Vascularization in Ornamented Osteoderms: Physiological Implications in Ectothermy and Amphibious Lifestyle in the Crocodylomorphs?

Vascularization in the core of crocodylian osteoderms, and in their superficial pits has been hypothesized to be a key feature involved in physiological thermoregulation and/or acidosis buffering during anoxia (apnea). However, up to now, there have been no quantitative data showing that the inner, or superficial, blood supply of the osteoderms is greater than that occurring in neighboring dermal tissues. We provide such data: our results clearly indicate that the vascular networks in both the osteoderms and the pits forming their superficial ornamentation are denser than in the overlying dermis. These results support previous physiological assumptions and indicate that vascularization in pseudosuchian (crocodylians and close relatives) ornamented osteoderms could be part of a broad eco-physiological adaptation towards ectothermy and aquatic ambush predation acquired by the crocodylomorphs during their post-Triassic evolution. Moreover, regressions demonstrate that the number of enclosed vessels is correlated with the sectional area of the cavities housing them (superficial pits and inner cavities). These regressions can be used to infer the degree of vascularization on dry and fossilized osteoderms and thus document the evolution of the putative function of the osteoderms in the Pseudosuchia. Anat Rec, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 301:175-183, 2018. © 2017 Wiley Periodicals, Inc.

Does skull morphology constrain bone ornamentation? A morphometric analysis in the Crocodylia.

Previous quantitative assessments of the crocodylians' dermal bone ornamentation (this ornamentation consists of pits and ridges) has shown that bone sculpture results in a gain in area that differs between anatomical regions: it tends to be higher on the skull table than on the snout. Therefore, a comparative phylogenetic analysis within 17 adult crocodylian specimens representative of the morphological diversity of the 24 extant species has been performed, in order to test if the gain in area due to ornamentation depends on the skull morphology, i.e. shape and size. Quantitative assessment of skull size and shape through geometric morphometrics, and of skull ornamentation through surface analyses, produced a dataset that was analyzed using phylogenetic least-squares regression. The analyses reveal that none of the variables that quantify ornamentation, be they on the snout or the skull table, is correlated with the size of the specimens. Conversely, there is more disparity in the relationships between skull conformations (longirostrine vs. brevirostrine) and ornamentation. Indeed, both parameters GApit (i.e. pit depth and shape) and OArelat (i.e. relative area of the pit set) are negatively correlated with snout elongation, whereas none of the values quantifying ornamentation on the skull table is correlated with skull conformation. It can be concluded that bone sculpture on the snout is influenced by different developmental constrains than on the skull table and is sensible to differences in the local growth 'context' (allometric processes) prevailing in distinct skull parts. Whatever the functional role of bone ornamentation on the skull, if any, it seems to be restricted to some anatomical regions at least for the longirostrine forms that tend to lose ornamentation on the snout.

Differentiation and growth of bone ornamentation in vertebrates: a comparative histological study among the Crocodylomorpha.

Bone ornamentation, that is, hollow (pits and grooves) or protruding (ridges) repetitive reliefs on the surface of dermal bones, is a frequent, though poorly studied and understood, feature in vertebrates. One of the most typical examples of this characteristic is given by the Crurotarsi, a taxon formed by the crocodilians and their closest allies, which generally display deep ornamentation on skull roof and osteoderms. However, the ontogenetic process responsible for the differentiation and development of this character remains controversial. This study was conducted to settle the question on histological and microanatomical evidence in several crurotarsan taxa. Observational and experimental data in extant and extinct crocodyliforms show that bone ornamentation is initially created, and later maintained during somatic growth (that is indefinite in crocodilians), by a complex process of bone remodeling comprising local resorption of superficial bone cortices, followed by partial reconstruction. The superficial reliefs of crocodilian dermal bones are thus permanently modified through pit enlargement, drift, stretching, shrinking, or complete filling. Ridges are also remodeled in corresponding ways. These processes allow accommodation of unitary ornamental motifs to the overall dimensions of the bones during growth. A parsimony optimization based on the results of this study, but integrating also published data on bone histology in non-crocodyliform crurotarsans and some non-crurotarsan taxa, suggests that the peculiar mechanism described above for creating and maintaining bone ornamentation is a general feature of the Crurotarsi and is quite distinct from that attributed by previous authors to other vertebrates.

Rostrum of a toothed whale: ultrastructural study of a very dense bone.

The rostral bones of the toothed whale, Mesoplodon densirostris, consist mainly of hypermineralized secondary osteons and have yielded among the highest values for density (2.6 g/cm3) and mineral content (86.7%) yet reported for any bone. Scanning and transmission electron microscopy show parallel rods of mineral oriented along the length of the rostrum. These consist of platey crystals of carbonated hydroxyapatite, which, judging from electron diffraction, are extremely well and coherently aligned. The collagen component of the rostral bone consists largely of very thin fibrils aligned in longitudinal register to form tubular networks. The collagen fibrils are also aligned with the lengths of the mineral rods, which are apparently accommodated in the tubular spaces of the collagenous network. This peculiar ultrastructure clearly differs from the densely packed mineralized fibrils commonly observed in vertebrate lamellar osseous tissues, although histological examination has indicated some vestiges of "normal" primary bone surrounding the secondary osteons. Thus, the bone tissue in the rostrum is characterized by a remarkably sparse collagenous component. This ultrastructure can explain the high density, stiffness, and brittleness of the rostrum that have been observed. It also raises interesting questions about possible modes of crystal growth during ongoing mineralization in normal bone, and may have some relevance in the mechanical behavior of dense bones in pathological conditions.

On how the periosteal bone of the delphinid humerus becomes cancellous: ontogeny of a histological specialization.

In cetaceans, the bones of the flippers lack a free medullary cavity and have a cancellous texture, with compact cortices reduced or absent. The present work discusses the ontogenetic basis of these characters in terms of the ontogeny of the structure and textural bone compactness (TBC) of the humeral diaphysis in a growth series of common dolphins (Delphinus delphis). The texture of the primary periosteal deposits is compact; soon after their accretion, the deposits undergo an extensive erosion that turns them into a cancellous tissue. A diffuse endosteal front of resorption expands in parallel with the growth of the cortex and acts as small units scattered within the cortices. Starting soon after birth and continuing throughout the life of the animals, the compactness of the periosteal cortex decreases at both general and local levels. This trend correlates strongly with the increase in size of the diaphyseal section and reflects the fact that relatively more bone is eroded than deposited during growth in the cancellous parts of the cortex. In the broad sense, this is basically an osteoporotic process, which is not identical, however, to senile or disuse osteoporoses.