F. Robin O'Keefe

Osteology of the Cryptocleidoid Plesiosaur Tatenectes laramiensis, with Comments on the Taxonomic Status of the Cimoliasauridae

ABSTRACT Recent field work in the Bighorn Basin, Wyoming has recovered significant new material of the plesiosaur Tatenectes laramiensis. The majority of cryptocleidoid plesiosaurs have been recovered from Middle and Upper Jurassic units (Oxford and Kimmeridge Clays, respectively) in the United Kingdom, but Tatenectes laramiensis is one of at least two cryptocleidoids known from the Upper Sundance Member of the Sundance Formation (Oxfordian) of North America. Although poorly known, they bear directly on both the phylogeny and biogeography of the cryptocleidoid plesiosaurs. Here we describe new fossil material of Tatenectes, and reevaluate the phylogenetic position of this genus based on all known material. New material includes a partial skeleton comprising cranial elements, axial column, and a partial pectoral girdle, as well as an isolated humerus and vertebrae. The pectoral girdle closely resembles that of Muraenosaurus belodis from the Oxford Clay, but is even shorter anteriorly. The cervical vertebrae are more compressed antero-posteriorly than in other Jurassic cryptocleidoids. The humerus is less derived, resembling that of Tricleidus seeleyi. Two most parsimonious trees were obtained, and the consensus tree solidifies the phylogenetic position of Tatenectes as being most closely related to the Oxford Clay taxon Kimmerosaurus.

Cranial Anatomy and Taxonomy of Dolichorhynchops Bonneri New Combination, a Polycotylid (Sauropterygia: Plesiosauria) from the Pierre Shale of Wyoming and South Dakota

Abstract The taxonomic identity of two well-preserved polycotylid plesiosaur skeletons from the Pierre Shale of far northern Wyoming and southern South Dakota has been controversial since their discovery. Originally referred to Dolichorhynchops osborni, the material was almost immediately christened Trinacromerum bonneri Adams 1997; more recently the material has been referred to Polycotylus. Recent preparation of the well-preserved skull of one specimen permits detailed examination of the cranial morphology of this animal for the first time, and allows for its inclusion in a cladistic analysis of the Polycotylidae. This analysis reveals a stable sister-taxon relationship with Dolichorhynchops osborni. However, the taxon possesses a bewildering mosaic of character states, superficially resembling Polycotylus in overall size and tooth morphology and Trinacromerum in details of the palate and lower jaw, while sharing several critical synapomorphies with Dolichorhynchops osborni. This wide-ranging homoplasy among characters previously diagnostic among polycotylid genera challenges the alpha taxonomy of forms from the western interior seaway. The conservative taxonomic course of referring the species T. bonneri to Dolichorhynchops is followed here, but a species-level review is necessary. Lastly, material described here reveals important new information concerning the polycotylid orbital region and lower jaw, allowing confident reconstructions of these controversial regions for the first time.

A Plesiosaur Containing an Ichthyosaur Embryo as Stomach Contents from the Sundance Formation of the Bighorn Basin, Wyoming

Herein we report the discovery of an ichthyosaur embryo from the Upper Member of the Sundance Formation (Oxfordian) of the Bighorn Basin, Wyoming. The specimen is the first known ichthyosaur embryo from the Upper Jurassic, and is the first Jurassic ichthyosaur embryo from North America. The embryo was discovered in close association with the abdomen of an articulated partial plesiosaur skeleton, and several lines of evidence support the interpretation of the embryo as plesiosaur stomach contents. The small size and extremely poor ossification of the embryo indicate that the animal was probably not a neonate. Although the taxonomic affinities of the fossil are unknown, the large ichthyosaurian (sensu stricto) Opthalmosaurus natans is the only known ichthyosaur from the Sundance Formation, and the embryo may belong to that taxon. The ichthyosaurs and plesiosaurs of the Sundance Formation of Wyoming (Oxfordian) have been known for over 100 years, having been first mentioned by Marsh (1891, 1893, 1895). The single currently recognized ichthyosaur taxon from the formation, Ophthalmosaurus natans, was described in a monograph by Gilmore (1906), but subsequently received little attention until recently (Massare and Young, 2004; Massare et al., 2006). Plesiosaurs are also known from the formation, comprising the large pliosaur taxon Megalneusaurus rex (Knight, 1898; Wahl et al., 2007) and the cryptocleidoid taxa Pantosaurus striatus (O’Keefe and Wahl, 2003a) and Tatenectes laramiensis (O’Keefe and Wahl 2003b; Mehl, 1912). These plesiosaur taxa have also received little attention in the scientific literature, despite their phylogenetic and biogeographic relevance to Jurassic plesiosaur evolution, and the context they may provide for the diverse and well-known plesiosaur fauna of the nearly coeval Oxford Clay of the United Kingdom (Andrews, 1910, 1913). In an effort to increase knowledge of this important fauna, the Sundance Plesiosaur Project has performed extensive fieldwork in the Upper Member of the Sundance Formation. Here we report on one discovery from this field program, a partial skeleton referable to Pantosaurus striatus. The skeleton is remarkable in preserving a gastric mass containing the partially digested remains of an embryonic ichthyosaur, and is the first evidence of the consumption of ichthyosaurs by plesiosaurs. Institutional Abbreviation—USNM, Smithsonian Institution, Washington, D.C. USA.

