Laurie C. Anderson

Naticid gastropod predation on corbulid bivalves : effects of physical factors, morphological features, and statistical artifacts

Naticid gastropod predation is generally stereotyped or fixed regardless of conditions. Naticid predation on corbulid bioalves is said to be anomalous: characterized by unstereotyped behavior, low success, and predation frequencies lower than those predicted by cost-benefit analysis. Anomalous predation is an important phenomenon because it may provide insight into the evolution of naticid behavior or prey functional morphology, and because stereotypy is important methodologically. Previous authors have hypothesized that predation on corbulids is anomalous because naticid predation is inhibited by the presence of conchiolin layers within corbulid valves

A comparative study of naticid gastropod predation on Varicorbula caloosae and Chione cancellata, Plio-Pleistocene of Florida, U.S.A.

Abstract Naticid gastropod predation on members of the bivalve family Corbulidae has been reported to be unusual in its lack of stereotypy, low rate of success, and lower frequency than predicted by a net energy maximization model. The cause behind these atypical patterns has been attributed to layers of conchiolin within the valve microstructure of corbulids, which are thought to inhibit shell penetration by naticids. The mechanism of shell penetration by drilling gastropods, as well as scattered instances of typical predation rates and patterns on corbulids suggest, however, that factors other than conchiolin layers should be investigated as potential causes of anomalous predation. We examined naticid-prey interactions by comparing naticid predation on Varicorbula caloosae and Chione cancellata from the Plio-Pleistocene of Florida. We tested for borehole site selectivity and prey size selectivity, two typically stereotyped behaviors. We compared the degree of predation success on both species, and compared observed prey-preference rankings with rankings predicted from cost/benefit analyses based on the net energy maximization model of Kitchell et al. (1981). We found that naticid predatory behavior was generally stereotyped in both species. The cases of unstereotyped predatory behavior on V. caloosae may be linked to its small size, rather than the presence of conchiolin layers. We also found that predicted rankings calculated from cost/benefit analyses were generally inconsistent with observed rankings; the size range of C. cancellata predicted to be the most profitable was not preferred. Size and size selectivity, however, are predictable from the net energy maximization model because they are behaviors that maximize net energy gain (Kitchell, 1986). Therefore, our results support the idea of naticids as energy maximizing predators, but we question the assumptions of the Kitchell et al. (1981) model, especially when applied to fossil assemblages.

Temporal and geographic size trends in Neogene Corbulidae (Bivalvia) of tropical America: using environmental sensitivity to decipher causes of morphologic trends

Abstract Because of their abundance and ubiquity, corbulid bivalves were significant components of the marine ecosystems affected by the environmental, oceanographic, climatic, and biotic changes that occurred during the Neogene in tropical America. Tropical American corbulids show dramatic changes in size over time and space. Eastern Pacific corbulids increase in size in the Holocene, whereas in the Caribbean and western Atlantic, size decreases from maxima in the middle and upper Miocene. As a result, size differences between living eastern Pacific and Caribbean/western Atlantic corbulids are statistically significant. Previously proposed causes of faunal trends in marine invertebrates of tropical America include changes in predation intensity, changes in productivity, and climatic cooling, all of which have been linked to the emergence of the Central American Isthmus (CAI). By comparing geographic and temporal patterns at both regional and local scales, it is possible to decipher the cause of corbulid size trends. Large corbulid species tend to occur in deposits that show evidence of increased nutrient availability, either from upwelling or coastal runoff, and tend to be excluded from units deposited under more oligotrophic conditions. Regional size trends in corbulid bivalves, therefore, are best explained by a general decrease in nutrient availability and productivity in the Caribbean/western Atlantic as the CAI emerged.

