Information about the early stages of the evolution of osteichthyans, known as bony fishhas been limited by the lack of primitive fossils. Now, a Chinese scientific team has managed to identify the oldest known remains, revealing their morphology and key anatomical features, such as jaws or teeth.
The investigation, which occupies the cover of the magazine Natureis led by researchers from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences and fills “an important gap” in evolutionary history, according to those responsible.
The scientists, including Min Zhu, Jing Lu and You-an Zhu, publish the results after more than a decade of field and laboratory work in two articles; in each one they describe a primitive species of bony fish.
Bony fish form the main trunk of the vertebrate tree of life. Its two surviving lineages, the ray-finned fishes and the lobe-finned fishes, have conquered a wide range of niches in aquatic and terrestrial ecosystems, respectively.
Ray-finned fishes include more than 30,000 extant species, comprising the majority of the fish we know today. A lineage of lobe-finned fish colonized the earth during the Devonian, giving rise to all tetrapods, including humans.
Fossils from before the Devonian (beyond about 419 million years ago) are scarce and many early specimens are fragmented and incomplete. The new fossil materials now presented from two sites in southwestern China help improve understanding of the sequence of the first evolutionary steps that shaped the osteichthyan lineage.
In the first article, the researchers describe a small, almost complete, articulated skeleton of a bony fish from the site in the Chongqing area, which dates back to the early Silurian (about 436 million years ago). It is, according to the authors, the oldest complete bony fish fossil known in the world, the Eosteus chongqingensis.
It lived approximately 436 million years ago and measured just 3 centimeters in total length, but is exceptionally complete, with the entire body from head to tail preserved.
This small fish displays a mosaic of primitive and derived features: its streamlined body, single dorsal fin, and special scales known as caudal fulcrums resemble early ray-finned fish, but it lacks the lepidotrichia (bony fin stripes) typical of bony fish.
The discovery demonstrates that the set of basic characteristics of bony fish evolved much earlier than previously thought, says a statement from the Institute of Vertebrate Paleontology and Paleoanthropology.
In the other article the prominence is for Megamastax amblyodus; The fossils were found in Yunnan province and date back to about 423 million years ago (Upper Silurian). At more than a meter in length, it was the largest vertebrate of its time.
Thanks to high-resolution computed tomography, the researchers reconstructed his complete cranial anatomy and dentition. The analysis solves a half-century-old enigma about the origin of their dental plaques.
Phylogenetic analyzes placed both fish in the trunk group of bony fish, which represents the primitive condition before the separation between ray-finned fish and lobe-finned fish.
Together, these discoveries advance the understanding of the transformations that shaped the osteichthyan lineage, the magazine summarizes.
The findings further demonstrate that southern China is the cradle of early vertebrate evolution.