Cetaceans in the ocean (e.g., whales, dolphins) evolved about 56 million years ago. It took another 50 million years for some of these species to become very large. Cetaceans, however, were not among the first marine behemoths.
About 249 million years ago, ichthyosaurs appeared in the oceans and dominated them for the next 150 million years. These Mesozoic fish-shaped marine reptiles have many similarities in size and lifestyle to modern cetaceans, which is in stark contrast to the fact that they are nearly 200 million years apart in time.
In a new study, researchers found a fossil skull the size of a modern cetacean. The well-preserved fossil skull is 2 meters long, along with parts of the spine, shoulders and arms. They found that the fossil belonged to a behemoth with a body length of more than 17 meters, which was a new species of Cymbospondylus in the order Ichthyosaurus.
Fossil skull of Cymbospondylus youngorum. | Image reference: Natalja Kent/NHM
The research team named it Cymbospondylus youngorum and published their findings in a recent journal of Science.
The fossil was excavated from the Augusta Mountains in Nevada, USA. Since 1902, paleontologists have been searching for fossils in some of Nevada's mountain ranges that open a window into the Triassic period, linking modern and ancient oceans.
The Augusta Mountains are buried with fossils. | Image: Lars Schmitz/NHM
There, paleontologists have discovered many ammonites, which are the shelled ancestors of modern cephalopods such as squids and octopuses; not only that, but there, paleontologists have also found many marine reptiles, specimens collectively known as the Fossil Hill Fauna.
The fossil mountain fauna contains many of Cymbospondylus youngorum's prey as well as competitors. Judging from Cymbospondylus youngorum's slender mouth and conical teeth, the researchers speculate that the species preys on squid and fish. At the same time, its large size suggests that it may also prey on smaller and younger marine reptiles.
In addition, they also think that this huge predator may have experienced some fierce competition. Using sophisticated computational models, the researchers examined possible energies in the food webs of fossil mountain fauna, recreating ancient environments and finding that in addition to Cymbospondylus youngorum, there may have been some larger carnivorous ichthyosaurs in the marine food webs at the time.
Studies have shown that the new marine behemoth Cymbospondylus youngorum, which lived in the ocean 246 million years ago, is the earliest known species of ichthyosaurs and is the beginning of a series of marine behemoths. They appeared only about 3 million years later than the first ichthyosaurs, which means they grew to such a large size in a very short period of time.
Viji shake, a volunteer at the Dinosaur Institute at the Natural History Museum of Los Angeles County, lies next to the skull of Cymbospondylus youngorum while preparing the specimen. | Image reference: Martin Sander/NHM
So, what kind of process drives the rapid expansion of body size? The researchers found that the common denominators of cetaceans and ichthyosaurs are not limited to having the same body size range, they also have similar body shapes, and these similarities make them scientifically valuable for comparative study.
Combining computer modeling with traditional paleontology, the researchers studied how these marine animals independently reached such large sizes. It turned out that while both Ichthyosaurs and cetaceans evolved very large sizes, their respective paths toward mega-evolution were different.
Ichthyosaurs seem to have experienced a period of physical flourishing and became behemoths early in their evolutionary history, which seems to have benefited from the emergence of a large number of tooth-shaped stones and ammonites to fill the ecological gap after the mass extinction at the end of the Permian. The mass reproduction of these creatures provided ample food for marine reptiles.
A fossil of an ichthyosaur surrounded by ammonites, which could have been a food source for them to grow into behemoths. | Photo credit: Georg Oleschinski / University of Bonn, Germany
Cetaceans have taken a different evolutionary route, and their body size expansion seems to be related to nutritional specialization, such as the loss of teeth in baleen whales (huge filter whales), and the evolution of toothed whales diving into the deep ocean for predation. It took them longer to reach the size of the behemoth.
Although the two have different evolutionary routes, they both use ecological niches in the food chain to make themselves bulky.
The researchers say that by studying completely extinct animals like ichthyosaurs, we can paint a richer picture of the complexity and commonalities of life history. As the new study found, this skull fossil not only gave scientists a new understanding of how marine reptiles rapidly grew into behemoths in the ocean during the age of dinosaurs, but also helped us better understand how modern cetaceans became such a large animal. In the future, using the dataset compiled by the new institute and the analytical methods tested, researchers will begin to consider including other secondary aquatic vertebrate populations in order to better understand their evolutionary history.
#创作团队:
Text: Sugar Beast
#参考来源:
https://nhm.org/stories/earths-first-giant
https://www.science.org/doi/10.1126/science.abf5787
#图片来源:
封面图：Illustrated by Stephanie Abramowicz, courtesy of the Natural History Museum of Los Angeles County (NHM).