Comparative data on the differentiation and growth of bone ornamentation in gnathostomes (Chordata: Vertebrata)

Bone ornamentation, in the form of rounded pits framed by a network of ridges, is a frequent feature among a great diversity of gnathostome taxa. However, the basic osteogenic processes controlling the differentiation and development of these reliefs remain controversial. The present study is a broad comparative survey of this question with the classical methods used in hard tissue histology and paleohistology. Distinct processes, unevenly distributed among taxa, are involved in the creation and growth of pits and ridges. The simplest one is mere differential growth between pit bottom (slow growth) and ridge top (faster growth). The involvement of several complex remodeling processes, with the local succession of resorption and reconstruction cycles, is frequent and occurs in all major gnathostome clades. Some broad, inclusive clades (e.g., Temnospondyli) display consistency in the mechanisms controlling ornamentation, whereas other clades (e.g., Actinopterygii) are characterized by the diversity of the mechanisms involved. If osteogenic mechanisms are taken into account, bone ornamentation should be considered as a character extremely prone to homoplasy. Maximum likelihood (ML) optimizations reveal that the plesiomorphic mechanism creating ornamentation is differential apposition rate over pits (slow growth) and ridges (faster growth). In some taxas e.g., temnospondyls vs lissamphibians or pseudosuchians, bone ornamentation is likely to be a homoplastic feature due to a convergence process driven by similar selective pressures. ML models of character evolution suggest that the presence of resorption in the development of ornamentation may be selectively advantageous, although support for this conclusion is only moderate.