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Revue de micropaléontologie 51 (2008) 167–181

Original article

Upper Tremadoc–Lower Arenig? Anisograptid–Dichograptid fauna fromthe Cabitza Formation (Lower Ordovician, SW Sardinia, Italy)

Faune a Anisograptides–Dichograptides du Tremadociensuperieur–Arenigien inferieur ? de la Formation de Cabitza

(Ordovicien inferieur, SW Sardaigne, Italie)

Gian Luigi Pillola a,∗, Sergio Piras a, Enrico Serpagli b

a Dipartimento di Scienze della Terra, Universita di Cagliari, Via Trentino 51, 09127 Cagliari, Italyb Dipartimento del Museo di Paleobiologia e dell’Orto Botanico, Universita di Modena e Reggio Emilia, Via Universita 4, 41100 Modena, Italy

bstract

The discovery of the anisograptid graptolite Araneograptus murrayi (Hall, 1865), the dichograptids Clonograptus (Clonograptus) cf. rigidusHall, 1858) and Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868) and of two unassigned species of Didymograptus in the Lateremadoc–Early Arenig? sediments of the Fluminese area (SW Sardinia) adds new data on the biostratigraphy and palaeoecological setting of theabitza Formation, as well as on the paleobiogeographical distribution of these taxa.2007 Elsevier Masson SAS. All rights reserved.

esume

La decouverte du graptolite Anisograptide Araneograptus murrayi (Hall, 1865), des Dichograptides Clonograptus (Clonograptus) cf. rigidusHall, 1858) et Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868) et de deux especes indeterminees de Didymograptus dans les sediments
u Tremadocien superieur–Arenigien inferieur? du Fluminese (SW Sardaigne) fournit des nouvelles donnees sur le contexte paleoecologique etur la biostratigraphie de la partie superieure de la formation de Cabitza, ainsi que sur la distribution paleobiogeographique de ces taxa.
2007 Elsevier Masson SAS. All rights reserved.

eywords: Graptolites; Caryocarids; Early Ordovician; Cabitza Formation; Sardic phase; Sardinia

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ots cles : Graptolites ; Caryocarides ; Ordovicien inferieur ; Formation de Ca

. Introduction

Graptolites have been known in Lower Ordovician sedimentsf southwestern Sardinia since the first half of the 20th century.ovarese (1920) reported the occurrence of Dictyonema sp. onloose rock specimen found, five years earlier, at Monte Cani

near Gonnesa). In the same year, Taricco (1920) described this
aterial, consisting of several incomplete colonies and, despite
heir fragmentation, assigned them to “Dictyonema flabelliformeichwald, 1840”. The Dictyonema-bearing levels were regarded

∗ Corresponding author.E-mail address: [email protected] (G.L. Pillola).

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035-1598/$ – see front matter © 2007 Elsevier Masson SAS. All rights reserved.oi:10.1016/j.revmic.2007.08.002

; Phase Sarde ; Sardaigne

y Taricco (in Novarese and Taricco, 1922: p. 320) as belongingo the Acadian (sic.) “Gruppo degli Scisti” (Cabitza Formation)nd he ingenuously extended the range of “D. flabelliforme” intohe Middle Cambrian.

Subsequently, the presence of “Dictyonema” in Sardinia wassually rejected, disregarded or questioned (Lecompte, 1949)nd the upper part of the Cabitza Formation continued to be con-idered Middle Cambrian in age (Cocozza, 1967, 1980; Rasetti,972).

More recent research carried out in the Gonnesa region
Monte Cani) led to the discovery of the Rhabdinopora flabel-iformis (Eichwald, 1840) bearing layers (Gandin and Pillola,985; Pillola, 1987; Barca et al., 1987). These in situ specimensere tentatively assigned to the morphotype “graptolithinum”
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r “forma typica” (Pillola and Gutierrez Marco, 1988: p. 559),hich, in turn, was included in the subspecies R. f. flabelliformis

Cooper et al., 1998). Additional poorly preserved specimensere also collected at Case Lai–Case Massidda outcrops, close

o the Cabitza Fm. type section (Loi et al., 1995, 1996) and new,ell-preserved rhabdosomes were recovered in the Monte Cani

rea.The occurrence of acritarchs from similar levels close to

onte Lisau, SSE of Gonnesa, corroborated the Tremadociange of the upper part of the Cabitza Formation (Barca et al.,987); later on, the recovery of Upper Cambrian faunas, includ-ng trilobites, hyolithids, echinoderms, brachiopods (Loi et al.,995, 1996; Pillola et al., 2002a and references therein) andonodonts (Serpagli et al., 1998), suggested the possibility ofcontinuous” sedimentation ranging from the Middle Cambrianp to the Early Tremadocian, including evidence of the Acero-are regressive event across the Cambrian/Ordovician boundaryLoi et al., 1995, 1996).

A stratigraphic sketch of the Cabitza Fm. type section (closeo Iglesias), with the position of the Rhabdinopora beds, wasrovided by Loi et al. (1995, 1996); further details on biostrati-raphic framework have been provided by Pillola et al. (2002bnd cited references).

Such discoveries have also had important tectonic implica-ions because the so-called “Sardic phase” of Stille (1939) wasefinitely considered as intra-Ordovician, located between earlyremadocian and Caradocian (Barca et al., 1987).

In the present paper, we document the first occurrence of anpper Tremadoc-basal Arenig? graptolitic assemblage includ-

ng the Anisograptid Araneograptus murrayi (Hall, 1865) andhe Dichograptids Clonograptus and Didymograptus, associ-ted with caryocarids, in the Early Ordovician sediments of SWardinia. The aims of the study are to use the new graptoliteata to revise and update the age of the upper part of the Cab-tza Formation and to evaluate them from the biostratigraphic,alaeogeographic and palaecological point of view.

