to systematic place-

ment) with little yolk followed by a multiwhorled shell formed

by a free-swimming veliger larva. Basal gastropod clades (e.g.

Vetigastropoda) lack planktotrophic larval development. The

great majority of Late Palaeozoic and Mesozoic ‘derived’ mar-

ine gastropods (Neritimorpha, Caenogastropoda and Hetero-

branchia) with known protoconch had planktotrophic larval

development. Dimensions of internal moulds of protoconchs

suggest that planktotrophic larval development was largely

absent in the Cambrian and evolved at the Cambrian–Ordovi-

cian transition, mainly due to increasing benthic predation.

The evolution of planktotrophic larval development offered

advantages and opportunities such as more effective dispersal,

enhanced gene flow between populations and prevention of

inbreeding. Early gastropod larval shells were openly coiled

and weakly sculptured. During the Mid- and Late Palaeozoic,

modern tightly coiled larval shells (commonly with strong

sculpture) evolved due to increasing predation pressure in the

plankton. The presence of numerous Late Palaeozoic and Tri-

assic gastropod species with planktotrophic larval develop-

ment suggests sufficient primary production although direct

evidence for phytoplankton is scarce in this period. Contrary

to previous suggestions, it seems unlikely that the end-Perm-

ian mass extinction selected against species with plankto-

trophic larval development. The molluscan classes with

highest species diversity (Gastropoda and Bivalvia) are those

which may have planktotrophic larval development. Extre-

mely high diversity in such groups as Caenogastropoda or eu-

lamellibranch bivalves is the result of high phylogenetic

activity and is associated with the presence of planktotrophic

veliger larvae in many members of these groups, although

causality has not been shown yet. A new gastropod species

and genus, Anachronistella peterwagneri, is described from the

Late Triassic Cassian Formation; it is the first known Triassic

gastropod with an openly coiled larval shell.

Key words: Gastropoda, Mollusca, Early Ontogeny, Proto-

conch, mass extinctions, selectivity.

THE biphasic life cycle of many marine invertebrates,

comprising a benthic adult and a swimming planktonic

larva, is an intriguing, widely discussed subject. The pos-

sible evolutionary implications of larval biology are far

reaching; for example, it has been proposed that the earli-

est eumetazoan was a planktonic organism that gave rise

to species with a feeding planktonic larva and a benthic

animal (J€agersten 1972; see Haszprunar et al. 1995 for a

discussion). For Gastropoda, it has been discussed

whether torsion (a pivotal apomorphy of the clade)

occurred to facilitate antipredatory larval retraction

(Garstang 1929; see e.g. Riedel 1996 for a discussion).

Also, the evolutionary implications of the larval biology

of living gastropods have been intensively investigated

and discussed (e.g. Thorson 1950; Scheltema 1971, 1986;

Shuto 1974; Bandel 1975, 1982; Jablonski and Lutz 1980,

1983; Lima and Lutz 1990). In most of these studies, the

planktotrophic/nonplanktotrophic dichotomy plays an

important role, influencing tempo and mode of evolution

as well as dispersal and gene flow. In palaeontology and

neontology, early ontogenetic shells have frequently been

used as a source of characters for taxonomy and system-

atics, especially during the last 20–30 years. However,

early ontogenetic pathways of fossil gastropods have

rarely been used for evolutionary and palaeoecological

studies (e.g. Hansen 1980, 1982; Jablonski 1986; Valentine

1986; Valentine and Jablonski 1986; N€utzel and Mapes

2001; N€utzel and Fr�yda 2003; N€utzel et al. 2006, 2007a;

Seuss et al. 2012). The present study focuses on the early

ontogeny of fossil gastropods, especially from the Palaeo-

zoic and early Mesozoic.

Early ontogenetic mollusc shells may be preserved in

fossils of great geological age. The oldest well-preserved

gastropod larval shells are of Late Silurian or Early

© The Palaeontological Association doi: 10.1111/pala.12104 1

[Palaeontology, 2014, pp. 1–25]

Devonian age (e.g. Dzik 1994; Fr�yda and Manda 1997;

Fr�yda 1998a, b, 1999, 2012). Earlier Palaeozoic proto-

conchs are known only from internal moulds (partly with

phosphatic coatings) or are strongly recrystallized. Proto-

conch preservation is more common in material from the

Late Palaeozoic and Mesozoic. However, although proto-

conchs of numerous Mid-Palaeozoic to Mesozoic species

have been reported, protoconch preservation is excep-

tional. Protoconchs are small, aragonitic and delicate.

Therefore, they are rarely preserved in sediments of great

geological age. In the Cenozoic, protoconch preservation

is much more common, but protoconchs are still

unknown even for many extant gastropod species.

The shell of marine conchiferan molluscs conserves

ontogeny to a very high degree when compared with

most other invertebrates (Jablonski and Lutz 1980; Lima

and Lutz 1990; Haszprunar et al. 1995). Most conchifer-

ans have a mineralized shell including an embryonic shell

as well as a postembryonic shell with comarginal accre-

tionary growth. Within Conchifera, there is considerable

variation in early shell ontogeny. Gastropods and bivalves

may have a planktonic veliger larva which commonly

feeds on plankton (planktotrophic). Cephalopods lack a

true larval stage and have a yolk-rich early ontogenetic

development. They are direct developers lacking meta-

morphosis, although juvenile cephalopods are called para-

larvae (Young and Harman 1988). Nautiloid eggs and

juveniles are rather large, whereas Ammonoidea and Bac-

tritoidea have smaller early juvenile shells: ammonitellas

and bactritellas (Mapes et al. 2007; Mapes and N€utzel

2009). Scaphopoda have tube-like bilateral symmetrical

larval shells (Engeser and Riedel 1996). They are usually

resorbed during adult growth. Scaphopod larvae are non-

feeding (Haszprunar et al. 1995).

THE GASTROPOD PROTOCONCH

The early ontogenetic shell of a gastropod is called a pro-

toconch. It consists of an embryonic shell (protoconch I)

built within the egg prior to hatching and a larval shell

(protoconch II) which is built during the larval phase of

gastropod species with planktotrophic larval development

(Fig. 1). The larval shell of gastropods has a horny oper-

culum (e.g. Ponder and Lindberg 1997). The embryonic

shell comprises about one whorl (see Schr€oder 1995 for

counting method; Fig. 2). At least the initial cap of the

embryonic shell is secreted by a shell gland within the egg

and shows no accretionary growth (Bandel 1982; Haszpr-

unar et al. 1995). Accretionary growth may start within

the embryonic shell according to Geiger (2012), but

growth lines are commonly not visible. Several of the

shells of the caenogastropod hatchlings illustrated by

Bandel (1975) show growth increments in their terminal

portions, indicating that accretionary growth may start

within the egg. The embryonic shell may be followed by a

larval shell and an adult shell; both are secreted in an

accretionary fashion by the mantle edge and therefore

show growth lines (increments).

The embryonic shell may be smooth, or ornamented

with micro-ornaments such as granules, pits and other

types of ornament (e.g. Robertson, 1971; Bandel 1975,

1982; Marshall 1983; Fig. 2C–D). In fresh shell material,

these ornaments can be studied readily with SEM under

high magnification. However, in fossil material of Meso-

zoic or Palaeozoic age, embryonic shells are usually re-

crystallized or corroded, and micro-ornaments are not

preserved. Thus, the transition from the embryonic shell

to the larval shell is rarely visible in the fossil protoconch.

The transition of the larval shell to the teleoconch is

commonly abrupt with a sudden change in ornament and

varices at the terminal edge of the larval shell (e.g. Figs 1,

2A, 5, 15B). In species with a planktonic larva, this

abrupt change reflects metamorphosis. Hickman (1995)

showed that metamorphosis including change in shell

morphology from the larva to the juvenile is not instanta-

neous but a staged process that may last several days.

Caenogastropods with planktotrophic larval development

may have a sinusigera, that is, the larval shell terminates

with two notches and a median larval beak (e.g. Hickman

1995, 1999a, b; Figs 1, 2A, see below).

Protoconchs in basal gastropod clades – obligatory

nonplanktotrophic

Extant members of basal gastropod clades of, for

example, the traditional Archaeogastropoda (excluding

F IG . 1 . Protoconch of Recent Cypraea sp. (Caenogastropoda,

off South Sulawesi, seagrass meadow, Indonesia) consisting of an

embryonic shell of about one whorl and a larval shell built by a

plankton-feeding veliger larva. The larval shell has a reticulate

ornament and terminates in a sinusigera consisting of a larval

beak and two velar notches. Scale bar represents 0.3 mm.

2 PALAEONTOLOGY

Neritimorpha), the Vetigastropoda (the most diverse

group of basal gastropods), Patellogastropoda (Docoglos-

sa) and Cocculiniformia do not have planktotrophic larvae

(e.g. Bandel 1982; Haszprunar 1995; Geiger et al. 2008;

Fr�yda 2012). Their planktonic larvae do not build a larval

shell (protoconch II), and therefore, the embryonic shell

(protoconch I) represents the entire protoconch (Fig. 3).

