A giant ichthyosaur lived in the oceans of the Triassic around 246 million years ago: the skull of Cymbospondylus youngorum alone measured two meters, its entire body over 17 meters. Researchers have now described the sea giant in detail for the first time using a well-preserved skull fossil. The analyzes also show that the giant developed surprisingly quickly on evolutionary timescales: the species only appeared about three million years after the first ichthyosaurs appeared. Whales, on the other hand, took over 50 million years to reach their gigantic proportions.
Around 252 million years ago, at the transition from the Permian to the Triassic, more than three quarters of the species living at the time died out after major volcanic eruptions. In the millions of years that followed, a new fauna emerged on earth, which, among other things, produced some of the largest living things that have ever lived on our planet. These included some of the dinosaurs living on land as well as representatives of the ichtysaurs, whose ancestors lived on land, but who themselves inhabited the oceans like today’s whales and dolphins.
Giant dinosaur from the ocean
A team led by Martin Sander from the University of Bonn has now described a very early sea giant: Cymbospondylus youngorum. “The ocean giant lived 246 million years ago, only about three million years after the appearance of the first ichthyosaurs,” the researchers report. They discovered the huge skeleton in a rock unit called Fossil Hill Member in the Augusta Mountains in Nevada (USA). The first eddies appeared there as early as 1998. “The significance of the find was not apparent for a long time, as only a few eddies were exposed at the edge of the gorge,” says Sander. “However, the anatomy of the vertebrae suggested that the front end of the animal could still be hidden in the rock.”
In fact, during excavations in 2011, his team found the well-preserved, two-meter-long skull of the giant marine reptile, along with remains of the forefins and chest area. The researchers dated the find to be 246 million years old. Based on the size of the preserved bones, especially the skull, they estimated the total length of the animal to be at least 17 meters – about the size of a modern sperm whale. The newly discovered marine dinosaur is probably the largest animal of its time. “As far as we know, it was actually the first giant living being that ever lived on earth,” says Sander.
Evolution at record speed
According to the researchers, the evolution to gigantic stature in Cymbospondylus youngorum was astonishingly fast – within only three million years since the appearance of the first, still very small, ichthyosaurs. “While the whales took around 90 percent of their 55 million year history to develop into giants, the ichthyosaurs – using the example of C. youngorum – succeeded in the first percent of their 150 million year history,” write Lene Delsett and Nicholas Pyenson of the National Museum of Natural History in Washington in an accompanying commentary on the study.
But how did such a huge species of ichthyosaurs develop so quickly? To find out, Sander and his colleagues modeled the ecosystem of the time on the computer and calculated how much energy the food web could have supplied. To do this, they included other fossil finds of potential prey species that were also preserved in Fossil Hill. As the researchers report, numerous ammonites – a group of cephalopods that are now extinct and related to modern octopuses – lived in the seas after the mass extinction. Together with other marine life, they apparently provided the ichthyosaurs with a lot of food. “We assume that the ichthyosaurs were able to develop so rapidly because they were the first larger creatures to populate the oceans and were exposed to less competition,” says Sander.
Similar development at different speeds
A comparison with today’s giants of the seas, the whales, shows that both ichthyosaurs and whales had to exploit new ecological niches in the food web in order to grow really big. “The results of our study illustrate how different groups of marine tetrapods evolved into epic sizes under somewhat similar circumstances, but at surprisingly different speeds,” says co-author Jorge Velez-Juarbe of the Natural History Museum in Los Angeles. “With the data set that we have compiled and the analytical methods tested, we can in the future also think about the inclusion of other groups of secondary aquatic vertebrates in order to understand this aspect of their evolutionary history.”
Source: Marin Sander (University of Bonn) et al., Science, doi: 10.1126 / science.abf5787