A BONE OF CONTENTION: OVERCOMING HISTORICAL MISINTERPRETATIONS OF THE FIFTH METATARSAL IN THEROPOD DINOSAURS

PALOMBI, DAMIANO; CANALE JUAN IGNACIO; BRINKMAN, DANIEL

General Roca

Congreso; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2023

The fifth metatarsal of theropod dinosaurs is often ignored because of its lack of demonstrable functionality, resulting in diverse interpretations of the articulation of this element and misleading reconstructions of the pes in some of the most iconic theropods known. The first representation of this element, in the reconstruction of Allosaurus (AMNH FARB 324), erroneously articulates the fifth metatarsal with the first metatarsal, essentially converting the fifth metatarsal into the first metatarsal’s proximal half. This interpretation contrasts starkly with our current knowledge of the pes of theropod dinosaurs, which adheres to a 2-3-4-5-0 phalangeal formula in which the hallux consists of a diminished first metatarsal together with two phalanges, while the fifth metatarsal is reduced and atrophic, typically with a semilunate morphology and a non-articular distal surface. Despite this, depictions of the first metatarsal in some theropods as consisting of proximal and distal halves still appear in scientific articles, books and in museum displays. Yet, we now know that the previously ascribed proximal half of the first metatarsal is actually the entire fifth metatarsal. Since the initial reconstruction of this element in AMNH FARB 324, the pedes in numerous Allosaurus specimens worldwide have been inaccurately reconstructed. Perhaps the most conspicuous example of this error is in the pedes of the mounted skeleton of Allosaurus (AMNH FARB 27457) situated at the main entrance of the American Museum of Natural History. Moreover, this misinterpretation was replicated by Osborn, then director of AMNH, during the original mounting of their Tyrannosaurus rex (AMNH FARB 5027). This error also affected the first illustration of Spinosaurus and the first osteological description of the holotype composite left pes of Deinonychus (YPM VP.005205, .005217, and .005240). In his description of Deinonychus, Ostrom (following Osborn) proposed that the latter specimen was the proximal half of the first metatarsal, though we here identify it as being the fifth metatarsal of the partial Tenontosaurus skeleton (YPM VP.005466) found at the D. antirrhopus type locality. Moreover, the proliferation of the fifth metatarsal misrepresentation has reached South America affecting the portrayals of some of Argentina's earliest described and most iconic theropods, such as Piatnitzkysaurus and Carnotaurus. These errors underscore the inadvertent propagation of misconceptions arising from inaccurate reconstructions of the pedes of some theropods and accentuate the need for meticulous examination of even seemingly insignificant elements; thus, lessening the lasting impact of the historical inaccuracies in some paleontological reconstructions.