According to a new study conducted by an international team of palaeontologists, living crocodiles evolved flatter and weaker skulls to enable them to swim more efficiently but at the cost of improved biting performance.
The research, published today in the journal Proceedings of the Royal Society B, addresses a long-standing hypothesis about the trade-offs between bite strength and aquatic adaptations in crocodiles and other aquatic predators that evolve from land-dwelling ancestors. It suggests that the dome-shaped skulls of terrestrial ancestral crocodile relatives were much stronger and more efficient during feeding, when compared to the flatter skulls of modern semi-aquatic species.
The study was based on an examination of fossils taken from the Cretaceous sedimentary rocks of the Bauru Basin in Brazil, which preserves one of the most diverse and informative records of the evolutionary history of ‘crocodyliforms’ – the wider group that includes modern crocodilians and their extinct relatives.
Three of the extinct species from the Bauru Basin were found to have dome-shaped (‘oreinirostral’) skulls. These were compared with three living species of modern crocodilians, which have broad, flattened (‘platyrostral’) skulls adapted for life in water.
Researchers used CT scans to digitally reconstruct the skulls of all six crocodyliforms. Engineering simulations were then used to explore the differences in feeding behaviours.
One of the study’s key findings is that the dome-shaped skulls of terrestrial ancestral crocs were much stronger and more efficient during feeding, when compared to the flatter skulls of modern semi-aquatic species. The study demonstrates just how diverse and adaptable these animals once were.
“We found that the dome-shaped extinct species were stronger and had more efficient jaw muscles than the flattened modern crocodiles, which supports a previous hypothesis of the effects of skull flattening in crocodiles,” said lead researcher Dr Ananth Srinivas, who conducted the study during his masters’ project in Palaeobiology at the School of Earth Sciences at the University of Bristol.
When crocodiles moved from land to water, the change in environmental conditions led them to evolve flatter skulls. “But this streamlining came at a cost,” added Dr Srinivas, now a postdoctoral researcher at the University of Alabama. “As modern crocodile skulls experience up to five times more stress when biting than their extinct, land-dwelling relatives.”
To adapt, modern crocodiles evolved several features including reinforced joints in the skull, bony armor, and enlarged jaw muscles that generate powerful bites to ambush prey, despite weaker skulls. In contrast, extinct terrestrial notosuchians were free from these constraints and were able to occupy diverse ecological roles, from herbivores to apex predators, demonstrating the evolutionary flexibility that allowed crocodyliforms to thrive for over 200 million years.
“These results reveal how crocodilian skulls evolved in response to new environments and feeding styles,” said researcher Dr Jen Bright from the University of Hull “It’s a powerful example of how evolution involves trade-offs, here, between strength and streamlining.”
Brazilian palaeontologist Sandra Tavares, part of the research team from the Monte Alto Museum of Paleontology, São Paulo State, Brazil, added: “This research not only helps explain the evolutionary pressures that shaped modern crocodiles, but also advances a more comprehensive understanding of the anatomical traits and behavioural ecology of these three key taxa from the Brazilian Cretaceous.”
Co-author Emily Rayfield, Professor of Palaeobiology at the University of Bristol School of Earth Sciences, added: “This study illustrates how we can use modern computational methods to estimate how extinct animals functioned and appraise the selective pressures shaping the evolution of the animals we see today.”
Paper
‘Constraints and adaptations in crocodyliform skull evolution’ by Ananth Srinivas et al in Proceedings of the Royal Society B [open access]