A tiny ornithodiran archosaur from the Triassic of Madagascar and the role of miniaturization in dinosaur and pterosaur ancestry

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.

Keywords: Dinosauria; Triassic; body size; evolution; phylogeny.

Fig. 1.

Anatomy of the femur and maxilla of Kongonaphon kely gen. et sp. nov. (UA 10618). (A) Right femur in anterolateral, (B) posteromedial, and (C) proximal views. (D) Right maxilla in right lateral and (E) palatal views. (F) Preserved elements in the holotype, UA 10618, presented in a silhouette of Kongonaphon. aof, antorbital fenestra; at, anterior trochanter; fht, tip of femoral head; fp mx, facial process of maxilla; ft, fourth trochanter; mx f, maxillary foramen; pf, palatine fossa; pmt, posterior medial tubercle; t, maxillary tooth. Illustrations credit: American Museum of Natural History/Frank Ippolito.

Fig. 2.

Body size of early avemetatarsalian (bird line) archosaurs mapped onto a consensus supertree, based on the current phylogenetic analysis (SI Appendix) and recent analyses (22). Silhouettes are scaled to estimated femoral lengths for the labeled nodes (SI Appendix, Table S1): A, base of Avemetatarsalia (represented by Teleocrater); B, base of Ornithodira (represented by Ixalerpeton); C, base of Dracohors (Silesauridae + Dinosauria) (represented by Silesaurus); and D, base of Saurischia (represented by Herrerasaurus). Silhouettes credit: Phylopic/Scott Hartman/Mathew Wedel, which is licensed under CC BY 3.0. Silhouette of Kongonaphon to the right of the taxon label is to scale (adapted from Fig. 1).