Histogenesis and growth dynamics of the tiny Vegagete rhabdodontomorph hindlimb (Ornithischia, Ornithopoda): Paleoecological and evolutionary implications

Abstract

The Vegagete rhabdodontomorph is an as-yet unnamed ornithopod known from tiny and fragmentary remains, and was collected in the upper Barremian–lower Aptian of the Salas de los Infantes municipality (Burgos Province, Spain). As the earliest known rhabdodontomorph and a stem member of the lineage leading to Rhabdodontidae, this animal is key to better understanding growth within basal Iguanodontia. To do so, we prepared histological thin-sections of a sample of differently sized femora and tibiae. Here we combined anatomical, micro-anatomical and histological observations to analyse the growth and life-history traits of this little animal, and reinterpreted growth and postural shifts within the broader evolutionary framework of Rhabdodontomorpha. We find that the largest Vegagete ornithopod individual was a late subadult, making it the smallest ornithopod ever recovered. This taxon would have shifted from a quadrupedal stance to a bipedal one at a juvenile stage. In sharp contrast, Late Cretaceous rhabdodontids would retain the juvenile quadrupedal stance of their ancestors, and maintain quadrupedality until their adulthood most probably through progenetic development. No sampled Vegagete ornithopod individual shows Lines of Arrested Growth (LAGs). However, a minimal age greater than one year is most likely. Given the many signs of histologic maturity, the absence of LAGs is here interpreted as uninterrupted, fast growth of the hindlimbs. This taxon would probably not have been able to endure long distance running. Instead, its small size and relatively long legs would have been beneficial to rapidly escape and hide from predators.