Evolution of the vertebrate skeleton: morphology, embryology, and development

Abstract

Two major skeletal systems—the endoskeleton and exoskeleton—are recognized in vertebrate evolution. Here, we propose that these two systems are distinguished primarily by their relative positions, not by differences in embryonic histogenesis or cell lineage of origin. Comparative embryologic analyses have shown that both types of skeleton have changed their mode of histogenesis during evolution. Although exoskeletons were thought to arise exclusively from the neural crest, recent experiments in teleosts have shown that exoskeletons in the trunk are mesodermal in origin. The enameloid and dentine-coated postcranial exoskeleton seen in many vertebrates does not appear to represent an ancestral condition, as previously hypothesized, but rather a derived condition, in which the enameloid and dentine tissues became accreted to bones. Recent data from placoderm fossils are compatible with this scenario. In contrast, the skull contains neural crest-derived bones in its rostral part. Recent developmental studies suggest that the boundary between neural crest- and mesoderm-derived bones may not be consistent throughout evolution. Rather, the relative positions of bony elements may be conserved, and homologies of bony elements have been retained, with opportunistic changes in the mechanisms and cell lineages of development.

Homology of the cranial vault in birds: new insights based on embryonic fate-mapping and character analysis

“…These new data bolster the oft-stated contention that bones of the cranial vault in the chicken (indeed, in all birds) may be identified and named incorrectly with respect to cranial vault bones in other tetrapods [15,16,20,33]. Moreover, the apparent difference in the embryonic origin of bones of the cranial vault of birds relative to other tetrapods can be eliminated by interpreting the avian ‘frontal’ instead as a fused frontal and parietal, i.e.…”

“…[38] locate it more posteriorly, along the suture between the parietal and supraoccipital. We compare our axolotl data to the former results [36,37], which have been validated in subsequent studies that use the same and alternative labelling techniques [19,39] and which constitute a growing consensus [33]. …”

“…We, therefore, caution against drawing definitive conclusions based on the limited taxon sampling that exists at this time. Nevertheless, the pattern of embryonic origin of skull bones we describe in the axolotl appears to be highly conserved phylogenetically, even when comparing distantly related species such as zebrafish and amniotes [25,33,63]. The only exception documented to date is the clawed frog, Xenopus laevis , which has a very different pattern of CNC-derivation of skull bones in comparison with the one shared by mouse, chicken, axolotl and zebrafish [25].…”

Paleontological Studies Integrated into a New Evolutionary Zoology

“…In its first two years, comparative morphology and embryology in evolutionary developmental contexts have been major themes of interest (ex. Hirasawa and Kuratani, 2015;Hojo et al, 2015;Tada and Kuratani, 2015;Hayashi et al, 2015;Nakano, 2015;Onai et al, 2015a, b;Oisi et al, 2015;Shigeno et al, 2015;Kaji et al, 2016;Takeuchi et al, 2016;Suzuki et al, 2016;Hirasawa et al, 2016), a clear trend dating to the launch year, as we reported in a previous review (Fukatsu and Kuratani, 2014). Nonetheless, ZL is open to all areas of basic zoology, and many of the top cited and most frequently accessed papers have been from fields other than evo-devo (Holland, 2015;Kishida et al, 2015;Inoue et al, 2015;Mizunami et al, 2015;Hosokawa et al, 2015;Moriyama and Numata, 2015).…”

A case of cranium bifidum with meningocele in Ayrshire calf

“…These bones develop from different ossification centers derived from tissues of different embryonic origins [1–5]. The squamous part of occipital bone develops from early fusion of interparietal and supraoccipital bones.…”