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WALKER AND BRETT—POST-PALEOZOIC PATTERNS IN MARINE PREDATIONbehavior, with the possible exception of putativecoprolites packed with crab parts, attributed to seals(Glaessner, 1960; Boucot, 1990).Sea otters.—Sea otters (Table 4) evolvedduring the cooling period of the late Miocene, andare restricted to temperate regions (Van Blaricomand Estes, 1988). Otters occur in shallow coastalareas, where they eat a variety of invertebrate prey(Haley, 1986; Van Blaricom and Estes, 1988). Insoft-sediment habitats, they are known to prey uponendobenthic bivalves (Kvitek et al., 1992, 1993).Sea otters ingest copious amounts of echinodermand molluscan prey—taking in up to 35% of theirbody mass in invertebrate prey every day (Hinesand Pearse, 1982)—but their predatory effects inthe fossil record remain unknown. Hines andPearse (1982) used the size, structure, and breakagecharacteristics of empty abalone shells to documentthe selectivity of the predator and the source ofabalone mortality in a rocky subtidal habitat off ofcentral California. Gormand sea otters preferabalones in California, and can consume about tenabalones a day (Costa, 1978). Similarly, crackedshells were used to infer otter predation on bivalveprey in southeastern Alaska (Kvitek et al., 1992).In this area, sea otters substantially impacted thepopulation of endobenthic bivalves and epibenthicurchins (Kvitek et al., 1992, 1993). Additionally,foraging pits dug by otters attracted predatory seastars, which then ate any exposed molluscs. Ottersdigging for clams also exhumed buried shells(Kvitek et al., 1992), suggesting that biologicalremainie is common in these areas. The reworkedshells then become settling sites for epibenthicinvertebrates. Curiously, sea otter populations maybe controlled by paralytic shellfish poisoning inthese areas (Kvitek et al., 1993).The predatory record of these creatures shouldbe discernible because sea otters have peculiarcarnassial teeth that are flat and rounded forcrushing prey, and their lower incisors are used toscoop meat out of shells. One thing is certain,however: where sea otters occur, their effects onpopulations of their favored food items should begreat. Sea otter predation on sea urchins has aconsiderable effect on nearshore communitystructure (Estes and Palmisano, 1974; Estes et al.,1982). It is also known that where humans havepreyed on sea otters for their furry pelts, theresultant fossil record is skewed towardherbivorous limpets and sea urchins; where seaotters are not preyed upon, the stratigraphic recordshows abundant kelp beds and fish populations(Simenstad et al., 1978).Cetaceans.—Cetaceans (whales and dolphins)originated from land-dwelling artiodactyls in theearly to middle Eocene (Gingerich et al., 2001;Thewissen et al., 2001). Early forms such asAmbulocetus (Fig. 11.7–11.9) probably wereamphibious and may have behaved like seals(Thewissen et al., 2001). It is not known what theseancient toothed whales fed on. By the late Eocenethe gigantic (20 m) and fully marine Basilosaurusseems to have occupied the guild of large Mesozoicmarine reptiles, such as mosasaurs (Fig. 11.10). Itssharp, multiply cusped, undifferentiated teeth wereapparently adapted to fish capture, although arelatively small head limited prey size (Benton,1997). The toothed whales (Suborder Odontoceti)diverged in the Oligocene and radiated during theMiocene into a large number of smaller, dolphinlikelineages (Barnes, 1984). These whales evolvedhighly sensitive echolocation and fast-swimmingbehaviors. They are well adapted for chasing downand capturing fish, sharks, and, in some cases, otherwhales. Apparently, these odontocetes re-evolvedmany of the adaptations of Mesozoic pursuitpredators, specifically the ichthyosaurs (seeMassare, 1987, 1997). The largest toothed whales,sperm whales, are of uncertain origin, but molecularstudies of Milinkovitch (1995) suggest that they mayactually have been derived, in the Oligocene Epoch,from the baleen whales rather than the odontocetewhales. Sperm whales are well adapted for deepdiving in pursuit of squid prey and perhaps occupythe guild of some Cretaceous mosasaurs.An unprecedented find of a walrus-like whaleskull from the Pliocene of Peru indicates that onerare form of whale may have been durophagouson molluscs and/or crustaceans (Muizon, 1993).Odobenocetops peruvianus did not have anelongated rostrum, but had large ventrally directed165