In Patellogastropoda, the protoconch is an uncoiled, short,

symmetrical tube that is sealed by a septum after teleo-

conch formation (Sasaki 1998). The patellid protoconch is

usually eroded through life and has not been reported

from Palaeozoic or Mesozoic members of the group. Actu-

ally, Palaeozoic crown group members of patellogastro-

pods are unknown according to Fr�yda et al. (2008a). In

Vetigastropoda, the protoconch is coiled and consists of

about one whorl (e.g. Bandel 1982). This is called the ‘tro-

choid condition’ (Haszprunar 1993) or ‘paucispiral type’

(Sasaki 1998). Bandel (1982) reported that this proto-

conch is initially organic and bilaterally symmetrical and

then mechanically deformed by muscles and subsequently

mineralized before hatching. The trochoid condition is

also found in Late Palaeozoic members of the extinct Eu-

omphaloidea (N€utzel 2002; Fig. 3D–E). It is also present

in several Mid-Palaeozoic gastropods, and the oldest

report of this type of protoconch is Early Devonian (Fr�yda

and Blodgett 2004). The size of vetigastropod protoconchs

varies from about 0.1 to 0.8 mm (e.g. Hickman 1992;

N€utzel et al. 2007a; Herbert 2012). This large variation

reflects variation in egg size and yolk or other nutrient

supply within Vetigastropoda. The protoconch may be

smooth or is ornamented with various types of micro-

ornaments (e.g. Geiger et al. 2008; Geiger 2012; Herbert

2012). Although lacking planktotrophic larval develop-

ment, vetigastropod species may be widely distributed.

The fact that all known extant basal gastropods lack plank-

totrophic larval development supports the suggestion that

a lecithotrophic larva represents the ancestral condition in

Gastropoda (Haszprunar 1995; Haszprunar et al. 1995;

Fr�yda 1999; N€utzel et al. 2006, 2007a; Fr�yda et al. 2008a).

There are a few reports of Palaeozoic gastropods with a

combination of a vetigastropod teleoconch and a possible

multiwhorled larval shell of the planktotrophic type,

among them putative members of slit-bearing pleurotom-

arioids (Dzik 1978, 1994; Yoo 1994; N€utzel and Mapes

2001; Kaim 2004). Geiger et al. (2008) discussed these few

cases and dismissed the possibility that Palaeozoic vetigas-

tropods had planktotrophic larval development. Of course,

it remains a matter of debate whether such gastropods

represent members of basal clades with planktotrophic lar-

val development or derived caenogastropods with a veti-

gastropod-like teleoconch, and there is a clear danger of

circular reasoning in this respect. Undoubted Late

A B

C D

F IG . 2 . Measured protoconch

parameters with Recent caenogastro-

pods Litiopa (A) and Iphitus (B–D)as examples. Scale bars represent

0.1 mm (A–B, D) and 30 lm (C).

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 3

Palaeozoic and Triassic members of slit-bearing vetigastro-

pods, trochoids and euomphalids have protoconchs of one

whorl which match the trochoid condition (Fig. 3). The

question of whether basal gastropods could have plankto-

trophic larval development is important for answering the

question of whether planktotrophy represents the original

state in gastropods. It is also possible, or even likely, that

disparity and the number of bauplans was higher in Early

gastropods than it is today, as has been proposed for ani-

mal clades in general by Hughes et al. (2013). If this is so,

the classification of modern gastropods cannot be applied

to ancient gastropods unequivocally and groups with a

vetigastropod-like teleoconch, but having a caenogastro-

pod-like larval shell might have formed extinct clades.

Derived clades – planktotrophy

Neritimorpha, Caenogastropoda and Heterobranchia are

considered to represent the more derived clades of Gas-

tropoda (e.g. Ponder and Lindberg 1997; Aktipis et al.

2008). However, it is still not entirely clear whether

planktotrophy is homologous in these derived clades or

whether it evolved independently. In most (but not in all)

of the published phylogenetic analyses, the mentioned

three groups form a monophylum (see Aktipis et al. 2008

for a summary), and thus, planktotrophy could be

homologous and form an apomorphy. In contrast to the

basal gastropod clades, marine members of the derived

clades can have planktotrophic larval development as well

as internal fertilization. All three clades are present in

seas, freshwater and on land. Freshwater and terrestrial

representatives generally have direct development, with

the exception of species from habitats between land and

sea (e.g. estuaries, coastal swamps), which may release

larvae into the sea (see below).

Neritimorpha

Neritimorpha (formerly included in Archaeogastropoda)

are of relatively low diversity in modern faunas (Lindberg

A

D

E

HG

F

B C

F IG 3 . Protoconchs of representa-

tives of basal clades (archaeogastro-

pods: vetigastropods and

euomphalids) consisting of about

one whorl reflecting nonplanktotro-

phy. A, trochoid Tricolia sp.,

Recent, off South Sulawesi, Indone-

sia, seagrass meadow. B–E, LateCarboniferous, north Central Texas,

Finis Shale, USA. B–C, pleurotom-

arioid Paragoniozona sp. D–E, eu-omphalid Amphiscapha sp. F–H,

Late Triassic Cassian Formation,

northern Italy. F, trochoid Eunem-

opsis sp. G–H, pleurotomarioid

Bandelium sp. Scale bars represent

50 lm (A), 0.5 mm (B, D), 0.1 mm

(C, E, F, H) and 0.2 mm (G).

4 PALAEONTOLOGY

2008). They are as old as Mid- or Late Palaeozoic and are

more diverse in Late Palaeozoic and Early Mesozoic fau-

nas than today. Extant species have multiwhorled highly

convolute, egg-shaped larval shells (Robertson 1971; Ban-

del 1982; Kaim and Sztajner 2005; N€utzel et al. 2007b;

Page and Ferguson 2013; Fig. 4A–B). This type of larval

shell separates them from the larval shells of Caenogastro-

poda which usually have well-separated whorls (Figs 1, 2,

5, 7, 9, 15, 16). In most modern neritimorphs (Neritidae),

larval and adult shells are internally resorbed (e.g. Bandel

2007; N€utzel et al. 2007b; Page and Ferguson 2013). This

condition is known from the Late Triassic (Bandel 2007),

but has not been reported from the Palaeozoic. However,

larval shells of Late Palaeozoic neritimorphs resemble cae-

nogastropod larval shells, suggesting a common origin of

planktotrophic larvae in Neritimorpha and Caenogastro-

poda (N€utzel et al. 2007b; Fig. 4C). Older supposed

members of the Neritimorpha have uncoiled larval shells

and are classified as Cyrtoneritimorpha (Fr�yda 1999,

2012; Figs 13, 14). The Palaeozoic family Platyceratidae is

probably related to Neritimorpha and contains both

tightly and openly coiled larval shells, the latter also

resembling caenogastropod larval shells (Fr�yda 1999).

Caenogastropoda

The Caenogastropoda is by far the most diverse clade of

the Gastropoda. Many of their extant marine members

have planktotrophic larval development. Thus, their pro-

toconch consists of an embryonic shell and a larval shell,

the latter a product of accretionary growth. The larval

shell consists of well-separated whorls which commonly

have intricate sculptures (Figs 1, 2, 5, 7, 9, 15, 16). In

many caenogastropod species with planktotrophic larval

development, the larval shell ends abruptly and terminates

A

B

C

F IG 4 . Protoconchs (planktotrophic type) of representatives of

Neritimorpha; from N€utzel et al. (2007b). A–B, Smaragdia sp.

Recent, off South Sulawesi, Indonesia, seagrass meadow, modern

type of neritimorph larval shell at metamorphosis (arrow marks

onset of teleoconch), strongly convolute, egg-shaped. C, Trachy-

spiridae sp., isolated larval shell with well-separated whorls (as

in caenogastropods) and strong ornament, Late Carboniferous,

north Central Texas, Finis Shale, USA. All scale bars represent

0.2 mm.

F IG 5 . Recent caenogastropod Horologica, sp. (Cerithiopsidae)

with high-spired slender smooth protoconch of the plankto-

trophic type; note that the protoconch comprises as many

whorls as the teleoconch; Lizard Island, Great Barrier Reef,

Queensland, Australia; from N€utzel (1998). Scale bars represent

0.3 mm (left), 0.1 mm (right).

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 5

with two notches and a median larval beak, the so-called

sinusigera (e.g. Hickman 1995, 1999a, b; Figs 1, 2A, 15B,

E). The notches accommodate the lobes of the velum and

the beak protects the larval head (Hickman 1995). The

notches can be very pronounced and are commonly

strengthened by a varix. The abapical notch is covered by

the first teleoconch whorl so that only the adapical notch

and part of the beak is visible (Figs 2A, 5).

By far the greater number of known Late Palaeozoic to

Cenozoic gastropod protoconchs represent caenogastro-

pod species. Most attempts to infer larval strategies

(planktotrophy vs nonplanktotrophy, see below) from

protoconch morphology and dimension refer to caeno-

gastropods. Caenogastropod protoconchs may comprise

as many as ten whorls. Their shape and ornament varies

considerably, but are conservative within species and

commonly also in higher categories. Thus, protoconchs,

especially larval shells of planktotrophic veligers, are taxo-

nomically diagnostic in many caenogastropods.

Planktotrophic larval development in combination with

internal fertilization represents an important evolutionary

innovation of Caenogastropoda (Ponder et al. 2008).

According to these authors, internal fertilization facili-

tated the production of encapsulated eggs and thus larvae

can undergo their early development in a protected envi-

ronment, to be released as veligers. As a consequence, a

planktonic larval stage was commonly abandoned in

many caenogastropod lineages and direct development

evolved, either within an external capsule or in capsules

or eggs retained in a brood pouch within the animal

(Ponder et al. 2008). In direct developers, nutrients are

provided to embryos by yolk, albumen, nurse eggs and

cannibalism (adelphophagy). According to the summary

given by Ponder et al. (2008), egg encapsulation and

intracapsular development facilitated invasion of marginal

marine and nonmarine habitats.

