Marine Life of the Triassic:
Recovery from the Permo-Triassic Extinction. Some groups survive at greatly reduced diversity (possibly only two genera of ammonoid survived extinction). Also, many survivors are quite small even in the same species (the Liliput Effect).
In earliest Triassic seas: return of stromatolites to shallow subtidal environments (perhaps grazers were so reduced that for a time they flourished).
Major adaptive radiation of groups begins: ammonoids up to 100 genera by end of the Early Triassic. Ammonoids are amazingly abundant throughout the Mesozoic.
Early Triassic seas dominated by bivalves, gastropods, ammonoids, some articulate brachiopods, and sea urchins (echinoids): the Modern Evolutionary fauna displaces the Paleozoic Evolutionary Fauna.
During Middle Triassic: return of reefs, built by a new group: scleractinian corals (or hexacorals).
Sharks diversify, including shellfish-eaters. Ray-finned fish radiate: most in Triassic have heavy armored scales.
Marine reptiles arise: most of the Triassic forms (thallatosaurs, placodonts, nothosaurs, pachypleurosaurs, etc.) are near-shore animals, but two pelagic clades also appear:
Terminal Triassic extinction event: wipes out last of the conodonts, many marine reptiles (other than ichthyosaurs and plesiosaurs), many species of bivalve and ammonoid.
Marine Life in the Jurassic:
Radiation of the dinoflagellates.
Origin of the coccolithophorids (chalk-makers).
Origin of the coiled oysters (soft sediment-dwelling oysters).
Radiation of belemnoids (internally-armored cephalopods, originated in later Paleozoic).
First skates and rays (flattened, bottom-dwelling sharks).
Radiation of the teleosts (advanced ray-finned fish with extrusible jaws: almost all living ray-fins are teleosts). Some reach 12 m or more: first giant plankton-gulping vertebrates (although the biggest placoderm was probably a plankton gulper, too.
Marine crocodilians appear in Early Jurassic. Heyday of the ichthyosaurs; high diversity of plesiosaurs (including forms over 14 m long!!).
Marine Life of the Cretaceous:
Radiation of the diatoms, benthic and planktonic forams, and (in Late Cretaceous) coccolithophorids. During Late Cretaceous: massive chalk deposits in epeiric seas.
Also radiations of advanced encrusting bryozoans, burrowing bivalves, predatory gastropods, echinoids, modern-style crabs, and huge diversity of teleosts during the Early Cretaceous. At the same time, huge decline in crinoids, articulate brachiopods, and the like: the Mid-Mesozoic Marine Revolution. Sea floor takes on modern appearance: lots of motile epifauna and infauna in the shallow waters, stalked forms limited to deeper ocean.
New sessile epifauna:
Ichthyosaurs rare in the Early Cretaceous, and die out before the Late Cretaceous. Plesiosaurs remain diverse, with both long-necked and short-necked forms. Rise of three new marine reptile groups:
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