A gliding lizard from the Early Cretaceous of China

Abstract

Gliding is an energetically efficient mode of locomotion that has evolved independently, and in different ways, in several tetrapod groups. Here, we report on an acrodontan lizard from the Early Cretaceous Jehol Group of China showing an array of morphological traits associated with gliding. It represents the only known occurrence of this specialization in a fossil lizard and provides evidence of an Early Cretaceous ecological diversification into an aerial niche by crown-group squamates. The lizard has a dorsal-rib-supported patagium, a structure independently evolved in the Late Triassic basal lepidosauromorph kuehneosaurs and the extant agamid lizard Draco, revealing a surprising case of convergent evolution among lepidosauromorphans. A patagial character combination of much longer bilaterally than anteroposteriorly, significantly thicker along the leading edge than along the trailing edge, tapered laterally to form a wing tip, and secondarily supported by an array of linear collagen fibers is not common in gliders and enriches our knowledge of gliding adaptations among tetrapods.

Fig. 1.

Xianglong zhaoi holotype (LPM 000666). Photograph (A) and line drawing (B) of LPM 000666. Abbreviations: cv, caudal vertebrae; ch, ceratobranchial process of hyoid apparatus; dr, dorsal ribs; lc, left clavicle; lf, left femur; lfi, left fibula; lt, left tibia; md I–V, manual digits I–V; pd I–V, pedal digits I–V; pls, pelvis; rh, right humerus; rr, right radius; rsc, right scapulocoracoid; ru, right ulna; sk, skull. (Scale bar: 2 cm.)

Fig. 2.

Close-up of LPM 000666. (A) Cranial region showing skull roof scales and gular flap. (B) Anterior trunk region showing scales along dorsal series, left arm, and leading edge of the patagium (counterpart). (C) Left manus. (D) Left pes. (E) Collagen fibers along trailing edge of the patagium (counterpart). (Scale bars: A–D, 3 mm; E, 1 mm.)

Fig. 3.

Phylogenetic position of Xianglong. (Left) The tree presented here is a strict consensus of the four most parsimonious trees (tree length, 1,058; consistency index, 0.47; retention index, 0.69) produced by an analysis on squamate interrelationship (see SI) with the addition of several squamate outgroups, the systematic positions of which are generally agreed. (Right) Several taxa possessing a patagium supported by elongate dorsal ribs are illustrated to highlight the convergent evolution of this unusual structure within the Lepidosauromorpha and to show some salient differences between the patagium of these taxa: kuehneosaur Icarosaurus (Top), Xianglong (Middle), and agamid Draco (Bottom).