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The late–middle Eocene Gosport Sand of the United States Gulf Coastal Plain preserves a spectacularly dense accumulation of fossil mollusks in which shells appear unusually large in comparison to congeners in adjacent units. We use size-frequency distributions of common taxa drawn from bulk samples to quantify size differences between the Gosport Sand and the underlying Lisbon and overlying Moodys Branch Formations. In all but one of the twelve lineages examined, Gosport Sand individuals are larger than those in adjacent units. As taphonomic processes such as sorting or winnowing cannot account for observed differences, the larger body sizes in the Gosport Sand must have resulted from differences in life history. We used sclerochronology and stable isotope geochemistry to assess growth trajectories for twelve lineages common to the three formations. Isotope analyses of microsamples collected along the ontogenetic trajectory reveal seasonal temperature changes, providing a chronometer for growth. Size-age data indicate that, while life spans vary somewhat among the three units, Gosport Sand taxa almost universally grew faster than those in the two adjacent formations. While growth rate can be enhanced by several factors, we hypothesize that rates of primary production were higher during deposition of the Gosport Sand. Ba/Ca ratios along the growth trajectories of shell carbonate exhibit spikes in Gosport Sand mollusks that are indistinguishable from those observed in modern shells that accrete in high productivity settings.
Trace fossils extend the history of early molluscan evolution because they have a higher preservation potential in siliciclastic strata, where body fossils are rare or absent, and also because they better record the activities of soft-bodied animals. The earliest mollusks are recorded by fan-shaped scratch arrays associated with death masks of the Ediacaran animal Kimberella. Cambrian traces––Climactichnites, Musculopodus, Radulichnus––and fecal pellets record giant, but shell-less mollusks that intermittently left the water to graze the biofilms of intertidal sand flats. After the Cambrian, these traces become progressively restricted. Their subsequent disappearance results from closure of a taphonomic window, rather than from extinction of these animals or their grazing behavior. Other Paleozoic groups of shell-less mollusks may have responded to increasing predation pressures by becoming infaunal. Unlike surface trails, their backfilled burrows––Psammichnites, “Aulichnites-Olivellites,” Dictyodora––are commonly preserved. These burrows reflect the evolution of sophisticated search programs in shallow-marine as well as deep-sea environments, but none of them continues into the Mesozoic.
The early Toarcian (Early Jurassic) global marine mass extinction is usually related to the development of organic-rich sediments preserved as black shales and interpreted as a global oceanic anoxic event—the Toarcian Oceanic Anoxic Event (T-OAE). In the Betic Cordillera, southern Spain, the deep-marine Fuente de la Vidriera section contains the T-OAE as recorded at the westernmost part of the European Tethys. Ichnological analysis of the section indicates a relatively abundant and moderately diverse trace-fossil assemblage composed of Alcyonidiopsis isp., Chondrites isp., Nereites isp., Palaeophycus heberti, Planolites isp., Teichichnus isp., Thalassinoides isp., and Trichichnus linearis. A well-developed endobenthic multi-tiered community is characterized by an upper tier represented by homogenized sediment—individual burrows difficult to discern, a middle tier with a relatively diverse trace-fossil assemblage of mainly vagile deposit feeders, and a lower tier with activities of semisessile deposit feeders. The ichnoassemblage indicates oxic or slightly dysoxic bottom waters that were relatively favorable for benthic organisms. The absence of anoxia is confirmed by previously published geochemical and isotopic data. The T-OAE did not induce extreme conditions for macrobenthic organisms inhabiting the seafloor in this area of the westernmost Tethys. Local factors probably limited the influence of the anoxic event in bottom waters but may have induced oxygen deficiency in upper water masses, producing unfavorable living conditions for pelagic biota and, consequently, a sudden decrease in ammonite abundance.
Modern stalked crinoids represent a relict fauna of once considerably higher diversity, as seen in their extensive fossil record. Comatulid crinoids, which lack a stalk and dominate modern crinoid diversity, have been interpreted as an evolutionary success story due to the increased mobility afforded by stalk loss. This mobility includes effective crawling and also swimming, often interpreted as anti-predatory escape strategies. Until recently it was assumed that stalked crinoids were incapable of active locomotion, but observations of an extant isocrinid have demonstrated that some can crawl relatively rapidly, perhaps in order to escape from benthic predators. Because the mechanics of crawling in stalked crinoids resemble the mechanics of swimming in comatulids, it is worth investigating whether a stalked crinoid would be capable of swimming. The feasibility of this scenario is tested using a biomechanical model of swimming in a stalkless crinoid and by applying the model to a stalked crinoid. The model demonstrates that the stalk imposes a heavy burden that limits the ability of a stalked crinoid to swim. Evolutionarily this might suggest that stalk loss was a key innovation that facilitated swimming; however, stalk loss alone is not sufficient to allow a crinoid to swim. Swimming would have allowed greater capability for escape from benthic predators than crawling. An evolutionary scenario is considered in which swimming evolved in a stalked crinoid to allow more effective escape from benthic predators subsequent to evolution of rapid crawling, precipitating eventual stalk loss.
The discovery of 11 Nautilus macromphalus shells in marine environments near New Caledonia constitutes the first opportunity for taphonomic analysis of empty shells of unburied, externally shelled cephalopods on the seafloor. Radiometric dating indicates specimen ages range from 14 to 42 years. These modern specimens provide a unique opportunity to examine the early, preburial taphonomy of this group of animals including shell condition, radiometric-age dating, epizoan encrustation, color degradation, and sediment infilling. The following conclusions are made: (1) given the limited sample available for study and assuming equal conditions where shells rested on the seafloor, the length of time the shell is unburied will not control the degree of epizoan encrustation or the external shell appearance; (2) shell boring is a major destructive process that probably impacts the potential of the shells to become fossilized; and (3) shells in the photic zone are impacted differently than those dredged from a deep water environment below the photic zone. A major part of this difference is probably the result of both boring and encrusting organisms, especially algae. By comparison, fossil cephalopods as a general group can be expected to vary considerably from the modern specimens because of evolutionary patterns of associated organisms, geographic distribution, and different environments with different paleoecological parameters through time. Caution in overreliance on the taphonomy of these modern shells should be exercised because of the limited sample of Nautilus specimens recovered. The need for additional taphonomic studies of modern externally shelled cephalopods with the recovery of more specimens from the marine environment is highly desirable.
Sequence stratigraphic analysis of Pennsylvanian coal-bearing strata suggests that glacial-interglacial fluctuations at high latitudes drove cyclic changes in tropical biomes. A literature review of plant assemblages in this paleoclimatic context suggests that coal forests dominated during humid interglacial phases, but were replaced by seasonally dry vegetation during glacial phases. After each glacial event, coal forests reassembled with largely the same species composition. This remarkable stasis implies that coal-forest refugia existed across the equatorial landscape during glacial phases, expanding to repopulate lowlands during and following deglaciation. One possibility is that refugia comprised small pockets of wetland forest strung out along valleys at some sites, but data are currently insufficient to test this hypothesis. The model presented here, if accepted, dramatically alters our understanding of the coal forests and helps explain aspects of their dynamics.