. Geological setting, location, palaeontological contentnd preservation

.1. Geological setting and location

The Cabitza Formation occurs in a wide area in the Flumineseubregion, from the Gutturu Pala to the Monte Argentu–Piscina

orta area (Fig. 1). In the latter locality, the northern limb ofhe easternmost portion of the “Cabitzan anticline” is mostlynconformably flanked by the Upper Ordovician deposits of theonte Argentu Fm., which are in turn overlain by the clastic

ocks of the Caradocian and Ashgillian Monte Orri and Por-ixeddu Formations and younger deposits (Hammann and Leone,997).

The deposits of the Cabitza Formation observed along theld road cut connecting the Riu Mannu valley to Buggerru show
evere tectonic disturbances and, except for minor portions,ppears rather monotonous. In spite of this, the four main lithofa-ies recognized in the uppermost Upper Cambrian and Tremadocn the type area of the Cabitza Fm. (Loi et al., 1995, 1996) are
Sat

léontologie 51 (2008) 167–181

bserved thorough the section. Locally, the widespread grey-reen siltstones and shales “graded rhythmites” (Facies 4) andhe rare hummocky cross stratification sandstones (Facies 3)Plate 1, Figs. 1 and 2) are replaced, towards the south, byaminated shales and fine sandstones (Facies 2) and by moreandy siltstones with ripple-drift cross-lamination (Facies 1).eoccurrences of rare sandstone beds, with HCS and pla-ar to weakly inclined laminations and of Facies 4, haveeen observed; in addition, Facies 1 and 2 with red bedsccur once along the transect, very close to the Piscina Mortaond.

Thus, despite the structural complexity of the transect andhe lack of the classical Middle Cambrian deposits, the faciesuccession of this portion of the Cabitza Fm. can be readily com-ared with many Upper Cambrian to Tremadocian successionsf southwestern Sardinia.

The fossiliferous beds crop out along a road cut, 1.25 kmouth of the junction with the Fluminimaggiore–Portixedduoad, not far from the locality “Piscina Morta” (Fig. 1). Thesere mainly composed of grey-green micaceous siltstones andhales “graded rhythmites” (above-mentioned Facies 4), withccasional thin sandy layers and rare, but more conspicuous,–30 cm-thick HCS sandstones beds (Plate 1, Fig. 1), suggest-ng an outer shelf sedimentary environment, not far from thetorm wave-base.

.2. Palaeontological content and preservation

The grey-green siltstones of the Cabitza Formation haveecently yielded several incomplete large rhabdosomes andmmature colonies of a robust anisograptid graptolite assignedo A. murrayi (Hall, 1865), as well as a large incomplete speci-

en of Clonograptus (Clonograptus) cf. multiplex (Nicholson,868), large portion of Clonograptus (Clonograptus) cf. rigidusHall, 1858) and several didymograptid colonies. CaryocaridCaryocaris cf. wrighti Salter, 1863) remains (Plate 1, Figs. 3nd 4) and few ichnofossils, which occur at different levels alonghe section, are being described in a separate paper.

Rhabdosomes are slightly flattened and tectonicallyeformed and are of Fe-oxides and/or black carbon; rare distalwisted portions of stipes are preserved as a faint carbon film.

Caryocarids occur as flattened, wrinkled carapaces and areostly preserved as very thin grey shiny imprints on the bedding

lane surfaces; occasionally, less deformed specimens showlackish carapaces (Plate 1, Fig. 3).

Preparation of the specimens involved mechanical removalf overlying matrix, by means of a vibro-tool, gentle chemicalissolution with hydrofluoric acid and final impregnation witharaloid resin. Photographs were taken under alcohol or waternd drawings were made via camera lucida.

. Systematic palaeontology

All studied specimens are housed in the Department of Earthciences, Cagliari University, Italy and are labelled with thecronym: DSTC. For abbreviations, we use the 2TRD (twohecae repeat distance).

G.L

.Pillola

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icropaléontologie51

(2008)167–181

169

Fig. 1. Geological sketch map of the Fluminimaggiore–Piscina Morta area. Fossiliferous locality described in this paper is indicated by asterisk (after Hammann and Leone, 1997, simplified).Fig. 1. Carte geologique schematique de la region de Fluminimaggiore–Piscina Morta. La localite fossilifere decrite dans ce travail est indiquee par un asterisque (d’apres Hammann et Leone, 1997, simplifiee).

170 G.L. Pillola et al. / Revue de micropaléontologie 51 (2008) 167–181

Plate 1. Fig. 1. Fossiliferous outcrop along the Piscina Morta (Fluminimaggiore) road cut section. The graptolite and caryocaridids level is located about 3.5 mtowards the right from the sandstone bed (HCS) indicated by arrow. Fig. 2. Oblique view of the sample figured on Plate 2, Fig. 1, showing the most common featuresof the fossiliferous beds represented by graded rhythmites. Figs. 3 and 4. Caryocaris cf. wrighti. Late Tremadoc/? Earliest Arenig, Cabitza Fm., Piscina Morta(Fluminimaggiore) SW Sardinia. 3, DSTCP 23180 lateral view. 4, strongly compressed, cleaved and wrinkled randomly disposed carapaces, sample DSTCP 23181.

G.L. Pillola et al. / Revue de micropaléontologie 51 (2008) 167–181 171

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ig. 2. (A) Proximal part of A. murrayi (Hall, 1865), cleavage-trace is indica. murrayi, slab DSTCP 23177.ig. 2. (A) Partie proximale de A. murrayi (Hall, 1865), la trace du clivage estartie apicale de A. murrayi, echantillon DSTCP 23177.

