José P. O'Gorman

Postcranial morphology of Aristonectes (Plesiosauria, Elasmosauridae) from the Upper Cretaceous of Patagonia and Antarctica

Abstract A partial, postcranial skeleton of a juvenile individual referred to Aristonectes cf. parvidens from the upper Maastrichtian López de Bertodano Formation, Isla Marambio (Seymour Island), Antarctica, is described. Additionally, two juvenile specimens, also referred to A. cf. parvidens from the Allen Formation (upper Campanian–lower Maastrichtian) and Jagüel Formation (upper Maastrichtian) (Río Negro province, Argentina), are redescribed. The analysis of the systematic value of the cervical centrum proportions of juvenile specimens of Elasmosauridae suggests that these elements can be used to differentiate juvenile specimens of A. cf. parvidens from juveniles of other Elasmosauridae. On this basis, the specimens described are referred to A. cf. parvidens. Based on the proportion of the cervical centra, the first South American plesiosaur described by Gay in 1848 is here referred to A. cf. parvidens. The coracoid of Aristonectes is described for the first time showing a cordiform fenestra, a feature only recorded in the Elasmosauridae among the Plesiosauria, therefore, these new data support the inclusion of Aristonectes within the Elasmosauridae. With the new material described in this paper, Aristonectes is one of the most frecuently recorded genera of Late Cretaceous plesiosaurs in the Southern Hemisphere.

Vegasaurus molyi, gen. et sp. nov. (Plesiosauria, Elasmosauridae), from the Cape Lamb Member (lower maastrichtian) of the Snow Hill Island Formation, Vega Island, Antarctica, and remarks on Wedellian Elasmosauridae

ABSTRACT A new elasmosaurid, Vegasaurus molyi, gen. et sp. nov., from Vega Island, James Ross Archipelago, Antarctica, is described. The holotype and only specimen of this species (MLP 93-I-5-1) was collected from the lower Maastrichtian Cape Lamb Member of the Snow Hill Island Formation. Vegasaurus molyi is the only Antarctic elasmosaurid and one of only a few Late Cretaceous elasmosaurids from the Southern Hemisphere whose postcranial anatomy is well known. Vegasaurus molyi is distinguished from other elasmosaurids by the following combination of characters: cervical region with 54 vertebrae with elongated centra, dumbbell-shaped articular faces and lateral ridge present in the anterior and middle parts of the neck but absent in the posterior-most cervical vertebrae; scapula with ventral ramus bearing a strong ridge in the anteromedial corner of its dorsal surface; ilium shaft with expanded distal end, divided into two parts forming an angle of 140° opening anteriorly; and humerus with anterior knee and prominent posterior projection with accessory posterior articular facet. Preliminary phylogenetic analysis places V. molyi within a clade that includes the Late Cretaceous Wedellian aristonectine elasmosaurids, Aristonectes and Kaiwhekea. This indicates a close relationship between Aristonectinae and non-Aristonectinae Late Cretaceous Weddellian elasmosaurids and suggests a Weddellian origin for the Aristonectinae.

The Oldest Lower Upper Cretaceous Plesiosaurs (Reptilia, Sauropterygia) from Southern Patagonia, Argentina

Abstract. Plesiosaurs are recorded for the first time from the lower section of Mata Amarilla Formation, Santa Cruz Province, Patagonia, Argentina. The stratigraphic succession consists of mudstones and siltstones interbedded with medium to fine-grained sandstone, deposited in a littoral environment during the Cenomanian-Santonian; therefore the material is the oldest record of plesiosaurs from the lower Late Cretaceous rocks of Argentina. The remains include teeth, some vertebrae, and one propodium assigned to Elasmosauridae indet. and Plesiosauria indet. The status of Polyptychodon patagonicus Ameghino, 1893, as well as its stratigraphic position are discussed, leading to the conclusion that the material described by Ameghino is probably from the Mata Amarilla Formation and can only be referred to Plesiosauria indet. Analysis of sedimentologic features suggests that the material described here was deposited in an estuarine environment, strongly influenced by tides. The characters of the inferred environment are consistent with the type of preservation of the materials.

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 Small Body Sized Non-Aristonectine Elasmosaurid (Sauropterygia, Plesiosauria) from the Late Cretaceous of Patagonia with Comments on the Relationships of the Patagonian and Antarctic Elasmosaurids

Abstract. The systematics of the Late Cretaceous non-aristonectine elasmosaurids from Argentinean Patagonia are poorly known as there is no valid species currently recognized. Here a new non-aristonectine elasmosaurid: Kawanectes lafquenianum nov. comb. from the late Campanian—early Maastrichtian Allen Formation is diagnosed. K. lafquenianum is a distinctively small-body sized non-aristonectine elasmosaurid characterized by caudal vertebrae with marked laterally projected parapophyses, presence of pelvic bar, high ratio (∼1.2) between humerus/femur length and a large posterodistal projection of the humerus which bears a posterior accessory articular facet. A phylogenetic analysis recovered K. lafquenianum closely related with Morenosaurus stocki, Vegasaurus molyi, and Aristonectinae, showing the relationships between the elasmosaurids from Patagonia, Western Antarctic, and the Pacific coast of the USA. K. lafquenianum is part of the fauna of the coeval Allen and La Colonia formations that also comprises indeterminate aristonectines and polycotylids. This relatively high diversity plesiosaur fauna includes the three main morphotypes (aristonectines, non-aristonectine elasmosaurids and polycotylids), which is remarkable because the depositational environments of the Allen Formation have been inferred as marginal marine to non-marine environments.

