A new genus and species of iguanodontian ornithopod dinosaur being named Calvarius rapidus has been described by a duo of paleontologists from the Universitat Autònoma de Barcelona.
Life reconstruction of the styracosternan dinosaur Hippodraco scutodens, a distant relative of Calvarius rapidus. Image credit: Sci.News / Lukas Panzarin / Bing Image Creator.
Calvarius rapidus lived on the Ibero-Armorican island, which was made of parts of what is now France, Spain, and Portugal, and was the biggest and westernmost island of the European Archipelago.
The species existed during the last 100,000 years before the end of the Cretaceous period.
It was one of the very last non-avian dinosaurs, which went extinct during the end-Cretaceous mass extinction event.
Calvarius rapidus belongs to Styracosterna, a subgroup of iguanodontian ornithopods that contains the hadrosaurids and all dinosaurs more closely related to them than to camptosaurids.
“Ornithopod dinosaurs form a clade representing one of the most evolutionarily successful radiations of Mesozoic vertebrates,” said Universitat Autònoma de Barcelona paleontologists Albert Prieto-Márquez and Albert Sellés.
“These herbivores colonized every continent and left a rich fossil record spanning the middle Jurassic through the latest Cretaceous.”
“One of the main evolutionary trends throughout ornithopod evolution is the progressive increase in body size.”
“In particular, the earliest ornithopods were relatively small (1-2 m in length), lightly built cursorial bipeds, exemplified by Hypsilophodon foxii from the Early Cretaceous of England or Orodromeus makelai from the Campanian of the Western Interior of North America.”
“With the evolution of Iguanodontia, and particularly Hadrosauriformes, ornithopods experienced a tenfold increase in body length, and at least 300-fold increase in body mass.”
“As ornithopods became more massive, they acquired a mediportal mode of locomotion.”
Calvarius rapidus represents a case of endemic small-bodied insular styracosternan with peculiar adaptations.
“The relatively small body size of this dinosaur, together with the morphological attributes of the metatarsal and inferred cursoriality, indicate evolutionary, and possibly ecological, convergence with distantly related clades of non-iguanodontian and early-diverging iguanodontian ornithopods,” the researchers said.
An isolated and highly modified fourth metatarsal of Calvarius rapidus was discovered in 2019 in the Talarn Formation of Spain.
The fossil came from the Masia de Ramón site, which is located 2.6 km south of Figuerola d’Orcau village in north-western Catalonia.
“The stratigraphic horizon at Masia de Ramón that yielded the specimen crops out at the top of the cliffs of the northern bank of the Conques River,” the scientists explained.
“Notably, Masia de Ramón occurs 25 m below the local Cretaceous-Paleogene boundary, thus placing it within the last 100,000 years of the Maastrichtian age.”
By analogy with the elongate metatarsals of lightly built, small-bodied cursorial ornithopods, it is likely that Calvarius rapidus was also capable of rapid locomotion.
“This is consistent with the differential distribution of intense bone remodeling in metatarsal IV, likely resulting from biomechanical stress produced during rapid hindlimb propelling,” the authors said.
“As a styracosternan, Calvarius rapidus represents an exception to the subcursoriality and mediportality, as well as the relatively large body size, that characterize other members of the clade.”
“The elongation of metatarsal IV, inferred cursoriality, and relatively small body size led us to hypothesize that this species evolved convergently in relation to the distantly related non-iguanodontian ornithopods and dryosaurid and elasmarian iguanodontians.”
“This also suggests that the postcranial skeleton of these animals was capable of substantial morphological plasticity.”
“In contrast to other regions of the world during the latest Cretaceous, where the small-bodied cursorial herbivore ecological niche was represented by non-iguanodontians and non-hadrosauriforms, in the Ibero-Armorican island of the Late Cretaceous European Archipelago this niche was occupied by a styracosternan,” they added.
“Calvarius rapidus is also probably another case of endemism in an insular terrestrial environment.”
The discovery is reported in a paper in the Journal of Vertebrate Paleontology.