Order GRAPTOLOIDEA Lapworth, 1875Family ANISOGRAPTIDAE Bulman, 1950Genus Araneograptus Erdtmann and Vandenberg, 1985Araneograptus murrayi (Hall, 1865) (Fig. 2A and B; Plate 2,

igs. 1–4 and Plate 3, Fig. 9)

1865. Dictyonema Murrayi Hall, p. 138, Pl. 20, Figs. 6nd 7.

1987. A. murrayi (J. Hall, 1865) – Gutierrez-Marco andcenolaza, p. 225, Pl. 1, Figs. 1–17 (with earlier synonymies).1991. A. murrayi (J. Hall, 1865) – Lindholm, p. 294, Figs. 6

nd 7,? 18c (with further synonymies).1994.? Araneograptus cf. murrayi (J. Hall) – Heuse et al.,

. 64, Pl. 9, Fig. 1.1994. A. murrayi (J. Hall, 1865) – Ortega and Suarez Soruco,

994 p. 230, Text-Fig. 3i, Pl. 2, Figs. 1 and 6.
2001. A. murrayi (J. Hall, 1865) – Maletz and Egenhoff,
. 58, Fig. 8.10.v 2004. R. flabelliformis norvegica (Kjerulf, 1865) – Piras et

l., 2004 p. 46.

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igs. 5 and 6. R. f. flabelliformis (Eichwald, 1840), Monte Cani, Cabitza Fm., GonnSTCP 21004a. 6, exceptionally well-preserved proximal portion of the rhabdosomeema.lanche 1 Fig. 1. Gisement fossilifere sur le talus de la route pour Piscina Morta (Flunvirons 3,5 m a droite du banc de gres (HCS) indique par la fleche. Fig. 2. Coupe oommun des niveaux fossiliferes, representes par les rythmites granoclasses. Figs. 3asal, Fm. de Cabitza, Piscina Morta (Fluminimaggiore) SW Sardaigne. 3, vue laterale

´chantillon DSTCP 23181. Figs. 5 et 6. R. f. flabelliformis (Eichwald, 1840), Monte Ctades astogenetiques, DSTCP 21004a. 6, partie proximale particulierement bien con’un nema robuste.

y parallel black lines, sample DSTCP 23176; (B) detail of the apical part of

uee par les deux lignes paralleles, echantillon DSTCP 23176 ; (B) detail de la

Material: Three large incomplete rhabdosomes DSTCP31161/62 and 23179, 10 fragments DSTCP 23163–23170,3178 and two early stages of juvenile colonies provisionallyssigned to this species DSTCP 23176/77; all specimens origi-ate from Piscina Morta, Fluminimaggiore.

Diagnosis: [after Gutierrez Marco and Acenolaza (1987) andindholm (1991)] – Large conical pendent rhabdosome, withn angle of 60–75◦ when flattened, decreasing distally in bigolonies. The rhabdosome may reach at least 30 cm in length.he shape of the meshes is variable from rectangular to oval.he stipes are generically 4–5 in 10 mm and the dissepimentsre 3–4 in 10 mm; however, their density can be well outsidehese ranges (cf. Lindholm, 1991). Lateral width of the stipesange from 0.5 to 1.25 mm. The thickness of the dissepiments isariable. The sicula is 1.8–2.5 mm long, bearing a short nema.ortical overgrowth is frequent in the proximal portion of the

habdosome.Description: The largest rhabdosome (Plate 2, Figs. 1a–c),

lthough incomplete, suggests that the total length exceeded ateast 15 cm, confirming the usual large size of this anisograptid.

esa, Early Tremadoc. 5, Rhabdosomes at different astogenetic stages, sampleDSTCP 23487a; note the full relief of the thecae and the occurrence of a robust

minimaggiore). Le niveau fossilifere a graptolites et caryocaridides est situe ablique de l’echantillon figure sur la Planche 2, Fig. 1, montrant l’aspect le pluset 4. Caryocaris cf. wrighti Salter, 1863, Tremadocien superieur/? Arenigien, DSTCP 23180. 4, carapaces comprimees et froncees, disposees aleatoirement,ani, Cabitza Fm., Gonnesa, Tremadocien inferieur. 5, rhabdosomes a differentsservee, rhabdosome DSTCP 23487a ; noter le relief des theques et la presence


Plate 2. Figs. 1–4. A. murrayi (Hall, 1865), Late Tremadoc/? Earliest Arenig, Cabitza Fm., Piscina Morta (Fluminimaggiore), SW Sardinia. 1a, largest incompleterhabdosome, associated with high order branches of Clonograptus (Clonograptus) cf. multiplex; DSTCP 23161b. 1b, counterpart, associated with Clonograptus(Clonograptus) cf. multiplex; DSTCP 23161a. 1c, detail of the mesh pattern, DSTCP 23161a. 2a, accumulation of rhabdosomes at different astogenetic stagesand several fragments; note the juvenile stage at the lower right and the twisted stipes of the larger rhabdosome; sample DSTCP 23163a. 2b, detail showing the laterally

