Several widespread and fundamental changes in the composition of marine faunas occurred in the early Cenozoic. Many major components of Mesozoic communities disappeared, including ammonites, belemnites, inoceramid and rudist bivalves and many previously abundant gastropods including Nerinea and Actaeonella. Other forms such as trigoniid bivalves and the gastropod Pleuro-tomaria, common in Mesozoic shelf areas, became very restricted in distribution during the Tertiary; Trigonia is now confined to southern Australia, and Pleurotomaria to deeper waters. Brachio-pods diminished considerably in importance, and many echinoid groups common during the Cretaceous also disappeared. The marine • reptiles, icthyosaurs, plesiosaurs and pliosaurs became extinct, while on the land the extinction of the dinosaurs was followed by a great expansion of the mammals.
The Cenozoic saw the rise in importance of many marine benthic groups; most of these originated in the Cretaceous, but were often present then in rather subordinate numbers. A good example of this are the predatory gastropods, mainly from the order Neogastropoda, which comprise about half the species of gastropods in Eocene assemblages. Groups which probably preyed on echinoids, polychaetes, sipunculids, bivalves and gastropods were present, and there is an abundance of polychaete feeding specialists which indicates a diverse and abundant polychaete fauna, although little of this is preserved. Forms with sophisticated predatory devices, such as Conus with its barbed, dart-like radula accompanied by toxic venom, appeared. All these families of predatory gastropods had evolved since the Albian, but reached significant numbers only in the latest Cretaceous. Many hetero-dont bivalve groups such as the large, extremely diverse, infaunal suspension-feeding superfamily Veneracea and the deposit-feeding Tellinacea, although present during the Cretaceous, underwent rapid development and diversified at the beginning of the Ceno-zoic to become very important elements in Cenozoic bottom communities. Many of the important groups of reef-building corals appeared in the Late Cretaceous, although they are not particularly common in the British Cenozoic, and diversified during the
Fig. q. The world during the Early Cenozoic. Positions of the continents after Briden et al. 1974.
Eocene, whilst many of the older coral groups became extinct during the early Cenozoic.
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