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. / Master of Science / Flight has only evolved three times within animals with backbones. The first of these three is a group of distant relatives of birds called pterosaurs, which evolved flight independently from birds and produced the largest ever flying animals. Flight requires a suite of specialized adaptations, and these can obscure our understanding of how flying animals evolved if all we have are poorly-preserved fossils or only specialized flighted forms. These problems can be found in the origins of pterosaur flight, with well-preserved (non-crushed or deformed) skeletons generally known only from very late forms. An exception to this rule is the early pterosaur Arcticodactylus cromptonellus, represented by three-dimensionally-preserved skeletal remains. Using CT imagery to study the skeletal anatomy obscured by rock, I describe the forelimb anatomy of Arcticodactylus cromptonellus and include it within an analysis of the evolutionary relationships of pterosaurs and other reptiles. Arcticodactylus is found to be the earliest-originating form of pterosaur known, and it possesses a mixture of features found in non-pterosaurs and in pterosaurs. Arcticodactylus demonstrates the gradual acquisition and timing of the assembly of pterosaur flight anatomy.

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