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he mesh pattern is characterised by a quite regular distributionf stipes and dissepiments giving rise to sub-rounded fenestrulaeith length/width ratio mostly close to one. Meshwork gener-

lly possesses five stipes in 10 mm in the proximal part, fourn the distal part of the rhabdosome. The dissepiments are con-tantly 3–4 in 10 mm. The lateral stipe width ranges from 0.5 to.25 mm and the thickness of the dissepiments varies between.5 and 1.4 mm. The thecae are 12 in 10 mm and seems to beimple straight tubes with dichograptid appearance. Sometimes,ue in part to tectonic distortion, the apertural area can simu-ate a denticulate shape in lateral view (Plate 2, Figs. 2a and b).ithecae or sicular bithecae have not been observed. Dissepi-ents that join the stipes to their dorsal side on large portions

f the specimen (Plate 2, Figs. 4a and b) show the characteristicorphology that are reminiscent of the famous Gaudı balconies

pouch-like “balconies”, sensu Lindholm, 1991). Locally, theseissepiments appear darker in comparison with adjacent stipeshich may reflect a primary structural/compositional differen-

iation. The oblique dissepiments are better preserved on coarseatrix, where flattening is less pronounced than in other colonies

tudied here.Finally, the observed siculae in the specimens assigned to

uvenile forms of A. murrayi, are straight, of about 2.2 mm long,ith a short thin nema (Fig. 2 and Plate 2, Fig. 3).Discussion: There is some doubt about the assignment of

his taxon to Araneograptus, but derives in part from theuspected occurrence of bithecae and from the inferred bi-r triradiality of the rhabdosome (see Lindholm, 1991: p.94, 295). The occurrence of three-vaned nemal structures inome colonies of A. murrayi, as observed by Acenolaza etl. (1996), may support a possible synonymy between Ara-eograptus and Rhabdinopora. The early astogenetic stagestudied here are suggestive of a primary biradiality (Fig. 2nd Plate 2, Figs. 2a and 3); however, the taphonomical effectf the mm thick accumulations as well as strong tectoniceformation, influence the preservation of the specimens. Inbsence of completely preserved examples, we are thereforebliged to maintain a doubt about the bi - or triradiality of thisaxon.

Despite this uncertainty, the Sardinian rhabdosomes do notiffer significantly from those assigned to A. murrayi in theossession of short meshes and relatively broad dissepiments,uch as the quadrangularis type specimen figured by Lindholm1991: Fig. 6) from Levis, Quebec, which represents the type
rea of this taxon (Hall, 1865: Pl. 20, Figs. 6 and 7).
In some Scanian specimens, the density of stipes and dissepi-ents have a lower per unit length (Lindholm, 1991), while the

ragmentary Thuringian specimen from the Schwarzburg anti-

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ompressed stipes with evidence of the thecal morphology. 3, Early astogenetic stage,f the meshwork pattern with the peculiar “Gaudı balconies” dissepiments DSTCP 2lanche 2. Figs. 1–4. A. murrayi (Hall, 1865), Tremadocien superieur/? Arenigienhabdosome incomplet et de grande taille, associe avec des branches d’ordre eleveontre-empreinte de la meme colonie, associee avec Clonograptus (Clonograptus)3161a. 2a, accumulation de rhabdosomes a differents stades astogenetiques et plusieateralement, du rhabdosome de plus grande taille, DSTCP 23163a. 2b, stipes comp3163a. 3, stade astogenetique precoce, DSTCP 23176. 4a, vaste portion d’une coloissepiments en « balcon de Gaudı » DSTCP 23162a.

léontologie 51 (2008) 167–181 173

line (Heuse et al., 1994), showing narrow dissepiments andobust stipes, are clearly affected by tectonical compression.ery similar measurements are shared by material from Sardinia,uelva province of Spain and from South America (Argentina

nd Bolivia) (Gutierrez Marco and Acenolaza, 1987: Pl. 1, Figs.–17; Acenolaza et al., 1996: Pl. 1, Figs. 1–12; Maletz andgenhoff, 2001: Fig. 8.10).

Cortical overgrowth of the dissepiments, placed obliquelyo the dorsal plane of the stipes (Gaudı balconies), is a notice-ble feature in many specimens of A. murrayi. According tocenolaza et al. (1996), this was supposed to improve the water

irculation through the rhabdosome for feeding and stabilizationurposes.

Distribution: A. murrayi has a worldwide distribution,ncluding Europe (Scandinavia, Sweden, Great Britain, Ger-

any, southern France, Spain and now Sardinia), North AfricaMorocco, Mauritania and? Algeria), eastern North AmericaQuebec, Canada), South America (Bolivia, Argentina) andW China. In addition, its biogeographic distribution can be

xtended to Australasia and western Canada if A. pulchellus isonsidered a synonym of A. murrayi as suggested by Gutierrezarco and Acenolaza (1987).Age: A. murrayi range from the Late Tremadoc (A. murrayi

one and Hunnegraptus copiosus zone) to the base of therenig (Tetragraptus phyllograptoides/Tetragraptus approxi-atus zone), equivalent to the Australasian stage La2 and basalart of La3.

Suborder DICHOGRAPTINA Lapworth, 1873Family DICHOGRAPTIDAE Lapworth, 1873Genus Clonograptus Nicholson, 1873Subgenus Clonograptus (Clonograptus) Nicholson, 1873Clonograptus (Clonograptus). cf. rigidus (Hall, 1858)

Fig. 3B and Plate 3, Figs. 1a and b)

cf. 1858. Graptolithus rigidus n. sp. J. Hall, pp. 121–22.l. 11, Figs. 1–5.

cf. 1989. C. rigidus (J. Hall) – Lindholm and Maletz, p. 720,ext-Figs. 2c–e; p. 721, Text-Figs. 3, 4g–j; p. 722, Text-figs. 5and b.

? 2003. Clonograptus cf. C. rigidus (J. Hall) – Jackson andenz, p. 152, Fig. 12f.