Heterobranchia

Heterobranchia are the second most diverse subclass of

gastropods. They originated in the sea, but gave rise to

terrestrial pulmonates later on. Marine Heterobranchia

are characterized by a heterostrophic (hyperstrophic) pro-

toconch (e.g. Robertson 2012). The larval shell is sinistral,

while the adult shell is usually dextral (Fig. 6). This con-

dition forms probably an apomorphy of Heterobranchia.

Commonly, the axis of the larval shell deviates markedly

from the teleoconch axis (Fig. 6B), but coaxial larval

shells do also occur (Fig. 6A, C). A change in coiling

direction during ontogeny is also present in other Palaeo-

zoic gastropod groups but occurs within the teleoconch

and therefore does not represent larval heterostrophy (e.g.

Fr�yda and Rohr 2006; Fr�yda and Ferrov�a 2011). Hetero-

branch larvae are generally smaller than caenogastropod

larvae (Page and Ferguson 2013). The larval shells of

planktotrophic species have relatively few whorls, usually

less than 2.5 whorls. Most larval shells of heterobranchs

are smooth. However, there are exceptions such as larval

shells of the Triassic–Jurassic families Tofanellidae with

A

B

C

F IG 6 . Protoconchs (plankto-

trophic type) of representatives of

Heterobranchia. A–B, Late Triassic

Cassian Formation, northern Italy.

A, mathildoid (tofanellid) heteros-

trophic coaxial larval shell with

strong axial ornament; from N€utzel

and Kaim (2013). B, tubiferid

(‘opisthobranch’) Sinuarbullina sp.

with heterostrophic sinistral, trans-

axial larval shell. C, donaldinid Do-

naldina sp., almost planispiral,

coaxial larval shell without visible

ornament, Moran Formation, Early

Permian, Wolfcampian, Texas,

USA. All scale bars represent

0.2 mm.

6 PALAEONTOLOGY

pronounced axial ribs (Bandel 1995; N€utzel and Kaim

2013; Fig. 6A) or Mathildidae (Gr€undel and N€utzel

2013). Undoubted heterobranchs are from the Mid-

Devonian (Bandel and Heidelberger 2002) and from the

Early Carboniferous (Yoo 1988, 1994; Fr�yda et al. 2008a).

Earlier records need confirmation (Fr�yda and Blodgett

2001). In the Late Palaeozoic, the heterobranch family

Donaldinidae had worldwide distribution (Anderson et al.

1990; Bandel 2002a). It comprises small, high-spired gas-

tropods with an almost planispiral heterostrophic larval

shell (Fig. 6C). As in caenogastropods, the majority of

Palaeozoic and Mesozoic heterobranch species with

known protoconch have planktotrophic larval develop-

ment. However, early ontogenetic traits are more difficult

to infer because the protoconch generally has fewer

whorls and the initial whorl is not visible in coaxial

protoconchs and because nonplanktotrophy obscures

heterostrophy and confusion with nonplanktotrophic

caenogastropods is possible.

At least by the Jurassic, marginal marine settings were

inhabited by archaeopulmonates which still release larvae

with sinistral (hyperstrophic) larval shells into the sea but

live in coastal swamps and estuaries as adults and are air

breathing (Pilkington and Pilkington 1982; Bandel 1991a;

Harbeck 1996). Subsequently, archaeopulmonates gave

rise to fully terrestrial pulmonates which have direct

development without larvae.

PLANKTOTROPHY AS INFERRED FROMPROTOCONCH MORPHOLOGY ANDDIMENSION

There are numerous publications on gastropod larval

ecology as inferred from protoconch morphology (e.g.

Thorson 1950; Shuto 1974; Jablonski and Lutz 1980,

1983; Lima and Lutz 1990). For operational reasons, the

dichotomy planktotrophy vs nonplanktotrophy is used in

the present study (see discussion by Jablonski and Lutz

1980, 1983). However, this is of course a simplification

because nonplanktotrophy encompasses nonfeeding (leci-

thotrophic) swimming larvae as well as direct develop-

ment. On the other hand, planktotrophy encompasses a

large variation in larval duration and amount of feeding.

Yet, in practice, in most fossil gastropods (caenogastro-

pods and neritimorphs) with known protoconchs, the

diagnosis of the developmental pathway (planktotrophic

or nonplanktotrophic) is clear.

Planktotrophy or nonplanktotrophy can be inferred

from dimensions and morphology of protoconchs of

marine gastropods. This was first elaborated by the pio-

neering work of Thorson (1950) who formulated the so-

called shell apex theory: ‘As a general rule, a clumsy, large

apex points to a nonpelagic development, while a nar-

rowly twisted apex, often with delicate sculptures, points

to a pelagic development’ (Thorson 1950). This theory

has been elaborated by Shuto (1974), Jablonski and Lutz

(1980, 1983), Lima and Lutz (1990) and others (Fig. 7).

Generally, a small embryonic shell (protoconch I) reflects

small eggs and thus a low amount of yolk. By contrast, a

large initial whorl indicates large yolk-rich eggs. Species

with nonplanktotrophic larval development generally have

fewer but larger eggs and hatchlings with a lower mortal-

ity. Species with planktotrophic larval development have

more abundant but smaller eggs and hatchlings with a

high mortality. Any comparison of egg size or sizes of

embryonic shells must also consider the systematic place-

ment of the gastropods studied. In a study of caenogas-

tropod hatchlings from the Caribbean, Bandel (1975)

showed that ‘lower mesogastropods’ (Cerithioidea, Littor-

inoidea) have generally smaller eggs and hatchlings than

‘higher mesogastropods’ and neogastropods (= Latrogas-

tropoda Riedel 2000; Fig. 8). Bandel (1975) also showed

that the variation of hatchling size is much higher in the

more derived clades of caenogastropods.

F IG . 7 . Protoconchs of Recent cerithiopsoid caenogastropods

in apical view at the same scale to show ‘Thorson Apex Theory’:

protoconchs of planktotrophic species (upper) have small initial

whorls and many larval whorls; protoconchs of nonplankto-

trophic species (lower) have large, clumsy initial whorls and few

protoconch whorls which are poorly demarcated from the teleo-

conch. Scale bar represents 0.1 mm.

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 7

The two most important variables to infer plankto-

trophy are the size of the embryonic shell and the

number of protoconch whorls (Fig. 2). Diameters of

embryonic and larval shells are best measured as maxi-

mum diameter as seen in apical views of SEM micro-

graphs. There are different methods of counting the

whorl numbers. I use the method given by Schr€oder

(1995) (see Fig. 2) which is similar to that of Pilsbry

(1939). This method uses the tip of the initial cap as

point of reference: whenever this point is passed by suc-

ceeding whorls, a new whorl is counted. Others use the

method introduced by Verduin (1977) who excluded the

‘nucleus’ from the count (Jablonski and Lutz 1980). It is

unimportant which method is used, but the method must

be explicit. However, one should keep in mind that the

various methods in use may differ from each other in 0.5

whorls.

In fresh shell material, the transition between embry-

onic shell and larval is mostly quite obvious in species

with planktotrophic larval development (e.g. Robertson

1971; Bandel 1975; Marshall 1983; Lima and Lutz 1990).

There is commonly a pronounced change in ornamenta-

tion and a suture or ledge at this transition. However, in

protoconchs of fossil gastropods, these delicate structures

are usually overprinted by re-crystallization or abrasion.

Therefore, the transition from embryonic to larval shell is

not visible in most of the fossil specimens. However, the

size of the hatchling is well reflected by the diameter of

the first whorl which is easy to measure even in re-crys-

tallized material (N€utzel 1998; N€utzel et al. 2007a). In

Early Palaeozoic internal moulds of gastropod (and other

conchiferan) protoconchs, ontogenetic boundaries cannot

be observed directly because the shell is dissolved. There-

fore, N€utzel et al. (2006, 2007a) used the diameter of the

whorl at 100 lm shell length as a proxy for the size of

the embryonic shell, assuming that at this shell length,

the animal was prior or close to hatching even in rela-

tively small early ontogenetic shells.

The embryonic shells (hatchlings) of planktotrophic

species range from somewhat less than 100 lm to up to

900 lm. However, extremely large values close to 1 mm

are only known from neogastropods (see also Riedel

2000). Bandel (1975) reported that the shell size of hatch-

lings of lower caenogastropods from the Caribbean that

have planktotrophic larval development ranges from 100

to 270 lm.

Besides the size of the embryonic shell reflecting the

amount of yolk, the second important variable for the

diagnosis of planktotrophy is the number of protoconch

whorls (Figs 9–11). The number of larval whorls may

reflect the duration of the larval life and phytoplankton

consumption. In caenogastropods, protoconchs of the

planktotrophic type consist of about two to more than

six whorls. As mentioned previously, heterobranch proto-

conchs usually have fewer larval whorls than caenogastro-

pods. Larval shells may differ considerably from the

teleoconch in shape and ornament although, as outlined

by Seuss et al. (2012), larval and adult shells commonly

resemble each other (see below).