The oldest elasmosaurs (Sauropterygia, Plesiosauria) from Antarctica, Santa Marta Formation (upper Coniacian? Santonian-upper Campanian) and Snow Hill Island Formation (upper Campanian-lower Maastrichtian), James Ross Island

Elasmosaurs are recorded for the first time in the Lachman Crags Member (Beta Member) of the Santa Marta Formation (lower Campanian) and in the Herbert Sound Member of the Snow Hill Island Formation (upper Campanian). These are the first elasmosaurids from James Ross Island, Antarctic Peninsula. These records greatly improve our knowledge of the taxonomic diversity of plesiosaurs of the Santa Marta Formation and Herbert Sound Member of the Snow Hill Island Formation, and extend the lower limit of the record of Elasmosauridae in Antarctica to the lower Campanian, making this the oldest record of an Antarctic elasmosaur.

A new species of Pliosaurus (Sauropterygia, Plesiosauria) from the Upper Jurassic of northwestern Patagonia, Argentina

Abstract. Pliosaurus patagonicus sp. nov., from the middle Tithonian, Vaca Muerta Formation, Neuquén Province, Argentina, is described on an autapomorphic trait —i.e., the distance between the posterior functional alveolus and coronoid eminence (300 mm) equivalent to the last fourteen alveoli— and a unique combination of characters: short and robust mandibular symphysis with six pairs of functional alveoli; in lateral view the dorsoventral height is almost constant along the symphysis; low coronoid eminence and mandibular rami almost straight. As in other species of Pliosaurus genus the new taxon has trihedral teeth but the typical lateral expansion of the mandibular symphysis is missing. This new record reinforces the hypothesis of a trend toward the shortening of the mandibular symphysis observed in late Kimmeridgian—Tithonian species from the northern hemisphere. The presence of the macropredator Pliosaurus patagonicus sp. nov. in a protected marine environment close to the coast may be interpreted as the result of predatory and/or reproductive behaviors.

Reassessment of the upper Maastrichtian material from Chile referred to Mauisaurus Hector, 1874 (Plesiosauroidea: Elasmosauridae) and the taxonomical value of the hemispherical propodial head among austral elasmosaurids

The femoral hemispherical articular head was regarded as autapomorphic of Mauisaurus haasti Hector, 1874 from the upper Campanian of New Zealand. Review of three Maastrichtian taxa, Aristonectes quiriquinensis from Chile, Aristonectes sp. from Antarctica and Kaiwhekea katiki from New Zealand, reveals that the femoral hemispherical head is a common character among aristonectines. This feature can therefore be discarded as an autapomorphy of Mauisaurus haasti. Propodials of A. quiriquinensis are shown to be the same as those in two coeval Chilean specimens previously referred to Mauisaurus sp.; in consequence, these are now referred to A. quiriquinensis. Additionally, specimens referred to A. quiriquinensis allow confirmation that during ontogeny the humerus and the femur change from a flat capitulum in juveniles to hemispherical heads of both the humeri and femora in the adult stage. The evidence shows that the presence of Mauisaurus along the southeastern Pacific margin cannot be verified to date.

NEW INSIGHTS ON THE ARISTONECTES PARVIDENS (PLESIOSAURIA, ELASMOSAURIDAE) HOLOTYPE: NEWS ON AN OLD SPECIMEN

Abstract. New observations on the holotype of the Maastrichtian aristonectine elasmosaurid Aristonectes parvidens Cabrera from Chubut Province, Argentina, reveals features that were not previously described while resulting in new interpretations. Quantitative comparison with non-aristonectine elasmosaurids demonstrates that the increase in the number of alveoli in the premaxilla, maxilla and mandible is not a consequence of skull size increase. Instead, decrease in alveolar size and interalveolar space, compared with that observed in non-aristonectine elasmosaurids, is at least as important. Increase in skull length compared with non-aristonectine elasmosaurids is not as marked as typically considered: skull length is equivalent to the summatory of length of the first 8–10 cervical vertebrae in non-aristonectines and the first 10–12 in aristonectines. The cervical region is characterized by a rapid increase of the relative broadness of the vertebral centra, which indicates a reduction in lateral mobility of the neck. Increase in alveoli number is achieved as a result of a number of changes that seem to indicate the importance of the biological role of the length of the alveolar row, this is probably related to changes in prey size and capture strategy such as trading for smaller fishes or invertebrates and/or a upgrading from ambushing one individual prey to simultaneously ambushing multiple prey individuals.

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.