Fossils & Evolution—Cnidaria1 Sponges Cnidarians Arthropods Brachiopods Bryozoans Segmented worms Molluscs Echinoderms Chordates Multicellular animals

Fossils & Evolution—Cnidaria 1

Spo

nges

Cni

dari

ans

Art

hrop

ods

Bra

chio

pods

Bry

ozoa

ns

Seg

men

ted

wor

ms

Mol

lusc

s

Ech

inod

erm

s

Cho

rdat

esMulticellular animals

Animals with nerve and muscle cells

Coelomates (true body cavity)

Deuterostomes (anus forms before mouth)


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


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


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)


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


Basic body forms


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


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


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


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)


Polyp andskeleton


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








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


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”


Septal insertion in Rugose coralsx = cardinal sector

y = alar sector

Fossulae = gaps between sectors

Six protosepta


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.


Septal insertion in scleractinians


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


Coralevolution


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

Documents

Fossils & Evolution—Cnidaria1 Sponges Cnidarians Arthropods Brachiopods Bryozoans Segmented worms Molluscs Echinoderms Chordates Multicellular animals