Early shape divergence of developmental trajectories in the jaw of galeomorph sharks

Abstract

Background The onset of morphological differences between related groups can be tracked at early stages during embryological development. This is expressed in functional traits that start with minor variations, but eventually diverge to defined specific morphologies. Several processes during this period, like proliferation, remodelling, and apoptosis for instance, can account for the variability observed between related groups. Morphological divergence through development is often associated with the hourglass model, in which early stages display higher variability and reach a conserved point with reduced variability from which divergence occurs again to the final phenotype. Results Here we explored the patterns of developmental shape changes in the lower jaw of two shark species, the bamboo shark (Chiloscyllium punctatum) and the catshark (Scyliorhinus canicula). These two species present marked differences in their foraging behaviour, which is reflected in their adult jaw morphology. By tracing the developmental sequence of the cartilage condensation, we identified the onset of cartilage for both species at around stage 31. Other structures that developed later without a noticeable anlage were the labial cartilages, which appear at around stage 33. We observed that the lower jaw displays striking differences in shape from the earliest moments, without any overlap in shape through the compared stages. Conclusions The differences observed are also reflected in the functional variation in feeding mechanism between both species. Likewise, the trajectory analysis shows that the main differences are in the magnitude of the shape change through time. Both species follow a unique trajectory, which is explained by the timing between stages.

Embryonic development in the bonnethead (Sphyrna tiburo), a viviparous hammerhead shark

“…Developmental staging series allows for the identification of emerging features for experimentation and provide a comparative reference for other published work. For unconventional (or non‐classical) model organisms, developmental staging can also aid in identifying the timing and ontogeny of morphological conservation and diversity between species, allowing the understanding of evolutionary changes that have resulted in differences in morphology among species 24 . In this sense, documenting the development of a hammerhead shark is crucial to our understanding of the timing and processes that lead to the elaboration and diversification of the chondrocranium and the unusual cephalofoil.…”

“…For unconventional (or non-classical) model organisms, developmental staging can also aid in identifying the timing and ontogeny of morphological conservation and diversity between species, allowing the understanding of evolutionary changes that have resulted in differences in morphology among species. 24 In this sense, documenting the development of a hammerhead shark is crucial to our understanding of the timing and processes that lead to the elaboration and diversification of the chondrocranium and the unusual cephalofoil. An understanding of the development of these more elusive species offers huge potential in which to uncover the trajectory of morphological novelty and extreme divergence of vertebrate form and function.…”