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

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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.