Forelimb and Pectoral Anatomy of Arcticodactylus cromptonellus, an Early Pterosaur from the Late Triassic, and the Origins of Pterosaurs

Pterosaurs represent the earliest appearance of only three clades of flying vertebrates, the pioneers of aerial vertebrate ecospace, and the lineage to produce the largest known flying organisms. The origins of the pterosaurian flight apparatus have been difficult to ascertain, in part, due to incomplete or two-dimensional preservation of the earliest (Triassic—Jurassic) pterosaur remains. An exceptional early pterosaur specimen that is preserved in three dimensions, the holotype and only known specimen of Arcticodactylus cromptonellus (Fleming Fjord Formation, Greenland) may help address these problems. However, it has remained mostly encased within matrix to protect the delicate elements, obscuring external study. Here I present new synchrotron tomographic scan data of the forelimb (wing-forming) elements of Arcticodactylus cromptonellus. I find that the forelimb of Arcticodactylus is a structural intermediate between the forelimb of early archosaurs and derived pterosaurs. In light of this intermediacy, I reexamined the phylogeny of early Pterosauromorpha, completely reviewing forelimb characters with additional consideration given to other important anatomical regions for pterosauromorph phylogeny. I find that the contents of Lagerpetidae represent a grade of non-pterosaur pterosauromorphs and that the pterosauromorph Scleromochlus taylori is actually closely-related to crocodylomorphs. I recover Arcticodactylus as the earliest-diverging pterosaur, with the pterosaurs of the early Mesozoic (Triassic—Early Jurassic) forming a highly-nested, gradational relationship around a monophyletic Late Mesozoic pterosaur clade with very few multispecific groups exclusive of this latter clade. The sum of this work is an understanding of the current pterosaur fossil record as preserving the gradual assembly of the pterosaur bauplan in exquisite detail.