OCEAN 320 Lecture 4: Unit 4C
Unit 4C
Corals
Earliest proliferation of organisms clearly ancestral Scleractinia are Middle Triassic;
consisted of at least 7 (possibly 9) suborders
oThese corals did not build reefs; were small solitary or phaceloid organisms of
shallow Tethys of southern Europe and Indo-China
Mesozoic
oMiddle-Late Triassic: corals widespread throughout Tethys regions and now
found in equatorial Panthalassa Ocean rim
o20-25 million years between earliest Triassic corals and earliest widespread coral
reefs
oTriassic corals: had wide range of skeletal micro-structures, suggests very remote
common ancestry; were large and poorly integrated, so phacelloid growth forms
were dominant
oPhacelloid growth forms: where branches are composed of individual coralites
o5-8 million year hiatus between collapse of Triassic reef development and
Jurassic reef onset
oLate Jurassic: probably all-time global max of Mesozoic coral diversity (150
genera recorded in European Tethys and 51 in Panthalassa)
oPaleobiogeographic procinces recognized to reflect continental plate movements,
esp increasing width of Protoatlantic
oLate Jurassic paleobiogeographic pattern was precursor to Cenozoic; dominated
by massive reef development throughout Tethys, Atlantic, far eastern Panthalassa
(probably a barrier to east-west dispersion, like Pacific is today)
oMost extant Scleractinia originate in Middle-Late Jurassic
Fossil record is unclear, few links in family tree
oJurassic: proliferation/origin of 2 of most major groups of corals
Fungiina: dominated much of Jurassic and Cretaceous; was greatly
diminished by end-Cretaceous and families attributes to it today have
uncertain affinities
Faviina: well-defined group; have remained a major family for 150 million
years
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oEarly Cretaaceous similar to Late Jurassic; little known about Mid-Cretaceous bc
reefs worldwide became dominated by rudist bivalves & environmental
perturbations (affect reef development)
Corals return to dominance only after unexplained total extinction in late
Cretaceous
Cenozoic
oFaviinade & Caryophylliidae are only families that survive to proliferate in
Cenozoic
oEvolutionary history of modern corals divisible into 3 geological intervals
1: Paleogene, when survivors of end-Cretaceous and Late Palaeocene
extinctions proliferate into diverse cosmopolitan fauna
2: Miocene, fauna that became subdivided into broad biogeographic
provinces we have today are precursors of most extant species evolved
3: Pilo-Pleisocene to present, when world went into full glacial mode and
modern distribution patterns emerged
oMiocene origin of non-Olligocene extant genera and immediate ancestors of
extant species; also time of obliteration of Tethys (extinction of non-
zooxanthellate corals from Mediterranean, and start of separate evolutionary
history in Indo-China and Atlantic
oHistory of corals most obvious in looking at visible taxonomy and distribution of
living corals; Pilo-Pleistocene fossil record better than Indo-Pacific; place in
Caribbean impacts Pleistocene glaciations greatest
Change spurred by closure of Central American Seaway: allowed
distinction to arise on each time, which makes a very similar species, very
different
oNo zooxanthellate scleractinian species common to indo-pacific and Caribbean
today
Palaeozoic Corals
Cambrian “corals:” small, cup-shaped, mostly solitary organisms with septa; some have
operculum over calice opening; 7 orders recognized, Tabulata and Rugosa most
important
oT & R have left good fossil records bc skeletons are calcitic (more stable than
aragonite skeletons of Scleractinia
Order Tabulata (Early Ordovician-Permian)
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oMuch less variable than rugose or scleractinian corals; all colonial and have
slender tube ike coralites (1-3 mm in diameter) crossed internally by transverse
partitions, called tubulae
oColonies are incrusting, flat or massive, may be branching
oIndividual coralites can be in contact or widely separated
oEach coralite has a theca (eveloping sheath) and groups of coralites are enclosed
in sheath-like epitheca
oMay be connected by fine tubules forming 3D structure; tubules may pierce
thecae as mural pores; tubulae usually found where coralites are in close contact
oDissepiments: external horizontal plates that occur when coralites are separated
oVertical structures usually form radially arranges spine-like septa
oNo physical evidence to suggest they were zooxanthellate, but colonial, have
similar growth rates to zooxanthellate Scleractinia, so they probably were
oNo evidence of common ancestry with Rugosa
oMid-Ordovician: radiation in Tabulata diversity; 6 families recognized, associated
with soft subrates (NOT carbonate platforms, like in high energy environments)
oSignificant frame builders during Silurian, relatively unimportant during intervals
of max reef development in Devonian, though; did not regain diversity during
Carboniferous or Permian, but periodically abundant
Order Rugosa (Mid-Ordovician-Permian)
oResemblance to Scleractinia, not Tabulata; array of growth forms, majority are
solitary or form colonies where individual coralites are large & dominant, basic
structural components similar
oMain difference: arrangement of septa; tubulae (similar to Tubulata) usually
abundant as well as array of dissepiments (similar to Scleractinia)
oEarliest where solitary and simply contruction
oComplex colonies evolve by Late Ordovician; skeletal detail of only evolutionary
change, and usually reversed one or more times
Bleaching kills third of coral in Great Barrier Reef’s north
Bleached corals that haven’t died can recover if the water temp drops, but older corals
can take longer and probably won’t get to before next bleaching event occurs
oCoral that has died has died for good
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Document Summary
Fossil record is unclear, few links in family tree: jurassic: proliferation/origin of 2 of most major groups of corals. Fungiina: dominated much of jurassic and cretaceous; was greatly diminished by end-cretaceous and families attributes to it today have uncertain affinities. Faviina: well-defined group; have remained a major family for 150 million years: early cretaaceous similar to late jurassic; little known about mid-cretaceous bc reefs worldwide became dominated by rudist bivalves & environmental perturbations (affect reef development) Corals return to dominance only after unexplained total extinction in late. Cenozoic: faviinade & caryophylliidae are only families that survive to proliferate in. Cenozoic: evolutionary history of modern corals divisible into 3 geological intervals. 1: paleogene, when survivors of end-cretaceous and late palaeocene extinctions proliferate into diverse cosmopolitan fauna. 2: miocene, fauna that became subdivided into broad biogeographic provinces we have today are precursors of most extant species evolved.
