Newly discovered pedes of Titanosaurian sauropod dinosaurs from Argentina yield novel data for phylogenetic analysis

GONZÁLEZ RIGA, BERNARDO J.; LAMANNA, MATHEW; ORTIZ DAVID, LEONARDO; CURRY ROGERS, KRISTI

Salt Lake City, Utah

Congreso; 76th Annual Meeting for the Society of Vertebrate Paleontology; 2016

Society of Vertebrate Paleontology

Studies of titanosaurian sauropod dinosaurs have historically been hampered by missing morphological data. In particular, the pedal osteology of titanosaurs has been poorly documented, though this situation is beginning to change with recent discoveries. At present, there are more than 60 valid taxa of titanosaurs, with five of these being known from complete, articulated pedes: Opisthocoelicaudia from Mongolia and Epachthosaurus, Notocolossus, and two unnamed species (from the Agua del Padrillo and La Invernada sites, respectively) from Argentina. Pedal elements are known for many other titanosaurs, but none of these preserve the hind foot in its entirety.Previous studies have recognized two titanosaurian pedal morphotypes, here termed ?long-footed? and ?short-footed.? In long-footed titanosaurs, the pes is strongly entaxonic, with the first four metatarsals exhibiting a significant increase in length and a concomitant decrease in robusticity from medial to lateral. This general pattern is present in medium-sized titanosaurs such as Epachthosaurus, Opisthocoelicaudia, and the Invernada and Padrillo forms, as well as in some larger taxa with slightly less complete pedes (e.g., ?Alamosaurus, Mendozasaurus). For example, in the Invernada titanosaur, the ratios of the minimum circumferences of metatarsals II?V to that of metatarsal I (here termed the ?robusticity index? or RI) varies from 0.82 to 0.64. The Padrillo taxon departs slightly from this pattern in having an unusually robust metatarsal V (RI = 0.91). Conversely, short-footed forms possess metatarsals that are all roughly the same length and robusticity, resulting in a hind foot that is comparatively shorter and more mediolaterally symmetrical than those of other titanosaurs. This morphology is currently observed only in the gigantic titanosaur Notocolossus, in which metatarsals II?V all have an RI of greater than 0.70.The phalangeal formulae of titanosaurs may also hold evolutionary significance, in that, among sauropods, phalangeal reduction reached its extreme in this group. The phalangeal formula of Epachthosaurus is 2-2-3-2-0, but in more derived taxa such as Notocolossus, the Padrillo and Invernada titanosaurs, and (probably) Mendozasaurus, this formula is reduced to 2-2-2-2-0 via the loss of a phalanx on digit III.In sum, recently discovered, complete titanosaurian pedes from Argentina exhibit substantial morphological variation, providing new sources of potentially informative characters relevant to the continued elucidation of titanosaur phylogeny.