Pliosaurus almanzaensis
Age
Jurassic（Tithonian）
144 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Order: Plesiosauria
Family: Pliosauridae
Genus: Pliosaurus
Species: Pliosaurus almanzaensis
Morphological description
The holotype of Pliosaurus almanzaensis, MOZ 3728P, preserves an almost complete mandible, the palate, basicranium, hyoid elements, and part of the postcranium. The mandible is triangular and measures approximately 90 cm from the posterior limit of the mandibular symphysis to the retroarticular process, with a width of about 43 cm between the glenoid cavities. The mandibular symphysis is relatively long, with nearly straight lateral margins. Its anterior portion is wider than high and lacks both conspicuous lateral expansion and a ventral keel. One of the most distinctive features of the species is the anterior extension of the angular into the mandibular symphysis, which was regarded by the researchers as an autapomorphy. A second autapomorphy occurs in the occipital condyle: although its surface shows some irregularities, it lacks a well-marked notochordal pit as well as the irregularly arranged grooves commonly seen in other species of Pliosaurus.
The mandibular symphysis contains at least nine pairs of alveoli, although the actual number may have been higher because part of the symphyseal region is missing. Posterior to the symphysis, the right mandibular ramus preserves 17 alveoli, whereas the incomplete left ramus preserves 15. The right mandible therefore had at least 26 alveoli in total, and the left at least 24. No tooth crown is completely preserved, but the surviving teeth clearly show the characteristic trihedral cross section. Their labial surface is flat and lacks striae. The first pair of anterior alveoli is relatively small and directed cranially, the second pair is larger, and the third to fifth pairs are larger still and inclined posteromedially. The parasphenoid lacks a sharp ventral median keel and is approximately triangular in cross section.
The basicranium preserves structures that are often obscured by the skull roof or poorly preserved in other Pliosaurus specimens, including a basisphenoid with a deep posterior notch and a well-preserved parasphenoid. The rugose surface of the basisphenoid suggests a relatively poor degree of ossification, and the sella turcica lies within a caudoventrally directed depression. The postcranial skeleton is more poorly preserved and includes a posterior cervical centrum, several dorsal centra, and elements of all four limbs. The femora are distinctly larger than the humeri, and the right forelimb is the best-preserved limb.
Etymology
Genus name, Pliosaurus: The Greek prefix plio- means "more" or "greater," while saurus means "lizard."
Species name, almanzaensis: Named after Cajón de Almanza in Patagonia, Argentina, where the holotype specimen MOZ 3728P was discovered.
Biological description
Pliosaurus almanzaensis was a large marine pliosaurid that lived during the Late Jurassic. Members of the family Pliosauridae were large predatory marine reptiles of Mesozoic seas, typically characterized by short necks and proportionally large skulls. Their fossil record extends from the Early Jurassic to the early Late Cretaceous. Most species of Pliosaurus known at the time of the study came from the Northern Hemisphere, particularly Europe, making the Patagonian specimen MOZ 3728P especially important for understanding the evolutionary history of the genus in the Southern Hemisphere.
The history of the specimen's discovery dates back to early 1986, when Sergio Cocca and Rafael Cocca, technicians at the Museo Provincial de Ciencias Naturales Prof. Dr. Juan A. Olsacher in Zapala, Argentina, discovered part of a pliosaur postcranium exposed at Cajón de Almanza in west-central Neuquén Province, approximately 10 km east of the town of Loncopué. Later that year, museum staff and researchers returned to the site in an attempt to determine the possible position of the skull. In February 1987, the MOZ team collected the entire specimen, MOZ 3728P. In January 1988, researchers subsequently determined its exact stratigraphic position. The specimen was later designated as the holotype of Pliosaurus almanzaensis.
The fossil was recovered from the Vaca Muerta Formation and dates to the late Tithonian of the Jurassic, placing this animal very close to the end of the Jurassic Period. The locality lies in the deeper western sector of the Neuquén Basin, where the Vaca Muerta Formation exceeds 500 m in thickness and is dominated by offshore facies. A widespread marine transgression began in the basin during the early Tithonian, transforming an area previously dominated by continental deposition into an extensive marine depositional system.
The strata containing the fossil record a distinctive marine environment. The lower part of the succession is dominated by fine-grained deposits ranging from siliciclastic mudstones to mixed siliciclastic-carbonate marls. These sediments accumulated under anoxic bottom conditions that favored the preservation of organic matter. Upsection, the succession grades into dark grey and greenish calcareous, laminated and bioturbated mudstones, interbedded with thin layers of very fine- to fine-grained sandstone and tuffaceous sandstone. A conspicuous floatstone bed composed predominantly of oyster shells is also present. This succession records a progradational depositional system interpreted as foreset-topset ramp deposits developed above anoxic bottomset facies.
Pliosaurus almanzaensis is particularly important for understanding the later evolutionary history of Pliosaurus. Earlier models based on other species suggested that the total number of mandibular alveoli declined from the Kimmeridgian into the Tithonian, with the most pronounced reduction occurring in the mandibular symphysis. By the Tithonian, several species were characterized by relatively low symphyseal alveolar counts.
Reference
O'Gorman JP et al. (2018). A new Pliosaurus species (Sauropterygia, Plesiosauria) from the Upper Jurassic of Patagonia: new insights on the Tithonian morphological disparity of mandibular symphyseal morphology. Journal of Paleontology.
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