Argentinonectes

Argentinonectes is an extinct genus of weddellonectian plesiosaur known from the Late Cretaceous (Maastrichtian age) Calafate Formation of Argentina. The genus contains a single species, Argentinonectes calafatensis, known from a partial skeleton.^[1]

The holotype specimen MPM-PV 1002 was found in October 2009 by Fernando Novas on the southeastern shore of the Argentino Lake, in sediments dating to the upper Maastrichtian age of the Late Cretaceous, and he spotted a section of the paddle and tail at the base of the lake. A section of the lake was then drained to excavate the specimen, and it was transported to Buenos Aires using a bulldozer.^[2] The holotype includes various post-cranial skeletal elements, including: four cervical vertebrae, two pectorals, eighteen dorsal vertebrae, three sacral vertebrae, twenty-eight caudal vertebrae, several articulated ribs, disarticulated gastralia, an almost complete pectoral girdle (the right scapula and both coracoids), the proximal end of the right ilium, an almost complete right forelimb and both hindlimbs. After the excavation, the specimen was brought to the Museo Argentino de Ciencias Naturales in Buenos Aires, where it was subsequently prepared,^[1] and it was available for study by December 2015.^[3]

The holotype was found with its distal appendicular elements, skull and neck all missing, the authors hypothesize that these elements were detached during a bloat-and-float decomposition. The authors also noted that the left coracoid presents parallel scratches on its dorsal surface, presumably made by scavengers.^[1] The authors also noted that several isolated teeth of sharks were found in close association to the holotype of Argentinonectes: the dogfish shark Protosqualus argentinensis, the cow shark Notidanodon dentatus (considered a species of Xampylodon by Cappetta et al. 2021),^[4] and Echinorhinus maremagnum.^[1]

The genus name Argentinonectes refers to the Argentino Lake, on the shore of which the holotype was found and the Greek word "nektes", meaning swimmer. The specific name refers to El Calafate town, located approximately 20 kilometers South-West from where the fossils where found.^[1]

Argentinonectes was a large-sized weddellonectian, distinguished from other members of Weddellonectia by multiple autapomorphies, those being: a strong concavity of the coracoid lateral margin; a boot-shaped posteromedial end of the coracoid; the distal ramus of the scapula being laterally short and dorsally rounded; the dorsal transverse processes having a T-shaped cross-section, with an almost flat dorsal surface; a well-developed sigmoid curvature in the humerus; spool-shaped phalanges with expanded polygonal articular facets; and, eight terminal caudal vertebrae being strongly interlocked, with overlapping neural and haemal pedicels forming a rigid caudal complex, the haemals are also unfused, not forming chevrons. The authors also noted multiple synapomorphies, including: having 17-19 dorsal vertebrae; the dorsal transverse processes in the middle dorsal region being approximately horizontal; the chevron facet being located equally on the anterior and posterior edges of the centrum; the anterior margin of the scapular blade having a pronounced posterodorsal inflection; the posterolateral cornua of the coracoid being projected laterally surpassing the level of the glenoid; the preaxial margin of the distal humerus being concave with a relatively small anterior expansion, much smaller than the posterior expansion; and, the tibia being wider than the fibula.^[1]

The authors also found the holotype to represent an osteologically mature individual. They observed that the coracoids, dorsal vertebrae and parts of the caudal vertebrae are fully fused, though the degree of fusion in caudal vertebrae 23-27 is not complete. The appendicular elements of Argentinonectes also show fully ossified propodials (upper segment of a limb).^[1]

The Calafate Formation, in which the holotype of Argentinonectes was found, is a geological formation that dates to the upper Maastrichtian age of the Late Cretaceous and it overlays the Chorrillo Formation. The Calafate Formation preserves a near-shore marine environment, abundant fossil remains from this formation include marine invertebrates such as pelecypods, gastropods and brachiopods.^[1][5] The Calafate Formation is also coeval with other shallow marine beds deposited in vast areas of Patagonia, such as the La Colonia, Dorotea and Los Alamitos Formation.^[6]

The phylogenetic analysis conducted by Novas et al. (2026), mostly based on the dataset of Benson and Druckenmiller (2014),^[7] found Argentinonectes to be nested within the clade Weddellonectia, in a sister taxon relationship with Chubutinectes.^[1]