Shuto (1974) plotted the number of protoconch

whorls against the quotient of protoconch diameter and

number of whorls. However, it is more informative to

plot the diameter of the first whorl (which reflects the

size of the protoconch I) vs the number of protoconch

whorls (Fig. 10). This results in a clear co-variation of

both variables, that is, the smaller the initial whorls, the

more protoconch whorls are present. The co-variation is

F IG . 8 . Hatchling size distribution according to systematic

placement (superfamily) of Recent caenogastropods from the

Caribbean redrawn from Bandel (1975). Hatchling (cf. size of

first whorl or embryonic shell) is relatively small (0.1–0.2 mm)

in ‘lower mesogastropods’ (Cerithioidea Littorinimorpha);

hatchling size including its variation distinctly larger in ‘higher

mesogastropods’ (Calyptraeoidea, Hipponicoidea, Tonnoidea,

Cypraeoidea, Naticoidea) and Neogastropoda (Muricoidea, Buc-

cinoidea, Volutoidea); largest hatchlings in nonplanktotrophic

Neogastropoda; this shows that for inference of larval traits and

for comparisons, the systematic placement needs to be consid-

ered. Most Late Palaeozoic and Mesozoic caenogastropods have

a diameter of the first whorl similar to that of Recent ‘lower

caenogastropods’.

8 PALAEONTOLOGY

geometric as becomes obvious in a log–log plot

(Fig. 11). As examples, I use data from N€utzel’s (1998)

study of Late Palaeozoic pseudozygopleurids from North

America (Figs 9, 10A, 11B) and from Marshall’s (1983)

monograph of the Recent triphorids of South Australia

(Fig. 10B). Both gastropod groups represent basal caeno-

gastropods. As outlined above, comparisons of larval

strategy need to take into account the systematic place-

ment of the gastropods (Bandel 1975; Fig. 8). The mod-

ern and the Late Palaeozoic examples show a strikingly

similar pattern. In planktotrophic species, the initial

whorl measures 0.1–0.2 mm in diameter and the num-

ber of protoconch whorls is greater than two to three

whorls. Marshall’s (1983) triphorids have more than two

whorls even if they were lecithotrophic. This probably

reflects the different counting method employed by this

author (see above). A third data set shows measure-

ments of various tropical caenogastropods representing

several genera from Australia (Queensland) and Indone-

sia (South Sulawesi) taken from my unpublished SEM

micrographs. It shows the same pattern as the other two

examples (Fig 10C). In ancient gastropods with openly

coiled larval shells, the diameter of the initial whorl is

not directly comparable with tightly coiled initial whorls.

N€utzel et al. (2006, 2007a) therefore used the diameter

of the whorls at a shell length of 100 lm which would

be prior to, or shortly after, hatching in species with

planktotrophic larval development. Small values (c. 40–80 lm) would indicate planktotrophy.

An abrupt termination of the larval shell may indicate

metamorphosis and, in connection with a sinusigera, sug-

gests the presence of a planktonic larva. The majority of

caenogastropods, neritimorphs and heterobranchs with

planktotrophic larval development have an abrupt transi-

tion from the larval shell to the juvenile teleoconch (in

the case of caenogastropods commonly as a sinusigera).

Species with nonplanktotrophic larval development show

a tendency to have a gradual, obscure transition to the

teleoconch (e.g. Marshall 1983), especially in direct devel-

opers. However, there are also species with nonplankto-

trophic larval development with abrupt transition,

especially in nonfeeding planktonic larvae. Moreover,

F IG . 9 . Comparison of proto-

conch dimensions of Recent Tri-

phoridae and Late Palaeozoic

Pseudozygopleuridae. Upper pair

planktotrophic, middle and lower

pairs nonplanktotrophic; middle

pair with abrupt transition from

protoconch to teleoconch; lower

pair with gradual, indistinct transi-

tion. Despite 300 myr difference,

dimensions and number of whorls

are similar and show the same asso-

ciations.

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 9

there are examples of species with multiwhorled larval

shells of the planktotrophic type which show a gradual

indistinct transition to the teleoconch, for example, in

Jurassic aporrhaids and cerithioids (Gr€undel 1999a;

Gr€undel et al. 2009; Schulbert and N€utzel 2013). Thus,

the transition from the larval shell to the teleoconch alone

should not be used as diagnostic for larval traits but is

useful in combination with other criteria. In fossil gastro-

pods having both a smooth larval and adult shell, the

transition is obscured in most cases. If it was abrupt, this

can only be seen in excellently preserved specimens

(N€utzel et al. 2000; N€utzel and Pan 2005).

THE GASTROPOD PROTOCONCH ASSOURCE OF MORPHOLOGICALCHARACTERS

The gastropod protoconch may form an important source

of morphological shell characters which can be diagnostic

on all hierarchical levels from subclass attribution to spe-

cies discrimination. This turned out to be most important

because the number of discrete shell characters is very

small in comparison with the huge number of gastropod

taxa. This imbalance forms a major handicap for phylo-

genetic studies of fossil Gastropoda. Wagner (2000, 2002)

showed that the number of characters and states in

Palaeozoic gastropods known only from teleoconchs is

sufficiently large to produce trees with good resolution,

especially in analyses of low hierarchical levels. At higher

levels, it is promising to code protoconch characters in

the same way as teleoconch characters (and not using a

specific protoconch type as a single characters), which will

increase the number of shell characters considerably.

Protoconchs are also very important for alpha taxon-

omy; especially the larval shells of marine caenogastropods

with planktotrophic larval development yield commonly

diagnostic features (e.g. Marshall 1978, 1983; Bandel 1992;

Gr€undel 1999a, b; Guzhov 2004; Kaim 2004).

WHAT DO THE LARVAE FEED ON?

According to a summary given by Richter (1987), small

gastropod larvae feed predominantly on phytoplankton

including nanno- and ultraplankton. Uptake of dissolved

organic matter and detritus has also been proposed.

Larger teleplanic (long lasting) larvae from the tropics

also feed on zooplankton (radiolaria, foraminiferids) and

even fish eggs, and parts of copepods have been found in

the stomachs of gastropod veliger larvae (Richter 1987).

Teleplanic larvae also feed on phytoplankton, for example

coccolithophorids, diatoms, silicoflagellates and especially

dinoflagellates. Richter (1987) showed that some veligers

are able to separate dinoflagellates in a special part of the

stomach and that the cellulose plates of the dinoflagellates

are digested aided by the enzyme cellulase.

PREDATION ON GASTROPOD LARVAE

There is consensus that mortality is very high in free-

swimming gastropod larvae, especially in planktotrophic

A

B

C

F IG . 10 . Scatter plots of diameter of initial whorl vs number

of protoconch whorls of Late Palaeozoic and Recent caenogas-

tropods. A, Carboniferous Pseudozygopleuridae, data from

N€utzel (1998). B, Recent Triphoridae, data from Marshall

(1983). C, various Recent caenogastropods. The patterns are

similar; inferred planktotrophic species (dashed ellipse) have a

diameter of the first whorl of 0.1–0.2 mm and the number of

protoconch whorls is greater than two to three whorls. Species

with nonplanktotrophic larval development have one to three

protoconch whorls and a diameter of the first whorl greater than

0.2 mm (larger values in B probably reflect a different method

of counting whorls used by Marshall, 1983).

10 PALAEONTOLOGY

larvae, and that among other factors such as salinity or

temperature, predation plays a pivotal role (Thorson

1950; Scheltema 1971, 1986; Jablonski and Lutz 1980,

1983; Pechenik 1999). There are few direct observations

of predation on gastropod veligers. For instance, Short

et al. (2012) observed various gastropod and other inver-

tebrate larvae in the guts of scyphozoans and fishes. The

fact that gastropod larvae are armoured (have a shell)

and have an operculum alone suggest that predation in

the plankton is pervasive. Hickman (1999a, b, 2001)

interpreted morphological features of gastropod larval

shells as defence, that is, the beak to protect the head,

varices and peripheral carinations or sculptures to

strengthen the shell. Moreover Hickman (1999a, b, 2001)

reported healed shell fractures at larval beaks of shells of

gastropod veligers and interpreted them as results of

failed predation attempts by unknown planktonic ani-

mals. I would expect that planktonic arthropods also prey

on gastropod larvae although I have found no reports of

this. Unsuccessful predation on Devonian planktonic

dacryoconarid tentaculites has been shown by Berkyov�a

et al. (2007).

ORIGIN OF PLANKTOTROPHY ANDTRENDS IN LARVAL MORPHOLOGYAND ECOLOGY

N€utzel et al. (2006, 2007a) showed that the early ontoge-

netic stages of Cambrian univalved molluscs (measured at

a shell length of 100 lm) are relatively large and that this

reflects nonplanktotrophic larval development in early

molluscs including possible representatives of the gastro-

pod stem line. So far, there is no direct evidence

for planktotrophy in Cambrian molluscs based on well-

preserved protoconchs. Direct observations of ontogenetic

borders (embryonic/larval and larval/teleoconch) are not

yet possible because Cambrian material is present as

steinkerns only (commonly phosphatic). Cambrian uni-

valves are difficult to assign to Gastropoda, and the earli-

est members of the clade are still disputed. For instance,

Aldanella and Pelagiella share more synapomorphies with

helcionellids than with gastropods according to Wagner

(2002). By contrast, Aldanella was assigned to Gastropoda

by Parkhaev (2006) and to Hyolitha by Dzik and

Mazurek (2013); the latter assignment is based (among

other arguments) on the presence of a cup-like, hemispher-

ical embryonic shell.

