Abstract
Early members of the dinosaur-pterosaur clade Ornithodira are very rare in the fossil record, obscuring our understanding of the origins of this important group. Here, we describe an early ornithodiran (Kongonaphon kely gen. et sp. nov.) from the Mid-to-Upper Triassic of Madagascar that represents one of the smallest nonavian ornithodirans. Although dinosaurs and gigantism are practically synonymous, an analysis of body size evolution in dinosaurs and other archosaurs in the context of this taxon and related forms demonstrates that the earliest-diverging members of the group may have been smaller than previously thought, and that a profound miniaturization event occurred near the base of the avian stem lineage. In phylogenetic analysis, Kongonaphon is recovered as a member of the Triassic ornithodiran clade Lagerpetidae, expanding the range of this group into Africa and providing data on the craniodental morphology of lagerpetids. The conical teeth of Kongonaphon exhibit pitted microwear consistent with a diet of hard-shelled insects, indicating a shift in trophic ecology to insectivory associated with diminutive body size. Small ancestral body size suggests that the extreme rarity of early ornithodirans in the fossil record owes more to taphonomic artifact than true reflection of the group's evolutionary history.
Highlights
Members of the dinosaur–pterosaur clade Ornithodira are very rare in the fossil record, obscuring our understanding of the origins of this important group
A lower bound on tetrapod body size is determined by physiological constraints [10, 11], but this limit was rarely approached by Mesozoic dinosaurs, which failed to occupy size ranges successfully inhabited by thousands of species of mammals, lepidosaurian reptiles, and lissamphibians [12]
In the lineage leading to birds did dinosaurs exhibit a sustained trend toward miniaturization [13], eventually approaching the lower extremes of tetrapod body size [14]
Summary
The Isalo II lagerpetid, K. kely, is represented by a partial skeleton, the holotype specimen UA 10618. The right maxilla of UA 10618 (Fig. 1 D and E) expands our knowledge of the lagerpetid cranium (part of the skull roof was previously known for Ixalerpeton, but no dentition) This element bears unserrated, simple, conical teeth, suggestive of an insectivorous diet. Unlike Kongonaphon, Scleromochlus is known from multiple specimens (seven individuals in total), all of comparable size, only two of which were found in association, so there is no evidence they represent a clutch or age-associated sibling group. We consider it most likely that Scleromochlus was small-bodied at maturity and that it and Kongonaphon represent a historically undersampled size class of early avemetatarsalians
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