A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds

Fabien Knoll,Nicholas P Edwards,Jose Luis Sanz,Russell J Garwood,Sophie Sanchez,William I Sellers,Phillip L Manning,Luis M Chiappe,Jesús Marugán-Lobón,Fernando Escaso,Francisco Ortega,Roy A Wogelius,Elena Cuesta,Francisco J Serrano

doi:10.1038/s41467-018-03295-9

Abstract

Fossils of juvenile Mesozoic birds provide insight into the early evolution of avian development, however such fossils are rare. The analysis of the ossification sequence in these early-branching birds has the potential to address important questions about their comparative developmental biology and to help understand their morphological evolution and ecological differentiation. Here we report on an early juvenile enantiornithine specimen from the Early Cretaceous of Europe, which sheds new light on the osteogenesis in this most species-rich clade of Mesozoic birds. Consisting of a nearly complete skeleton, it is amongst the smallest known Mesozoic avian fossils representing post-hatching stages of development. Comparisons between this new specimen and other known early juvenile enantiornithines support a clade-wide asynchronous pattern of osteogenesis in the sternum and the vertebral column, and strongly indicate that the hatchlings of these phylogenetically basal birds varied greatly in size and tempo of skeletal maturation.

Highlights

Fossils of juvenile Mesozoic birds provide insight into the early evolution of avian development, such fossils are rare
The new fossil we report here, MPCM-LH-26189 a/b
The ossification pattern seen in the sternum of MPCM-LH-26189 (Figs. 2c, d and 4b) is complex and characteristic of early juvenile enantiornithines: both the caudomedial ossification and the trabeculae are present, the craniomedial disc had yet to ossify

Summary

Introduction

Fossils of juvenile Mesozoic birds provide insight into the early evolution of avian development, such fossils are rare. We suggest that this unusually high number of free caudal vertebrae is due to the immature stage of the specimen (Fig. 5); one or several cranial-most and/or caudalmost vertebrae would have been fused with the synsacrum and/or pygostyle as the skeleton of MPCM-LH-26189 matured into adulthood.

Results

Conclusion