Ordovician–Silurian shelly assemblages (larval fall

assemblages) from conodont samples (Fig. 12) may yield

numerous small gastropods with small initial parts indi-

cating planktotrophic larvae. Thus, N€utzel et al. (2006,

2007a) concluded that in molluscs including gastropods,

planktotrophic larval development was acquired at the

Cambrian–Ordovician transition and was connected with

the Ordovician radiation. Others argued in the same

direction based on the phylogenetic distribution of plank-

totrophy, that is, based on the fact that basal clades lack

planktotrophic larval development (Haszprunar 1995;

A BF IG . 11 . Log–log plot of same

data used in Fig. 10A, C showing

that the diameter of initial whorl

and the number of protoconch

whorls follows a power function. A,

Recent caenogastropods from Indo-

nesia and Australia. B, Late Carbon-

iferous Pseudozygopleuridae.

F IG . 12 . Ordovician fossil assemblage consisting predomi-

nantly of minute internal moulds (steinkerns) of gastropod

protoconchs. Gastropods larger than 0.5 mm are lacking, sug-

gesting that this assemblage represents a planktonic community

of gastropod larvae (larval fall). Residue of an acetic acid-etched

Ordovician limestone glacial erratic boulder northern Germany

derived from Baltica, Darriwilian. Scale bar represents 0.5 mm.

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 11

Haszprunar et al. 1995; Hickman 1999a). This was also

suggested based on molecular studies which revealed

polyphyly of planktotrophy in invertebrates (Peterson

2005). Chaffee and Lindberg (1986) argued that Early

Cambrian univalved molluscs as known from small shelly

assemblages were in most cases too small for maintaining

high fecundities that correlate with planktotrophy, and

they concluded that planktotrophy evolved at the Late

Cambrian – Ordovician transition. However, Mus et al.

(2008) found a c. 1-mm-large helcionellid-like apex on

the tip of limpet as large as 3 cm from the Early

Cambrian of Spain. They discussed the possibility of

Cambrian small helcionellids in fact being larval shells of

larger limpets which happen to be rarely preserved. This

case was also used by Nielsen (2013) who wanted to

prove the well-known hypothesis that the eumetazoan

ancestor was planktonic and represented a plankton-

feeding gastraea. However, it must be stated that the apex

of the very interesting limpets reported by Mus et al.

(2008) are by no means clear evidence for planktotrophy.

First, marked ontogenetic and abrupt change does also

occur within the teleoconch of molluscs, and second, any

inference of planktotrophy must also relate to egg size as

reflected by the size of protoconch I, which is obviously

not preserved in the material reported by Mus et al.

(2008). Thus, the helcionellid-like apex of these Cambrian

limpets is by no means clearly a product of a plankto-

trophic larva; it could as well be a larval shell of the leci-

thotrophic type or an early juvenile shell.

TRENDS IN LARVAL SHELLMORPHOLOGY

The protoconchs of many Ordovician to Mid-Palaeozoic

gastropods of various systematic placements are openly

coiled, tube-like and resemble fishhooks (Figs 13, 14;

e.g. Bockelie and Yochelson 1979; Dzik 1994; Fr�yda and

Manda 1997; Fr�yda 1999, 2012). Some of these

protoconchs are entirely uncoiled (sometimes even

stretched) or form a single loose whorl (Figs 13A, 14);

this is the cyrtoneritimorph type of larval shell. Some of

these forms may have a small (<50 lm), sometimes bul-

bous initial part which probably represents the embry-

onic shell (Fig. 14A–B, E). The following uncoiled tube

represents an early type of planktotrophic larval shell

which shows no or little ornamentation. In other gastro-

pods, uncoiling is restricted to a narrow first whorl and

the following larval shell is tightly coiled (Fig. 13B); this

is found in Devonian perunelomorphs and Late Palaeo-

zoic imoglobids, both of caenogastropod affinity (Fr�yda

1999; N€utzel et al. 2000; N€utzel and Cook 2002).

Uncoiled protoconchs are not restricted to representa-

tives of derived clades; some members of basal clades

with nonplanktotrophic development may also have

uncoiled protoconchs, for example macluritoids (Fr�yda

and Rohr 2006) and euomphalomorphs (Yoo 1994;

Bandel and Fr�yda 1998).

Entirely and partially uncoiled protoconchs can still be

found in a few Late Palaeozoic gastropods (Fig. 14E, H, J;

e.g. N€utzel and Cook 2002, Fr�yda 2012), but have not

been reported from the Mesozoic until now. This mor-

phology is reported from the Mesozoic for the first time

in Anachronistella peterwagneri gen. et sp. nov. from the

Late Triassic Cassian Formation (Fig. 14K–N) which is

newly described herein (Appendix). But this is an excep-

tion because this protoconch type has not generally been

found in the Mesozoic.

N€utzel and Fr�yda (2003) quantified the decline of the

openly coiled protoconch morphology and showed a clear

trend throughout the Palaeozoic. It is likely that the

openly coiled larval shell morphology was especially vul-

nerable to predation and that increasing predation pres-

sure selected against this morphology (N€utzel and Fr�yda

2003). In analogy with Vermeij’s (1977) Mesozoic Marine

Revolution, this was called the Palaeozoic Plankton

A B C

F IG . 13 . Palaeozoic evolution of tightly coiled whorls from an ancestral openly coiled larval shell of planktotrophic type as adapta-

tion to increasing predation pressure in the plankton. A, openly coiled larval shell (cyrtoneritimorph type) as is common in various

Ordovician to Devonian gastropod groups. B, larval shell consisting of openly coiled initial whorl and subsequent tightly coiled larval

shell (peruneloid/imoglobid type) as reported from the Devonian and Late Palaeozoic. C, larval shell entirely tightly coiled which is

the dominant type from the Late Palaeozoic onward. Such larval shells may also be ornamented or carinated.

12 PALAEONTOLOGY

Revolution (N€utzel and Fr�yda 2003) and is consistent

with predation-driven evolution (escalation) (Vermeij

1987, 2013). A similar phenomenon has been reported in

Palaeozoic Ammonoidea: early representatives have

openly coiled initial whorls or are entirely loosely coiled,

whereas younger forms are tightly coiled (Klug et al.

2010). This is part of the Devonian Nekton Revolution

which is characterized by an escalatory rapid occupation

of the marine water column during the Devonian (e.g.

fish and ammonoids). Generally, predation pressure in

plankton and nekton increased during the Palaeozoic, and

the disappearance of uncoiled larval shells reflects this

trend. Seuss et al. (2012) investigated the morphology of

Late Palaeozoic larval shells of Neritimorpha and Caeno-

gastropoda in comparison with related adult shells. They

found that larval shells commonly resemble the

teleoconch in shape, that is, a caenogastropod with a

slender high-spired shell commonly also possesses a high-

spired slender larval shell. The same is found in larval

and adult ornament: a caenogastropod with a smooth

teleoconch is more likely to possess a smooth larval shell

than can be expected by chance. In principal, this seems

to be counterintuitive because the planktonic habitat dif-

fers strongly from benthic habitats, and thus, they should

require different shell morphological adaptations. The

similarity of adult and larval shells was explained by Seuss

et al. (2012) as the result of increasing predation pressure

in the plankton: early larval shells were uncoiled and thus

rather vulnerable. When predation increased, it selected

against open coiling (N€utzel and Fr�yda 2003) and the

tightly coiled adult shell morphology was transferred to

the larval stage (Seuss et al. 2012). It is also in the logic

of this escalation in the plankton that the first gastropod

larval shells with strong ornaments and carinations and

A

D

G

J

M N

K L

HI

EF

B CF IG . 14 . Gastropods with openly

coiled larval shells. They have small

initial parts which are bulbous in

some cases; these are presumably

embryonic shells which are followed

by openly coiled larval shells proba-

bly of planktotrophic type. A–C,internal moulds (steinkerns) with

openly coiled larval shells from the

Silurian of the Kosov quarry, Czech

Republic; specimens from a larval

fall assemblage. D–I, cyr-toneritimorphs with openly coiled

larval shell (from Fr�yda et al. 2009).

D, G, Vltaviella, Early Devonian,

Czech Republic. E, H, Pseudorthony-

chia, Pennsylvanian, USA. F, I, Eifel-

cyrtus, Mid-Devonian, Germany. J,

cyrtoneritimorph, early juvenile

including openly coiled larval shell,

Mississippian, Imo Formation,

Arkansas, USA. K–N, Anachronistel-la peterwagneri gen. et sp. nov.,

holotype (BSPG 2010 III 6) from

the Late Triassic Cassian Formation;

this is the only known Mesozoic

gastropod with a widely uncoiled

first whorl of the larval shell. L,

arrow marks termination of larval

shell. M, shows detail of faint colla-

bral ornament of larval shell. Scale

bars represent 0.1 mm (A–B, K–M),

0.2 mm (C–E, N), 0.5 mm (F–G, I),1 mm (H) and 0.3 mm (J).

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 13

strengthened sinusigeras appeared in the Late Palaeozoic

(Seuss et al. 2012; Figs 4C, 9, 15).

GASTROPOD LARVAL ECOLOGYTHROUGH TIME

I estimate the number of Palaeozoic and Mesozoic gastro-

pod species with known protoconch to be between 1000

and 2000. This represents a small percentage (�10%) of

the total number of fossil species described from this period

of time. Thus, protoconch preservation is exceptional,

especially in the Palaeozoic and Early Mesozoic. In the fol-

lowing section, important monographs and other publica-

tions documenting gastropod species with known larval

shells are considered. The Palaeozoic and Mesozoic fossil

record of gastropod larval shells is spotty; for instance, in

the Late Triassic, most species have been reported from a

single formation, and the same is true for a large portion of

the genera: nearly all protoconchs that have been reported

from the Late Triassic are from the Cassian Formation in

northern Italy. Under these circumstances, global or large

regional studies of larval ecology are hardly possible.

