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| Calvarius rapidus Prieto-Márquez and Albert Sellés, 2023 |
ABSTRACT
A metatarsal IV from a hitherto unknown endemic small-bodied styracosternan is described from uppermost Maastrichtian strata of NE Spain, part of the Ibero-Armorican island of the Late Cretaceous European Archipelago. This element is unique among the fourth metatarsals of all other ornithopods in combining the greatly elongated proportions of basally branching members of the clade with the prominent medial flange seen in other styracosternans. This specimen becomes the holotype of a new genus and species. Histological data indicate that this individual was a late subadult at the time of death, but not far from reaching somatic maturity. By analogy with the elongate metatarsals of lightly built, small-bodied cursorial ornithopods, it is likely that this animal 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. Notably, this styracosternan represents an exception to the mediportality and large body size that characterize all other members of the clade. The elongation of metatarsal IV, inferred cursoriality, and small body size constitute a case of evolutionary convergence with the distantly related non-iguanodontian ornithopods and dryosaurid and elasmarian iguanodontians. Unlike other regions of the world where, during the latest stages of the Cretaceous, the small-bodied cursorial herbivore ecological niche was represented by non-iguanodontians and non-hadrosauriforms, in the Ibero-Armorican island this niche was likely occupied by a styracosternan.
SYSTEMATIC PALEONTOLOGY
DINOSAURIA Owen, Citation 1842
ORNITHISCHIA Seeley, Citation 1887
ORNITHOPODA Marsh, Citation 1881
IGUANODONTIA Baur, Citation 1891
STYRACOSTERNA Sereno, Citation 1986
CALVARIUS RAPIDUS, gen. et sp. nov.
Type [and Only] Species— Calvarius rapidus.
Holotype and Only Known Material— MCD-8734, a left metatarsal IV.
Diagnosis—Styracosternan iguanodontian ornithopod dinosaur having a metatarsal IV with the following autapomorphies: maximum length/mediolateral width at mid-length ratio greater than nine; segment distal to medial flange longer than combined length of medial flange and proximal extent of metatarsal; prominent medial flange that is dorsoplantarly compressed and with a prominent plantar component in its medioplantar projection; and relatively enlarged medially projected dorsomedial corner of distal surface.
Etymology—The genus name refers to a nearby hill known as Serrat del Calvari. It is also a derivation of the Catalan ‘calvari’ (‘suffering’), alluding to the chronostratigraphic proximity of MCD-8734 to the K-Pg extinction event. The species name means ‘quick’ or ‘fast’ in Latin, referring to the likely cursorial adaptations of this animal.
CONCLUSION:
We describe a new genus and species of a relatively small styracosternan iguanodontian ornithopod from the Ibero-Armorican domain of south-western Europe. This animal is represented by a highly modified metatarsal IV nowhere to be found among ornithopods. The fossil comes from the Masia de Ramón locality, corresponding to uppermost Maastrichtian strata of the Talarn Formation, Lleida province, NE Spain. Within Ornithopoda, this metatarsal IV is unique in combining the greatly elongated proportions of early-diverging members of the clade with the prominent medial flange seen in more derived styracosternans such as hadrosaurids. Histological data indicate that this specimen was a late subadult at the time of death, approaching somatic maturity. By analogy with the elongate metatarsals of lightly built, small-bodied cursorial ornithopods, it is likely that this animal 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.
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. C. rapidus is also probably another case of endemism in an insular terrestrial environment.
Albert Prieto-Márquez and Albert Sellés. 2023. Evolutionary Convergence in A Small cursorial styracosternan ornithopod Dinosaur from western Europe. Journal of Vertebrate Paleontology. 42(5); e2210632. DOI: 10.1080/02724634.2023.2210632
https://www.icp.cat/index.php/en/press-room/noticies-icp/item/3328-calvarius-rapidus-the-new-dinosaur-species-that-almost-saw-the-asteroid-falling