Reduced seasonality of Holocene climate and pervasive mixing of Holocene marine section: Northeastern Gulf of Mexico shelf

The large porcelaneous foraminifers Cyclorbiculina compressa, Parasorites orbitolitoides , and Peneroplis proteus are conspicuous in death assemblages from Holocene marine sediments of the Alabama and Florida panhandle shelf. The species inhabited the northeastern Gulf of Mexico in the Holocene (ca. 6.4–1.9 ka) but do not live in the region today. These foraminifers require warm, clear waters, and thus are important paleoclimatic and paleoenvironmental indicators. They apparently were derived from sublittoral seagrass habitats and indicate reduced seasonality in the region during the middle to late Holocene. In addition, a mixed foraminiferal fauna, a hydrodynamically reworked macrofaunal assemblage, and stratigraphic disorder in foraminiferal 14 C dates indicate extensive reworking of the entire Holocene marine transgressive package. Evidence that mollusks are indigenous but large foraminifers are transported supports the generalization that out-of-habitat transport of macrofauna is negligible in most marine settings.

EVOLUTION AND PHYLOGENETIC RELATIONSHIPS OF NEOGENE CORBULIDAE (BIVALVIA; MYOIDEA) OF TROPICAL AMERICA

Abstract We used 24 fossil and Recent species to construct character states of both composite and exemplar taxa for phylogenetic analyses of Neogene genera and subgenera of the Corbulidae from tropical America. All characters were conchologic and two matrices, which differed in the manner that commarginal-rib characters were coded, were analyzed using branch and bound searches and maximum parsimony. Character polarity was determined using Corbula sensu strictu as an outgroup. These analyses produced a limited number of robust and well-resolved cladograms that require only one ghost lineage. Such stable results indicate a high level of congruency among characters, and demonstrate that conchologic data sets can yield highly resolved cladograms. Tropical American corbulids are not monophyletic, and include two major clades. Crown groups within these clades are endemic, and all genera endemic to tropical America first appear in the Miocene. In fact, generic diversity and body size peak in the upper Miocene of the Caribbean/western Atlantic. Range restrictions and extinctions of large-bodied genera from both corbulid clades contribute to a post-Miocene decline in body size in this region. The eastern Pacific does not experience a similar decline in diversity and body size. Diversity and morphologic trends in Caribbean corbulids coincide with regional environmental changes, in particular decreases in seasonality and productivity. Except for the extinction of Bothrocorbula, however, corbulid extinctions apparently predate faunal turnover reported for other molluscs. Associated with these extinctions, we found evidence of geographic range restriction, but not range shifts, in corbulid genera, indicating that the geologic development of environmental refugia contributed more to survival than eurytopy. Large-bodied genera of the southern Caribbean Gatunian Province (Tenuicorbula, Panamicorbula, and Hexacorbula) became restricted to the eastern Pacific. Range restriction to this high productivity refugium (i.e., paciphilic genera), rather than origination of new taxa, produced several corbulid genera now endemic to the region. Large-bodied genera from the Caloosahatchian Province of the southeastern U.S. (Bothrocorbula and Bicorbula), however, underwent global extinction. These subtropical and warm temperate taxa are presumably more eurytopic than their tropical counterparts, but do not exhibit range shifts in response to Neogene environmental change.

Distinguishing Natural and Archaeological Deposits: Stratigraphy, Taxonomy, and Taphonomy of Holocene Shell-Rich Accumulations from the Louisiana Chenier Plain

