The origin of jawed vertebrates dates back to the early Silurian period

A great biotic catastrophe marked the end of the Ordovician (485.4 to 443.8 million years ago), immediately followed by the “Great Ordovician Biodiversity”. Ecological niches emptied of their occupants do not remain vacant for long: the frenzy of innovations produces many new species. The four fossil organisms just described by three teams based in China illustrate how at the beginning of the Silurian Period (443.8 to 419.2 million years ago), the period immediately after the Ordovician, this explosion in biodiversity revealed in a scattershot order several key characteristics of modern vertebrates.

Zhikun Gai’s team from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing described it Tujiaspis vividus, from the Huixingshao rock formation, dating back 436 million years, in Xiushan District. This organism is part of a lineage of ostracoderms – jawless fish that already existed during the Ordovician period. Its rounded, dome-shaped head is extended by a scale-covered articulated tail and bears three dorsal fins and an erect caudal fin.

In the same geological formation it was also Xiushanosteus mirabilis, the first fully described early Silurian coel type. So, two types of primitive fish coexisted, with and without jaws. president SheochanostiusAnd the Dome-shaped, its armored thorax makes it placoderm – an armored fish with an external skeleton composed of plates. Part of it evokes a kind of spine, and two dorsal fins. This first jawed fish also featured a caudal fin similar to that of some modern fish such as the fruit bat.

It was thought that placoderms were the origin of other jawed fishes, but two complementary discoveries have led to a revision of this filiation.

The fossilized jawbone of a 439-million-year-old fish has already been discovered in southern China by a team from Qujing Normal University. Spiral-shaped, set with overlapping rows of teeth, it evokes the mouth of a shark. The fish to which it belongs, such as sharks, have been identified as cartilaginous, a lineage characterized by a cartilaginous internal skeleton.

In the same place, the researchers discovered more than a thousand pieces of another chondrichthy with an armored head, with a body equipped with plates forming fins. These two specimens postpone the emergence of jawed vertebrates by 14 million years towards the beginning of the Silurian. The skeleton of the second jaw is reminiscent, according to the researchers, of what was observed in the first bony fish, bony fish – our distant ancestors. Thus, cartilaginous and osteophyte would have the same antiquity, even if the first known fossils of the latter date only to the end of the Silurian period. Thus jawed vertebrates, which today account for 99% of vertebrates, would have appeared concurrently in blastoderm, chondrogenic, and eustachian forms at the beginning of the Silurian.