A team of palaeo-biologists from Germany and the USA have unearthed a Middle Triassic fossil of an ichthyosaur (Cymbospondylus youngorum) from the Favret Canyon in Nevada, USA.
The animal was 18 m long, with the skull alone more than 2 m. The discovery is important as it shows that the ichthyosaur evolved 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 at around 60 mya, took nearly 90 per cent of their evolutionary history to evolve into the giants they are today. But the ichthyosaur, which dominated ocean waters from 252-94 million years ago, took only 1 per cent of its evolutionary history to evolve gigantism.
This is well documented in the fossil record. The Cartorhynchus, the closest relative of Cymbospondylus, which lived 248.5 million years ago, had a skull length of only 55 mm. But Cymbospondylus youngorum had a skull length of 1890 mm – a mere 2.5 million years later.
The study employed conventional palaeontological tools of fossil recovery and identification using established protocols of comparative morphology. This was further supplemented by computational phylogeny, and energy flux modelling to better situate the ichthyosaur in the food web.
Phylogenetic analyses, in conjugation with fossil studies, reveal that C.youngorum and its close relatives account for a great deal of ‘morphological disparity of Early and Middle Triassic ichthyosaurs’. This points towards adaptive radiation, a phenomenon in evolution whereby organisms evolve, adapt, and diversify very quickly from a common ancestor in tandem with a changing environment.
The study further argues that the presence of fossils from the Cymbospondylus genus across the Northern Hemisphere adds weight to the adaptive radiation theory in the case of the ichthyosaur.
The oldest relative of the ichthyosaur – the Cartorhynchus hailing from China – is no bigger than a few inches long. The researchers argue whether the conditions in the proto-Pacific were conducive to gigantism as opposed to the warm-and-shallow Tethys sea, where the fossils of the Chinese ichthyosaur relative are found.
What constituted the ichthyosaur diet?
This is an important question to answer if we seek to explain the short burst of evolution that led to gigantism. The fossil assemblage recovered from the site is dominated by cephalopods, which constituted much of the marine invertebrate fauna of that time.
The ichthyosaur, therefore, fed on these cephalopods, the now-extinct conodonts, and even small ichthyosaurs. These provided the ichthyosaur with a sufficient and stable food reservoir.
A morphological analysis of ichthyosaur dentition shows that C.youngorum had a fairly generalist diet of fish and squid, and studies have consistently found cephalopods and fish in the stomach contents of fossilised ichthyosaur remains.
A case for convergent evolution
The comparison between the ichthyosaurs that lived during the Mesozoic and present-day cetaceans (whales, dolphins and porpoises) is an interesting case study for convergent evolution.
Both ichthyosaurs and cetaceans returned to the sea – ‘transitioning from full-time life on land to full-time life in the ocean’. Both have remarkably similar body shapes and lifestyles. Being tail propelled swimmers, not only are they similar in terms of body shape, but also size – both evolved gigantic bodies. The only difference is that they existed nearly 200 million years apart.
This leads to another question: did ichthyosaurs and cetaceans follow similar trajectories of evolution, regardless of the speed at which the process took place?
As things stand, despite so many similarities, the evolutionary pathways to both were quite different. The end-Permian mass extinctions led to a proliferation of conodonts and ammonoids that served as food to the ichthyosaur. Cetacean gigantism also evolved from trophic specialisation but the pathway was different.
“This discovery and the results of our study highlight how different groups of marine tetrapods evolved 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 release. He is from the Department of Mammalogy, Natural History Museum of Los Angeles County.
“Moving forward, with the dataset we’ve compiled and analytical methods we’ve tested, we can start thinking about including other groups of secondarily aquatic vertebrates to understand this aspect of their evolutionary history,” he adds.
– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)