Crocodiles have evolved more than we think

If you’ve ever looked at a crocodile and felt like you were looking at a window to the Jurassic Period, you aren’t alone – but you aren’t right. 

New analysis suggests that the ancestors of modern crocodiles and alligators have changed habitat and form multiple times since they shifted from land to aquatic animals in the early Jurassic. 

“Crocodiles are not living fossils,” says Christopher Brochu from the University of Iowa in the US. “Transitions between land, sea, and freshwater were more frequent than we thought, and the transitions were not always land-to-freshwater or freshwater-to-marine.” 

Previous research has indicated that crocodylomorphs evolved on land roughly 200 million years ago before moving to freshwater. The prevailing wisdom has been that they stayed there. 

However, the new analysis by Bronchu and co-authors Eric Wilberg and Alan Turner, of Stony Brook University in New York, US, “suggests a complex evolutionary history”.

“Those living at sea had paddles instead of limbs, and those on land often had hoof-like claws and long legs. These did not all evolve from ancestors that looked like modern crocodiles, as has long been assumed,” the authors write in a paper published in the journal Scientific Reports

In the analysis, Bronchu and his colleagues perform a phylogenetic analysis of 100 taxa, with character sampling of 407. They designate each species as terrestrial, freshwater semiaquatic, or marine. 

“Reconstructing ancestral habitats across the tree results in a large number of independent transitions between environments,” they write. 

The researchers pinpoint “at least three independent shifts from terrestrial to aquatic”, as well as other transitions. They also find nine shifts between freshwater and marine habitats. 

The study points out that crocodylomorphs have exhibited convergent evolution across the millennia, meaning that different species have changed in the same ways – for instance, from webbed to unwebbed feet. 

“Unsurprisingly, much of this convergence accompanies ecological shifts, when relatively distantly related lineages adapt to the same habitats,” the authors write. 

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Phylogenetic and temporal pattern of habitat shifts within Crocodylomorpha. Colored arrows within circles indicate habitat transitions. Horizontal arrows indicate habitat transitions that occurred within collapsed groups. Combined green and black colouring of Calsoyasuchus represents uncertainty regarding its assigned habitat.

The Royal Institution of Australia has an education resource based on this article. You can access it here.

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