Evolutionary simplification of the dermatocranium in early tetrapods
Final Report Abstract
Although tetrapods exhibit impressive variation in cranial morphology, all extant groups are characterized by having far fewer bones in their dermatocranium than the earliest forms. The extreme end to this trend is exemplified in modern lissamphibians, in which only 19 out of the original 43 dermal skull elements of their early tetrapod ancestors are retained. Remarkably, this cranial simplification does not have a single origin but has evolved repeatedly in a convergent manner. On the other hand, certain bones have always been retained despite the functional diversity of the tetrapod skull. Clearly, this is a significant trend in tetrapod evolution, however, little work has been done examine the overall pattern(s) of simplification and to elucidate the functional adaptations and mechanisms behind it across tetrapods as a whole. In the present DFG-project, we focused on the bones of the dermatocranium, mainly on bones of the median series (nasal, frontal, parietal, and postparietal), which are among the most stable dermal bones of the skull, and the temporal series, which is variably composed of intertemporal, supratemporal and tabular bones. Our results indicate that bones of the temporal series do not form a developmental module and that the evolutionary loss of these bones is quite variable. In most extant and extinct tetrapod groups, the intertemporal bone was lost first, followed by the supratemporal, and then the tabular and/or postparietal. The supratemporal was repeatedly lost and regained and was found to be the most evolvable element of all bones examined. No obvious relationship was found between skull simplification and lifestyle. Moreover, no indication was found that early tetrapods (temnospondyls and lepospondyls) possessed cells of large size and that these are correlated with the presence or absence of dermal bones. Microstructure of bones of the temporal series did not provide conclusive evidence for bone loss via fusion of adjacent bones. Ongoing research will examine if bone microstructure differs between bones of the median series (most evolutionary stable dermal bones) and those of the temporal series (least stable dermal bones). We are currently also investigating the morphological disparity of dermal skull bones to detect if those dermal bones that are lost multiple times are more evolvable compared to more stable ones.
Publications
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Patterns and processes of skull simplification in the temporal series of tetrapods. 82nd Annual Meeting of the Society of Vertebrate Paleontology, Toronto, Canada
Kean, K.J., Pérez-Ben, C., Danto, M. & Fröbisch N.B.
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Using ancestral state reconstruction to understand the simplification of the tetrapod skull. 67th Annual Meeting of The Palaeontological Association, 2023
Kean, K.J., Pérez-Ben, C., Danto, M. & Fröbisch N.B.
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Evolution of the tetrapod skull: a systematic review of bone loss. Fossil Record, 27(3), 445-471.
Kean, Kim Julia; Danto, Marylène; Pérez-Ben, Celeste & Fröbisch, Nadia Belinda
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Investigating the patterns and mechanisms of skull simplification in tetrapodomorphs. 21st European Association of Vertebrate Paleontology, NGF Abstracts and Proceedings, no. 1, 2024, p.29
Kean, K.J., Pérez-Ben, C., Canoville, A., Danto, M. & Fröbisch, N.B.
