Sally E. Walker

Rates of burial and disturbance of experimentally-deployed molluscs; implications for preservation potential

Rates of burial and transport of molluscan remains are essentially unknown for deeper continental shelf and slope environments, especially over periods of years. An understanding of the rates of taphonomic loss are critical to paleoecological analyses and to paleoenvironmental studies in general. The post-depositional history of organic remains is highly dependent on the length of time the material remains at or near the sediment/water interface. In order to measure these rates, 100 gastropod and bivalve shells were scattered over a marked area of sea bottom at 21 sites in seven environments of deposition (EOD9s) in the Gulf of Mexico and at five EOD9s on the Bahamas platform edge. A total of over 2600 shells were deployed. Each site was thoroughly documented with video photography. After one year in the Bahamas and after two years in both the Gulf of Mexico and Bahamas, these sites were re-photographed and video-taped to measure rates of burial and movement of shells. Shell condition (e.g., articulation, encrustation, and color loss) for those shells that remained exposed was also determined. Shells deployed in Gulf of Mexico petroleum seep sites, on the open continental shelf, and on the continental slope experienced high rates of burial (0.5-3.0 cm) within two years. Shells at these sites generally were not transported or disturbed, and disarticulation rates were low. In the Bahamas, shells on the platform shelf were completely buried within one year. On the steep platform edge from 70 to 300 m, shells on hardground ledges remained exposed, whereas shells in carbonate sands were buried by up to 3.5 cm of sediment. Transport was more common on the steep slopes of the platform edge. Net sedimentation rates for the outer continental shelf and slope of 0.01-0.06 cm yr-1 are well below our observed burial rates of 31 cm yr (super -1) . Thus, burial rate may be somewhat independent of sedimentation rate due to local reworking of sediments by storms at shallower depths and mechanisms such as deep bottom currents or bioturbation at deeper sites. Therefore, the potential for fossil preservation in offshore areas with low sedimentation rates may be much greater than previously assumed.

Taphonomic tiering: experimental field taphonomy of molluscs and foraminifera above and below the sediment–water interface

Abstract A review and synthesis of field-based taphonomic experiments using molluscs and foraminiferans in marine systems reveal differing patterns of hardpart accumulation and destruction for these two major groups of fossil organisms. For benthic molluscs, most destruction appears to occur at the sediment–water interface in siliciclastic settings. Mollusc preservation is suggested to be high once the shell is buried below the Taphonomically Active Zone although this observation has not been experimentally addressed. Foraminiferal preservation, however, depends on microhabitat, biogeochemical conditions within the sediments, and test composition. The Taphonomically Active Zone for foraminiferans appears to be the upper 10 cm in most depositional settings (from shallow to deep sea). The preservation of tests or shells, however, has not been examined below the sediment–water interface using field-based experiments. Pore water geochemistry and associated microbial processes may be the most important factors in which to examine subsurface taphonomy. The term, taphonomic tiering, is applied to variations in taphonomic activity within the sediments that differentially affect hard parts. In essence, the subsurface region where hardparts are presumed to be preserved, is one of the new frontiers in experimental field taphonomy.

Criteria for recognizing marine hermit crabs in the fossil record using gastropod shells

ABSTRAcr-Hermit crabs have left a rich fossil legacy of epi- and endobionts that bored or encrusted hermit crab-inhabited shells in specific ways. Much of this rich taphonomic record, dating from the middle Jurassic, has been overlooked. Biological criteria to recognize hermitted shells in the fossil record fall within two major categories: 1) massive encrustations, such as encrusting bryozoans; and 2) subtle, thin encrustations, borings, or etchings that surround or penetrate the aperture of the shell. Massive encrustations are localized in occurrence, whereas subtle trace fossils and body fossils are common, cosmopolitan, and stratigraphically long-ranging. Important trace fossils and body fossils associated with hermit crabs are summarized here, with additional new fossil examples from the eastern Gulf Coast. Helicotaphrichnus, a unique hermit crab-associated trace fossil, is reported from the Eocene of Mississippi, extending its stratigraphic range from the Pleistocene of North America and the Miocene of Europe.

