View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002molluscs are then transported to nests often farfrom their original habitat (Teichert and Serventy,1947; Smith, 1952; Lindberg and Carlton, 1969;Lindberg and Kellogg, 1982). Coastal birds suchas the oyster catchers and eiderducks can preydirectly on large numbers of molluscs and otherinvertebrates (Schäfer, 1972; Cadée, 1989,1995).Oyster catchers penetrate molluscan prey bystabbing slightly gaping valves with their beaks,resulting in broken mollusc shells (Drinnan, 1957;Carter, 1968; Cadée, 1995). Oyster catcherpredation often produces distinctive shell damage,with one valve fragmented and the otheruntouched (Drinnan, 1957). However, at least halfof the prey shells may be left intact as these birdsalso insert their beak between the valves,supposedly without harming the shells.The diet of herring gulls may consist of up to70% marine molluscs. Prey are directly scoopedup by the gull from the marine environment andtransported to the shore where the shells aredropped over tidal flats or other hard surfaces,which fragments the shells so that the meat can beextracted (Cadée, 1995). Often, birds selectparticular sizes of prey, which can affect theresultant fossil record of coastal molluscanassemblages (Cadée, 1989). Cadée (1995) hasestimated for the Dutch Wadden Sea that shellcrushingshore birds may fragment up to 35% ofthe annual shell carbonate production. Because theDutch Wadden Sea benthos has up to 75% shellfragments, many of these fragments may be ofbiological rather than physical origin (Cadée,1995). Thus, in shallow Cenozoic seas, coastal andsea birds would have been important agents of shellfragmentaion (Cadée, 1995). These fragments mustoccur in the fossil record (Trewin and Welsh, 1972),but it is generally impossible to pinpoint exactlywho fragmented the shells.Shore birds may also leave benthic feedingtraces in soft sediment. For example, gulls such asLarus ridibundus may make troughs up to 3 m long,15 cm wide, and 3 cm deep in soft shore sedimentsas they forage for food (Cadée, 1990). Shelducksmake smaller pits (60 cm in diameter and 10 cmdeep). These feeding traces are similar to thoseproduced by foraging rays and flatfish (Cadée,1990). However, rays excavate sand around thecircumference of the foraging pits, whereasshelducks excavate to only one side (Cadée, 1990).These distinguishing characteristics could beobliterated by the tides, and therefore it may bedifficult to distinguish bird foraging pits from thoseof benthic fish predators. Foraging pits of aquaticbirds are known only from the Holocene.Pinniped Mammals.—Pinnipeds—seals, sealions, and walruses (Fig.11.11)—in modern seashave a global distribution, occur in enormousnumbers in some regions of the world, and are ableto dive to great depths in the ocean in search offood (Table 4). Therefore, some forms must havehad a significant impact on hardshelledcommunities, although many eat fish. Some of thepinnipeds evolved molar crowns with hypermineralizedcutting edges for crushing and piercingthe hard exoskeletons of crustaceans and molluscs(Haley, 1986). Pinnipeds evolved in the Eocene, andthus have had over 40 million years to affect theevolutionary history of molluscs and arthropods;however, there is no indication that they did so.Walruses (Family Odobenidae) feed mainly onbenthic invertebrates, and have a peculiar feedingstyle: they suck out the siphon or foot of bivalvesusing their piston-like tongue while their mouthworks as a vacuum pump (Muizon, 1993).Walruses have large and deep palates, a wide, bluntsnout with strong muscular insertions, and areduction of maxillary dentition (Muizon, 1993).The tusks are thought to have a primarily social,rather than foraging function. Walruses also leavelong, narrow feeding tracks or small excavated pitsthat can be seen in side-scan sonar (Oliver et al.,1983; Nerini, 1984, her fig. 3).All known odobenine odobenids (walruses) arebottom feeders and are first known from the Miocene(Repenning, 1976). Several extinct species from thePliocene are known to have been molluscivorous(Repenning, 1976) and were widespread at that timein the northern hemisphere (Muizon, 1993).However, the modern walrus (Odobenus) has a fossilrecord only from the Pleistocene (Repenning, 1976).There is no direct fossil record of pinniped predatory164