Probable basal allosauroid from the early Middle Jurassic Ca\xf1ad\xf3n Asfalto Formation of Argentina highlights phylogenetic uncertainty in tetanuran theropod dinosaurs

Oliver W M Rauhut,Diego Pol

doi:10.1038/s41598-019-53672-7

Abstract

Tetanurae, the most successful clade of theropod dinosaurs, including modern birds, split into three major clades early in their evolutionary history: Megalosauroidea, Coelurosauria, and Allosauroidea. The oldest tetanurans occur in the earliest Middle Jurassic, but the early fossil record of the clade is still poor. Here we report one of the oldest known and most complete pre-Late Jurassic tetanuran, the probable allosauroid Asfaltovenator vialidadi gen. et sp. nov., which has an unusual character combination, uniting features currently considered to be apomorphic of different tetanuran lineages. A phylogenetic analysis resulted in a monophyletic Carnosauria (Allosauroidea + Megalosauroidea), and the inclusion of the new taxon significantly changes topology within carnosaurs. The analysis shows concentrated homoplasy in proximal nodes at the base of Tetanurae, and a temporal peak at the Pliensbachian-Toarcian extinction event, recently identified as a potential driver of tetanuran radiation. These results highlight the complex morphological evolution in the early radiation of tetanuran theropods, in which convergences and parallelisms were extremely common. This pattern seems to be a common feature in rapid radiation events of major clades of vertebrates and might explain the common difficulties to unravel phylogenetic relationships of important lineages at the base of major clades.

Highlights

The Tetanurae represent the most diverse clade of theropod dinosaurs, which includes most of the well-known Mesozoic theropods, such as Allosaurus or Tyrannosaurus, and modern birds[1]
Large basal tetanuran diagnosed by the following character combination: premaxillary teeth with well-developed distal, but only minute mesial serrations*; postorbital with small cornual dorsal process; exoccipital with pronounced horizontal ridges between paroccipital processes and foramen magnum*; ossified antarticular in the mandible; platycoelous cervical vertebrae; neural spines of cervical vertebrae three and four triangular and backswept*; anterior cervical epipophyses tab-like and elongated; mid-cervical vertebrae with median pit between parapophyses ventrally; ventral keel absent in posterior cervical and poorly www.nature.com/scientificreports developed in anterior dorsal vertebrae; well-developed paradiapohyseal lamina in middle and posterior dorsal vertebrae; dorsals 11 and 12 with small additional anterior centrodiapophyseal lamina*; articulated metacarpus broader than long; manual digit III significantly more slender and shorter than digits II and III
The jugal expands anteriorly, as in most basal tetanurans, and the posterior, quadratojugal process is notably high at its base, higher than the jugal body below the orbit

Summary

Introduction

The Tetanurae represent the most diverse clade of theropod dinosaurs, which includes most of the well-known Mesozoic theropods, such as Allosaurus or Tyrannosaurus, and modern birds[1]. The supratemporal fossa does not extend onto the posterior process of the postorbital, in contrast to the situation in megalosaurids[2], www.nature.com/scientificreports and, as in allosauroids, the dorsal lamina of the squamosal is continuous between the lateral and medial ramus, not invaginated by the supratmeporal fenestra.

Results

Conclusion