Ecological determinants of clinal morphological variation in the cranium of the North American gray wolf

Abstract The gray wolf (Canis lupus) exhibits both genetic and morphologic clinal variation across North America. Although shape variation in wolf populations has been documented, no study has been made to exhaustively quantify it, or to correlate morphologic variation with environmental variables. This study utilizes a large historical database of wolf skull linear measurements to analyze shape, and attempts to correlate it with wolf ecology. A variety of statistical tests are employed; size and shape are examined through a principal component analysis and a calculation of allometry vectors. Multiple regression analysis (both global and stepwise) are then used to test the resulting principal components against various biotic and abiotic factors. In addition, the effects of sexual dimorphism and taxonomy on morphology are explored through 1-way analysis of variance and canonical variates analysis, respectively. Several patterns are revealed, including size increase with latitude in accord with Bergmanns rule. Static allometry is significant, the fundamental pattern being a decrease in the robusticity of the basicranium relative to the viscerocranium. Sexual dimorphism, allometry, and a correlation with precipitation are other key factors driving morphological variation. Examination of these patterns has allowed us to make conclusions about the direct and indirect ways the environment has affected clinal variation in wolves.

Evidence of pachyostosis in the cryptocleidoid plesiosaur Tatenectes laramiensis from the Sundance Formation of Wyoming

In this paper we present evidence for pachyostosis in the cryptocleidoid plesiosaur Tatenectes laramiensis Knight, 1900 (O’Keefe and Wahl, 2003a). Pachyostosis is not common in plesiosaurs and is particularly rare in non-pliosaurian plesiosaurs, although enlarged gastralia were first recognized in Tatenectes by Wahl (1999). This study aims to investigate the nature of the disproportionately large gastralia of Tatenectes in greater depth, based on new material. A recently discovered partial skeleton consisting of a dorsal vertebral series, ribs, gastralia, and a complete pelvic girdle was collected from the Jurassic-aged Sundance Formation of the Bighorn Basin in Wyoming during the summer of 2006. The gastralia of this specimen are disproportionately large considering the small size of the taxon (about 3 meters total length), and we therefore investigated the size of these elements quantitatively. Polished cross-sections were also prepared to explore the histology of the ribs and gastralia. The ribs of Tatenectes are not pachyostotic, whereas the gastralia exhibit a novel condition of pachyostosis while lacking osteosclerosis. Skeletal tissue modification is common among secondarily marine tetrapods. These modifications can follow one of two major trends: toward a lighter skeleton or toward a heavier skeleton. The skeleton can be made lighter by reduction in number or size of skeletal elements. Bones themselves can also be reduced in density (de Buffrénil et al., 1990; de Ricqlès and de Buffrénil, 2001). This condition, known as osteoporosis, occurs when the cortical bone layer, usually the most compact and dense region of a bone, is reduced, and the marrow cavity or regions of cancellous bone are increased (de Ricqlès and de Buffrénil, 2001). Osteoporosis is a common pathology in humans, but in secondarily marine tetrapods the osteoporosis is adaptive, having occurred in several lineages including ichthyosaurs, pliosaurs, and cetaceans (de Buffrénil et al., 1990; de Ricqlès and de Buffrénil, 2001). At the other end of the spectrum, the skeleton of secondarily marine tetrapods can become heavier. Bones can become enlarged via pachyostosis. Pachyostosis is another adaptive condition through which the periosteal cortex of the bone undergoes hyperplasy (Francillon-Viellot et al., 1990). In such cases hyperplasy indicates that the cortical bone layer grows to a greater thickness, either through increased amount of time in the growth stage, or accelerated growth rate. The periosteal cortex of the bone is therefore thicker, thus enlarging the entire bone (de Ricqlès and de Buffrénil, 2001). The total weight of the skeleton can also be multiplied through increased bone density. Osseous tissue may also become more dense via a condition termed osteosclerosis. This condition involves disruption of endochondral ossification, resulting in less resorption of endochondral tissue and a lack of endosteal development. Therefore, cancellous bone tissue or marrow cavities do not form and the bones are instead filled with dense calcified cartilage. Pachyostosis and osteosclerosis can occur concurrently, and this combined condition

