View - Kowalewski, M. - Virginia Tech

PALEONTOLOGICAL SOCIETY PAPERS, V. 8, 2002FIGURE 9—Shark predation on Pennsylvanian nautiloids. Bottom—reconstruction of shark Symmoriumshown attacking a small coiled nautiloid. Top—sequence of events involved in producing punctures ina nautiloid shell rotated over tooth row; adapted from Mapes and Hansen (1990).other aspects of ecology as a consequence of thePermo-Triassic extinction. Hence, we herein referto the purported middle Paleozoic escalation phasesimply as the “Middle Paleozoic Revolution.”Possible responses (aptations sensu Gould andVrba, 1982; Vermeij, 1987; Gould, 2002) to predationpressure may follow two patterns: a) changes inbehavior and mode of life, and b) changes inmorphology. Both types of aptations can be inferredfor middle to late Paleozoic organisms. Theseresponses may represent direct adaptations toincreased predation pressure, such as thepreferential survival of spiny organisms, as wellas exaptions that spring from pre-existing skeletalfeatures that can be co-opted for a different function(Gould and Vrba, 1982; Gould, 2002). In a majorityof cases it is not possible to distinguish betweenthese modes of origin.Micro-architecture.—One such exaptation thatmay impede predation is shell micro-architecture. Forexample, micro-architecture such as pseudopunctaein brachiopod shells may help to prevent propagationof shell fractures (Alexander, 1986a, 2001). Perhapsthe advantage of this micro-architecture providesone of the reasons that pseudopunctate brachiopods,especially productids and chonetids, becomedominant in the late Paleozoic.Shell Architecture.—In some groups ofgastropods, the presence of an open umbilicusweakens the shells and makes them more easilycrushed (Vermeij, 1983, 1987). Therefore, one mightpredict a decline in umbilicate forms in the face ofincreasing predation pressure. In a sample of some60 genera of bellerophontids, Signor and Brett(1984) found a substantial decline in umbilicateforms, beginning in the Silurian Period (Fig. 10).Ribbing and fluting also render shells moreresistant to crushing; again Signor and Brett foundan increased incidence of sculpture in post-Silurian nautiloids. Alexander (1986b) observeda parallel decrease in the incidence of smoothimplicate shells and increase of coarse ribs in post-Silurian brachiopods. Similarly, he observed adeclining proportion of rectimarginate shells infavor of stronger uniplicate and strongly ribbedshells. Such morphological features could aid inthe resistance to shell breaking and crushingpredators. Strongly plicate brachiopods, such as106