A quantitative assessment of bone area increase due to ornamentation in the Crocodylia

Abstract

Bone ornamentation, in the form of highly repetitive motives created by pits and ridges, is a frequent feature on vertebrate skull roofs and osteoderms. The functional significance of this character remains a matter of controversy and speculation. The many diverging hypotheses proposed to explain it all share a common logical prerequisite: bone ornamentation should increase significantly the surface area of the bones that bear it. In order to test this assumption in the Crocodylia, we developed a method for quantifying the gain in area due to ornamentation using a three-dimensional-surface scanner. On crocodylian osteoderms, the gain in area can be up to 40%, and on the cranial table, it ranges between 10 and 32% in adult specimens (in both cases, it shows substantial differences between the adults of the various species included in the sample). Area gain on the snout is lesser (0-20% in adults), and more variable between species. In general, bone ornamentation is less pronounced, and results in fewer area gains in juvenile specimens. The main morphometric results yielded by this study are discussed in reference to the few comparative data available hitherto, and to the functional interpretations proposed by previous authors.

Photogrammetry for 3D digitizing bones of mounted skeletons: Potential and limits

“…After the photographs, the specimen was disassemble and the isolated right femur was scanned with a branded light-fringe scanner Breukmann Stereoscan 3D (AICON 3D Systems Gmb, Biberweg 30 C, 38114, Braunschweig, Germany). Given the high resolution and the high quality of this surface scanner (scope: 720 mm, resolution: 22 m), and its regular use for quantitative studies (e.g., Bonneau et al, 2012;Clarac et al, 2015;Fabre et al, 2015;Polly et al, 2013), the 3D model obtained was then considered the reference 3D model, or reference object (RO).…”

“…For example, synchrotron X-ray micro-tomography (Laloy et al, 2013;Pradel et al, 2009;Rücklin et al, 2014) enables us to see through rock and to have access to internal structures. Conversely, three dimension (3D) surface analyses are generally acquired with surface scanners (e.g., Clarac et al, 2015;Falkingham, 2012) or photogrammetry (e.g., Belvedere et al, 2013;Brassey et al, 2015;Castanera et al, 2013;Falkingham, 2012;Petti et al, 2008). Photogrammetry is a method for reconstructing an object in 3D from photographs.…”

Cranial bone histology ofMetoposaurus krasiejowensis(Amphibia, Temnospondyli) from the Late Triassic of Poland

“…The function of the ornamentation is still unclear. The best supported hypotheses suggest that they increase the surface area for skin supports, increase the strength of the bone, protect blood vessels or assist in thermal exchange (summarized in Coldiron, 1974; Witzmann, 2009; Rinehart & Lucas, 2013; Clarac et al, 2015; Clarac et al, 2016). …”

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

“…It can display diverse aspects, the most common of which, observed in a considerable series of forms, from the Devonian arthrodire placoderms (Miles, ; Downs and Donoghue, ) to extant archosaurs (Buffrénil et al, ), is a pattern of densely‐packed pits separated by a network of ridges. These reliefs then form a repetitive motif showing either a honeycomb‐like pattern, e.g., the postorbital part of skull roof in crocodiles (Clarac et al, ) and the carapace of some turtles, or a partly radiating structure formed by both pits and sub‐parallel or slightly divergent grooves framed by ridges, like on the dermal bones of actinopterygians (Lehmann, ) and temnospondyls (Bystrow, ; Schoch and Milner, ; Witzmann et al, ). Considering its striking morphological consistency through time and taxa, this particular type of ornamentation could be viewed as the typical example of a plesiomorphic trait, highly conservative in its morphology.…”