Blue whales are now the largest living beings on Earth — or, more precisely, in the seas. But has life in the oceans always favored a giant size or did it only appear later in the evolutionary history of these animals?
Whale-like body shape and size appeared a few times in vertebrate evolutionary history, the first being a group of marine reptiles — which are not dinosaurs — known as ichthyosaurs. And in the case of these animals, a giant body evolved early on and very quickly in the group.
This is what reveals a new species of ichthyosaur found in rocks of the Fossil Hill formation, in Nevada, in the United States. christened of Cymbospondylus youngorum (in honor of the couple Tom and Brenda Young, owners of a brewery in the region and who financed the excavation), the article describing the find was published in December in the scientific journal Science.
The fossil was excavated by a team of paleontologists at the Los Angeles Museum of Natural History. The work had the collaboration of researchers from the Universities of Bonn and Mainz, in Germany, and from the Catholic University of the Santissima Conception, in Chile.
With an estimated age of 246 million years, the ichthyosaur may be the oldest “giant” ever discovered, with a skull about two meters long from the tip of the snout to the base of the head, in connection with the neck, and an estimated body size of 17 meters, or nine meters less than an adult blue whale.
Despite the phylogenetic distance —whales are mammals, the fossil is a reptile—and geological distance—the most recent common ancestor of current cetaceans, a group that includes whales and dolphins, lived 56 million years ago, in the Eocene period, while the reptile is at least 190 million years older—the bodies of ichthyosaurs and whales are similar in both shape and size.
In the study, scientists looked at how these two very distinct groups of vertebrates evolved with similar body shapes (basically, ichthyosaurs look like a primitive dolphin).
“Our question was how did the body size evolve in these animals and at what speed, because we already had evidence of an explosive diversification of the group in the early Triassic. [há 249 milhões de anos]”, explains Claremont College professor and also research associate at the Museum of Natural History in Los Angeles, Lars Schmitz.
The surprise came, first, to see that the new fossil was unrelated to one described by the group in 2013, known only from the back of the skull — the “snout” was lost.
“We were really surprised to find that it was another species of a large ichthyosaur from the same locality and we started to ask ourselves: ‘what does this giant size mean in these reptiles so early in their origin?'”, asks the first author of the study. study, paleontologist Martin Sander, from the University of Bonn, Germany.
In addition to size, the researchers also wanted to understand the function and dynamics of a large body in the sea, and also what the diet of these animals was like, because large organisms, in general, need a lot of energy to survive.
“We started with a question of ‘who eats whom’, and then we carried out analyzes to estimate the possible relationships of the food chain in that environment, also considering the morphology of the skull, teeth and body of these animals”, explains Eva Maria Griebeler, specialist in ecology and evolution at the University of Mainz.
In the ichthyosaur, the conical and relatively backward-curved teeth indicated a diet based on fish and other aquatic vertebrates, such as smaller reptiles. On the other hand, large whales (known as mysticetes), such as the blue whale and the humpback, lose their teeth and feed on large amounts of krill (or plankton).
Dolphins and orcas, which are called odontycetes, appeared later in the evolutionary history of these mammals. The curious thing is that even in other aquatic mammals, such as manatees and (extinct) elephant seals, the shape and size similar to those of whales did not appear, explains Velez-Juarbe.
Scientists calculated the size and weight of known ichthyosaur fossils and ran evolutionary models of these animals.
The result was that the new reptile is considered basal in the evolutionary history of the group (that is, it came from a lineage that diverged early from the terrestrial common ancestor), but already had a giant body, indicating a very rapid evolutionary rate. The lifetime that separates the C. youngorum of its terrestrial ancestor was 3 million years old.
Already in whales, giant forms appeared much more slowly and gradually. This came from quadrupedal terrestrial ancestors that looked more like a cross between tapirs and dogs (but had nothing to do with either the first or the second), 56 million years ago.
Jorge Velez-Juarbe, from the Department of Mastozoology at the Museum of Natural History in Los Angeles, explains that whales grew at a much slower rate than ichthyosaurs.
“We only started to see large whale species very late in their evolutionary history, when they lost their teeth in the group known as mysticetes,” he says.
The physical changes of this group, such as the disappearance of the hind legs and then the front legs, with fins in place, and the position of the nostril that migrated from the front to the top of the skull, favored an aquatic life of these animals little by little.
But not all would be calm waters in the evolution of ichthyosaurs. About 50 million after their origin, these marine reptiles went through a process of extinction in the late Triassic, and the lineages that survived into the Jurassic and lasted until the end of the Cretaceous, 90 million years ago, were generally smaller and more similar to current dolphins — some exceptions, such as a giant ichthyosaur found in England and announced last Monday (10), had large sizes.
The team of paleontologists, in fact, seeks to understand a little more about what the marine ecosystem of the Triassic was like, much less diverse in terms of organisms called primary producers, which are abundant today.
“The only way to get so big and so fast is to feed at the bottom of the food chain, where there is more abundance, and that’s not what happened with ichthyosaurs. Some species were even what we call raptorial, that is, they actively hunted other vertebrates, maybe even other ichthyosaurs. Which is also intriguing, because many of these animals became extinct in the transition from the Triassic to the Jurassic, and then the forms that continue to feed essentially on fish and molluscs, such as ammonites”, says Sander .
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