Morphological association between muscle attachments and ossification sites in the late cartilaginous skull of tuatara embryos

Abstract

During development, the embryonic cartilaginous skull in most vertebrates is partially replaced by bones with endochondral and perichondral ossifications.Muscle attachments are thought to influence the patterns of ossification and, hence, the differentiation of the skull. To investigate the association between muscle attachments and early ossifications of reptilian embryos, we conducted digital 3D reconstructions of the cranium, the head, and the neck musculature from a histological section series of a late term embryonic tuatara, Sphenodon punctatus, with a total body length of 52 mm. As the sole living rhynchocephalian species, it is an important outgroup in comparative studies of squamate evolution. We found that head and neck muscles are largely associated with early ossification of the basal plate and the palatoquadrate, and with three other ossifications in an older specimen with a total body length of 72 mm. These results suggest that tensile forces resulting from embryonic muscle contraction are largely, but not exclusively, correlated with the area of endochondral ossification in the chondrocranium and palatoquadrate in tuatara. Beyond little-known genetic factors, the complexity of chondrocranial architecture, the progress of its development, and the effect of multiple muscle transmitting forces in the chondrocranium must be considered to provide a more comprehensive discussion of the mechanical properties of the embryonic skull.

Exceptional In Situ Preservation of Chondrocranial Elements in a Coniacian Mosasaurid from Colombia

“…The chondrocranium in IGMp879524 is comparable to that of nearly all described extant lepidosaurs [2,5,9,[19][20][21]; it has similar components arranged in the nasal and orbitotemporal regions. This condition, previously unknown in mosasaurids, allows us to project some of the morphofunctional studies carried out on the chondrocranium of recent lepidosaurs, towards the functionality of the chondrocranium in mosasaurids.…”

“…According to [21], in the embryos of Tuatara punctatus, there are some muscles for moving the eye that are inserted mainly into the interorbital septum, but also into the planum supraseptale and the pila metoptica. The latter is highly fused with the pila antotica and pila accesoria, forming a continuous plate in T. punctatus.…”

Deep‐time invention and hydrodynamic convergences through amniote flipper evolution

“…This was in order to see whether the same morphofunctionally interpretable results can be obtained by a simplified dataset or if the more complex one is necessary. As the two parts of the musculoskeletal system, bones and muscles, depend on and influence each other during their ontogenetic development and evolution (see a recent discussion on that topic in Zhang et al, 2022, pp. 928–929), we coded both, only seemingly independent, entities as one functional system (sensu Ziermann et al, 2021).…”