The group of early jawed fishes traditionally referred to the Acanthodii is emerging as pivotal in our understanding of our own beginnings. The oldest history of these animals is poorly known, mostly because our understanding of this species - described from the early Paleozoic Era (before the Devonian Period) - is based on isolated scales, fin spines, tooth whorls, and dentigerous jaw bones. Recent research has revealed that this group of spiny fishes, known collectively as the acanthodians, were the first ancestors of modern-day sharks. Last month, scientists discovered a new species of acanthodian from China, predating previous findings by about 15 million years. This makes it the earliest acanthodian body fossils - and the oldest undisputed jawed fish. "Until this point, we've picked up hints from fossil scales that the evolution of jawed fish occurred much earlier in the fossil record, but have not uncovered anything definite in the form of fossil teeth or fin spines," University of Birmingham paleobiologist Ivan Sansom explained.
Life reconstruction of Fanjingshania renovata.
A team of researchers from the Chinese Academy of Sciences, Qujing Normal University, and the University of Birmingham painfully reconstructed this animal from thousands of tiny skeletal fragments, piecing together slowly what this new animal might look like. Named after a famous UNESCO World Heritage Site, a nearby mountain known as Fanjingshan, it truly is a bizarre fish sporting an external bony "armor" and multiple pairs of fin spines. The fossil remains of Fanjingshania were originally recovered from a site in the Guizhou Province of South China. Road excavations in China's Guizhou Province unearthed a part of the renowned fossil site in Guizhou province known as the Rongxi Formation. Beneath lay a new fossil bed filled with never-before-seen species, including Fanjingshania renovate.
Previously, the earliest known jawed animal was a fish that lived some 423 million years ago. With this discovery, science can present the first tangible evidence of a diversification of major vertebrate groups tens of millions of years before the beginning of the Devonian Period. Fanjingshania's shoulder girdle, with its array of fin spines, was obvious to the scientists from the beginning as a key to determining the new species' evolutionary position. Fanjingshania's shoulder spines are identical to those of acanthodians known as climatiids. Moreover, Fanjingshania and climatiids have modified trunk scales instead of normal dermal plate development. This is a specialization of jawed vertebrates, where bone plates develop from a single ossification center. Fossil remnants from Fanjingshania hint at extensive resorption and remodeling, which would normally be associated with the development of skeletal structures 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."
Life reconstruction of Fanjingshania renovata.
A phylogenetic hypothesis for Fanjingshania that uses a numeric matrix derived from observable characters confirmed the researchers' initial hypothesis that this new animal represents an early evolutionary branch of primitive chondrichthyans. Dr. Ivan J. Sansom from the University of Birmingham explains that because these results correspond with morphological clock estimates of the age of the common ancestor of cartilaginous and bony fish, around 455 million years ago (during the Ordovician period) they have profound implications for our understanding of when jawed fish originated. "This is the oldest jawed fish with known anatomy," said Professor 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."
The findings were published online in the journal Nature.
