Transcript septa

Chordates
Echinoderms
Molluscs
Segmented worms
Bryozoans
Arthropods
Cnidarians
Sponges
Brachiopods
Deuterostomes (anus
forms before mouth)
Coelomates (true body cavity)
Animals with nerve and muscle cells
Fossils &animals
Evolution—Cnidaria
Multicellular
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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
Fossils & Evolution—Cnidaria
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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
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•
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•
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
Phylum Cnidaria
Class Hydrozoa
(unimportant)
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
Note: Permian scleractinian-like
forms 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 and
skeleton
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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 corals
Six protosepta
x = cardinal sector
y = alar sector
Fossulae = gaps between
sectors 18
Fossils & Evolution—Cnidaria
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
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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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Coral
evolution
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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
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