Material: One incomplete specimen on the slab DSTCP
3172a and counterpart DSTCP 23172b from Piscina Morta,luminimaggiore.
Diagnosis: A narrow-stiped clonograptid, with several orderf stipes, with somewhat equal thickness throughout of the

DSTCP 23176. 4a, large portion of a mature colony DSTCP 23162a. 4b, detail3162a.basal, Fm. de Cabitza, Piscina Morta (Fluminimaggiore), SW Sardaigne. 1a,(? 4–6e) de Clonograptus (Clonograptus) cf. multiplex ; DSTCP 23161b. 1b,cf. multiplex ; DSTCP 23161a. 1c, detail de la maille tres reguliere, DSTCPurs fragments ; noter le stade juvenile en bas a droite et les stipes, comprimees

rimees lateralement montrant le detail de la morphologie des theques. DSTCPnie mature, DSTCP 23162a. 4b, detail de la morphologie de la maille avec les


Plate 3. Graptolites of the Late Tremadoc/? Earliest Arenig, Cabitza Fm., Piscina Morta (Fluminimaggiore), SW Sardinia. Fig. 1. Clonograptus (Clonograp-tus) cf. rigidus (Hall, 1858). 1a, Rhabdosome showing overlapping branches and stipes DSTCP 23172a. 1b, counterpart DSTCP 23172b. Figs. 2 and 3.Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868). 2, fragment of the high order branches preserved on the slab 23161b. 3, incomplete rhabdosome asso-ciated with A. murrayi, DSTCP 23161a. Figs. 4 and 5 “Didymograptus” sp. 2. 4, cleaved rhabdosome, DSTCP 23173. 5, detail of thecal morphology, DSTCP 23175b.


Fig. 3. (A) Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868), composite reconstruction after counterparts DSTCP 23171a-b; (B) Clonograptus (Clono-gF ructio( 2317

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raptus) cf. rigidus (Hall, 1865), drown after the sample DSTCP 23172a.ig. 3. (A) Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868), reconstClonograptus) cf. rigidus (Hall, 1865), dessine d’apres le rhabdosome DSTCP

habdosome. Thecae are seldom visible, about 10–14 in 10 mm.ortical thickening present.

Description: Rhabdosome biradial, large, horizontal, withixth order of branching and an extrapolated diameter of about00 mm. The first order branches make a 6-mm-long funicle, but
he sicula is not visible. The second order branches is 2.5–3.5-
m-long and the third is about 6-mm-long. The stipes arehickened by cortical tissue and, in the proximal part, measure

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igs. 6–8, 10. “Didymograptus” sp. 1. 6, poorly preserved accumulation of severastogenetic stage, slab 23175. 10, strongly deformed rhabdosome, slab 23175a. Fig.lanche 3. Graptolites du Tremadocien superieur/? Arenigien basal, Fm. de CabiClonograptus). cf. rigidus (Hall, 1858). 1a, rhabdosome montrant la superposition digs. 2 and 3. Clonograptus (Clonograptus) cf. multiplex (Nicholson, 1868). 2, frassocie avec A. murrayi, DSTCP 23161a. Fig. 4 et 5 « Didymograptus » sp. 2. 4, rhab

´chantillon DSTCP 23175b. Figs. 6–8, 10. “Didymograptus” sp. 1. 6, accumulation de´chantillon 23175. 8, stade astogenetique juvenile, echantillon 23175. 10, rhabdosostogenetique juvenile DSTCP 23177.

n d’apres les contreparties de l’echantillon DSTCP 23171a-b ; (B) Clonograptus2a.

bout 1 mm, the thickness decreasing up to 0.5 mm in the higherrder stipes. Thecae are rarely visible on the sixth order twistedtipes (Fig. 3B and Plate 3, Figs. 1a and b); their shape, in com-ressed lateral view, is that of straight tubes, with occasionallyentrally prolonged apertures. Thecal inclination to the axis of
he stipes is 15–20◦. Thecal length is 1.4–1.8 mm and the widtharies between 0.6 and 0.9 mm. Overlap is one half or less andhe 2TRD ranges from 1.25 to 1.4 mm (10–14 in 10 mm).
l rhabdosomes, slab 23175a. 7, Early astogenetic stage, slab 23175. 8, Early9. ? A. murrayi (Hall, 1865), early astogenetic stage, DSTCP 23177.tza, Piscina Morta (Fluminimaggiore), SW Sardaigne. Fig. 1. Clonograptuse branches et de stipes, DSTCP 23172a. 1b, contre-empreinte DSTCP 23172b.gment de branches d’ordre eleve (? 4–6e), 23161b. 3, rhabdosome incompletdosome endommage, DSTCP 23173. 5, detail de la morphologie des theques,plusieurs rhabdosomes, mal conserves, 23175a. 7, stade astogenetique juvenile,me tres deforme, echantillon 23175a. Fig. 9. ? A. murrayi (Hall, 1865), stade

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Discussion: The aspect of the rhabdosome and the degree ofranching of the specimen from Piscina Morta is comparableo those figured by Hall (1865) from Levis (Quebec, Canada);n particular with the Figs. 1 and 2 of Pl. 11. The coloniesf C. rigidus studied by Lindholm and Maletz (1989) fromuebec (including the material studied by Hall), do not differ

ignificantly from the Sardinian specimen, except for its thickertipes and slightly wider thecae. Clonograptus cf. C. rigidusrom Yukon Territory of Canada, described by Jackson andenz (2003), differs only in having a shorter funicle. Although

he differences with the type material are negligible, the lackf completely preserved Sardinian specimens prevents positivedentification.

Occurrence: Upper Cabitza Fm., Late Tremadoc–? basalrenig.

Clonograptus (Clonograptus) cf. multiplex (Nicholson,868) (Fig. 3A and Plate 3, Figs. 2 and 3)

cf. 1989. C. multiplex (Nicholson) – Lindholm and Maletz,. 727, Pl. 83, Figs. 1–6; p. 729, Text-figs. 7a and b.

cf. 1997. C. multiplex (Nicholson) – Toro, p. 397, Pl. 1,ig. 1.

