Systemic distribution of medullary bone in the avian skeleton: ground truthing criteria for the identification of reproductive tissues in extinct Avemetatarsalia

Abstract Background Medullary bone (MB) is an estrogen-dependent, sex-specific tissue produced by female birds during lay and inferred to be present in extinct avemetatarsalians (bird-line archosaurs). Although preliminary studies suggest that MB can be deposited within most skeletal elements, these are restricted to commercial layers or hormonally treated male pigeons, which are poor analogues for wild birds. By contrast, studies in wild bird species noted the presence of MB almost exclusively within limb bones, spurring the misconception that MB deposition is largely restricted to these regions. These disparate claims have cast doubt on the nature of MB-like tissues observed in some extinct avemetatarsalians because of their “unusual” anatomical locations. Furthermore, previous work reported that MB deposition is related to blood supply and pneumatization patterns, yet these hypotheses have not been tested widely in birds. To document the skeletal distribution of MB across Neornithes, reassess previous hypotheses pertaining to its deposition/distribution patterns, and refine the set of criteria by which to evaluate the nature of purported MB tissue in extinct avemetatarsalians, we CT-scanned skeletons of 40 female birds (38 species) that died during the egg-laying cycle, recorded presence or absence of MB in 19 skeletal regions, and assessed pneumatization of stylopods. Selected elements were destructively analyzed to ascertain the chemical and histological nature of observed endosteal bone tissues in contentious skeletal regions. Results Although its skeletal distribution varies interspecifically, we find MB to be a systemic tissue that can be deposited within virtually all skeletal regions, including cranial elements. We also provide evidence that the deposition of MB is dictated by skeletal distribution patterns of both pneumaticity and bone marrow; two factors linked to ecology (body size, foraging). Hence, skeletal distribution of MB can be extensive in small-bodied and diving birds, but more restricted in large-bodied species or efficient flyers. Conclusions Previously outlined anatomical locations of purported MB in extinct taxa are invalid criticisms against their potential reproductive nature. Moreover, the proposed homology of lung tissues between birds and some extinct avemetatarsalians permit us to derive a series of location-based predictions that can be used to critically evaluate MB-like tissues in fossil specimens