Comparative ossification sequence and skeletal development of the postcranium of palaeognathous birds (Aves: Palaeognathae)

Abstract

Palaeognaths constitute one of the most basal lineages of extant birds, and are also one of the most morphologically diverse avian orders. Their skeletal development is relatively unknown, in spite of their important phylogenetic position. Here, we compare the development of the postcranial skeleton in the emu (Dromaius novaehollandiae), ostrich (Struthio camelus), greater rhea (Rhea americana) and elegant crested-tinamou (Eudromia elegans), focusing on ossification. All of these taxa are characterized by element loss in the appendicular skeleton, but there are several developmental mechanisms through which this loss occurs, including failure to chondrify, failure to ossify and fusion of cartilages prior to ossification. Further evidence is presented here to support a reduction in size of skeletal elements resulting in a delay in the timing of ossification. This study provides an important first look at the timing and sequence of postcranial ossification in palaeognathous birds, and discusses the influence of changes in the pattern of skeletal development on morphological evolution.

Insight into the early evolution of the avian sternum from juvenile enantiornithines

“…Unfortunately, no early juvenile ornithurines are recognized; however, given the derived appearance of the sternum we infer that development may have proceeded similarly to that of living birds. This is supported by the ossification of the sternum from a single pair of plates in most paleognaths 8,14 , indicating the plesiomorphic dinosaurian condition was retained in the lineage leading to modern birds (Fig. 4d).…”

“…In most neornithine taxa, this process is relatively small and forms from an extension of the sternal body 8 . The enantiornithine condition resembles Rhea; besides the main pair of ossifications that form the sternum in other ratites, Rhea possesses an additional pair that form distinct craniolateral processes (pro-osteon, Parker) 8,14 . However, in this taxon the ossifications also form the costal margin and are fused to the sternal body 8 .…”

“…This pair appears during embryological development, widely separated mediolaterally, and do not fully expand to meet and fuse into a single element until late in ontogeny 8,14 . The absence of additional sternal ossifications among ratites is inferred to be secondary, lost when the group evolved to be flightless 1,8 .…”

“…We also recognize that the developmental pathways that lead to a superficial morphotype may have changed through time and in clade-specific ways. Furthermore, a majority of recent research on living birds focuses on embryological development 4,5,[14][15][16][17][18] , which is poorly known for extinct taxa. However, a review of literature dating back over a century creates the basis for comparison with enantiornithines 5,6,8,13,14,16,19 .…”

Twenty-first century advances in knowledge of the biology of moa (Aves: Dinornithiformes): a new morphological analysis and moa diagnoses revised

“…In Megalapteryx, the scapula is reduced to a very variable small extension of the coracoid, on which we could not discern a vestigial claviculae. Maxwell & Larsson (2009) found an ossified 'furcula' (claviculae) in stage 40 ' Dromaius embryos, but in Struthio and Rhea they termed the homologous cartilaginous flange on the coracoid a procoracoid and noted that it only ossified after hatching. A specimen of Dromaius novaehollandiae (AM O.73767) uniquely has a distinct sliver of bone seemingly derived from a distinct ossification joined to the dorsal margin of the scapula, which appears to justify the interpretation of the enlarged flange in this region as being derived from the claviculae.…”