Abstract Shell-rich deposits of the Louisiana chenier plain (LCP) include three types: (1) cheniers, Holocene relict shorelines of 10–100 km in length that typically are composed of laterally continuous, planar beds primarily containing poorly sorted bivalves in a sandy matrix; (2) mounds, 3–5 m high, ∼10 m across, anthropogenic structures composed of redeposited sediment that have been formed primarily through depositing basket-loads of nearby chenier sediment; and (3) middens, thin, unstratified lenses that are typically1–3 m long,, although they can be up to 100 m across, often lacking topographic expression, and composed primarily of large Rangia cuneata in a black clay matrix. Middens are deposited on cheniers by humans as refuse. LCP mounds often are recognizable using criteria such as topographic expression, and both middens and mounds often are identifiable through faunal composition and the presence of artifacts. However, efforts to record the cultural history of southwestern Louisiana have been hampered because many LCP mounds are now remnants (mounds that have been excavated for road fill and have lost their original diagnostic topographic expression from mound construction). Further, these archaeological deposits contain many of the same species found in cheniers, may be composed primarily of chenier sediment, and directly overlie cheniers. But, LCP cheniers, middens, and mounds have different depositional histories and it may be possible to develop additional stratigraphic, taxonomic, and taphonomic criteria to distinguish them. Preliminary tests indicate that the following criteria can be used to distinguish among these deposits. LCP cheniers typically are composed of alternating coarse- to fine beds of shell and fine- to medium sand. LCP middens examined are small lenses mainly of one molluscan species and black clay. The LCP mounds examined have a complex stratigraphy of mainly unlayered redeposited chenier sediment. Both mounds and middens exhibit somewhat greater taphonomic alteration (particularly chemical) than cheniers and contain evidence of human occupation, such as pottery fragments, vertebrate remains, and charcoal.

Taphonomic and paleoenvironmental evidence of Holocene shell-bed genesis and history on the northeastern Gulf of Mexico Shelf

Subsurface shell beds that are common on the eastern Alabama/western Florida Panhandle shelf provide paleoenvironmental and taphonomic data that demonstrate a dynamic depositional history with reactivation and amalgamation to the base of the Holocene marine transgressive package. In most of the ten shell beds we examined, shallow-marine mollusks, large soritid foraminifera, and cupularid bryozoans are common. Bioclasts occur in all preservation states, but a majority are pristine or only slightly altered. Normal grading, and concave-up, stacked, and random fabrics are common. In three of these shell beds, a relict estuarine component characterized by poorly-preserved Chione cancellata can be detected. Three other shell beds contain 1 of 2 estuarine molluscan assemblages. One assemblage is characteristic of fine-grained and the other of coarser-grained substrates. In all estuarine shell beds, bioclast preservation ranges from excellent to mixed, and bioclasts have random fabrics and locally are normally graded. Most molluscan remains are indigenous and assemblages range from within-habitat time-averaged to environmentally condensed. Shell beds are thick (up to about 75 cm) and overlie bay or shoreface ravinement surfaces, indicating that shell beds first accumulated as coarse transgressive deposits, and are composite concentrations modified by multiple events. Shell beds, however, are amalgamated. Only fabrics reflecting the final modifying events are preserved, and evidence of previous accumulation processes are obliterated. For marine shell beds especially, these final agents were, and possibly continue to be, highenergy events such as storms (winter cold fronts, hurricanes) or currents associated with Loop Current eddies. Episodically high sedimentation rates associated with these events may have acted as a buffer, preventing a long history of reworking and exposure (thus reducing shell alteration) typical of transgressive lags. Our results indicate that: 1) within-habitat time-averaged and environmentally condensed assemblages can be distinguished by combining taphonomic and environmental data, even when the ranges of environmentally disparate species overlap, 2) transgressive-lag deposits can be composed of well-preserved bioclasts, and 3) transgressive lags can be reworked and amalgamated but still be recognizable on the basis of their stratigraphic context.

A phylogenetic and morphologic context for the radiation of an endemic fauna in a long-lived lake: Corbulidae (Bivalvia; Myoida) in the Miocene Pebas Formation of western Amazonia

