fossils & evolution—cnidaria1 sponges cnidarians arthropods brachiopods bryozoans segmented...
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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)
Fossils & Evolution—Cnidaria 2
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
Fossils & Evolution—Cnidaria 3
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
Fossils & Evolution—Cnidaria 4
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)
Fossils & Evolution—Cnidaria 5
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
Fossils & Evolution—Cnidaria 6
Basic body forms
Fossils & Evolution—Cnidaria 7
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
Fossils & Evolution—Cnidaria 8
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
Fossils & Evolution—Cnidaria 9
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
Fossils & Evolution—Cnidaria 10
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)
Fossils & Evolution—Cnidaria 11
Polyp andskeleton
Fossils & Evolution—Cnidaria 12
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
Fossils & Evolution—Cnidaria 13
Skeletal morphology (cont.)
Fossils & Evolution—Cnidaria 14
Skeletal morphology (cont.)
Fossils & Evolution—Cnidaria 15
Skeletal morphology (cont.)
Fossils & Evolution—Cnidaria 16
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
Fossils & Evolution—Cnidaria 17
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”
Fossils & Evolution—Cnidaria 18
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
Fossils & Evolution—Cnidaria 21
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
Fossils & Evolution—Cnidaria 22
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