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. 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. 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.

中文翻译：

---

象形鲨颌部发育轨迹的早期形状分化

可以在胚胎发育的早期阶段跟踪相关组之间形态差异的发生。这表现在功能性状中，从微小的变化开始，但最终分化为定义的特定形态。在此期间的几个过程，例如增殖、重塑和细胞凋亡，可以解释相关组之间观察到的变异性。发育过程中的形态学差异通常与沙漏模型相关，在沙漏模型中，早期阶段表现出更高的变异性，并达到一个保守点，变异性降低，从这个点再次发生最终表型的分歧。在这里，我们探讨了竹鲨 (Chiloscyllium punctatum) 和猫鲨 (Scyliorhinus canicula) 两种鲨鱼下颌发育形状变化的模式。这两个物种的觅食行为存在显着差异，这反映在它们的成年颌骨形态上。通过追踪软骨凝结的发育顺序，我们确定了这两个物种的软骨在第 31 阶段左右开始出现。后来没有明显原基的其他结构是唇软骨，它们出现在第 33 阶段左右。我们观察到较低的下颌从最早的时刻就表现出明显的形状差异，在比较的阶段中没有任何形状重叠。观察到的差异也反映在两个物种之间摄食机制的功能变化上。同样，轨迹分析表明，主要区别在于形状随时间变化的幅度。这两个物种都遵循独特的轨迹，