OSTEOLOGICAL DESCRIPTION OF AN EMBRYONIC SKELETON OF THE EXTINCT ELEPHANT BIRD, AEPYORNIS (PALAEOGNATHAE: RATITAE)

Abstract The embryonic skeleton of the most massive bird ever discovered, the extinct elephant bird Aepyornis, has remained until this time essentially unknown. This state is due, in large part, to the reluctance to open complete, unbroken eggs. However, the advent of high-resolution X-ray computed tomography (HRCT) provides a non-destructive means of looking inside such eggs to observe the contents that they conceal. This study uses HRCT technology to digitally extract an embryonic skeleton from a complete Aepyornis egg and subsequently to aid in the systematic description of this specimen. Skeletal elements digitally retrieved from the egg include portions of the braincase, palate, rostrum, vertebral column, and both the fore- and hindlimbs. Because this specimen is at a relatively early stage of ontogenetic development, the bones that were recovered reveal morphology that typically is obliterated in the adult due to the highly fused state of the skeleton, especially seen in the skull. The usefulness of this information, however, is limited unless a level of ontogenetic maturity can be assigned to the specimen. Therefore, we use an extant phylogenetic bracket in a novel analysis comparing the morphological developmental of living ratites and chickens with this specimen of Aepyornis to assess its level of ontogenetic maturity.