A New Small-Bodied Azhdarchoid Pterosaur from the Lower Cretaceous of England and Its Implications for Pterosaur Anatomy, Diversity and Phylogeny

Abstract

BackgroundPterosaurs have been known from the Cretaceous sediments of the Isle of Wight (southern England, United Kingdom) since 1870. We describe the three-dimensional pelvic girdle and associated vertebrae of a small near-adult pterodactyloid from the Atherfield Clay Formation (lower Aptian, Lower Cretaceous). Despite acknowledged variation in the pterosaur pelvis, previous studies have not adequately sampled or incorporated pelvic characters into phylogenetic analyses.Methodology/Principal FindingsThe new specimen represents the new taxon Vectidraco daisymorrisae gen. et sp. nov., diagnosed by the presence of a concavity posterodorsal to the acetabulum and the form of its postacetabular process on the ilium. Several characters suggest that Vectidraco belongs to Azhdarchoidea. We constructed a pelvis-only phylogenetic analysis to test whether the pterosaur pelvis carries a useful phylogenetic signal. Resolution in recovered trees was poor, but they approximately matched trees recovered from analyses of total evidence. We also added Vectidraco and our pelvic characters to an existing total-evidence matrix for pterosaurs. Both analyses recovered Vectidraco within Azhdarchoidea.Conclusions/SignificanceThe Lower Cretaceous strata of western Europe have yielded members of several pterosaur lineages, but Aptian pterosaurs from western Europe are rare. With a pelvis length of 40 mm, the new animal would have had a total length of c. 350 mm, and a wingspan of c. 750 mm. Barremian and Aptian pterodactyloids from western Europe show that small-bodied azhdarchoids lived alongside ornithocheirids and istiodactylids. This assemblage is similar in terms of which lineages are represented to the coeval beds of Liaoning, China; however, the number of species and specimens present at Liaoning is much higher. While the general phylogenetic composition of western European and Chinese communities appear to have been approximately similar, the differences may be due to different palaeoenvironmental and depositional settings. The western Europe pterodactyloid record may therefore be artificially low in diversity due to preservational factors.

The first pterosaur pelvic material from the Dinosaur Park Formation (Campanian) and implications for azhdarchid locomotion

“…The preacetabular tubercle is relatively smaller in Vectidraco and does not protrude as far laterally. Although the postacetabular process of Vectidraco is pneumatized, Naish et al (2013) did not describe any preacetabular pneumaticity, which is extensive in UALVP 56200. It is likely that the difference in degree of pneumatization is tied to body size (O'Connor 2009), but it may also indicate differences in pelvic air sac organization.…”

“…Furthermore, whereas most pterodactyloid preacetabular processes are spatulate and thin, UALVP 56200 tapers anteriorly and is relatively robust. The robust muscle attachment sites are unlike those of most pterosaurs, which have poorly developed muscle scars for hindlimb musculature (Molnar 1987;Wellnhofer 1988;Bennett 1990;Naish et al 2013;Frigot 2017). The ilium is similar to those of pteranodontids, but differs in several respects.…”

“…The anterior end of the preacetabular blade of Vectidraco is missing, and although it tapers anteriorly in lateral view, it does not taper transversely. The origin of ITB in Vectidraco is not as well developed, nor is it rugose, but Naish et al (2013) described a sharp lateral ridge, which is present in UALVP 56200 separating the origins of ITC and PIFI-1. The preacetabular tubercle is relatively smaller in Vectidraco and does not protrude as far laterally.…”

“…The preacetabular process contacts and is fused indistinguishably with a medial plate of bone formed from the fused distal sacral ribs, a condition termed a "fenestrated sacral shield" by Naish et al (2013). The preacetabular process is also fused to the sacral shield in Tropeognathus .…”

“…Furthermore, Bennett (1990) speculatively reconstructed the preacetabular process of AMNH 22569 as extending far past the first sacral rib, which is not the case in UALVP 56200. Naish et al (2013) described a small, nearly complete azhdarchoid pelvis (Vectidraco daisymorrisae) from the Early Cretaceous of the Isle of Wight, which provides important comparative material. Overall, the morphology of Vectidraco is similar to that of UALVP 56200, but there are important differences, including the scale.…”