Joseph G. Carter

A synoptical classification of the Bivalvia (Mollusca)

Preface This classification summarizes the suprageneric taxonomy of the Bivalvia for the upcoming revision of the Bivalvia volumes of the Treatise on Invertebrate Paleontology, Part N.

A new crocodylomorph archosaur from the Upper Triassic of North Carolina

Abstract A new taxon of sphenosuchian crocodylomorph, Dromicosuchus grallator, is described on the basis of a well-preserved, largely articulated partial skeleton from Late Triassic strata in the Durham sub-basin of the Deep River basin (Newark Supergroup) of Durham County, North Carolina. The holotype was preserved directly beneath the skeleton of a rauisuchian archosaur; this association, along with apparent bite marks to the head and neck of the crocodylomorph, suggests that the two animals died and were buried together during the act of predation. Dromicosuchus grallator is most closely related to Hesperosuchus agilis from the Petrified Forest Member of the Chinle Formation (late Carnian or early Norian) of Arizona and New Mexico and Saltoposuchus connectens from the Middle Stubensandstein (Löwenstein Formation; middle Norian) of Württemberg, Germany. The monophyly of Sphenosuchia is only weakly supported at present.

A new suchian archosaur from the Upper Triassic of North Carolina

Abstract A partial but largely articulated skeleton of a ‘rauisuchian’ archosaur from Late Upper Triassic strata of the Durham sub-basin, Deep River basin, Newark Supergroup, North Carolina, represents a new species of Postosuchus Chatterjee, 1985. It represents the first record of this taxon from eastern North America. The well preserved specimen includes cranial bones, a largely articulated right manus, right and left pedes, pubes, axis, several postaxial cervical, dorsal, and caudal vertebrae, chevron bones, osteoderms, interclavicle, clavicles, cervical ribs, a sacral rib, and a complete set of gastralia. The skeletal elements are described and compared to those of other ‘rauisuchians’. An apparant autapomorphy of Postosuchus alisonae includes a well developed flange on the proximal portion of metacarpal II fitting into strongly proximally grooved metacarpal I. The new specimen includes many bones previously unknown for Postosuchus and it allows a more complete differentiation of Postosuchus from other ‘rauisuchian’ genera. Diagnostic features of this genus include an axis with two ventral keels; postaxial cervical centra with strongly developed single ventral keels that are anteriorly and/or posteriorly extended into hypapophyses; short ribs on the anterior cervical vertebrae; heart-shaped cervical neural ‘spine tables’; a subrectangular, relatively short coracoid; and proportionately short manus with dorsoventrally compressed, reduced, blunt unguals on manual digits III and IV. Morphological comparisons indicate a close relationship between Postosuchus and Batrachotomus and possibly also Tikisuchus.

Thermal potentiation and mineralogical evolution in the Bivalvia (Mollusca)

The most important factor controlling the timing of Phanerozoic mineralogical evolution in the Bivalvia appears to be thermal potentiation of calcite deposition in colder marine and estuarine environments. Cold temperature has promoted mineralogical evolution in the Bivalvia by kinetically facilitating (potentiating) initially weak biological controls for calcite, thereby exposing their genetic basis to natural selection. Calcite has evolved in bivalve shells for a variety of selective advantages, including resistance to dissolution; resistance to chemical boring by algae and gastropods; reduced shell density in swimming and soft-bottom reclining species; enhanced flexibility in simple prismatic shell layers; and fracture localization and economy of secretion in association with certain foliated structures. Endogenous calcite in bivalve shells varies from biologically induced to weakly and strongly biologically controlled. Biologically controlled calcite generally first appears in bivalve shells as an impersistent component of the outer shell layer, only later, in some groups, expanding to include the entire outer and then part or all of the middle and inner shell layers. The initial stages of mineralogical evolution are shown by certain modern Mytilidae, Veneridae and Petricolidae. In the latter two families, the calcite occurs as conellae in the outer part of the outer shell layer. Calcitic conellae in the inner shell layer of Pliocene Mercenaria are not barnacle plates, as previously indicated, but endogenous calcite comparable in origin to other venerid conellae. Their occurrence in Mercenaria may reflect thermal potentiation of weak biological controls for calcite, as well as local detachment of the secretory mantle epithelium near the pallial and adductor musculature.