Alexandronectes zealandiensis gen. et sp. nov., a new aristonectine plesiosaur from the lower Maastrichtian of New Zealand

ABSTRACT A fragmentary plesiosaur skull from lower Maastrichtian levels of the Conway Formation, New Zealand, is redescribed. Originally regarded as pertaining to two separate individuals, we argue that they represent a single individual belonging to a new aristonectine elasmosaurid, Alexandronectes zealandiensis gen. et sp. nov. This new taxon has common morphologies with other aristonectines such as expansion of the pterygoids extending posteriorly beyond the occipital condyle (as observed in Ar. quiriquinensis and probably in Kaiwhekea katiki) and the presence of an ‘A’-shaped squamosal arch in dorsal view. Otherwise, it is distinguished from these latter species by having different paraoccipital processes, a different mandibular glenoid, and an adult skull comparatively smaller than K. katiki and Aristonectes spp. The new taxon is a morphologically intermediate form between the dorsoventrally high skull of K. katiki and the mediolaterally expanded skulls of Aristonectes spp. The studied specimen is the second genus and species and the third report of an aristonectine recovered from lower Maastrichtian beds of New Zealand, emphasizing the diversity of this group in New Zealand and also indicating that aristonectines could include smaller species than those already known.

A Virtual Phytosaur Endocast and its Implications for Sensory System Evolution in Archosaurs

ABSTRACT Many recent studies have detailed the morphology of archosaurian endocrania. However, the outgroup to Archosauria, Phytosauria, has yet to be studied with modern techniques that would allow reconstruction of their internal anatomy. Pseudopalatus mccauleyi is a derived phytosaur from the Upper Triassic Chinle Formation, Arizona. A skull of P. mccauleyi, USNM 15839, was imaged using computed tomography in order to create the first high-quality, digitally reconstructed phytosaur endocast. Pseudopalatus mccauleyi exhibits overall endocast morphology that is similar to that of an extant crocodylian. These clades, phytosaurs and extant crocodylians, exhibit convergent Baupläne and similar inferred ecologies. A notable difference between the endocasts of the two clades is a considerable dural expansion in P. mccauleyi that denotes a large pineal body. This expansion, and the overall morphology of the endocast, is consistent with the historic endocranial reconstructions of the phytosaurs Pseudopalatus buceros, Smilosuchus gregorii, and Parasuchus hislopi. A comparison with phylogenetically diverse archosaurian endocasts reveals that endocast morphologies are highly conserved within Pseudosuchia, regardless of Bauplan or ecology. This conservatism is in contrast to the diversity of endocast morphology observed within Theropoda and Sauropodomorpha, or between members of those clades and Pseudosuchia. The most pronounced variability in pseudosuchian endocast morphology is a trend in size reduction of the pineal region, from a large basal condition to a reduced derived condition wherein the pineal region is indistinguishable from the rest of the endocast. A similar trend in pineal reduction is also seen in theropods and sauropods. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline/UJVP

A new partial skeleton of a cryptocleidoid plesiosaur from the Upper Jurassic Sundance Formation of Wyoming