? 2003. Clonograptus (Clonograptus) cf. multiplex (Nichol-on) – Jackson and Lenz, p. 152, Figs. 10a, c, d.

Material: One incomplete specimen on the slab DSTCP3171a and some fragments in the counterpart DSTCP 23171b
rom Piscina Morta, Fluminimaggiore.
Description: Rhabdosome large, horizontal, rigid, diameterf about 15 cm, at least four orders of branching. Funicle notisible, length of the second order stipes is variable between

ig. 4. (A) “Didymograptus” sp. 1, rhabdosome on the slab DSTCP 23175; (B) “Didymp. 2, after the sample DSTCP 23173.ig. 4. (A) « Didymograptus » sp. 1, rhabdosome sur l’echantillon DSTCP 23175 ; (BDidymograptus » sp. 2, d’apres l’echantillon DSTCP 23173.

léontologie 51 (2008) 167–181

7–20 mm, of the successive orders is 15–25 mm, 20–24 mm,2–20 mm, respectively. The stipes are of normal dichograptidppearance and apparently without bithecae (Fig. 3A and Plate, Figs. 2 and 3). Stipe width ranges between 1.0 and 1.4 mmn the second order stipe, 0.5–0.6 mm or less in higher orders.he branching angle of the second dichotomy, is 80–90◦, at the

hird order is 45–60◦ and for the fourth and the fifth orders isbout 60◦. Thecae is not visible, probably due to the corticalhickening.

Discussion: The material from England and Scandinaviaescribed by Lindholm and Maletz (1989) is closely similar tohe Piscina Morta specimen, for example the same length andidth of the stipes and angle of branching. The asymmetrical

ength of the stipes, also observed in the Sardinian rhabdosomeFig. 3A and Plate 3, Figs. 2 and 3), is considered by Lindholmnd Maletz (1989) as a peculiarity of Clonograptus multiplex.he specimen described and figured by Jackson and Lenz (2003)s Clonograptus cf. multiplex from Yukon Territories of Canada,as a similar dimension as our specimen, but the Sardinianolony possesses more slender stipes. C. multiplex figured byoro (1997: Fig. 1.1) from Argentina has the same width oftipes and a similar angle of branching; however, the materialrom the Acoite Fm. shows a more regular length of stipes afterach dichotomy. The absence of thecal parameters on the incom-lete Sardinian specimen, do not permit any comparisons withther species.


Subfamily DICHOGRAPTINAE Lapworth, 1873Genus Didymograptus McCoy, 1851

ograptus” sp. 1, rhabdosome on the slab DSTCP 23175; (C) “Didymograptus”

) « Didymograptus » sp. 1, rhabdosome sur l’echantillon DSTCP 23175 ; (C)

cropa

6

M

1

M

dTt

dtntwc1o32T1

saAipsaoiKdds(s

A

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2

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Subgenus Didymograptus (Didymograptus) McCoy, 1851“Didymograptus” sp. 1 (Fig. 4A and B and Plate 3, Figs.

–8, 10)

cf. 1988. Didymograptus cf. sinensis Lee and Chen –olyneux and Rushton, p. 66, Figs. 9a and b.? 1991. Didymograptus sp. 1 – Lindholm, p. 314, Text-figs.

4a and b.Material: Eight specimens from slab DSTCP 23175, Piscina

orta, Fluminimaggiore.Diagnosis: Rhabdosome of didymograptid type, with two

eclined or deflexed stipes, sicular length of about 1–1.25 mm.hecae are straight tubes inclined of about 25–30◦ to the axis of

he stipe. Thecae are 10–12 in 10 mm.Description: The rhabdosome is slightly declined or

eflexed, with a stipe divergence angle of 100–145◦. Due tohe flattening of all the specimens, it is not possible to recog-ize detail of the proximal part of the rhabdosome, especiallyhe possible presence of bithecae along the stipes. The stipesiden from about 0.53 proximally, reach 0.7 mm and remain

ostantly 0.7 mm in the distal part. The sicula is straight, about–1.25-mm-long and 0.3–0.35 wide at the aperture; presencef a little nema of about 0.35 mm (Fig. 4A and B and Plate, Figs. 6–8, 10). The thecae are straight tubes, inclined of5–30◦ to the axis of the stipe. The overlapping is about 1/4.hecae 10–12 in 10 mm. The 2TRD2 is 1.6 mm, 2TRD5 is.95 mm.

Discussion: The general features of the studied rhabdo-omes (including the Didymograptus sp. 2 described below)re common in didymograptids from the upper part of therenig and from the lower part of the Llanvirn, but are rare

n the Upper Tremadoc, where there are few species com-arable to the Sardinian material. The assignment of thesepecimens to Didymograptus is based, among others, on thepparent lack of bithecae along the stipes; in fact, the absencef bithecae is a distinctive feature of early Didymograptusn contrast to similar rhabdosomes retaining bithecae (e.g.,iaerograptus Spjeldnaes, 1963 and allied taxa). The Sar-inian species is very close to Didymograptus cf. sinensis,escribed by Molyneux and Rushton (1988) from England; aimilar form described as Didymograptus sp. 1 by Lindholm1991) has a larger sicula in comparison with the Sardinianpecimens.


“Didymograptus” sp. 2 (Fig. 4C and Plate 3, Figs. 4 and 5)

cf. 1988. Didymograptus sp. – Molyneux and Rushton, p. 66,ig. 9c.

Material: Two specimens from the slabs DSTCP 23173 and3174, Piscina Morta, Fluminimaggiore.

