Richard R. Alexander

Influence of sediment grain size on the burrowing of bivalves; correlation with distribution and stratigraphic persistence of selected Neogene clams

Burrowing ability of 21 species of clams from Oregon, New Jersey, and Scotland was determined in each sieved sediment ranging from fine gravel to mud in half phi increments. A burrowing rate index (BRI), which normalizes burrowing time for specimen mass, was calculated for each species in each sediment. Skewness and kurtosis of the profile of average BRIs across the range of grain sizes was used to categorize species as substrate generalists, substrate sensitive, or substrate specialists. Substrate generalists (Anadara ovalis, Mercenaria mercenaria, Astarte sulcata, Venerupis decussata, Venus striatula, Venus casina, Mya arenaria] burrowed slowly into a wide range of grain sizes

The Fossil Record of Shell-Breaking Predation on Marine Bivalves and Gastropods

Any treatment of durophagous (shell-breaking) predation on bivalves and gastropods through geologic time must address the molluscivore’s signature preserved in the victim’s skeleton. Pre-ingestive breakage or crushing is only one of four methods of molluscivory (Vermeij, 1987; Harper and Skelton, 1993), the others being whole-organism ingestion, insertion and extraction, and boring. Other authors in this volume treat the last behavior, whereas whole-organism ingestion, and insertion and extraction, however common, are unlikely to leave preservable evidence. Bivalve and gastropod ecologists and paleoecologists reconstruct predator-prey relationships based primarily on two, although not equally useful, categories of pre-ingestive breakage, namely lethal and sublethal (repaired) damage. Peeling crabs may leave incriminating serrated, helical fractures in whorls of high-spired gastropods (Bishop, 1975), but unfortunately most lethal fractures are far less diagnostic of the causal agent and often indistinguishable from abiotically induced, taphonomic agents of shell degradation.

Post-Miocene Shift in Stereotypic Naticid Predation on Confamilial Prey from the Mid-Atlantic Shelf: Coevolution with Dangerous Prey

Abstract Of More than 4000 specimens of the naticid gastropods Euspira heros (Say) and Neverita duplicata (Say) from southern New Jersey, the lower Pleistocene of North Carolina, upper Miocene of Maryland, and the lower Miocene of Delaware, subsamples with complete and incomplete boreholes (n = 613) were compared for borehole-site stereotypy, prey size-selectivity, prey profitability, and prey effectiveness. In confamilial encounters, adaptation of naticid predators is evidenced by a shift in borehole-site stereotypy on the body whorl toward the umbilicus during the last 18 my, particularly for N. duplicata. Inferentially, an umbilical drilling position enabled the base of the predators foot to occlude the prey aperture and prevent egress of the dangerous preys foot, thereby reducing the preys escape potential. The tradeoff was that the umbilical site required the predator to drill through a thicker shell location. Prey effectiveness, the ratio of incomplete boreholes to total attempts, was initially low (0.03) for both species in the lower Miocene, but increased appreciably from the Pleistocene to the Recent for N. duplicata (0.32). Such increase in successful prey escape indirectly may reflect prey adaptation since the Miocene. Cost/benefit curves, i.e., log of the ratio of apertural lip thickness /internal shell volume regressed on whorl diameter (WD), have significantly greater negative slopes for Miocene versus Recent conspecifics. Lower cost/benefit ratios for successive size classes of modern naticids suggest that confamilial prey have become increasingly profitable molluscan options as they increased in size, despite increased risk of fatality to the predator. Decreasing naticid prey size-selectivity, as evidenced by lower regression correlation coefficients since the Miocene, reflects increasing mismatches between predator and prey size. Outcomes of size mismatches in predatory encounters between E. heros and N. duplicata were not predictable necessarily given potential differences in species agressiveness and foot size. This unpredictability fueled coevolution between these cannibalistic moonsnails and their dangerous intraspecific and interspecific prey.

Shell Repair Frequencies in New Jersey Bivalves: A Recent Baseline for Tests of Escalation with Tertiary, Mid-Atlantic Congeners

Abstract Articulated and disarticulated shells of Anadara ovalis, Anomia simplex, Argopecten irradians, Astarte castanea, Crassostrea virginica, Divalinga quadrisulcata, Donax variabilis, Ensis directus, Geukensia demissa, Mercenaria mercenaria, Mya arenaria, Mytilus edulis, Petricola pholadiformis, Spisula solidissima, and Tagelus plebeius collected from New Jersey intertidal life and death assemblages were measured, and the frequency, type (scalloped, divoted, cleft, embayed), location (ventral, anterior, posterior), and shell size at inception of repair determined. Repair frequency ranges from zero (D. quadrisulcata) to 0.30 (M. arenaria). Size refuge from sublethal breakage was attained by S. solidissima and C. virginica. Posterior repairs necessitated by siphon-nipping characterize M. mercenaria and E. directus. Ventral repairs associate with species subjected to valve-wedging (S. solidissima, M. mercenaria) or with ventral egress of the foot (A. ovalis). Ventral repairs among deep infaunal clams (M. arenaria, E. directus, T. plebeius) may have been induced by sediment-loading stress during burrowing. Anterior repairs are characteristic of deep infaunal species with foot exposure (M. arenaria, T. plebeius). Repair frequencies were compared statistically with valve surface area, ventral margin thickness, burrowing rate, depth of sediment penetration, shell microstructure, shell ornament, and tissue exposure when valves adducted. Of these comparisons, the only significant correlation was between frequency of embayed repairs and valve surface area. The complex relationship between repair frequency and these variables is underscored by factor analysis. New quantitative approaches are employed that advocate phyletic rather than assemblage-level tests of escalation involving Tertiary congeners from the Atlantic Coastal Plain.