ABSTRACT Cryptocleidoid plesiosaurs from the Upper Jurassic are well known from the Oxford Clay (Callovian) of the United Kingdom. The plesiosaurs of the nearly coeval Sundance Formation (Oxfordian) of North America are poorly known, but are thought to include two cryptocleidoid taxa: Pantosaurus striatus and Tatenectes laramiensis. Here we present two specimens recently recovered from the Bighorn Basin of Wyoming. The first specimen comprises three articulated adult cervical vertebrae and fragments of a fourth. This specimen preserves a posteriorly directed cervical neural spine, a character diagnostic of Pantosaurus striatus. It also resembles Pantosaurus in the morphology of its cervical rib articulations. The second specimen is a partial articulated skeleton comprising a complete pelvic girdle, dorsal, sacral, and caudal vertebrae, and numerous ribs and gastralia. This specimen displays a number of unique characters, including posteriorly directed dorsal, sacral, and caudal neural spines, highly autapomorphic illia, and a pathologically asymmetric pelvic girdle. Despite the lack of overlapping material, it is tentatively referred to Pantosaurus on the basis of posteriorly directed neural spines. These specimens represent the first significant adult material provisionally referable to Pantosaurus striatus, as well as the first posterior axial column and pelvic girdle.

The occurrence of vertebrate and invertebrate fossils in a sequence stratigraphic context: the Jurassic Sundance Formation, Bighorn Basin, Wyoming, U.S.A.

ABSTRACT Previous studies of the sequence stratigraphic distribution of fossils have focused on the record of relatively abundant marine invertebrates. Only a handful of studies have examined how sequence stratigraphic architecture influences the occurrence of vertebrates, particularly large and rare tetrapods. The Jurassic Sundance Formation of the Bighorn Basin, Wyoming, USA, contains a rich suite of invertebrate and vertebrate fossils, including large and rare marine reptiles, and this allows the sequence stratigraphic controls on the distribution of these groups to be compared. The Sundance Formation consists of four depositional sequences, with the lower two being carbonate dominated and the upper two siliciclastic dominated. Two incised valley fills are also present. The presence of multiple depositional sequences and strongly erosional sequence boundaries is the likely cause of the complicated lithostratigraphic nomenclature of the Sundance. Invertebrates (mollusks and echinoderms) in the Sundance conform to well-established patterns of occurrences, including strong facies control and fossil concentrations at maximum flooding surfaces, in the upper portion of parasequences, and within lags overlying sequence boundaries. As expected from their rarity, marine reptiles (ichthyosaurs, plesiosaurs, and pliosaurs) show a weaker connection to sequence stratigraphic architecture. Nonetheless, they do display facies control and are found primarily in offshore mudstone, rather than shoreface and estuarine sandstone. They are also more common at hiatal surfaces, including a zone of concretions at the maximum flooding surface and in lag deposits overlying sequence boundaries. These associations suggest that sequence stratigraphic architecture may be a useful approach for discovery of marine vertebrates and that sequence stratigraphic context should be considered when making paleobiological interpretations of marine vertebrates as well as invertebrates.

Cranial Anatomy of Morturneria seymourensis from Antarctica, and the Evolution of Filter Feeding in Plesiosaurs of the Austral Late Cretaceous

ABSTRACT This paper redescribes the holotype skull of the aristonectine elasmosaur Morturneria seymourensis from the upper Maastrichtian of Seymour Island, Antarctica. This description supports the validity of the genus Morturneria, distinct from the genus Aristonectes from Chile and Argentina. The paroccipital process of Morturneria is plesiomorphic, similar to Alexandronectes and unlike the autapomorphic occiput of Aristonectes. The palate of Morturneria is autapomorphic in possessing a strongly developed midline keel. The cranium of Morturneria is about 60% complete and preserves the anterior skull roof and palate; both regions were previously unknown in any aristonectine. The combination of the Morturneria holotype and recent research on other aristonectines allows the first confident cranial reconstruction of an aristonectine elasmosaur. The cranial anatomy of both Morturneria and its close relatives is derived relative to all other plesiosaurs, possessing a novel suite of dental and oral cavity adaptions. The suspensorium extends far behind the occipital condyle, and the jaw is long and hoop-like; together these features allowed a large gape and oral cavity volume. The palate of Morturneria is strongly keeled, forming arched lateral oral chambers that further increased oral cavity volume. The dentition of Morturneria is similar to that of Aristonectes, and all share autapomorphic interlocking combs of needle-like teeth that occluded outside the mouth and did not meet tip to tip. The upper and lower dentition formed an oral battery that may have functioned like a sieve in straining food particles from substrate ejected from the oral cavity. We theorize that this highly derived suite of adaptations is convergent with extant gray whales and archaic mysticetes and hypothesize that it functioned similarly in sieve feeding following suction. This is the first identification of whale-like filter feeding in any marine reptile, a condition once claimed to be anatomically impossible.