Diagnosis: Rhabdosome of didymograptid type, with twoeclined stipes, sicular length of 1.5–1.65 mm. Thecae aretraight tubes inclined of about 30◦ to the axis of the stipe. Thecaere 14 in 10 mm.

Tncb

léontologie 51 (2008) 167–181 177

Description: The rhabdosome is declined, with a stipe diver-ence angle of 60◦. Due to the flattening of the specimens, its not possible to recognize details of the proximal part of thehabdosome, nor the possible presence of bithecae along thetipes. The stipes widens from 0.8 mm proximally to 1 mm athe third thecae, then width remains constant in the distal partf the rhabdosome. The sicula (Fig. 4C and Plate 3, Figs. 4 and) is straight, 1.5–1.6 mm long and the aperture is 0.25–0.35ide; a short nema about 0.17 mm long. The thecae are straight

ubes ventrally prolonged, inclined of 25–30◦, the overlappings about 1/4. The thecae are 14 in 10 mm. The 2TRD2 is 1.4 mm,TRD5 is 1.6 mm, distally is about 1.6 mm.

Discussion: The closest taxon comparable with the Sar-inian Didymograptus sp. 2 is represented by Didymograptusp. described by Molyneux and Rushton (1988) from the Lateremadoc–Early Arenig Watch Hill Grits of Bitter Beck 3

ocality (Lake District, England). The specimens studied hereiffer only in having a shorter sicula; however, the discrep-ncy observed in the sicular size may also be related to tectoniceformation.


. Age and palaeoenvironments of the upper Cabitzaormation

.1. Age

In the upper part of the Cabitza Fm., two biostratigraphicallyignificant levels can be distinguished: R. f. flabelliformis and. murrayi levels.

The epipelagic graptolite R. f. flabelliformis is an Earlyremadoc taxon distributed in all biofacies, ranging from

he base of Anisograptus matanensis zone, through the R. f.nglica zone and extending into the Adelograptus tenellusone (see Fig. 5). Other fossil remains from these lev-ls comprise acritarchs, rare Proteuloma geinitzi (Barrande,868) echinoderms plates, brachiopods (unassigned lingulates)nd ichnofossils (Tomaculum problematicum Groom, 1902,lanolites s.s. pp., Cruziana cf. bagnolensis Moriere, 1878).critarchs from Monte Lisau (close to the R. f. flabelliformis

evel of Monte Cani, Gonnesa) include, among others, Acantho-iacrodium angustum (Downie) Combaz, 1967, Cymatiogaleauvillieri (Deunff) Deunff, 1964, Dactylofusa squama (Deunff)ombaz et al., 1967 (Barca et al., 1987); this microflora is indica-

ive of a general Early Tremadocian (zones d and e of Martin,982).

The second level is characterised by A. murrayi, a taxonhat appears in the Late Tremadoc (A. murrayi zone and Hun-egraptus copiosus zone) and ranges into the base of therenig (Tetragraptus phyllograptoides/Tetragraptus approxi-atus l.c. zones). The same range is demonstrated for C.ultiplex (Nicholson, 1868) and C. rigidus (Hall, 1858) (Fig. 5).
hus, even if the Sardinian clonograptids, now with tentativeomenclature, are positively identified at the species level, alear differentiation between Late Tremadoc and earliest Arenig,ased on these three taxa, would not be possible. Finally, appar-


Fig. 5. Correlation chart for the Tremadoc and Early Arenig of Scandinavia, Eastern North America South America (Bolivia) and Australasia (after Egenhoff et al.,2004; Cooper, 1999; Maletz and Egenhoff, 2001) and SW Sardinia (this paper). On right column is shown the range of the most significant taxa discussed in thispaper; La = Lancefieldian; Be = Bendigonian.F e Scan( de lc Be = B

eaf

4

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ttaci

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ig. 5. Schema de correlation pour le Tremadocien et l’Arenigien inferieur dd’apres Egenhoff et al., 2004; Cooper, 1999; Maletz et Egenhoff, 2001) et SWites dans ce travail est representee dans la colonne droite. La = Lancefieldien ;

nt didymograptids, present in the upper Cabitza Fm. biota,ssociated with Caryocaris cf. wrighti and unassigned ichno-ossils, are of little use in resolving this problem.

.2. Depositional environments

The palaeoenvironments of the Cabitza Fm., formerly con-idered as shallow open-shelf or mainly distal turbidite depositsRasetti, 1972; Cocozza, 1980), have recently been more pre-isely defined following examination of a composite sectionropping out in the type area (Loi et al., 1996) and in severalreas within the Sulcis and Iglesiente subregions (SW Sardinia)Fig. 6). It is now more clear that the Cabitzan successionegan with deep shelf siliciclastics deposited during the Mid-le Cambrian (CAB 1–4a local informal zones) followed byapid shallowing above the uppermost “Paradoxides beds” tohe Upper Cambrian (CAB 4b, 5a and 5b), as displayed by
iagnostic features of deposition on shoreface and delta tide-ominated environments. A rapid deepening (mid-distal upperffshore) followed these shallow Furongian deposits and, imme-iately above, the HCS “Oryctoconus beds” accumulated on
si

n

dinavie, Nord-Ouest de l’Amerique, Amerique du Sud (Bolivie) Australasiea Sardaigne (ce travail). La distribution verticale des taxa les plus significatifsendigonien.

he shoreface at the Cambrian–Ordovician boundary (peak ofhe ARE event). The lower part of the Cabitza Fm. reaching

thickness of up to 180 m, is overlain by the Early Ordovi-ian “monotonous” upper portion representing the stratigraphicnterval of this study.