Evolution and phylogenetic significance of cardioidean shell microstructure (Mollusca, Bivalvia)

Abstract The shell microstructure of Carboniferous and Triassic permophorids; Triassic and Recent carditids; Devonian, Carboniferous, and Triassic crassatelloideans; and Jurassic through Recent cardioideans is examined in a phylogenetic context, using separate microstructural and morphologic data sets, as well as a combined data set. The microstructural and morphologic data sets are significantly incongruent, but the combined data set suggests that modiomorphoideans (modiomorphids and permophorids) are basal to crassatelloideans; crassatelloideans are basal to carditids (including Septocardia), and carditids are basal to cardiids. On the other hand, the possibility of direct permophorid ancestry for the carditid-cardiid clade cannot be excluded, as suggested by the retention of permophorid-like matted (transitional nacreous-porcelaneous) structure in some early carditids and cardiids. In the absence of stratigraphic data and other evidence for phylogenetic relationships, shell microstructure offers limited potential for assessing subfamily-level phylogenetic relationships within the Cardioidea. This is because of microstructural convergences reflecting biomechanical adaptations for fracture control and abrasion resistance, and possibly also selection for metabolic economy of secretion in tropical, oligotrophic habitats. General evolutionary trends in cardiid shell microstructure are nevertheless apparent: Cretaceous cardiids completely replaced an ancestral laminar, matted structure in their inner shell layer with non-laminar porcelaneous structures; evolved better defined CL structure, stronger reflection of the shell margins, and increased thickness or secondary loss of the ancestral prismatic outer shell layer; and, in Protocardia (Pachycardium) stantoni, added inductural deposition. Some Cenozoic cardiids then evolved wider first-order crossed lamellae, non-denticular composite prisms, composite fibrous prisms, ontogenetic submergence of a juvenile non-denticular composite prismatic outer shell layer into the CL middle shell layer, or ontogenetic submergence of the inner part of a juvenile fibrous prismatic outer shell layer into the CL middle shell layer. The shell microstructure of Hemidonax donaciformis is unusual for a cardioidean, and suggests closer affinities with the superfamily Tellinoidea than with the superfamily Cardioidea. Extensive inductural deposits in Protocardia (Pachycardium) stantoni raise the possibility that photosymbiosis evolved among some Mesozoic members of the Protocardiinae, thereby increasing the likelihood that this feature has evolved several times independently in the Cardiidae. Cemented, calcareous periostracal granules or spines are known to occur in modiolopsoideans, mytiloideans, modiomorphids, permophorids, trigonioids, astartids, cardiids, myoids, pholadomyoids, and septibranchoids. Consequently, the presence of these structures is not necessarily indicative of close anomalodesmatan affinities.

δ18O in mollusk shells from Pliocene Lake Hadar and modern Ethiopian lakes: Implications for history of the Ethiopian monsoon

Abstract Two of the five lacustrine intervals in the largely fluvial Hadar Formation, Afar, Ethiopia, occur in the Sidi Hakoma Member deposited 3.4–3.2 Ma. In a perspective of the δ18O of 11 modern Ethiopian lakes and their shells, the δ18O of the Hadar fossil shells provide a snapshot of the nature of ancient Lake Hadar and Ethiopia’s climate in the Pliocene. Ethiopia’s modern lakes both in the Rift and on the Western Plateau are fed by drainage of Plateau rain with its well established barely negative δ18OSMOW of −1.3‰. Except for the man-made Lake Koka reservoir, all other Ethiopian lakes are isotopically quite positive ranging from +5.4 to +16.0‰, indicating how significant evaporation is in their water budget. Shells from lakes with extant mollusk populations are mostly in isotopic equilibrium with the δ18O and temperature of their lake water. The upper transgressive interval in the Sidi Hakoma Member is the largest one in the Formation beginning at its base with the ‘Gastropod Beds’ beach deposits. Mollusks from shell beds other than the ‘Gastropod Beds’ show more positive and more variable δ18O between shells, with internal variations within shells as much as 7‰. At these times the site must have been underlain by a shallow partially isolated embayment of Lake Hadar which underwent rapid expansions and then contractions by evaporation, within the few year lifetimes of the individual mollusks. The results from the ‘Gastropod Beds’ are of most significance for interpreting the overall paleoclimate at Hadar. Their uniformly negative δ18OPDB shell values that average −6.7‰ represent a much less evaporated stage of Lake Hadar when its δ18OSMOW was 8‰ lower than the spectrum of modern lakes in Ethiopia, and indeed even 3‰ or more lower than average modern Plateau rain. To explain such negative values we hypothesize that the Atlantic-derived air mass component to the Ethiopian monsoon was persistently strengthened during Pliocene summers, which intensified the amount and the negative isotopic character of rainfall onto both the Afar and the Ethiopian Plateaus that drained to Lake Hadar. A similar phenomenon characterized the brief periodic pluvial episodes of the Quaternary, including the latest in the early Holocene, known as the African Humid Period. In contrast to the hot semi-desert steppe conditions of today’s western Afar, the diverse abundant terrestrial fossil fauna at Hadar, including the early hominid Australopithecus afarensis, is explained by the wetter, and probably cooler, summers that persisted throughout the Late Pliocene.