Taphonomic significance of hermit crabs (Anomura: Paguridea): Epifaunal hermit crab — Infaunal gastropod example

Abstract As secondary inhabitants of gastropod shells, hermit crabs (Anomura: Paguridea) can influence the preservation potential and resulting fossil record of gastropod shells and can modify the paleoecological information left on the shells. The fossil record of hermit crabs is almost non-existent (occasional chelipeds) and does not illuminate the role of hermit crabs as taphonomic agents. However, observations and experiments on bionts associated with modern hermit crab-inhabited gastropod shells demostrate a series of specific criteria by which bionts can be used to infer the presence of pagurids in the fossil record, their behavior, and the taphonomic consequences for the gastropod record since the origin of Paguridea in the Early Jurassic. This study shows that the infaunal snail shell, Olivella biplicata , when occupied by the epifaunal hermit crab, Pagurus granosimanus , has a unique set of filter-feeding bionts present in the aperture region of the shell. These bionts are not found on the living snail, do not settle on experimental tethered shells, and, because they leave bore holes or etchings on the shells, provide trace fossil evidence of hermit crab occupation of fossil shells. The taphonomic experiments revealed that empty tethered shells are more likely to be crushed by predators than buried. Experiments on the burial of previously hermitted shells show that some bionts are more likely to be destroyed early in the burial history (e.g., barnacles) than others (e.g., encrusting bryozoa).

Gastropod shell repair in the intertidal of Bahía la Choya (N. Gulf of California)

Abstract Models of evolutionary escalation between gastropods and their shell-breaking predators rely on the presence of a strong relation between predation intensity and repair frequency. Some previous work has suggested that both predation intensity and repair frequency have increased through geologic time. Repair frequency (the percentage of shells with at least one repair scar) in four Recent gastropods from the northern Gulf of California shows both high interspecific (7.6% in Cerithium stercusmuscarum to 87.9% in Turritella gonostoma ) and interhabitat variation (11.9–30.7% in Theodoxus luteofasciatus and 26.8–64.9% in Cerithidea albonodosa ). Habitat-mixing, time-averaging and collecting practices might diminish variation in shell repair in fossil populations. Nevertheless, the high microhabitat variation observed here indicates that trends in shell repair through geologic time should consider the variation in shell repair at any one time. Reliable estimates of repair frequencies in fossil gastropods requires samples of several species and several habitats. Measuring shell repair should be better standardized, published data are now often difficult to compare.

Taphonomy on the continental shelf and slope: two-year trends ^ Gulf of Mexico and Bahamas

Abstract The Shelf and Slope Experimental Taphonomy Initiative was established to measure taphonomic rates in a range of continental shelf and slope environments of deposition (EODs) over a multiyear period. We deployed experiments on the forereef slope off Lee Stocking Island, Bahamas, and on the continental shelf and slope of the Gulf of Mexico for 2 yr in 18 distinctive EODs at depths from 15 to 530 m. Overall, most shells deployed at most sites had relatively minor changes in shell condition. Most EODs generated relatively similar taphonomic signatures. A few sites did produce taphonomic signatures clearly distinguishable from the central group and these sites were characterized by one or more of the following: high rates of oxidation of reduced compounds, presence in the photic zone, and significant burial and exhumation events. Thus, unique taphonomic signatures are created by unique combinations of environmental conditions that include variables associated with regional gradients, such as depth and light, and variables associated with edaphic processes, such as the seepage of brine or petroleum or the resuspension and redeposition of sediment. Most sites, however, showed similar taphonomic signatures, despite the variety of EOD characteristics present, suggesting that insufficient time had elapsed over 2 yr to generate a more diverse array of taphonomic signatures. Discoloration and dissolution were by far the dominant processes over the 2-yr deployment period. Periostracum breakdown, loss of shell weight, and chipping and breakage was less noticeable. EODs were chosen based on the expectation that the process of burial and the influence of depth and sediment type should play the greatest roles in determining between-EOD differences in taphonomic signature. EOD-specific edaphic factors often overrode the influence of geographic-scale environmental gradients. Taphonomic alteration was greater on hardgrounds and in brine-exposed sites than on terrigenous muds. Dissolution was less effective at sites where burial was greatest. Discoloration occurred most rapidly at shallower sites and on hardgrounds. Water depth was less influential in determining taphonomic signature than burial state or sediment type. The limited influence of water depth is likely due to the presence of shallow sites that, for one reason or another, were protected from certain taphonomic processes and deeper sites that were characterized by unusually strong taphonomic signals.