Early and Mid-Palaeozoic

As outlined above, there is no evidence for planktotrophic

larval development in Cambrian univalved molluscs

including putative members of Gastropoda. The size of

the initial parts of Cambrian shells suggests either lecitho-

trophic or direct development. Well-preserved shells

showing clear ontogenetic borders have not been reported

from the Cambrian. The same is true for the Ordovician

and Silurian with few exceptions. However, Ordovician–Silurian assemblages of gastropod protoconchs preserved

as internal moulds (probably representing larval fall

assemblages; Fig. 12, 14A–C) consist of specimens with

small initial parts. Bockelie and Yochelson (1979)

reported such an assemblage from the Ordovician of

Spitsbergen, but interpreted the specimens as representing

worms because their openly coiled shape differs markedly

from the shape modern gastropod protoconchs. Later, it

turned out that these types of tiny openly coiled shells

represent gastropod larval shells (Dzik 1994; Fr�yda 1998a,

b, 1999). N€utzel et al. (2006) reported assemblages of

openly coiled protoconchs from glacial erratics of north-

ern Germany derived from Baltica (Darriwilian), the

Canadian Arctic (Llandovery Laurentia – Arctic Platform)

and the Czech Republic (Wenlock–Ludlow Perunica

microcontinent). These assemblages were found in etched

residues (usually conodont samples; Fig. 12) and consist

of abundant protoconch steinkerns <1 mm, suggesting

that larval fall was common and gastropod larvae were

abundant in Ordovician and Silurian seas. Many of these

protoconchs were most probably formed by plankto-

trophic veliger larvae (N€utzel et al. 2006).

Quite a number of protoconchs of Devonian gastro-

pods have been reported, mainly from the Prague Basin

but also from Alaska and other regions thanks to the

A

D E

B C

F IG . 15 . The first caenogastropod

larval shells of the planktotrophic

type with strong ornaments (A, C),

strengthened velar notches and beaks

(B) and carinations (D–E) appear inthe Late Palaeozoic. A, isolated larval

shell of a pseudozygopleurid, Missis-

sippian, Arkansas, USA, from N€utzel

and Mapes (2001). B, Cerithioides

sp., larval shell and first teleoconch

whorl, Pennsylvanian, Buckhorn

Asphalt deposit, Oklahoma, USA. C,

isolated larval shell of Nuetzelina stri-

ata Bandel 2002b, Mississippian,

Arkansas, USA, from N€utzel and

Mapes (2001). D, Permocerithium

nudum N€utzel 2012, Mid-Permian,

Akasaka Limestone, Japan, from

N€utzel and Nakazawa (2012); Erwini-

spira jucunda (Pan and Erwin 2002),

Late Permian, South China, from

N€utzel and Pan (2005). Scale bars

represent 0.1 mm (A) and 0.2 mm

(B–E).

14 PALAEONTOLOGY

research activity of J. Fr�yda (e.g. Fr�yda and Manda 1997;

Fr�yda 1998a, b, 2012; Fr�yda and Blodgett 1998, 2001,

2004; Fr�yda and Heidelberger 2003; Fr�yda et al. 2009).

Although recrystallized, many of these protoconchs are

well preserved, but ontogenetic boundaries are commonly

obscured and larval traits are difficult to infer in such

cases. Devonian protoconchs are smooth or at least lack

strong ornament which, together with recrystallization,

makes it hard to locate ontogenetic boundaries. However,

several Early Devonian gastropods obviously had plankto-

trophic larval development. This is the case for unorna-

mented uncoiled to almost straight tube-like protoconchs

with small initial part (Fig. 14D–I) such as Vltaviella

(Fr�yda and Manda 1997), Eifelcyrtus (Fr�yda and Heidel-

berger 2003) or Praenatica (Fr�yda et al. 2009) which

belong to Cyrtoneritimorpha. The modern type of caeno-

gastropod larval shell with tightly coiled larval whorls (in

the Palaeozoic sometimes with openly coiled initial

whorl) has also been reported from the Devonian, in

Pragoscutula with a limpet-shaped teleoconch (Fr�yda

2001; Cook et al. 2008) and in the subulitoid genera

Balbiniconcha and Prokopiconcha (Fr�yda 2001). The

diverse high-spired gastropod family Palaeozygopleuridae

is characterized by relatively large paucispiral protoconchs

(Horn�y 1955) which clearly reflect nonplanktotrophic lar-

val development (N€utzel 1998; Fr�yda et al. 2008b, 2013).

In addition, the paucispiral, relatively large protoconchs

of several other Devonian gastropods such as Murchisonia

and Ruedemannia have been reported. They are clearly of

the nonplanktotrophic type, match the trochoid condition

(see above) and are of vetigastropod affinity.

Carboniferous

Numerous Carboniferous gastropod protoconchs have

been reported, some of them superbly preserved. Some

caenogastropod and neritimorph larval shells have strong

ornaments and pronounced sinusigeras for the first time

in the Phanerozoic probably due to increasing predation

in the plankton as previously outlined. This makes it eas-

ier to observe ontogenetic boundaries, especially the tran-

sition from the larval shell to the teleoconch. About 75

per cent of the Carboniferous caenogastropod and neri-

timorph species with known protoconchs had plankto-

trophic larval development (estimated 200 species with

known protoconch). By far the majority of these species

are known from the Pennsylvanian of the USA (e.g.

Knight 1930; Hoare and Sturgeon 1978, 1985; N€utzel

1998; N€utzel et al. 2000; Bandel 2002b; Bandel et al. 2002;

N€utzel and Pan Hua-Zhang 2005). The best preserved

Palaeozoic protoconchs have been reported from the

Pennsylvanian Buckhorn Asphalt deposit in Oklahoma,

one of the very few occurrences with aragonite preserva-

tion (N€utzel 1998; Bandel et al. 2002; Seuss et al. 2009).

Mississippian gastropod protoconchs have also been

reported (e.g. N€utzel and Pan Hua-Zhang 2005) including

an assemblage of very well-preserved larval shells and

microgastropods from the Chesterian of Arkansas repre-

senting a larval fall association (Fig. 15A, C;N€utzel and

Mapes 2001; Mapes and N€utzel 2009). Gastropod larval

fall assemblages have also been reported from the Car-

boniferous of Germany (Herholz 1992) and Japan (Isaji

and Okura 2014). Such assemblages may occur in oxy-

gen-deficient facies that are entirely or largely devoid of

benthos but contain nektonic organisms and mollusc lar-

vae (N€utzel and Mapes 2001). They document the rich

and abundant veliger assemblages in Late Palaeozoic seas

and are indirect evidence for primary production even if

primary consumers are not found.

Most of the Carboniferous caenogastropods and

neritimorphs have the modern type of larval shell which

is tightly coiled throughout (N€utzel and Fr�yda 2003;

Seuss et al. 2012). However, some species representing

Cyrtoneritimorpha still have entirely uncoiled larval shells

(Fig. 14E, H, J; Bandel and Fr�yda 1999) which were

probably built by planktotrophic larvae. Moreover, several

caenogastropods have uncoiled initial whorls but tightly

coiled larval shells (N€utzel et al. 2000; N€utzel and Cook

2002).

In the Carboniferous, there are examples of very well-

preserved larval shells with clear ontogenetic borders,

especially between the larval shell and the teleoconch

(Figs 9, 15A–C). Several of the species in question have

pronounced sinusigeras and, for the first time in the

Phanerozoic, larval shells with strong armour such as

axial ribs occur (Fig. 15A, C). The best example repre-

sents the caenogastropod family Pseudozygopleuridae

which comprises c. 100 known Late Palaeozoic species.

Most of them have been described from the Pennsylva-

nian of the USA (e.g. Knight 1930; Hoare and Sturgeon

1978, 1985; N€utzel 1998). The family ranges from the

Early Carboniferous to the Late Permian. It comprises

small- to medium-sized caenogastropods with a larval

shell that is ornamented with pronounced collabral sinu-

ous axial ribs which reflect the course of the sinusigera

(Figs 9, 15A). Pseudozygopleuridae is to my knowledge

the first gastropod family that was primarily based on

protoconch characters (Knight 1930). Most pseudozygo-

pleurids had planktotrophic larval development having a

smooth initial whorl with a diameter of 0.1–0.2 mm and

up to four larval whorls (Figs 9, 10A, 11B; N€utzel 1998).

However, some species had nonplanktotrophic larval

development. Pseudozygopleuridae had a wide distribu-

tion with representatives from the USA, Europe (N€utzel

1998), China (Pan and Erwin 2002), Australia (Yoo 1988,

1994) and Japan (Isaji and Okura 2014). Besides the

Pseudozygopleuridae, several other diverse caenogastro-

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 15

pod groups with characteristic larval shells are present in

the Carboniferous (and in the Permian), for example

ancient cerithimorphs (Goniasmatidae: Fig. 15B, Ortho-

nematidae, Propupaspiridae) and some subulitoids

(N€utzel 1998; N€utzel and Bandel 2000; N€utzel et al. 2000;

N€utzel and Pan 2005). Other Carboniferous larval shells

are entirely smooth, for example, in Soleniscidae and

some Naticopsidae which also have a smooth teleoconch

(N€utzel et al. 2000, 2007b; N€utzel and Pan 2005).