Escalation in Late Cretaceous-early Paleocene oysters (Gryphaeidae) from the Atlantic Coastal Plain

Abstract More than 1600 valves of Late Cretaceous and early Paleocene Northern Atlantic Coastal Plain gryphaeid oysters (Exogyrinae and Pycnodonteinae) were examined for breakage-induced shell repair and morphologic variability to evaluate the hypothesis of escalation. The Exogyrinae show disproportionately higher average repair frequency (0.41) relative to the ecologically and functionally similar unornamented pycnodonts (0.19). An increase in repair frequency (independent evidence of the action of a selective agent, e.g., predation) through the stratigraphic interval supports escalation. Variation in repair frequencies may reflect differences in oyster morphology and in the strength and diversity of shell crushers across an onshore-offshore gradient. Escalation of antipredatory adaptation characterized the evolutionary response of gryphaeid oysters to their durophagous predators. Adaptation generally occurred by the enhancement of existing traits in both oyster lineages. Characters that confer a selective advantage against predators are not all expressed or improved concurrently in both oyster lineages. Morphologic adaptations to minimize shell breakage include the development of expansive, broad commissural shelves, thickened valves, and surface ornamentation (Exogyrinae). Surface ornament in the Exogyrinae gradually increased with time. For some characters, such as thickness, conflicting functional demands (e.g., valve stabilization) may have limited adaptation to predators.

Mechanical strength of shells of selected extant articulate brachiopods: Implications for paleozoic morphologic trends

Dried shells of Terebratalia transversa, Laqueus californianus, Hemithyris psittacea, and T. unguicula and alcohol‐soaked, tissue‐lined shells of Terebratulina retusa, Dallina septigera, Cryphus vitreus, and Liothyrella uva were crushed in an apparatus that facilitated measurement of the force (newtons) against the valves at the instant of fracture. The results revealed that the costate shells of T. transversa and T. retusa were the strongest. Force is correlated with valve thickness, but not with size (length). When normalized for valve thickness, the force required to fracture shells is correlated with shell biconvexity (height/length) among pooled species of dried specimens. Geniculate specimens of T. retusa were not stronger than the intraspecific variants with a constant radius of curvature to their valves. The percent‐frequency of plicate, spinose, lamellose and rugate genera increase significantly in the successive stages, Caradocian (Late Ordovician) through Famennian (Late Devonian) at the expens...

Epizoans on late Ordovician brachiopods from Southeastern Indiana

The frequency of epizoans (cornulitids, inarticulate brachiopods, bryozoans, solitary and colonial rugosan corals) on over 8000 specimens of articulate brachiopods (four strophomenids, five orthids, one rhynchonellid) was calculated for four stratigraphic horizons in the Dillsboro Formation of southeastern Indiana. Frequency of shells encrusted correlates significantly with the surface area of the valves. Punctae in brachiopod shells (Onniella meeki) may have deterred larval settlement of epizoans. Coarse ribbing on articulates deterred encrustation by the inarticulate brachiopod. The horn coral shows a preference for attachment to the anterior of Hiscobeccus capax. Bryozoans show a preference for the incurrent lateral margins of inferred living hosts, suggesting rheotropic behavior by settling larvae. Inarticulate brachiopods are concentrated around the sloping commissure of the brachial valve of strophomenids, suggesting geotropic behavior and/or selective survival of settling larvae. Inarticulates dete...

Life orientation and post-mortem reorientation of Chesterian brachiopod shells by paleocurrents