Taphonomic Trends Along a Forereef Slope: Lee Stocking Island, Bahamas. I. Location and Water Depth

Abstract The Shelf and Slope Experimental Taphonomy Initiative (SSETI) Program was established to measure taphonomic rates in a range of continental shelf and slope environments. Experiments were deployed on the forereef slope off Lee Stocking Island, Bahamas, for one and two years along two transects (AA and BA) in seven distinctive environments of deposition (EODs) along each transect: in sand channels on the platform top (15 m) and the platform edge (30 m), on ledges down the wall (70–88 m), on the upper (183 m—transect BA only) and lower (210–226 m) talus slope below the wall, and on the crest (256–264 m) and in the trough (259–267 m) of large sand dunes. Discoloration was by far the dominant taphonomic process over the two-year deployment period, with dissolution or maceration of shell carbonate a close second. Periostracum breakdown was not significant, nor was loss of shell weight. Chipped edges and breakage (assayed by the edge alteration variable) were much less common, but were important in some species. The degrees of edge alteration and dissolution were correlated with discoloration more frequently than expected by chance, emphasizing that the process of discoloration progressed in a coordinated fashion with the other two over time. The degree of burial or the interaction between degree of burial and water depth explained most of the trends observed in discoloration. The deep water sites, below the photic zone, including the talus slope and dune EODs, had very similar taphonomic signatures. Shells were characterized by a low degree of discoloration, little edge alteration, and varying degrees of dissolution. Photic zone sites, including the platform top and wall locations, followed the opposite trends, with the shallowest site, on the platform top, typically attaining the most extreme degree of alteration. The wall location was most similar to the platform top despite the greater depth and less rigorous physical and sedimentological regime. The platform edge occupied an intermediate position, likely due to the greater degree of burial that resulted in shells at this site being at least as frequently under aphotic conditions as under photic conditions. The data indicate that similar taphonomic signatures can be attained in distinctly different ways over a two-year exposure period, complicating the interpretation of taphofacies and the taphonomic process.

ISOTOPIC ECOLOGY OF THE MODERN LAND SNAIL CERION, SAN SALVADOR, BAHAMAS: PRELIMINARY ADVANCES TOWARD ESTABLISHING A LOW-LATITUDE ISLAND PALEOENVIRONMENTAL PROXY

Abstract The isotopic ecology of terrestrial snails from tropical island settings is not known despite the importance of such data sets for paleoenvironmental reconstructions. In this study, variations in carbon (13C/ 12C) and oxygen (18O/16O) isotope ratios obtained during whole-shell and sequential-rib analyses of the modern land snail, Cerion, reveal a record of diet and local environment on San Salvador, Bahamas. The mean δ13C value of adult Cerion shells collected from C4 vegetation is higher by 1.0‰ relative to snails collected from C3 plants, suggesting that carbon isotopes in shell carbonate reflect the dominant plant type in the diet, though the broad range of shell carbonate δ13C confirms a varied diet for this genus. The mean δ18O values of adult Cerion shells collected from the west coast of San Salvador are 0.8‰ higher than those collected from the east coast of the island. This difference may reflect the incorporation of water vapor derived from 18O-rich hypersaline lakes located in the islands western interior. Sequential-rib analysis of one adult Cerion shell reveals variations in δ13C and δ18O values through ontogeny that may reflect changes in food source and rainfall seasonality, respectively. This study lays the groundwork for future studies to establish fossil Cerion as a valuable paleoenvironmental proxy for San Salvador and the Bahamas.