Oxygen and carbon isotopic composition and shell microstructure of the bivalve Laternula elliptica from Antarctica

Oxygen and carbon isotopic measurements were obtained from microsamples of an adult Laternula elliptica, an infaunal, aragonitic, Antarctic bivalve. The δ 18 O values of samples from the exterior surface are, on average (∼ 4.5‰), similar to calculated values inferred to represent precipitation in equilibrium with ambient environmental conditions. This indicates that bulk samples as well as many microsamples from the exterior surface would provide reliable isotopic estimates of paleotemperatures and paleosalinities. Nevertheless, both oxygen and carbon isotopic values from discrete shell areas may be influenced by vital effects

A Late Triassic traversodont cynodont from the Newark Supergroup of North Carolina

ABSTRACT A new traversodont cynodont, Plinthogomphodon herpetairus, is described on the basis of a partial snout from Late Triassic strata of the Newark Supergroup exposed in the Deep River basin (Durham sub-basin) of North Carolina. In the structure of its upper postcanine teeth, Plinthogomphodon most closely resembles Boreogomphodon from the Upper Triassic (Carnian) of Virginia and Luangwa from the Middle Triassic (Anisian) of Zambia. It differs from these taxa only in some features of these teeth.

Nacre in an early gryphaeid bivalve (Mollusca)

McRoberts (1992, figs. 4.13, 4.14, 6.8) illustrated the shell microstructure of late Triassic Gryphaea (Gryphaea) arcuataeformis Kiparisova, 1936, and Gryphaea (Gryphaea) nevadensis McRoberts, 1992. McRoberts (1992, p. 33) described the left valve of G. arcuataeformis as showing “neomorphosed calcite with multiple laminae of ?prismatic structure perpendicular to [the] outer shell surface ….” He described the left valve of G. nevadensis as consisting of two distinct layers of neomorphosed calcite:“…an outer layer with ?prismatic structure occasionally with bands of dark material (?micritic matrix), and a much thinner inner layer with ?cross-foliated structure ….” Subsequent study has shown these microstructural diagnoses to be inaccurate. They are revised as follows.

Molluscan biostratigraphy of the lower River Bend Formation at the Martin Marietta quarry, New Bern, North Carolina

The lower River Bend Formation at the Martin Marietta New Bern quarry in Craven County, North Carolina, contains a diverse and abundant moldic molluscan fauna. This fauna, reconstructed by latex casts, suggests a Vicksburgian or a post-Vicksburgian, pre-Chickasawhayan age for the New Bern exposure. Forty-one molluscan species and subspecies are presently identified from the lower River Bend Formation, 11 of which are new: Turritella caelatura alani, Turritella neusensis, Galeodaria britti, Phalium newbernensis, Cymatium planinodum, Oocorys vadosus, Ecphora wheeleri, Lyria concinna, Scaphella saintjeani, Turricula ( Orthosurcula ) aequa , and Lucina ( Stewartia ) micraulax . This fauna is virtually identical at the generic level and similar at the species level to the Vicksburgian faunas of the Gulf Coastal Plain. About 37 percent of the New Bern species also occur in the Vicksburgian of Mississippi, although many of these species reach considerably larger sizes at New Bern. Apparent evolutionary transitions between previously known Vicksburgian and Chickasawhayan mollusks suggest a time of deposition intermediate between these two Oligocene stages. Moderately high molluscan diversity, the abundance of characteristically warm-water genera, and associated carbonate-rich sediments suggest that the lower River Bend Formation represents a subtropical, open-marine, predominantly carbonate environment immediately seaward of a nearshore lagoonal or barrier island complex. The lower River Bend Formation at New Bern differs faunally, climatically, and sedimentologically from the upper River Bend Formation in quarry exposures near Belgrade, North Carolina. The upper River Bend Formation contains a lower diversity molluscan fauna with marked dominance diversity and few warm-water taxa. It represents a slightly cooler nearshore, open-marine environment in a transitional siliciclastic-carbonate sedimentary regime. The considerable taxonomic and sedimentologic differences between the lower and upper parts of the River Bend Formation corroborate microfossil evidence suggesting that they represent temporally distinct depositional cycles.