An ancient shark may be the human jaw ancestor - here's why
The discovery of a 439-million-year-old shark by Chinese paleontologists prompts us to reevaluate the timeframe of vertebrate evolution.
Scientists were surprised by China's antiquity of a newly discovered acanthodian species. This discovery also shows that this is the oldest undisputed jawed fish and precedes the first acanthodian body fossils by roughly 15 million years.
The results were published in Nature on September 28.
As stated, paleontologists could identify fossils of extinct ancestors that date back hundreds of millions of years ago, called the Paleozoic era. These primitive sharks, known as acanthodians, were covered in spines. Unlike modern sharks, they evolved bony "armor" around their paired fins.
Fanjingshania — called after the well-known UNESCO World Heritage Site Fanjingshan — is a peculiar fish that distinguishes itself from living jawed fish, cartilaginous sharks and rays, bony ray, and lobe-finned fish by having numerous pairs of spines on its fins and an exterior bony armor.
Examination of Fanjingshania was performed by a team of researchers from the Chinese Academy of Sciences, Qujing Normal University, and the University of Birmingham.
The fossil remains of Fanjingshania were recovered from bone bed samples of the Rongxi Formation at a site in Shiqian County of Guizhou Province, South China.
These discoveries provide verifiable proof that key vertebrate groupings underwent diversification tens of millions of years before the 420 million-year-old start of the so-called "Age of Fishes."
Fanjingshania has dermal shoulder girdle plates that fuse as a unit to the pectoral, prepectoral, and prepelvic spines. Furthermore, the ventral and lateral portions of the shoulder plates were discovered to extend to the posterior edge of the pectoral fin spine.
Dentine development has been recorded this time
The species has distinct trunk scales and crowns that are made up of a row of tooth-like elements (odontodes) adorned by discontinuous nodose ridges. Dentine development is unusually recorded in the scales.
"This is the oldest jawed fish with known anatomy," said Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences.
"The new data allowed us to place Fanjingshania in the phylogenetic tree of early vertebrates and gain much-needed information about the evolutionary steps leading to the origin of important vertebrate adaptations such as jaws, sensory systems, and paired appendages."
"Skeletal development, including humans"
One of the most astonishing things about Fanjingshania is showing evidence of extensive resorption and remodeling that are typically associated with skeletal development in bony fish, including humans.
"This level of hard tissue modification is unprecedented in chondrichthyans, a group that includes modern cartilaginous fish and their extinct ancestors," said lead author Dr. Plamen Andreev, a researcher at Qujing Normal University.
"It speaks about greater than currently understood developmental plasticity of the mineralized skeleton at the onset of jawed fish diversification."
"The new discovery puts into question existing models of vertebrate evolution by significantly condensing the timeframe for the emergence of jawed fish from their closest jawless ancestors. This will have profound impact on how we assess evolutionary rates in early vertebrates and the relationship between morphological and molecular change in these groups," said Dr. Ivan J. Sansom from the University of Birmingham.
Modern representatives of chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods) have contrasting skeletal anatomies and developmental trajectories that underscore the distant evolutionary split of the two clades. Recent work on upper Silurian and Devonian jawed vertebrates has revealed similar skeletal conditions that blur the conventional distinctions between osteichthyans, chondrichthyans, and their jawed gnathostome ancestors. Here we describe the remains (dermal plates, scales, and fin spines) of a chondrichthyan, Fanjingshania renovates gen. et sp. nov., from the lower Silurian of China that pre-date the earliest articulated fossils of jawed vertebrates. Fanjingshania possesses dermal shoulder girdle plates and a complement of fin spines that have a striking anatomical similarity to those recorded in a subset of stem chondrichthyans (climatiid ‘acanthodians’). Uniquely among chondrichthyans, however, it demonstrates osteichthyan-like resorptive shedding of scale odontodes (dermal teeth) and an absence of odontogenic tissues in its spines. Our results identify the independent acquisition of these conditions in the chondrichthyan stem group, adding Fanjingshania to an increasing number of taxa nested within conventionally defined acanthodians. The discovery of Fanjingshania provides the strongest support yet for a proposed early Silurian radiation of jawed vertebrates before their widespread appearance5 in the fossil record in the Lower Devonian series.
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