For a while, crocodile: crocodylomorph resilience to mass extinctions
by: Keegan M. Melstrom, Kenneth D. Angielczyk, Kathleen A. Ritterbush, and Randall B. Irmis
Summarized by: Emily Rosenberger, a 3rd year undergraduate student studying geology at Binghamton University. In her free time, she enjoys hiking, learning guitar, and reading whatever she can get her hands on.
What data were used? Researchers collected data concerning the skulls of 99 extinct crocodylomorphs, which is the group that includes currently living crocodilians and their ancestors, 20 skulls of modern crocodilians, 81 skulls of mammals, and 47 skulls of reptiles. Traits such as tooth shape, snout and skull length, and jaw size were measured. By comparing the shapes of extinct crocodylomorph skulls to the skulls of animals with known diets and habitats, the researchers reconstructed the diets and habitats of the extinct crocodylomorphs. Researchers also collected data on when each extinct crocodylomorph group was alive and an estimate of how abundant each group was ecologically.
What was the hypothesis being tested: The goal of the study was to determine what physical characteristics, present in the skull, and ecological traits allowed crocodylomorphs to survive two mass extinctions, the end-Cretaceous and the end-Triassic, which caused all other members of their clade, Pseudosuchia, to die out.
Methods: Scientists took photos of the top and side of all of the skulls included in the study. After digitizing the images, the researchers were able to compare different measurements from the crocodylomorph skulls, such as eye socket size and snout length to the skulls of the other animals. The similarity in skull and tooth shape between extinct crocodylomorphs and modern animals was used to reproduce the diets and lifestyles of the extinct crocodylomorphs. Researchers created the following five categories separated by diet and habitat: aquatic carnivore, terrestrial hypercarnivore, terrestrial herbivore, semiaquatic generalist and terrestrial generalist, as typically, these diets and habitats are associated with specific traits (e.g. carnivores have sharper teeth than herbivores). Using the five categories they created and the time period each was known to be alive during, the researchers charted during what time periods the categories had more or less species surviving (Fig 1.)
Results: Researchers found that there have been many crocodylomorph lifestyles over the past 200 million years. In this study, crocodylomorphs with similar skull shapes were assumed to have lived similarly (i.e. in their diet, habitat). Crocodylomorphs had very similar skulls during the Late Triassic, which researchers interpreted as having similar lifestyles. During the end-Triassic extinction, the most heavily populated category was the terrestrial generalist category. After this extinction, crocodylomorph skulls showed an increase in lifestyle diversity through an increase in skull shape variety of crocodylomorphs throughout the rest of the Mesozoic. This variation in crocodylomorph lifestyle was highest in the Late Cretaceous. After the end-Cretaceous extinction, only two of the five types of crocodylomorph lifestyles survived. Aquatic carnivores eventually died out, and only semiaquatic generalists survived into modern times. Researchers believed the two categories of crocodylomorphs that survived and thrived after extinction events, first the terrestrial generalists and later the semiaquatic generalists, were able to do so because they did not require as specific of diets as the specialist categories did.
Why is this study important? Researchers in this study compared skull shape data of crocodylomorphs taken during this study and skull shape data from past studies to create lifestyle categories for extinct crocodylomorphs. Using this lifestyle data, they were able to add to the knowledge about what traits allowed crocodylomorphs to survive mass extinctions and how the current niche of semiaquatic generalists in crocodilians emerged.
Broader Implications beyond this study: Crocodylomorphs used to have many different lifestyles in both land and water. By understanding the past trends of crocodylomorph extinctions, the current threats faced by living crocodilians can be better understood and current conversation efforts can be better focused. Looking at what lifestyles crocodylomorphs were able to fill after the end-Triassic and end-Cretaceous extinctions may provide a view into the overall effect these extinctions had on the ecosystem as a whole.
Citation: Melstrom, K. M., Angielczyk, K. D., Ritterbush, K. A., & Irmis, R. B. (2025). For a while, crocodile; crocodylomorph resilience to mass extinctions. Palaeontology, 68(2), Article 70005. https://doi.org/10.1111/pala.70005