Crocodylomorph cranial shape evolution and its relationship with body size and ecology

Crocodylomorpha, which includes living crocodylians and their extinct relatives, has a rich fossil record, extending back for more than 200 million years. Unlike modern semi-aquatic crocodylians, extinct crocodylomorphs exhibited more varied lifestyles, ranging from marine to fully terrestrial forms. This ecological diversity was mirrored by a remarkable morphological disparity, particularly in terms of cranial morphology, which seems to be closely associated with ecological roles in the group. Here, I use geometric morphometrics to comprehensively investigate cranial shape variation and disparity in Crocodylomorpha. I quantitatively assess the relationship between cranial shape and ecology (i.e. terrestrial, aquatic, and semi-aquatic lifestyles), as well as possible allometric shape changes. I also characterise patterns of cranial shape evolution and identify regime shifts. I found a strong link between shape and size, and a significant influence of ecology on the observed shape variation. Terrestrial taxa, particularly notosuchians, have significantly higher disparity, and shifts to more longirostrine regimes are associated with large-bodied aquatic or semi-aquatic species. This demonstrates an intricate relationship between cranial shape, body size and lifestyle in crocodylomorph evolutionary history. Additionally, disparity-through-time analyses were highly sensitive to different phylogenetic hypotheses, suggesting the description of overall patterns among distinct trees. For crocodylomorphs, most results agree in an early peak during the Early Jurassic and another in the middle of the Cretaceous, followed by nearly continuous decline until today. Since only crown-group members survived through the Cenozoic, this decrease in disparity was likely the result of habitat loss, which narrowed down the range of crocodylomorph lifestyles.

鳄形超目（Crocodylomorpha）涵盖现生鳄目类群及其灭绝近亲，拥有可追溯至2亿余年以前的丰富化石记录。与现代半水生的真鳄类不同，已灭绝的鳄形超目类群展现出更为多样的生存策略，涵盖海洋、完全陆生等多种生态类型。这种生态多样性与显著的形态分异相互呼应，其中以颅骨形态分异尤为突出，而颅骨形态似乎与该类群的生态位紧密相关。
本研究采用几何形态测量法（geometric morphometrics），对鳄形超目的颅骨形状变异与形态分异展开全面探究。本研究定量评估了颅骨形状与生态类型（即陆生、水生及半水生生存模式）之间的关联，同时分析了可能存在的异速生长形状变化（allometric shape changes）。此外，本研究还刻画了颅骨形状的演化模式，并识别出演化体制转变事件。
研究结果显示，颅骨形状与体型大小之间存在显著关联，且生态类型对观测到的颅骨形状变异具有显著影响。陆生类群（尤其是诺托鳄类（notosuchians））的形态分异度显著更高，而演化至长吻型的体制转变多与大体型的水生或半水生类群相关。这一发现揭示了鳄形超目演化历史中，颅骨形状、体型大小与生存模式之间复杂的相互关系。
此外，形态分异随时间演化的分析结果对不同的系统发育假说极为敏感，这表明需基于多套系统发育树对整体演化模式进行描述。针对鳄形超目的多数研究结果均支持：其形态分异在早侏罗世出现首个峰值，在白垩纪中期出现第二个峰值，随后近乎持续下降直至现代。由于新生代仅冠群（crown-group）类群得以存续，这种形态分异的下降大概率是栖息地缩减所致——栖息地缩减缩小了鳄形超目类群的生存策略范围。