Was A Cold-blooded Strategy Key To Crocodile Survival Across Mass Extinctions?

现存动物的内部骨骼微结构记录了生活史、代谢和生态学的关键信息，为重建灭绝物种的这些特征提供了关键框架。我们可以直接观察到，现存的鳄鱼物种是生长速度缓慢的冷血动物，它们通常被归类为“活化石”，自恐龙时代以来几乎没有经历过进化变化。然而，丰富的化石记录的鳄鱼和他们的灭绝亲属（=Crocodylomorpha）揭示了更大的生态形态多样性，高度分化的生态指示，因此可能非常不同的生活史轨迹。此档案保存：（1）几个血统的完全海洋，鳍状肢的群体能够动力游泳;（2）活跃的食肉动物，直立的四肢适合跑步;（3）草食性物种;（4）巨人，达到约12米长;和（5）物种生活在高纬度比亚热带分布的当前形式。一些已灭绝的海洋鳄形类可能比现存物种有更高的代谢率，而鳄形类辐射之外的已灭绝物种的生长率更接近哺乳动物和鸟类等温血动物。此外，生长率发生沿着一个频谱，而不是代表二元末端成员，只有详细的骨组织学研究可以揭示的范围内的增长策略。关键的是，在这个群体的进化树中，大部分信息都很缺乏。因此，我们不知道大多数灭绝的鳄形类是否具有缓慢生长的特征，或者这是否发生在它们进化史的后期，我们也不知道生长速度是否在身体大小、生态或纬度上是均匀的。生长速率策略的变化可能是为什么一些鳄形类谱系在最近两次大规模灭绝中幸存下来的关键，而其他群体则消失了，包括它们的大多数近亲（例如非鸟类恐龙），但这从未被测试过。令人惊讶的是，大量的知识差距也影响了我们对现存物种的采样，限制了我们将这些物种作为理解灭绝动物的类似物的能力。结合古生物学，生物学和生态学领域，再加上组织学方法，从内部骨骼显微结构中解锁以前无法获得的数据，我们将重建鳄鱼在2.3亿年进化历史中的生长速率和热生理学。我们将利用这一点，以确定如何通过时间对应于气候，环境和生态变化的差异crocodylomorph分支，解决三个关键的研究问题：（1）如何增长率和热生理学变化的crocodylomorph树？(2)鳄形类的生长速率策略是否随生态、体型和纬度而变化？不同的生长率策略是否对应于鳄形类在主要环境和气候事件中的灭绝选择性？我们的研究将填补有关活物种和化石物种的重要知识空白，阐明热生理学策略的演变，并阐明鳄鱼如何在近亲灭绝时幸存下来，并对活物种的恢复力产生潜在影响。

The internal bone microstructure of living animals records key information on life history, metabolism, and ecology that provides a critical framework for reconstructing these traits in extinct species. We can directly observe that extant crocodile species are cold-blooded animals with slow growth rates, and they are often categorized as 'living fossils' that have experienced little evolutionary change since the time of the dinosaurs. However, the rich fossil record of crocodiles and their extinct relatives (=Crocodylomorpha) reveals far greater ecomorphological diversity, indicative of highly divergent ecologies and thus perhaps very different life history trajectories. This archive preserves: (1) several lineages of fully marine, flippered groups capable of powered swimming; (2) active predators with upright limbs adapted for running; (3) herbivorous species; (4) giants that reached around 12 metres in length; and (5) species living at much higher latitudes than the subtropical distribution of current forms. Some extinct marine crocodylomorphs probably had higher metabolic rates than living species, and extinct species just outside of the crocodylomorph radiation had growth rates that were closer to those of warm-blooded animals such as mammals and birds. Furthermore, growth rates occur along a spectrum, rather than representing binary end members, and only detailed osteohistological study can reveal the range of growth strategies. Critically, there is a dearth of information across most of the group's evolutionary tree. Consequently, we do not know if most extinct crocodylomorphs were characterized by slow growth rates or if this occurred later in their evolutionary history, nor do we know whether growth rates were homogeneous across body sizes, ecologies, or with latitude. It is possible that variation in growth rate strategies is key to why some crocodylomorph lineages survived the last two mass extinctions, when other groups disappeared, including most of their closest relatives (e.g. non-avian dinosaurs), but this has never been tested. Surprisingly, a substantial knowledge gap affects our sampling of extant species too, limiting our ability to use these as analogues for understanding extinct animals. Combining the fields of palaeontology, biology, and ecology, coupled with histological approaches to unlock previously inaccessible data from internal bone microstructure, we will reconstruct growth rates and thermophysiology for crocodylomorphs over their 230 million-year evolutionary history. We will use this to determine how differences between crocodylomorph clades and through time correspond to climatic, environmental, and ecological variation, addressing three key research questions: (1) How do growth rates and thermophysiology vary across the crocodylomorph tree? (2) Do growth rate strategies in crocodylomorphs vary with ecology, body size, and latitude? and (3) Do different growth rate strategies correspond to extinction selectivity in crocodylomorphs across major environmental and climatic events? Our research will fill important knowledge gaps pertaining to both living and fossil species, shed light on the evolution of thermophysiological strategies, and elucidate how crocodylomorphs survived two mass extinctions, when close relatives went extinct, with potential implications for the resilience of living species.