Ichthyosaurs were a successful group of large marine reptiles for most of the Mesozoic Era.
These creature first appeared in the oceans after the Permian mass extinction, about 248 million years ago (Early Triassic epoch), and became extinct in the early Late Cretaceous epoch, 90 million years ago.
They breathed air like dolphins and whales, had a streamlined body for moving through the water, large eyes for improved vision at depth, an elongated skull with jaws full of conical teeth, suited for catching fish and squid.
Like modern orca or great white sharks, they may have been apex predators of their ecosystems, but until recently there has been little direct evidence of this.
“Ichthyosaurs derive from an as yet unknown group of land-living reptiles and were air-breathing themselves,” said Dr. Martin Sander, a paleontologist at the University of Bonn and the Dinosaur Institute at the Natural History Museum of Los Angeles County.
The well-preserved skull of Cymbospondylus youngorum — along with part of the backbone, shoulder, and forefin — were recovered from the Fossil Hill Member in the Augusta Mountains of Nevada, the United States.
Based on the age of the newly-identified species, it appears that ichthyosaurs evolved to such a vast size in just 3 million years — far faster than whales did.
The fossil remains of C. youngorum were first discovered in the Augusta Mountains in 1998 – specifically in the form of fragments of the creature’s spine.
Professor Sander said: “The significance of the discovery was not immediately clear, as it only showed a few vertebrae.”
“However, the anatomy of the vertebrae suggests that the anterior end of the animal may still be hiding in the rocks.”
However, researchers didn’t validate this idea until September 2011 – they dug out of the rock a well-preserved fossil skull (6.6 feet long), forelimbs and chest bones from the large ichthyosaur.
Faster evolution of large body size in ichthyosaurs than in cetaceans
According to Professor Sander, the marine reptile was certainly the largest animal discovered in this time period.
“As far as we know, it was even the first giant creature to ever inhabit Earth,” the paleontologist added.
But what’s even more interesting about the size of C. youngorum is the fact that the creature appeared only 3 million years after the first ichthyosaurs evolved from their wild ancestors, evolved fins and shaped hydrodynamics.
“That’s an amazingly short time to grow this big,” Sander explained.
To see how C. youngorum became so large, the researchers used ecosystem modeling to explore the energy flows of the local food web at that time.
“One unique aspect of this project is the integrative nature of our approach,” said paper author and paleobiologist Lars Schmitz, of Scripps College in California.
After describing in detail the anatomy of the giant skull and thus understanding how this animal related to other ichthyosaurs, we wanted to understand the significance of the new discovery in the context of the large-scale evolutionary pattern of ichthyosaurs and whale body sizes.
To do this, we needed to figure out how the fossil ecosystem preserved in the Fossil Hill Member may have functioned.
The analysis suggested that Nevada’s waters were once suitable for the evolution of such a giant – being rich in eel-like conifers and ammonites – and may have supported a larger ichthyosaur as well.
In contrast to the evolution of modern whales, which have slowly risen in size throughout their history, the ichthyosaurs appear to have undergone a sudden boom, the team said.
“We assume that ichthyosaurs were also able to evolve very quickly because they were the first larger creatures to inhabit the world’s oceans and had less competition,” Sander added.
In contrast, the evolution of whales has been driven by the availability of different types of plankton – as well as the adoption of different feeding disciplines.
However, both ichthyosaurs and whales relied on exploiting niches in the good chain to reach such enormous sizes.
With the study completed, C. youngorum entered the paleontology collections of the Los Angeles County Museum of Natural History, where it is currently on display.
Sources:
https://www.science.org/doi/10.1126/science.abf5787