Platypuses belong to an ancient group of mammals, the monotremes. Today, these are the only living mammalian species to lay eggs, and the group also includes echidnas. But, as the x-rays of Dr Larry Vogelnest show, the platypus retains several skeletal features which link it to its distant ancestors (often called reptiles, but more technically referred to as synapsids or early tetrapod amniotes) that gave rise to mammals. Particularly striking is the anatomy of the pectoral girdle, the bones which hold the forelimbs in place around the anterior spine. This anatomy shares several features (including the interclavicle and two coracoid bones, absent in ‘true’ Eutherian mammals) with modern reptiles such as lizards, but bears closest resemblance to a group of fossil land vertebrates which form part of the evolutionary lineage of mammals. The limbs are splayed wide as seen in modern reptiles, but they still move with some motions that are more similar to mammalian limbs, and the neck vertebrae show small rib-like projections absent in other mammals but found in many reptiles and other land vertebrates.
Image courtesy of Dr Larry Vogelnest and CSIRO Publishing.
Anterior: At the head end of the head-tail or head-anus body axis.
Eutherian: Belong to the ‘true’ mammals, producing live young without a marsuipial pouch and feeding them milk through nipples. Further distinguished from other mammals through characters of the feet and ankles.