The upper part of the Cabitza Fm. has been poorly investi-ated, both from palaeontological and sedimentological pointf view; the thickness is still unknown but, in all likelihood,xceeds 200 m. The succession is remarkably monotonouslyniform and the structural setting adds further complications.n this context, the rare palaeobiological information assumesigh values for age bracketing and for palaeoenvironmentalnterpretations. Laminated siltstones and shales and subordinateandstones largely dominate the succession; sporadically morer less weathered ellipsoidal carbonatic Fe-rich concretions (upo 50 × 20 cm) and extremely rare thin calcareous beds haveeen found. Graded rhythmites and laminated sandy-micaceous
iltstones and shales represent the overwhelming facies that isndicative of low energy environments below storm wave action.
Palaeontological data supports the suggestion of the conti-uity of deep water environments through the upper Cabitza


Fig. 6. Schematic log of the autochtonous late–Early Cambrian to Ordovician succession of SW Sardinia. CP1, CP2: informal biozonation of the Campo PisanoFm.; CAB 1 to CAB 6: informal biozonation of the Cabitza Fm.(after Pillola et al., 2002a, 2002b, modified and updated).Fig. 6. Log schematique de la succession autochtone du Cambrien inferieur sommital a Ordovicien du SW de la Sardaigne. CP1, CP2 : biozonation informelle de laFormation de Campo Pisano ; CAB 1 a CAB 6 : biozonation informelle de la Formation de Cabitza (d’apres Pillola et al., 2002a, 2002b, modifie et mis a jour).

1 cropa

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ormation; furthermore, the environmental settings for the tworaptolitic associations discussed above are in agreement withhe ecostratigraphic model proposed by Cooper (1999). In fact,f the Sardinian outcrops of Early Tremadoc age, only three yieldhabdinopora; two of them are composed of deposits belonging

o the “Facies 4” described by Loi et al. (1996), indicating depo-ition in a distal upper offshore, while in the third (Case Laiutcrops, close to Cabitza), the R. flabelliformis-bearing levels represented by red beds with indications of possible shallowater deposition (this latter occurrence may represent a peculiar

ase in its usual distributional pattern).The present Piscina Morta findings also suggest an outer

helf depositional environment, dominated by A. murrayi andaryocarids, with subordinate clonograptids and didymograp-ids. Usually caryocarids are very common in outer shelf or slopeeposits and are almost invariably associated with graptolites;heir massive occurrence provides valuable indications of theepositional setting and bathymetry (Vannier et al., 2003).

In all likelihood, the Sardinian Araneograptus levels accu-ulated during the transgression that followed the Ceratopyge

egressive event (CRE), a time when graptolites became abun-ant and taxonomically diverse, especially in deep watereposits (Cooper, 1999). Similar graptolitic accumulations inuter shelf environments around Gondwana have been inter-reted as a displacement of mesopelagic faunas outside theirriginal biotope, triggered by upwelling phenomena and, inome cases, concomitant with massive death caused by an oxy-en deficiency in the water masses (see Gutierrez Marco andcenolaza, 1987; Cooper, 1999 and references therein).

. Conclusions

The discovery of the A. murrayi-dominated levels in the upperpart of the Cabitza Fm. permits correlation with Hunnebergian(Late Tremadoc–earliest Arenig) and implies that there was“continuous” sedimentation through the Tremadocian of SWSardinia. Structural complexity makes stratigraphical stud-ies difficult and useful biostratigraphical data are very few;however, the complete Tremadocian is documented by:◦ the occurrence of the “Oryctoconus beds” at the Cam-

brian/Ordovician boundary;◦ the presence of trilobite P. geinitzi and of the graptolite R.

f. flabelliformis in the lower part of the sequence;◦ the presence of sparse but chronostratigraphically signifi-

cant acritarchs communities indicating an undifferentiatedTremadocian age;

◦ the Late Tremadoc–Early Arenig? graptolitic associationdescribed herein in the upper Cabitza Fm.

Environmental conditions at that time in SW Sardinia seem tohave been rather monotonous, oscillating between shorefaceand mainly outer shelf siliciclastic platform.Palaeoecological information indicated by the accumula-
tion of an epipelagic-mesopelagic a caryocarid/graptoliticassociation, strongly supports the outer shelf depositionalenvironments inferred for the bulk of the upper portion ofthe Cabitza Formation.
E

E

léontologie 51 (2008) 167–181

Until now, there was no clear evidence of shallow waterdeposits prior to the emersion related to the so-called Sardictectonic phase, nor of major eustatic events in the uppermostCabitzan deposits.

cknowledgments

We are deeply grateful to Mr. Luciano Menghi, Fluminimag-iore, who kindly supplied the specimens DSTCP 23161 andSTCP 23172 and to Prof. Annalisa Ferretti (Modena Univer-

ity) for a critical reading of the manuscript. Prof. Alfredo LoiCagliari University) gave us useful suggestions. Prof. B.-D.rdtmann (Technical University, Berlin) and J.C. Gutierrez-arco (University of Madrid) are deeply acknowledged for

indly supplying the papers on the topic.We are indebted to J. Maletz and A.C. Lenz, for the critical

eview of the manuscript and the improvement of the text.This research was supported by MIUR grants ex 60% G.L.

illola “Paleobiodiversita: applicazioni nella ricerca di base eella valorizzazione dei beni culturali geo-paleontologici” andRIN A. Ferretti “L’inizio dopo la fine: comparse e riprese evo-

utive durante e dopo l’estinzione di massa del tardo Ordovicianoel Nord-Gondwana”. This paper is a contribution to the IGCroject 503 “Ordovician palaeogeography and palaeoclimate”.

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