Medullary bone in an Early Cretaceous enantiornithine bird and discussion regarding its identification in fossils

Jingmai O’Connor,Mark Norell,Gregory M Erickson,Alida M Bailleul,Zhonghe Zhou,Han Hu

doi:10.1038/s41467-018-07621-z

Abstract

Medullary bone is an ephemeral type of bone tissue, today found only in sexually mature female birds, that provides a calcium reservoir for eggshell formation. The presence of medullary bone-like tissues in extant birds, pterosaurs, and dinosaurs distantly related to birds shows that caution must be exercised before concluding that fossils bear medullary bone. Here we describe a new specimen of pengornithid enantiornithine from the Lower Cretaceous Jiufotang Formation. Consisting of an isolated left hindlimb, the three-dimensional preservation contrasts with the crushed preservation characteristic of most Jehol specimens. Histological examinations suggest this resulted from the presence of a thick layer of highly vascular bone spanning the medullary cavities of the femur and tibiotarsus, consistent with expectations for medullary bone in extant birds. Micro-computed tomographic scans reveal small amounts of the same tissue extending into the pedal phalanges. We consider the tissue to be homologous to the medullary bone of Neornithines.

Highlights

Medullary bone is an ephemeral type of bone tissue, today found only in sexually mature female birds, that provides a calcium reservoir for eggshell formation
Medullary bone (MB) is always endosteal in origin (Table 1; Criterion 2), but as previously mentioned, it forms in cancellous spaces of bones without medullary cavities sensu stricto (Criterion 1)
The new specimen IVPP (Institute of Vertebrate Paleontology and Paleoanthroplogy, Chinese Academy of Sciences, Beijing, China) V15576 is an isolated, articulated left hindlimb preserved in three dimensions

Summary

Introduction

Medullary bone is an ephemeral type of bone tissue, today found only in sexually mature female birds, that provides a calcium reservoir for eggshell formation. In addition to the three previously mentioned criteria, MB should have the following additional six characteristics (Table 1): (4) the periosteal surfaces should be free from indication of pathology [unlike in Allosaurus UUVP (Utah Museum of Natural History, Salt Lake City, UT, US.) 5300 in which the periosteum bears a callus]18; (5) the MB should line the majority of the medullary cavity [unlike the condition in Dysalotosaurus SMNS (Staatliches Museum für Naturkunde, Stuttgart, Germany) T3];10 (6) the MB should be clearly demarcated from the CB without a graded transition allowing the MB to often show separation from the cortex during diagenesis;[2,6] (7) the MB should occur in multiple skeletal elements [in neornithines MB is most commonly seen in the long bones of the hindlimb[23], most prevalently expressed in the femur[1], and is universally present in the tibiotarsus[22]. In neornithines MB can form extensively throughout the entire skeleton, reportedly occurring in the skull bones and pedal phalanges of some taxa[2,24]; (8) MB should coincide with a reduced growth rate associated with the onset of reproduction making it unlikely to occur in young, rapidly growing individuals [MB in juvenile Tyrannosaurus BMRP

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