Fossils & Evolution—Cnidaria 1

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esMulticellular animals

Animals with nerve and muscle cells

Coelomates (true body cavity)

Deuterostomes (anus forms before mouth)

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Cnidarians—Stuff to know

• All bold font morphologic terms in text

• Classification and stratigraphic ranges of paleontologically important groups

• Skeletal mineralogy

• Septal insertion patterns in rugosans and scleractinians

• Hermatypic vs. ahermatypic ecology

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Cnidarians—Be able to identify:

• Order Scleractinia– Genus Diploria; Genus Montastrea;

Genus Dichocoenia

• Order Rugosa– Genus Hexagonaria; Genus Pachyphyllum

• Order Tabulata– Genus Favosites; Genus Halysites;

Genus Aulopora

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Cnidaria—Phylum overview

• Colonial and solitary invertebrates• Examples include hydroids, jellyfish, sea anemones, corals• Two body layers (ectoderm and endoderm) separated by

middle, non-cellular (“jelly”) layer (mesogleoa)• No coelom (no true body cavity)• No organs• Primary radial symmetry• Possess specialized stinging structures (nematocysts)

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Cnidaria—Phylum overview (cont.)

• Body is a polyp (mouth up) or medusa (mouth down)

• Digestive system is a central mouth that leads to a digestive cavity (enteron)

• Mouth may be surrounded by tentacles

• Muscle cells and nerve cells facilitate simple movements

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Basic body forms

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Cnidaria—Phylum overview (cont.)

• Skeleton may be absent, internal, or external– If present, organic or calcareous

• Aquatic (fresh and marine)• Suspension feeders• Sessile, planktonic, or nektonic• Stratigraphic range is Late Proterozoic

(Ediacaran) to Recent

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Classification

• Class Hydrozoa (“hydroids,” unimportant as fossils)

• Class Scyphozoa (jellyfish, unimportant as fossils)

• Class Anthozoa (true corals and others)– Exclusively marine– Polyp stage only; no medusa– Free-swimming larvae

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Cnidaria classification

C lass H yd rozoa(un im portan t)

C lass S cyphozoa(un im portan t)

S u b c lass O c toco ra llia(re la tive ly un im p orta n t)

O rder Tabulata(tabulates)

Early Ordovician-Perm ian

O rder Rugosa(rugose)

M iddle Ordovician-Perm ian

O rder Scleractinia(m odern corals)

M iddle Triassic-Recent

Subclass Zoantharia

C lass An tho zoa(co rals et a l.)

Phylum C nidaria

Note: Permian scleractinian-likeforms are now known

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Skeletal morphology

• Coral skeletons are external and calcareous– Aragonite or calcite

• Skeleton is secreted by the epidermis at the base of the polyp

• Skeleton consists of basal plate, radial septa, and outer wall (theca)

• As skeleton grows upward, new basal plates may be added– Tabulae (transverse plates)– Dissepiments (smaller, curved plates)

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Polyp andskeleton

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Skeletal morphology (cont.)

• Polyp occupies the calice, the part of the skeleton above the last-formed tabula or dissepiments

• Skeleton of one coral (solitary or colonial) is a corallum

• Skeleton of one polyp in a colony is a corallite• Skeletal tissue between corallites in a colony is

coenosteum

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Skeletal morphology (cont.)

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Skeletal morphology (cont.)

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Skeletal morphology (cont.)

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Septal insertion

• Tabulates lack septa or possess only minor septa

• Order of septal insertion is the most important aspect of classification in the rugose corals and scleractinians

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Septal insertion in Rugose corals

• First six septa are “protosepta”– Stage 1: cardinal and counter septa– Stage 2: alar septa (on either side of cardinal septum)– Stage 3: counterlateral septa (on either side of counter

septum)

• All subsequent septa (metasepta) are added on either side of cardinal septum and on counter side of alar septa

• Septa cluster into four quadrants hence, “Tetracorals”

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Septal insertion in Rugose coralsx = cardinal sector

y = alar sector

Fossulae = gaps between sectors

Six protosepta

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Septal insertion in scleractinians

• Stage 1: six protosepta

• Stages 2 and higher: metasepta added in the center of spaces between existing septa– Metasepta added in groups of 6, 12, 24, 48, etc.

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Septal insertion in scleractinians

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Coral evolution• Among the common corals, tabulates (Early Ordovician-

Permian) were first to originate• Rugose corals (Middle Ordovician-Permian) might have

evolved from tabulates or they might have a separate ancestor

• Scleractinians might have evolved from rugosans (?), or from a naked sea anemone– Permian “scleractinian-like” forms are known– Late Paleozoic aragonitic rugosans are known– No Early Triassic corals are known– Pattern of septal insertion is quite different in rugosans and

scleractinians

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Coralevolution

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Coral ecology and reefs• Hermatypic = reef corals that possess

zooxanthellae (symbiotic algae)– Shallow, tropical water (25–29°C; < 90 m depth)– Rapid skeletal growth– Oligotrophic (low nutrient) environments

• Ahermatypic = non-reef corals without zooxanthellae– Wide environmental range (all latitudes)– Up to 6000 m depth; down to 1°C temp