Jurassic Shark: Shark from the Jurassic Period was Already Highly Evolved
Published:12 Mar.2023    Source:University of Vienna

Cartilaginous fishes (sharks, rays, and ratfish) are an evolutionarily very old group of animals that already lived on earth before the dinosaurs more than 400 million years ago and have survived all five mass extinctions. Their fossil remains can be found in large numbers all over the world -- however, usually only the teeth remain, while the cartilaginous skeleton decays together with the rest of the body and does not fossilize.

In the Solnhofen archipelago, a so-called "Konservat Lagerstätte" in Bavaria, Germany, skeletal remains and even imprints of skin and muscles of Late Jurassic vertebrates (including cartilaginous fishes) have been preserved due to special preservation conditions. The research team used this circumstance to take a closer look at the previously unclear role of the already extinct species Protospinax annectans in the evolution of sharks and rays, also with the help of modern genetic evidence.
"Protospinax carried features that are found in both sharks and rays today," explains study author Patrick L. Jambura. Protospinax lived some 150 million years ago and was a 1.5-m-long, dorso-ventrally flattened cartilaginous fish with expanded pectoral fins and a prominent fin spine in front of each dorsal fin. Although known from well preserved fossils, the phylogenetic position of Protospinax has puzzled researchers ever since it was first described in 1918. "Of particular interest," Jambura continued, "is whether Protospinax represents a transition between sharks and rays as a 'missing link' -- a hypothesis that has gained considerable appeal among experts over the past 25 years." Alternatively, Protospinax could have been a very primitive shark, an ancestor of rays and sharks, or an ancestor of a certain group of sharks, the Galeomorphii, which includes the great white shark today -- all of which are exciting ideas whose plausibility has now been clarified by scientists.
Even though cartilaginous fishes as a group have survived to this day, most species disappeared during its evolution, including Protospinax. Why Protospinax became extinct at the Jurassic-Cretaceous boundary some 145 million years ago and why there is no comparable shark species today, while the ecologically similarly adapted rays exist relatively unchanged to this day, remains a mystery at this point.