Cartilaginous fish have changed much more in the course of evolution than previously assumed. Evidence for this thesis was provided by molecular biological data on fossil remains of Protospinax annectans, an already highly evolved shark from the late Jurassic. This is the result of a recent study by an international research group led by paleobiologist Patrick L. Jambura at the Department of Paleontology at the University of Vienna, which was recently published in the journal Diversity.
Cartilaginous fishes like sharks and rays are evolutionarily a very old group of animals that already lived on earth before the dinosaurs more than 400 million years ago and have so far survived all five faunal changes due to mass extinctions. Their fossil remains can be found in large numbers all over the world - but usually only the teeth remain, while the cartilaginous skeleton decays together with the rest of the body.
A unique window to the past
However, in the Solnhofen archipelago, a so-called preserved deposit in Bavaria, skeletal remains and even imprints of skin and muscles of the Jurassic cartilaginous fish have been preserved due to the special storage 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 molecular biological methods.
"Protospinax carried features that are found today in both rays and sharks," explains study first author Patrick L. Jambura. The fossils of Protospinax are 150 million years old. A 1.5-m-long, flat-topped cartilaginous fish with outspread pectoral fins and two prominent spines in front of each dorsal fin, Protspinax’s phylogenetic position 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 traction 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 particular group of sharks, such as the great white shark - all exciting ideas whose plausibility has now been clarified by scientists.
A mystery solved - a mystery remained
Incorporating the latest finds, Jambura and his international team reconstructed the family tree of sharks and rays still living today using genetic data (mitochondrial DNA) and embedded fossil groups - including Protospinax annectans - using morphological data. The result of the analysis was astonishing: Protospinax was neither a "missing link" nor a ray nor an original shark - but a highly evolved shark.
"We tend to think of evolution like a hierarchical ladder, with older groups at the top of this system. But in fact, evolution never stopped even for these primitive representatives; they too evolved day by day via changes in their DNA to adapt to an ever-changing environment and survive to this day," says paleobiologist Jambura.
Even if cartilaginous fishes have survived as a group until today, most species disappeared in the course of evolution, so did Protospinax. Why Protospinax died out at the border between the Jurassic and Cretaceous periods and there is no comparable shark species today - while the ecologically similarly adapted rays exist relatively unchanged to this day - remains a mystery at the present time.
Publication in Diversity:
Patrick L. Jambura, Eduardo Villalobos-Segura, Julia Türtscher, Arnaud Begat, Manuel Andreas Staggl, Sebastian Stumpf, René Kindlimann, Stefanie Klug, Frederic Lacombat, Burkhard Pohl, John G. Maisey, Gavin J. P. Naylor and Jürgen Kriwet: Systematics and Phylogenetic Interrelationships of the Enigmatic Late Jurassic Shark Protospinax annectans Woodward, 1918 with Comments on the Shark-Ray Sister Group Relationship. In: Diversity, 2023.
DOI: 10.3390/d15030311
Illustrations:
Fig. 1: Fossil of the Late Jurassic shark Protospinax annectans from Solnhofen and Eichstätt, Germany (C: Sebastian Stumpf) Fig. 2: Paleoreconstruction of the Solnhofen Archipelago 150 million years ago showing Protospinax annectans and the Jurassic ray Asterodermus platypterus (C: Manuel Andreas Staggl).
