Limusaurus and bird digit identity

Alexander Vargas,Jacques Gauthier,Günter Wagner

doi:10.1038/npre.2009.3828.1

Abstract

AbstractLimusaurus is a remarkable herbivorous ceratosaur unique among theropods in having digits II, III and IV, with only a small metacarpal vestige of digit I. This raises interesting questions regarding the controversial identity of avian wing digits. The early tetanuran ancestors of birds had tridactyl hands with digital morphologies corresponding to digits I, II & III of other dinosaurs. In bird embryos, however, the pattern of cartilage formation indicates that their digits develop from positions that become digits II, III, & IV in other amniotes. Limusaurus has been argued to provide evidence that the digits of tetanurans, currently considered to be I, II and III, may actually be digits II, III, & IV, thus explaining the embryological position of bird wing digits. However, morphology and gene expression of the anterior bird wing digit specifically resemble digit I, not II, of other amniotes. We argue that digit I loss in Limusaurus is derived and thus irrelevant to understanding the development of the bird wing.

Highlights

Limusaurus is a remarkable herbivorous ceratosaur unique among theropods in having digits II, III and IV, with only a small metacarpal vestige of digit I1
We argue that digit I loss in Limusaurus is derived and irrelevant to understanding the development of the bird wing
Quantitative analysis only favors the II,III,IV identification of tetanuran digits when bird digits are coded as II,III,IV, a category assumption based on embryological position alone[1]

Summary

Introduction

Limusaurus is a remarkable herbivorous ceratosaur unique among theropods in having digits II, III and IV, with only a small metacarpal vestige of digit I1. The early tetanuran ancestors of birds had tridactyl hands with digital morphologies corresponding to digits I, II & III of other dinosaurs[2]. The pattern of cartilage formation indicates that their digits develop from positions that become digits II, III, & IV in other amniotes[3].

Results

Conclusion