Thursday, February 1, 2018

[Paleontology • 2018] Osteology of the Late Cretaceous Argentinean Sauropod Dinosaur Mendozasaurus neguyelap: Implications for Basal Titanosaur Relationships

Mendozasaurus neguyelap Gonzàlez Riga, 2003

in Gonzàlez Riga, Mannion, Poropat, David & Coria. 2018. 

The titanosaurian sauropod dinosaur Mendozasaurus neguyelap is represented by several partial skeletons from a single locality within the Coniacian (lower Upper Cretaceous) Sierra Barrosa Formation in the south of Mendoza Province, northern Neuquén Basin, Argentina. A detailed revision of Mendozasaurus, including previously undocumented remains from the holotype site, allows us to more firmly establish its position within Titanosauria, as well as enabling an emended diagnosis of this taxon. Autapomorphies include: (1) middle and posterior cervical vertebrae with tall and transversely expanded neural spines that are wider than the centra, formed laterally by spinodiapophyseal laminae that are not connected with the pre- or postzygapophyses; (2) anterior caudal vertebrae (excluding anteriormost) with ventrolateral ridge-like expansion of prezygapophyses; and (3) humerus with divided lateral distal condyle on anterior surface. New remains demonstrate that the presacral vertebrae of Mendozasaurus were not unusually short anteroposteriorly, with this compression instead resulting from taphonomic crushing. Comparative studies of articulated pedes of other taxa allow us to interpret that the pedal formula of Mendozasaurus was 2-2-2-2-0, based on disarticulated bones that form a right hind foot. Mendozasaurus was incorporated into an expanded version of a titanosauriform-focussed phylogenetic data matrix, along with several other contemporaneous South American titanosaurs. The resultant data matrix comprises 84 taxa scored for 423 characters, and our phylogenetic analysis recovers Mendozasaurus as the most basal member of a diverse Lognkosauria, including Futalognkosaurus and the gigantic titanosaurs Argentinosaurus, Notocolossus, Patagotitan and Puertasaurus. Lognkosauria forms a clade with Rinconsauria (Muyelensaurus + Rinconsaurus), with Epachthosaurus and Pitekunsaurus recovered at the base of this grouping. A basal lithostrotian position for this South American clade is well supported, contrasting with some analyses that have placed these taxa outside of Lithostrotia or closer to Saltasauridae. The sister clade to this South American group is composed of an array of near-global taxa and supports the hypothesis that most titanosaurian clades were widespread by the Early–middle Cretaceous.

Figure 3. Life restoration of Mendozasaurus neguyelap based on the largest adult individual (femur length: 1530 mm). Artwork by Bernardo Gonzaléz Riga.

A detailed description of all remains pertaining to the early Late Cretaceous Argentinean titanosaurian sauropod dinosaur Mendozasaurus neguyelap enables a revised diagnosis for the genus. An expanded phylogenetic analysis recovers Mendozasaurus and several other taxa as part of a rich Lognkosauria that is placed within a diverse clade of South American lithostrotian titanosaurs. The sister clade to this South American group is composed of a near-global array of titanosaurs, which supports recent work that has argued for a widespread distribution of most titanosaurian clades by the Early–middle Cretaceous.

Bernardo J. Gonzàlez Riga, Philip D. Mannion, Stephen F. Poropat, Leonardo D. Ortiz David and  Juan Pedro Coria. 2018. Osteology of the Late Cretaceous Argentinean Sauropod Dinosaur Mendozasaurus neguyelap: Implications for Basal Titanosaur Relationships. Zoological Journal of the Linnean Society. zlx103.  DOI: 10.1093/zoolinnean/zlx103
Bernardo J. González Riga. 2003. A new titanosaur (Dinosauria, Sauropoda) from the Upper Cretaceous of Mendoza, Argentina. Amehginiana. 40: 155-172.

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