Permian

For the Permian, many fewer caenogastropods and neri-

timorphs with known protoconch have been reported

(N€utzel 1998; Bandel 2002b; N€utzel et al. 2002, 2007b;

Pan and Erwin 2002; N€utzel and Nakazawa 2012). For

the first time, there are larval shells with pronounced

carinations probably as antipredatory adaptation

(Fig. 15D–E). As in the Carboniferous, most of the spe-

cies with known protoconch had planktotrophic larval

development. The systematic inventory is basically similar

to that present in the Carboniferous.

Triassic – implications for the end-Permian mass extinction

There are c. 100 nominate Early Triassic gastropod spe-

cies; for gastropods, this standing diversity is extremely

low (N€utzel 2005). Protoconchs are only known for very

few Early Triassic gastropod species from the Sinbad

Limestone (Utah), the Salt Range (Pakistan) and Far East

Russia (Batten and Stokes 1986; N€utzel and Erwin 2002;

N€utzel and Schulbert 2005; Kaim 2009; Kaim et al. 2013).

The published protoconch images suggest that Early Tri-

assic caenogastropods, neritimorphs and heterobranchs

with preserved protoconchs had planktotrophic larval

development, although more detailed studies are needed.

The fact that the majority of Late Palaeozoic as well as

Early Triassic caenogastropods (and others) had plankto-

trophic larval development suggests that the end-Permian

mass extinction did not select against species with plank-

totrophic larval development. This is in contrast to previ-

ous suggestions by Valentine (1986) and Valentine and

Jablonski (1986) who assumed that the end-Permian

extinction might have selected against planktotrophy

based on indirect evidence. They argued that Palaeozoic

stalked crinoids, articulate brachiopods and archaeogas-

tropods (vetigastropods) probably had predominantly

planktotrophic larval development based on their pre-

dominantly tropical distribution and abundance in the

Late Palaeozoic and that planktotrophy is not present in

modern representatives of these groups. Accordingly,

vetigastropods, stalked crinoids and articulate brachio-

pods with planktotrophic larval development became

extinct at the end-Permian mass extinction event. How-

ever, Valentine provided no direct evidence for this

hypothesis by protoconch morphology. As outlined previ-

ously, archaeogastropods (Vetigastropoda) probably never

had planktotrophic larval development. Moreover, it was

argued that modern representatives of invertebrate groups

that originated in the Early Triassic have nonplankto-

trophic larval development because environmental condi-

tions were unfavourable during origination due to

prevailing disturbances in the planktic realm. Selectivity

against planktotrophy would be plausible if low produc-

tivity was assumed for the Early Triassic (e.g. Twitchett

2006). Other scenarios propose high Early Triassic pro-

ductivity including eutrophication causing euxinia (Meyer

et al. 2011). The presence of Early Triassic gastropods

with planktotrophic larval development argues against

selectivity and Early Triassic low productivity. However,

only a few larval shells have been reported and those are

of Olenekian age and thus not very close to the Permian–Triassic boundary.

In the Late Triassic, few formations have yielded gas-

tropods with preserved protoconchs. Nearly all Late

Triassic gastropods with documented protoconch come

from the Carnian Cassian Formation (Figs 3F–H, 4A–B,

14K–N, 16). More than 500 named gastropod species

have been reported from this exceptional occurrence, and

I estimate that the protoconch is known for 200 to 300

species. Of those, more than 100 species belong to

derived clades (neritimorphs, caenogastropods, hetero-

branchs; e.g. Bandel, 1991b, 1992, 1995, 2007; N€utzel

1998). Nearly all (>90%, maybe even 100%) of these pro-

toconchs reflect planktotrophic larval development (see

also N€utzel 1998). This reflects the tropical, shallow warm

water habitats of the biota of the Cassian Formation

(N€utzel et al. 2010). This percentage of planktotrophic

species is exceptionally high even when compared with

modern tropical settings. Only a few preserved caenogas-

tropod protoconchs from the Late Triassic of other

regions have been reported: from the Norian of Idaho

(N€utzel and Erwin 2004) and the Carnian of Slovenia

(Kaim et al. 2006).

Jurassic

A large number of gastropods with preserved protoconchs

have been reported from the Jurassic of Germany, Poland,

Russia and England. Data from outside Europe are scarce,

for example from New Zealand (Bandel et al. 2000). Most

Early Jurassic caenogastropods with preserved proto-

conchs have been reported from Germany (Schr€oder

1995; Gr€undel and N€utzel 1998; Gr€undel 1999a, b, 2003a,

2007; Gr€undel et al. 2009; Schulbert and N€utzel 2013).

16 PALAEONTOLOGY

Again, the great majority had planktotrophic larval

development. The same is true for Early Jurassic species

from England (Todd and Munt 2010; Gr€undel et al.

2011) and for the Mid-Jurassic caenogastropod species

from Germany reported by Gr€undel (1999b, 2001, 2003b),

from Poland (e.g. Kaim 2004) and Russia (Guzhov 2004;

Gr€undel 2005). The great majority of Mid- and Late

Jurassic caenogastropod species from Russia with known

protoconchs had planktotrophic larval development

(Guzhov 2002a, b, 2004, 2005, 2006).

Cretaceous and Palaeogene

The great dominance of Jurassic caenogastropod species

with planktotrophic larval development continues in

Early and Mid-Cretaceous faunas from Russia (Guzhov

2004), England (Gault Clay; Tracey 2010) and Poland

(Kaim 2004). Kiel (2006) studied an Albian microgastro-

pod assemblage from Madagascar and noted that species

had planktotrophic larval development (except one coc-

culinid which cannot have planktotrophic larval develop-

ment). Well-preserved protoconchs have been reported

from Late Cretaceous caenogastropods of the US Gulf

Coast (Jablonski 1986; Dockery 1993; Bandel and

Dockery 2012). According to Jablonski (1986), 55 per

cent of the investigated taxa from the US Gulf Coast

which are present both in the Late Cretaceous and in the

Palaeogene, and thus survived the end-Cretaceous mass

extinction, had planktotrophic and 45 per cent had non-

planktotrophic larval development (n = 51). Thus, as in

the end-Permian mass extinction, mode of development

was neutral according to selectivity according to the cur-

rent state of knowledge. Jablonski and Hunt (2006) stated

that geographical range was more important to explain

survivorship, that is, widespread species have a higher

chance of survival. Accordingly geographical range size

was heritable at the species level. According to Hansen

(1980), some Palaeogene neogastropod species with

planktotrophic larval development had wider geographi-

cal ranges; supposedly free gene flow and resistance to

isolation gave species with planktotrophic larval develop-

ment an evolutionary stability which resulted in greater

species longevity. A shift from a strong dominance of

Paleocene neogastropod species with planktotrophic larval

development to species with nonplanktotrophic develop-

ment in the Eocene has been reported from the Gulf

Coast by Hansen (1982).

There is a rich body of literature for post-Palaeogene

gastropods with preserved protoconchs. Future analyses

of ontogenetic patterns are promising, especially because

well-preserved material is much more common than in

the Palaeozoic and Mesozoic. Moreover, many Neogene

gastropod species are known from several occurrences,

A

D E

B C

F IG . 16 . Examples of Late Triassic

caenogastropod protoconchs from

the Cassian Formation (northern

Italy); all were formed by plankto-

trophic larvae. A, Protorcula. B,

Zygopleura. C, Ampezzopleura. D,

Ptychostoma (sensu Bandel 1992). E,

Prostylifer. Scale bars represent

0.5 mm (A) and 0.2 mm (B–E).

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 17

whereas the Palaeozoic and Mesozoic fossil record is

spotty as was outlined previously.

DISCUSSION

It is likely that the discussion of whether planktotrophy

was original in Eumetazoa and Mollusca, or not, will con-

tinue. Judging from the size and shape of the initial parts

of Cambrian univalved molluscs, it is likely that they had

nonplanktotrophic larval development (N€utzel et al. 2006,

2007a). However, Cambrian material with preserved early

ontogenetic shells showing clear ontogenetic boundaries

is needed to corroborate this hypothesis. Ordovician–Silurian specimens (internal moulds) from larval fall and

juvenile gastropod assemblages with small initial

parts, reflecting small amounts of yolk indicate that

planktotrophy arose at the Cambrian–Ordovician transi-

tion, probably as an escape of unprotected hatchlings from

increasing predation pressure in benthic habitats as a con-

sequence of the Ordovician radiation (N€utzel et al. 2006,

2007a). This corroborates one of the hypotheses formu-

lated by Strathmann (1986, 1993) that it is the function of

a larval stage to transform a small hatchling into a larger

juvenile animal and that escape from benthic predation is

generally favourable for unprotected (nonencapsulated)

larvae or hatchlings. On the other hand, planktotrophy

also offered evolutionary opportunities, for example, it is

very plausible that dispersal capacity is another advantage

of planktotrophy (e.g. Jablonski and Lutz 1980, 1983;

Strathmann 1986) and implications for gene flow have

been widely discussed. Besides predation, the evolution of

the meroplankton was certainly also constrained by the

evolution of primary production which was generally

increasing during the Phanerozoic (Bambach 1993; Signor

and Vermeij 1994). Predation was probably not only a

major driving force for the evolution of planktotrophy but

also for the subsequent evolution of planktonic gastropod

larval shells. There is consensus that today’s mortality of

planktonic larvae is very high (e.g. Thorson 1950; Schelte-

ma 1971, 1986; Jablonski and Lutz 1980, 1983; Pechenik

1999; Short et al. 2012). Larval shells, ornaments including

carinations, larval beaks and strengthened sinusigeras as

well as the larval operculum serve as antipredatory armour

(Bandel et al. 1994; Hickman 1999a, b, 2001). Larval orna-

ments could also lower the Reynolds number to prevent or

slow sinking, especially when the velum is retracted.

