Deuterostomes (anus forms before mouth)
Coelomates (true body cavity)
Animals with nerve and muscle cells
1

• 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 2

• 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 3

• 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 4

• 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 5

Fossils & Evolution—Cnidaria 6

•
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 7

• 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 8

Class Hydrozoa
(unimportant)
Phylum Cnidaria
Class Scyphozoa
(unimportant)
Class Anthozoa
(corals et al.)
Subclass Octocorallia
(relatively unimportant)
Subclass Zoantharia
Order Tabulata
(tabulates)
Early Ordovician-Permian
Order Rugosa
(rugose)
Middle Ordovician-Permian
Order Scleractinia
(modern corals)
Middle Triassic-Recent
Fossils & Evolution—Cnidaria
Note : Permian scleractinian-like forms are now known
9

• 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 10

Fossils & Evolution—Cnidaria 11

• 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 12

Fossils & Evolution—Cnidaria 13

Fossils & Evolution—Cnidaria 14

Fossils & Evolution—Cnidaria 15

• 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 16

• 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 17

Six protosepta x = cardinal sector y = alar sector
Fossils & Evolution—Cnidaria
Fossulae = gaps between sectors
18

• 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.
Fossils & Evolution—Cnidaria 19

Fossils & Evolution—Cnidaria 20

• 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 21

Fossils & Evolution—Cnidaria 22

• 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
Fossils & Evolution—Cnidaria 23