Abstract The Corbulidae are one of a handful of a primarily marine bivalve clades that exhibit a remarkable radiation, marked by increased species richness and divergent morphologies, within a long-lived lake. For corbulids, this diversification occurred within the lower to middle Miocene Pebas Formation of western Amazonia. Only one taxon associated with this radiation (Anticorbula) remains extant. We conducted a series of phylogenetic analyses to characterize diversification of Corbulidae within the Pebas Formation and relate that diversification to geologically older freshwater corbulids from the Paleocene Fort Union Formation of the northern Great Plains (United States). We used these results, as well as a quantitative examination of morphospace occupation, to infer whether Pebasian corbulids represent a true species flock, and whether the lacustrine system represented by the Pebas Formation represents a cradle of, or reservoir for, freshwater corbulid diversity. We conducted two sets of phylogenetic analyses using shell morphology characters. A genus-level data set incorporated type species of freshwater corbulid genera, any Paleocene representatives of these genera, and selected brackish and marine corbulid genera. A species-level analysis added all described freshwater corbulid taxa to the genus-level matrix. Our results were highly resolved (few most-parsimonious trees), but not particularly robust (low branch support). For the genus-level matrix, we used a taxon jackknife procedure to explore the effects of taxon sampling on tree stability and topology. Jackknife results recover a subclade of freshwater taxa (including both Anticorbula and Pachydon species and the Paleocene Ostomya sp.) in 92.4% of trees, although placement of this subclade across the ingroup varies, as do the topologic positions of other freshwater species. Freshwater and marine corbulids also are morphologically distinct from each other, a factor that likely reduced the robustness of our phylogenetic results. By combining these results with paleoecologic, stratigraphic, and morphologic data, we infer that freshwater corbulids arose once within the family, prior to the Cenozoic, with three distinct freshwater lineages present at their first appearance in the late Paleocene of North America. Within the Miocene Pebas system of South America, we reconstruct supralimital morphologic evolution within three lineages as freshwater taxa became variously adapted to the fluid, dysoxic muds characterizing lake-bottom facies representative of the Pebas lacustrine system. In addition, corbulids apparently successfully coped with high predation pressures from co-occurring shell-crushing predators. Finally, we consider that freshwater Corbulidae were primarily fluvial taxa throughout their geologic history, with a relatively ephemeral radiation within the Pebasian lake system, thus making the Pebasian system a cradle of diversity for several corbulid lineages.

Minimal incorporation of Deepwater Horizon oil by estuarine filter feeders

Natural abundance carbon isotope analyses are sensitive tracers for fates and use of oil in aquatic environments. Use of oil carbon in estuarine food webs should lead to isotope values approaching those of oil itself, -27‰ for stable carbon isotopes reflecting oil origins and -1000‰ for carbon-14 reflecting oil age. To test for transfer of oil from the 2010 Deepwater Horizon spill into estuarine food webs, filter-feeding barnacles (Balanus sp.) and marsh mussels (Geukensia demissa) were collected from Louisiana estuaries near the site of the oil spill. Carbon-14 analyses of these animals from open waters and oiled marshes showed that oil use was <1% and near detection limits estimated at 0.3% oil incorporation. Respiration studies showed no evidence for enhanced microbial activity in bay waters. Results are consistent with low dietary impacts of oil for filter feeders and little overall impact on respiration in the productive Louisiana estuarine systems.

Late Holocene record of community replacement preserved in time-averaged molluscan assemblages, Louisiana chenier plain

Late Holocene relict shorelines of the southwestern Louisiana chenier plain contain molluscan assemblages that vary greatly in taxonomic composition and bioclast preservation. Taxonomic composition varies with ridge age: older ridges are oyster rich, whereas younger ridges are dominated by infaunal bivalves. Taphonomic features can be separated into those caused by biostratinomic and those caused by pedogenic processes. Pedogenic alteration generally increases as ridge age increases, whereas biostratinomic alteration reflects the prevalence of reworked bioclasts in assemblages. These molluscan assemblages are extensively time averaged, causing temporal overcompleteness of depositional units (i.e., amount of time averaging for bioclasts within a unit is much greater than the time it took for that unit to form). Chenier-plain progradation over the last 3,000 years both caused and preserved the observed trend in community composition. This trend was caused by community replacement related to changing substrate stability and by changes in the source of reworked bioclasts, both of which operated in response to progradation. Net progradation also allowed this trend to be preserved because time averaging occurred episodically and shorelines were effectively separated into discrete generations. Although coastal deposits are not typically viewed as ideal sites for high-resolution paleoenvironmental studies, millennial-scale community trends can be detected in this setting.