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An ancient Jurassic shark fossil reveals a surprising evolutionary fact

A recent study by an international research team led by paleobiologist Patrick L. Jambura of the Department of Paleontology at the University of Vienna has shown that cartilaginous fish have undergone more evolutionary changes than previously thought. Confirming this conclusion, new fossils of a shark-like stingray, Protospinax annectans, which indicates that sharks were highly developed during the Late Jurassic period. The results of the research were published in the journal Variation.

Sharks, stingrays, and mousefish are an ancient group of animals known as cartilaginous fish that predate the dinosaurs and have existed on Earth for over 400 million years. They also survived all five mass extinctions, and fossil remains can be found in abundance all over the world. However, as the cartilaginous skeleton decays with the rest of the body and does not fossilize, as a rule only its teeth remain intact.

A unique window into the past

In the Solnhofen Archipelago, called “Konservat Lagerstätte” in Bavaria, Germany, the skeletal remains and even skin and muscle traces of late Jurassic vertebrates (including cartilaginous fish) have been preserved, thanks to special conservation conditions. The research team used this to take a closer look at the previously obscure role of the now-extinct Protospinax species. annectans in the evolution of sharks and rays, also with the help of modern genetic evidence.

protospinax had features found today in both sharks and stingrays,” explains study author Patrick L. Jambura. protospinax It was a 1.5 m long, dorsoventrally flattened cartilaginous fish that lived about 150 million years ago, with enlarged pectoral fins and a distinctive fin spine in front of each dorsal fin. Despite the fact that the phylogenetic position of the protospinax Known from well-preserved fossils, it has baffled researchers since it was first described in 1918.

Dzhambura continued: “This is particularly interesting if it represents. protospinax Crossing between sharks and stingrays as the “missing link” – a hypothesis that has gained considerable popularity among experts over the past 25 years.” protospinax It could have been a very primitive shark, the ancestor of rays and sharks, or the ancestor of a particular group of sharks, including the modern-day great white Galeomorphii, all fascinating ideas now proven to be plausible. scientists determine.

One mystery solved, another remains

Taking into account the latest fossil findings, Jambura and his international team reconstructed the family tree of existing sharks and rays using genetic data (mitochondrial DNA) and embedded fossil groups. Protospinax annectans, using morphological data. The results of the analysis were striking: protospinax It was not a “missing link,” not a stingray, nor a primitive shark, but a highly evolved one.

“We tend to think of evolution as a hierarchical, ladder-like system. Elderly groups are at the bottom, and humans are at the top as a very young species in Earth’s history. But in reality, evolution has never stopped even for these primitive creatures, and they, just like us, are changing from day to day with changes in their DNA. they continue to evolve. It is the only way they have adapted to the ever-changing environment and survived to this day,” says Jambura.

Although cartilaginous fish survive as a group, most species have disappeared during their evolution. protospinax. From where protospinax It went extinct about 145 million years ago on the border between the Jurassic and Cretaceous periods, and why there are no similar shark species today, while ecologically similar adapted rays have existed relatively unchanged to this day, is currently a mystery.

Source: Port Altele

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