Evolution of the Vascular Pattern in Mammals : Using Synchrotron Imaging to Visualise Hidden 3D Structures

Abstract: The morphology and histology of long bones are important tools for understanding tetrapod locomotion and growth. In mammals, the epiphyses of long bones ossify from a secondary centre of ossification. The consensus is that vascular canals are closely tied to the formation of these centres. However, the epiphyseal vascular organisation differs significantly between each major clade of mammals: Eutheria, Metatheria and Monotremata. Previous studies on the distribution of canals mainly examined 2D thinsections, which has led to inaccurate interpretations of their three-dimensionality. This study uses propagation phase-contrast X-ray microtomography to visualise the epiphyseal vascularisation of humeri from the eutherian Dasypus novemcinctus, the metatherian Didelphis marsupialis and the monotreme Tachyglossus aculeatus in 3D, to update on the known variability in extant mammals. Additionally, humeri from two stem-mammals, Galesaurus planiceps and Morganucodon sp., are included to infer the ancestral condition for Mammalia. Two types of canals are considered; piercing canals that pierce through the metaphysis, and cartilage canals that are connected to the perichondrium. Cartilage canals are reported in D. marsupialis, where these were previously thought to be absent. Traces of cartilage canals are also found in Morganucodon sp. Both fossil taxa exhibit piercing canals, as in monotremes and eutherians, although the canals are more numerous in the fossils. The distribution of cartilage canals therefore is wider than previously assumed. This new fossil data shows that both types of canals have evolved within the mammalian stem-group, thereby suggesting that the process involving both types of canals for ossifying epiphyses is relatively ancient.