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How marine reptiles divided up the spoils in the Mesozoic oceans

During the Mesozoic era, while dinosaurs dominated the land, the oceans were ruled by various predators, including crocodiles, giant lizards, ichthyosaurs, and plesiosaurs. Researchers from the University of Bristol have now modeled the changing ecologies of these marine reptiles. These oceans hosted a diverse array of fossil reptiles, many reaching lengths of over 10 meters. Feeding on fish, molluscs, and even each other, these toothy monsters mostly disappeared by the end of the Cretaceous period, around 66 million years ago, coinciding with the extinction of dinosaurs. Some marine crocodiles, snakes, and turtles still exist today, but the ecological roles are now occupied by sharks, seals, and whales.

In her study, Jane Reeves, a Ph.D. student at the University of Manchester and a former MSc student at the University of Bristol's School of Earth Sciences, utilized modern computational methods to analyze the ecological traits of 371 well-known Mesozoic marine tetrapods. She categorized them into six ecological groups based on their movement, habitat, and feeding habits. These groups included pursuit predators, ambush predators (in deep and shallow waters), reptiles capable of walking on land, shallow-water shell-crushers and foragers, and marine turtles with various life modes.

To reconstruct the behavior of these ancient , Jane relied on established ecological traits such as tooth shape indicating diet preferences, sharp teeth being fish-eating, and broad, flat teeth suggesting shell-crushing capabilities. Professor Mike Benton, who co-supervised the study, emphasized the significance of using ecological characters to understand the form and function of fossilized creatures.

This research sheds light on the fascinating world of Mesozoic marine reptiles and helps us understand how they once thrived and interacted within their unique ecosystems.

Mesozoic marine tetrapod ecospace. Animals in each group share ecological characteristics. They came in many shapes and sizes and had great variation in feeding apparatus. Credit: Tom Stubbs, University of Bristol

Co-supervisor Dr. Ben Moon expressed their interest in ichthyosaurs and the idea of their migration through ecospace during the Mesozoic era, spanning 186 million years. The study conducted by Jane revealed a clear in their ecological preferences, starting as semi-terrestrial beings in the Triassic and transitioning to various ecologies, including ambush hunting and pursuit predation in the Jurassic and Cretaceous.

Co-supervisor Dr. Tom Stubbs highlighted their aim to investigate whether these marine reptiles competed with each other. Surprisingly, the analysis showed that they avoided direct competition. For instance, after a significant extinction event at the end of the Triassic, surviving ichthyosaurs and plesiosaurs displayed conservatism, not expanding their ecological roles. This left many niches empty until new groups of crocodiles and turtles emerged in the Jurassic, taking over those roles.

Jane Reeves shared her enthusiasm for studying a wide variety of creatures and reconstructing the ecological lifestyles of extinct animals solely from their . While acknowledging the need for caution in making assumptions, the collected data reflects the common day-to-day behaviors of each animal. These results provide valuable insights into the submerged world of the Mesozoic seas, shedding light on what truly transpired beneath the surface during that era.

Source: University of Bristol

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