Meet C. youngorum, the giant ichthyosaur that ruled the oceans 250 million years ago

A team of paleobiologists from Germany and the United States unearthed a Middle Triassic fossil of an ichthyosaur (Cymbospondylus youngorum) of Favret Canyon in Nevada, USA.

The animal was 18 m long, with the skull alone over 2 m. The discovery is significant because it shows that the ichthyosaur developed gigantism very early in its evolutionary history.

The ichthyosaur was the largest tetrapod of its time, whether on land or at sea. To put this in perspective, whales (cetaceans), which emerged in the Cenozoic around 60 million years ago, took almost 90 % of their evolutionary history to become the giants they are today. But the ichthyosaur, which dominated ocean waters 252-94 million years ago, took only 1% of its evolutionary history to develop gigantism.

This is well documented in the fossil record. The Cartorhynchus, the closest relative of the Cymbospondylus, which lived 248.5 million years ago, had a skull length of only 55 mm. Corn Cymbospondylus youngorum had a skull length of 1,890 mm – barely 2.5 million years later.

The study used conventional paleontological tools to recover and identify fossils using established protocols of comparative morphology. This was supplemented by computational phylogeny and energy flow modeling to better locate the ichthyosaur in the food web.

Phylogenetic analyzes, combined with fossil studies, reveal that C. youngorum and its close relatives are at the origin of a great “morphological disparity of the ichthyosaurs of the lower and middle Triassic”. This points to adaptive radiation, an evolving phenomenon by which organisms evolve, adapt and diversify very rapidly from a common ancestor in tandem with a changing environment.

The study further argues that the presence of fossils of the genus Cymbospondylus across the northern hemisphere adds weight to the theory of adaptive radiation in the case of the ichthyosaur.

The ichthyosaur’s oldest relative – the Cartorhynchus native to China – is no more than a few inches in length. Researchers wonder if conditions in the Proto-Pacific were conducive to gigantism as opposed to the warm, shallow Sea of ​​Tethys, where the fossils of the Chinese ichthyosaur parent are found.

What was the diet of the ichthyosaurs?

This is an important question which must be answered if one seeks to explain the brief burst of evolution which led to gigantism. The fossil assemblage recovered from the site is dominated by cephalopods, which constituted a large part of the marine invertebrate fauna of that time.

    ichthyosaur fossil An ichthyosaur fossil surrounded by shells of ammonites, the food source that may have fueled their growth. (Georg Oleschinski, courtesy University of Bonn, Germany.)

The ichthyosaur therefore fed on these cephalopods, conodonts now extinct, and even small ichthyosaurs. These provided the ichthyosaur with a sufficient and stable food reservoir.

Morphological analysis of ichthyosaur dentition shows that C. youngorum had a fairly general diet of fish and squid, and studies have consistently found cephalopods and fish in the stomach contents of fossilized ichthyosaur remains.

A case for a convergent evolution

The comparison between ichthyosaurs that lived in the Mesozoic and current cetaceans (whales, dolphins and porpoises) is an interesting case study for a convergent evolution.

Both ichthyosaurs and cetaceans have both returned to the sea – “going from a full-time life on land to a full-time life in the ocean.” The two have remarkably similar body shapes and lifestyles. Being tail-propelled swimmers, not only are they similar in body shape, but also in size – the two gigantic bodies evolved. The only difference is that they existed almost 200 million years apart.

This leads to another question: Did ichthyosaurs and cetaceans follow similar evolutionary trajectories, regardless of how fast the process took place?

As it stands, despite so many similarities, the evolutionary paths of the two were quite different. The massive extinctions of the late Permian led to a proliferation of conodonts and ammonoids which served as food for the ichthyosaur. The gigantism of cetaceans also evolved from trophic specialization but the path was different.

“This finding and the results of our study highlight how different groups of marine tetrapods developed body sizes of epic proportions under somewhat similar circumstances, but at surprisingly different rates,” says one of the authors, Dr. Jorge Velez-Juarbe in a press release. He is from the Department of Mammalogy, Los Angeles County Museum of Natural History.

“In the future, with the dataset we have compiled and the analytical methods we have tested, we can start to think about including other groups of secondary aquatic vertebrates to understand this aspect of their evolutionary history,” he adds.

– The author is an independent scientific communicator. (to post[at]ritvikc[dot]with)

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