Pediculate, biconvex brachiopods from the Chesterian (Late Mississippian) Chainman Formation of the Confusion Range, Westcentral Utah, lived with their commissures steeply inclined to the sediment surface. This inference is based on 1) commensal epizoans localized anteriorly on both valves, 2) abrasion equally well developed on the posterior of both valves, 3) preservation of specimens attached to skeletal clasts, and 4) flume experiments that reveal the advantages of the erect orientation for lateral inhalant flow between the valves. Experiments show that concavo-convex brachiopods (Linoproductus, Inflatia) are hydrodynamically most stable in their reclining life orientation. Winged taxa (Punctospirifer) and non-winged taxa (Anthracospirifer, Reticulariina) with broad interareas are next in order of stability in their life orientation. Brachiopods with a small interarea (Schizophoria) or no interarea (Rhipidomella) show moderate to very low stability, respectively. Brachiopods with curved hinge lines (Composita, Cleiothyridina, Eumetria, Hustedia) also show low stability in life orientation. Taphonomic data are consistent with these results. Concavo-convex and bioconvex, winged shells, as well as non-winged forms with broad interareas, were most often found anterior-posteriorly crushed by sediment compaction, indicating burial in life orientation. Forms with small interareas and with curved hingelines are often dorsoventrally flattened or laterally compressed, indicating reorientation by currents before burial and sediment compaction. This Chainman brachiopod fauna experienced moderate currents, competent to destabilize the shells selectively from the array of shapes. Frequencies of preservation of shells in life orientation are expected to vary with the hydrodynamic stability of the shape. This is a potentially reliable indicator of paleocurrent competencies in brachiopod community transitions across the paleoslope or from proximal to distal portions of tempestites.

Non-Predatory Shell Damage in Neogene Western Atlantic Deep-Burrowing Bivalves

Abstract Few shell-crushing predators can excavate prey living deep within the sediment. Despite this infaunal refuge from predators, many deep-burrowing bivalves display a strikingly high incidence of shell damage when compared with species living at shallower depths. A non-predatory origin is necessary to explain this high incidence of injury. To evaluate the generality of this hypothesis, repair frequency (number of repairs per shell), along with position and geometry, were determined for 68 Neogene bivalve species from the US Atlantic and Gulf Coastal Plain, seven of which are classified as deep burrowers (>20 cm depth of sediment penetration). Panopea bitruncata, Panopea floridana, Panopea reflexa, and Panopea americana, which are among the deepest burrowers (up to 100 cm), have repair frequencies (2.8– 6.2) that are higher than any other species, while the value for Cyrtopleura costata (0.76) is exceeded only by semi-infaunal Atrina species (1.12). However, not all deep-burrowing species had such high incidences of damage. Two moderately deep-burrowing species (Anodontia alba and Tagelus plebeius; 20–50 cm) showed repair frequencies (0.04–0.30) that overlapped the range (0.001–0.52) for shallow-burrowing, semi-infaunal, and epifaunal species. Therefore, any generalizations about the relationship between depth of sediment penetrated and frequency of shell damage are inadvisable. Most scars on deep burrowers were not attributable to the direct activity of shell-crushing predators because: (1) injury occurred late in life when individuals were positioned in the safety of a deep burrow; (2) scar types consistent with a predatory origin were rare; and (3) scars frequently occurred on the anterior of the shell, which is least accessible to digging predators.

Influence of Ambient Flow Around the Horseshoe Crab Limulus polyphemus on the Distribution and Orientation of Selected Epizoans

Cumulatively scored distributions of the slipper shell gastropodCrepidula sp., the sand-building polychaete wormSabellaria vulgaris, the calcareous meandering tubed polychaeteFilograna implexa, the barnacleSemibalanaus balanoides, and the lacy bryozoanSchizoporella sp. were mapped on a cm-square gridded silhouette of the prosoma, opisthosoma, and movable spines, and telson of the horseshoe crabLimulus polyphemus stranded on Reeds Beach, Cape May County, New Jersey in the Delaware Bay. Statistically tested and contoured epizoan frequencies for 81 males and 59 females revealed that slipper shells and agglutinated worm tubes are concentrated near the movable spines on the opisthosoma, but rare on recessed chilarial or opercular pleurites, the prosoma, and telson. Barnacles are disproportionately concentrated in entapophyseal pits on the opisthosoma and in the longitudinal furrows flanking the cardiac lobe of the prosoma. Bryozoans are concentrated near movable marginal spines of the opisthosoma, but also found on all areas of the prosoma, opisthosoma, and hinge area in between, as well as the telson. Different morphologic sites for their respective modal concentrations may result from competitive exclusion of bryozoans by barnacles. Genal angles have lower statistically predicted frequencies of bryozoans, barnacles, and sand worm tubes. Apertural openings of calcareous tubes are predominantly posteriorly oriented whereas the anterior magin of slipper shells are randomly oriented on the opisthosoma. Eschewing differential mortality, nonrandom distributions indicate possible: rugophilic (groove-seeking) larval settlement behavior for barnacles that preferred the recessed hinge area, longitudinal grooves, and pits; rheophilic (current-seeking) larval settlement behavior for the gastropods and tube-building polychaetes that filter feed in the eddies generated near the movable spines of the downcurrent-sloping opisthosoma; and geotactic (gravity-influenced) larval settlement or post-settlement migration to the vaulted crest (cardiac lobe). Deficiencies of protruding epizoic skeletons on the genal angles and anterior margin of the host may be due to removal by abrasion as the horseshoe crab plowed through sediment or shearing of the epizoans from the carapace substratum in strong currents or waves.