Relatively large gastropod larvae of certain higher caeno-

gastropods may also have rather complex periostracal

appendages to hinder sinking (Richter 1987; Bandel et al.

1994, 1997). Of course, such appendages do not preserve

in fossil larval shells.

During the Palaeozoic, there is a clear trend against

openly coiled protoconchs, both in gastropods with

planktotrophic and nonplanktotrophic development

(N€utzel and Fr�yda 2003). By the end of the Palaeozoic,

the openly coiled morphology is not present any longer

with a single exception from the Late Triassic Cassian

Formation which is reported herein. The protoconchs of

modern holoplanktonic pteropods may be stretched (Ban-

del and Hemleben 1995), but this is clearly a modern,

derived state not inherited from Palaeozoic gastropods. It

is very plausible to hold increasing predation pressure

responsible for this trend against open coiling because the

openly coiled and sometimes even stretched larval shell

morphology is certainly more vulnerable to predation

than the tightly coiled morphology. The first strongly

ornamented larval shells have been reported from the Late

Palaeozoic indicating a continuing escalation of predation

in the planktic realm. The first reports of carinated larval

shells from the Permian point in the same direction.

However, it must be emphasized that the number of

reported protoconchs from the Early and Mid-Palaeozoic

is still low and new findings may change the picture con-

siderably. The evolution of the modern tightly coiled lar-

val shell which is commonly ornamented, and commonly

resembles the adult shell, was probably the result of a het-

erochronic shift of adult characters onto the larval stage

as suggested by Seuss et al. (2012).

It is still not entirely clear to me why some marine

invertebrate species (including gastropods) have plankto-

trophic larval development, whereas others have non-

planktotrophic larval development. It is beyond the scope

of the present contribution to explore the possible expla-

nations of ontogenetic traits, and there is a rich body of

neontological literature about this issue. However, some

possible adaptational advantages of planktotrophy and

nonplanktotrophy should be mentioned. Both traits may

occur in closely related (commonly congeneric) species

and in species sharing similar habitats. For instance, Mar-

shall (1983) stated that among the 67 Southern Australian

Recent species of the Triphoridae for which he reported

the protoconch, 33 have lecithotrophic development (=nonplanktotrophic) and 34 have planktotrophic develop-

ment. In Triphoridae and Cerithiopsoidea, both highly

diverse groups of small spongivorous gastropods, plankto-

trophic and nonplanktotrophic larval development occur

throughout the family, often in the same genus or species

pair (Marshall 1978, 1983). In triphorids, such species

pairs (one planktotrophic and one nonplanktotrophic)

are separated by the Bass Strait. Marshall (1983) sug-

gested that temperature and nutrients were the governing

oceanographical factors for the evolution of this pattern.

Planktotrophic and nonplanktotrophic development

within the same species (poecilogeny) is very rare in gas-

tropods (Bouchet 1989; Krug 2007) so that the larval trait

is usually diagnostic at the species level. Possible

advantages and disadvantages of ontogenetic traits were

18 PALAEONTOLOGY

summarized by Pechenik (1999): advantages including

avoidance of inbreeding, benthic competition and preda-

tion and disadvantages including high mortality or dis-

persal away from suitable habitats. It seems plausible that

species with feeding larvae have a higher dispersal capac-

ity, a higher gene flow between populations, potentially a

wider geographical range and thus a lower extinction

probability (e.g. Strathmann 1986, Jablonski and Lutz

1980, 1983). However, species with nonplanktotrophic

larval development can also have wide distributions.

Moreover, if nonplanktotrophy was generally disadvanta-

geous, its existence is hard to explain. As outlined previ-

ously by various authors, it is most likely that a trade-off

of fecundity and larval or juvenile mortality connected to

r/K or slow/fast life strategies. Predation, nutrition, tem-

perature and seasonality must be the main parameters.

I estimate that the protoconchs of 1000–2000Palaeozoic and Mesozoic gastropod species have been

reported, certainly a small portion of the known species

from this period of time. Basal clades such as vetigastro-

pods always had nonplanktotrophic larval development.

The majority of species of the other, more derived gastro-

pod clades had planktotrophic larval development, and

most data are known from the Caenogastropoda. This

shows that gastropod veliger larvae have been an impor-

tant part of the zooplankton (meroplankton) since the

Palaeozoic and is also indirect evidence for continuous

primary productivity in the oceans. It is strange that

according to the current state of knowledge, neither the

end-Permian nor the end-Cretaceous mass extinction

selected against planktotrophy, although a productivity

decline has been proposed for both events. In the case of

the end-Permian extinction event, more data (species

with known protoconch) from the Early Triassic are

needed close to the boundary. Larval traits of gastropods

have not been discussed in the context of other big mass

extinction events.

It is almost certainly no coincidence that the molluscan

classes which display by far the highest species diversity

(Gastropoda and Bivalvia; see Appeltans et al. 2012) are

those which may have planktotrophic larval development.

Cephalopods have no true larvae (paralarvae) but are

direct developers. Extremely high diversity in such groups

as the Caenogastropoda or eulamellibranch bivalves is

result of high phylogenetic activity and is associated with

the presence of planktotrophic veliger larvae. Though,

causality has not been shown yet. This association of evo-

lutionary success (manifest as species diversity) and

planktotrophy seems obvious although nonplanktotrophy

must also have evolutionary advantages because this strat-

egy is still widespread in Mollusca including Gastropoda.

Acknowledgements. I thank Christian Klug (Z€urich) and the

Palaeontological Association for inviting me to the symposium

‘Fossilised Ontogenies and Evolution’ held in Z€urich in December

2013. I would like to thank my collaborators Klaus Bandel (Ham-

burg), Jiri Fr�yda (Prag), Joachim Gr€undel (Berlin), Andrzej Kaim

(Warszawa), Christian Klug (Z€urich), B. Marshall (Wellington)

and Barabara Seuss (Erlangen). This paper benefitted from reviews

by Peter Wagner (Washington) and Jiri Fr�yda (Prag). The Deut-

sche Forschungsgemeinschaft (DFG) is acknowledged for financial

support (project numbers NU 96/10-1, NU 96/10-2).

Editor. Andrew Smith

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APPENDIX: DESCRIPTION OF THE FIRSTMESOZOIC GASTROPOD WITH UNCOILEDLARVAL SHELL

SYSTEMATIC PALAEONTOLOGY

This published work and the nomenclatural acts it con-

tains, have been registered in Zoobank: http://zoobank.

org/References/6E9F28B6-1E49-48AA-BF80-440426D52E29

Subclass CAENOGASTROPODA Cox, 1960

Family unknown

ANACHRONISTELLA gen. nov.

LSID. urn:lsid:zoobank.org:act:AFBF6E3F-0F3A-4D49-8CC5-

F0BF9909357F

Derivation of name. The openly coiled protoconch is anachro-

nistic in this Triassic gastropod.

Type species. Anachronistella peterwagneri sp. nov.

Diagnosis. Protoconch widely openly coiled with faint

axial larval ornament and an abrupt transition to the tele-

oconch; teleoconch high-spired with indistinct wavy axial

ribs.

Anachronistella peterwagneri sp. nov.

Figure 14K–N

LSID. urn:lsid:zoobank.org:act:680E3B11-C213-466A-A417-

5CAA855003DC

Derivation of name. After the palaeontologist Peter J. Wagner

(Washington, DC) for his contributions to invertebrate palaeon-

tology and our knowledge of Palaeozoic gastropods.

Holotype. The only specimen is housed in the Bayerische Staats-

sammlung f€ur Pal€aontologie (BSPG), M€unchen under the num-

ber BSPG 2009 III 6.

Diagnosis. As for genus which contains only the type spe-

cies so far.

Type locality. Misurina Skilift, Near Cortina d’Ampezzo, north-

ern Italy (see N€utzel et al. 2010).

24 PALAEONTOLOGY

Type horizon. Late Triassic, Early Carnian, Cassian Formation.

Description. The fragmentary specimen consists of about 2.5

whorls with two protoconch and a half teleoconch whorl. The

specimen is 0.42 mm high and 0.40 mm wide. It has a blunt

apex and is high-spired subsequently so that it has a barrel

shape. The protoconch consists of two whorls and is 0.35 mm

wide and 0.24 mm high; the first whorl is narrowly tube-like,

widely openly coiled and smooth. The initial tip of the proto-

conch is broken off. The earliest preserved part of the shell tube

has a diameter of 70 lm. The second larval whorl is only slightly

wider than the first whorl. It is convex and has a collabral orna-

ment of fine sigmoidal threads. It terminates abruptly in a sig-

moidal opisthocyrt suture. The teleoconch is high-spired with an

indistinct wavy axial ribbing.

Discussion. One may ask whether it is wise to base a new

taxon on a single specimen with only part of the teleoconch

preserved. The answer is that the present specimen is so

characteristic that it can be re-identified easily if a more

complete shell were found. It is also so unusual and remark-

able that a name is desirable to communicate about this spe-

cies and genus. As was discussed above, it represents the

only known Mesozoic gastropod with an uncoiled larval

shell. The protoconch separates it from all other gastropods

described from the Cassian Formation. The small initial part

of the tube-like larval shell (similar to those measured by

N€utzel et al. (2006) from Palaeozoic specimens) and the

abrupt termination of the protoconch suggest that this

species had planktotrophic larval development.

N €UTZEL : LARVAL ECOLOGY IN FOSS IL GASTROPODS 25