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Evolutionary structure & timing of major habitat shifts in Crocodylomorpha
New study says modern-day crocodiles and alligators came from variety of surroundings

January 24, 2019, University of Iowa

[Image: crocodile.jpg]
Credit: CC0 Public Domain

A new study throws into question the notion that today's crocodiles and alligators have a simple evolutionary past.

Previous research has pointed to crocodiles and alligators starting with a land-based ancestor some 200 million years ago and then moving to fresh water, becoming the semi-aquatic ambush predators they are today.

But a new analysis, published online today in the journal Scientific Reports, offers a different story. Modern crocodiles and alligators came from a variety of surroundings beginning in the early Jurassic Period, and various species occupied a host of ecosystems over time, including land, estuarine, freshwater and marine.

As University of Iowa researcher and study co-author Christopher Brochu says, "Crocodiles are not living fossils. 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."

Brochu and colleagues from Stony Brook University pieced together crocodile and alligator ancestry by analyzing a large family tree showing the evolutionary history of living and extinct crocodylomorphs (modern crocodiles and alligators and their extinct relatives). The team was then able to predict the ancestral habitat for several divergence points on the evolutionary tree.

This suggests a complex evolutionary history not only of habitat, but of form. 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 paper is titled, "Evolutionary structure and timing of major habitat shifts in Crocodylomorpha."

Study co-authors are Eric Wilberg and Alan Turner, from Stony Brook University.

Journal Reference:
Eric W. Wilberg et al, Evolutionary structure and timing of major habitat shifts in Crocodylomorpha, Scientific Reports (2019). DOI: 10.1038/s41598-018-36795-1

Extant crocodylomorphs are semiaquatic ambush predators largely restricted to freshwater or estuarine environments, but the group is ancestrally terrestrial and inhabited a variety of ecosystems in the past. Despite its rich ecological history, little effort has focused on elucidating the historical pattern of ecological transitions in the group. Traditional views suggested a single shift from terrestrial to aquatic in the Early Jurassic. However, new fossil discoveries and phylogenetic analyses tend to imply a multiple-shift model. Here we estimate ancestral habitats across a comprehensive phylogeny and show at least three independent shifts from terrestrial to aquatic and numerous other habitat transitions. Neosuchians first invade freshwater habitats in the Jurassic, with up to four subsequent shifts into the marine realm. Thalattosuchians first appear in marine habitats in the Early Jurassic. Freshwater semiaquatic mahajangasuchids are derived from otherwise terrestrial notosuchians. Within nearly all marine groups, some species return to freshwater environments. Only twice have crocodylomorphs reverted from aquatic to terrestrial habitats, both within the crown group. All living non-alligatorid crocodylians have a keratinised tongue with salt-excreting glands, but the lack of osteological correlates for these adaptations complicates pinpointing their evolutionary origin or loss. Based on the pattern of transitions to the marine realm, our analysis suggests at least four independent origins of saltwater tolerance in Crocodylomorpha.
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