Taphonomic Trends Along a Forereef Slope: Lee Stocking Island, Bahamas. II. Time

Abstract The Shelf and Slope Experimental Taphonomy Initiative (SSETI) Program was established to measure taphonomic rates in a range of continental shelf and slope environments over a long period of time. For this report, mollusk shells were deployed for one and two years at seven different environments of deposition (EODs) along two onshore-offshore transects off Lee Stocking Island in the Bahamas. The experimental sites were located: in sand channels on the platform top (15 m) and the platform edge (33 m); on ledges down the wall (70–88 m); on the upper (183 m) and lower (210–226 m) talus slope below the wall; and on the crest (256–264 m) and in the trough (259–267 m) of large sand dunes. Shell condition was assessed using a range of taphonomic attributes including dissolution, abrasion, edge alteration, discoloration, and changes in shell weight. After two years, taphonomic alteration was not particularly intense in any EOD. No species was particularly susceptible or resistant to taphonomic alteration. Taphonomic processes were unexpectedly complex. Effects of location, transect, water depth, and degree of exposure all had significant effects. On average, shells deployed in shallow sites were altered significantly from the controls more frequently than shells deployed at deeper sites. However, the number of significant interaction terms between time and the other main effects indicates a complex interaction between taphonomic processes and the local environment that, over the short term, defies any attempt at delineating taphofacies over a broader spatial area than a single deployment site. Some locations attained the same taphonomic signature in different ways making discrimination of taphonomic rules difficult. For example, deeper-water sites and shallow sites where burial rates were high yielded similar taphonomic signatures because shells were in the aphotic zone in both cases, and this limited the rate and range of taphonomic interactions. Taphonomic processes were strongly nonlinear in time for all taphonomic attributes in all species and all EODs. Nonlinear taphonomic rates hinder the interpretation of single-point-in-time studies in understanding the taphonomic process and buttress a commitment to long-term experiments.

Paleoecology of gastropods preserved in turbiditic slope deposits from the Upper Pliocene of Ecuador

Abstract Upper Pliocene gastropods preserved in turbiditic deposits from the Upper Onzole Formation of northwestern Ecuador provide an excellent window into the past for understanding paleoecological dynamics in deep-water habitats, such as shells used as substrata by epi- and endobionts (encrusters and bioeroders), shell use by hermit crabs, and predation (shell repair and drilling) just prior to the closing of the Isthmus of Panama. Previously, paleoecological and taphonomic information was lacking for deep-water (mid-to-outer shelf) and bathyal fossil gastropods in contrast to shallow-water habitats that have been relatively well studied. This study showed that while epibionts were relatively rare on the fossil shells because of curatorial bias, endobionts (bioeroders) occurred on 51% of the 59 species examined. Of the bioeroders, the trace fossil Helicotaphrichnus commensalis was found on 27% of the species. The presence of Helicotaphrichnus indicates that hermit crabs were also a part of the deep-water assemblage prior to burial by the turbidity flow although there are no body fossils of hermit crabs yet reported from this area. Using epi- and endobiont criteria preserved on the shells, the hermit crabs appeared to prefer shells that had large last whorl volumes. Recycling and retention of Upper Pliocene shells by Recent hermit crabs (biological remanie) on the coast of Ecuador also occurs, which could lead to potential temporal anomalies in the fossil record. In addition, the trace fossil Helicotaphrichnus is now reported for the first time from the Upper Pliocene of Ecuador. This trace fossil also occurred on bathyal shells, suggesting that its bathymetric range may be much deeper than previously reported. Records of predation retained on the deep-water fossils were common (66% of the species had shell repair; 68% of the species had drilled shells) and represented a variety of molluscivorous predators. The durophagous predators (e.g. crabs) and drilling predators (naticids, octopods, and muricids) appeared to be selective in their choice of prey items. High-spired gastropods (especially of the family Turridae) were statistically more likely to be affected by predators than all other prey species and groups examined. Shell architecture for other shell types examined, however, did not appear to affect the frequency of shell repair occurrence. Although predation is thought to be a generalist activity in deep-water environments, it may be that in certain areas, specialist predators may be more common than previously considered. Thus, fossil deep-water communities may contain a rich legacy of paleoecological interactions that can then be used for evolutionary ecological questions pertaining to selective predation with depth, variation in encrusters and bioeroders with depth, and to compare differences in paleoecological structure of these molluscan communities before and after the closure of the Isthmus of Panama.