A new paper in Nature suggests that before we were apes we were fish and not just any old fish. The primitive fish which predates the split between sharks and bony fish, Acanthodes bronni was the common ancestor of all jawed vertebrates on Earth – including mankind, according to the paper. The split with bony fish occurred some 420 million years ago and Acanthodes bronni disappeared some 250 million years ago.
The researchers re-examined fossils of Acanthodes bronni, the best-preserved acanthodian species. They created highly detailed latex moulds of specimens revealing the inside and outside of the skull, generating a new data set for assessing cranial and jaw anatomy as well as the organizations of sensory, circulatory and respiratory systems in the species. The analysis of the sample combined with recent CT scans of skulls from early sharks and bony fishes led to the reassessment of what Acanthodes bronni tells us about the history of jawed vertebrates.
Acanthodes bronni : Wikipedia
Acanthodes and shark-like conditions in the last common ancestor of modern gnathostomes
Samuel P. Davis, John A. Finarelli & Michael I. Coates
ABSTRACT:
Acanthodians, an exclusively Palaeozoic group of fish, are central to a renewed debate on the origin of modern gnathostomes: jawed vertebrates comprising Chondrichthyes (sharks, rays and ratfish) and Osteichthyes (bony fishes and tetrapods). Acanthodian internal anatomy is primarily understood from Acanthodes bronni because it remains the only example preserved in substantial detail, central to which is an ostensibly osteichthyan braincase. For this reason, Acanthodes has become an indispensible component in early gnathostome phylogenies. Here we present a new description of the Acanthodes braincase, yielding new details of external and internal morphology, notably the regions surrounding and within the ear capsule and neurocranial roof. These data contribute to a new reconstruction that, unexpectedly, resembles early chondrichthyan crania. Principal coordinates analysis of a character–taxon matrix including these new data confirms this impression: Acanthodes is quantifiably closer to chondrichthyans than to osteichthyans. However, phylogenetic analysis places Acanthodes on the osteichthyan stem, as part of a well-resolved tree that also recovers acanthodians as stem chondrichthyans and stem gnathostomes. As such, perceived chondrichthyan features of the Acanthodes cranium represent shared primitive conditions for crown group gnathostomes. Moreover, this increasingly detailed picture of early gnathostome evolution highlights ongoing and profound anatomical reorganization of vertebrate crania after the origin of jaws but before the divergence of living clades.
Nature 486, 247–250, doi:10.1038/nature11080
e! Science News reports:
… created highly detailed latex molds of specimens revealing the inside and outside of the skull, providing a valuable new data set for assessing cranial and jaw anatomy as well as the organizations of sensory, circulatory and respiratory systems in the species. …
The analysis of the sample combined with recent CT scans of skulls from early sharks and bony fishes led the researchers to a surprising reassessment of what Acanthodes bronni tells us about the history of jawed vertebrates.
“For the first time, we could look inside the head of Acanthodes, and describe it within this whole new context,” Coates said. “The more we looked at it, the more similarities we found with sharks.” ….
However, analysis of the evolutionary relationships of Acanthodes bronni — even with these new data added — still connected this species to early bony fishes. Meanwhile, some acanthodian species turned out to be primitive sharks, while others were relatives of the common ancestor of sharks and bony fishes. ….
Using more than 100 morphological characters, the researchers quantified the mutual resemblance among the earliest jawed fishes. Acanthodians as a whole, including the earliest members of humans’ own deep evolutionary past, appear to cluster with ancient sharks.
“The common ancestors of all jawed vertebrates today organized their heads in a way that resembled sharks,” said Finarelli, PhD, Lecturer in Vertebrate Biology at University College Dublin. “Given what we now know about the interrelatedness of early fishes, these results tell us that while sharks retained these features, bony fishes moved away from such conditions.”
Furthermore, the analysis demonstrated that all of these early members of the modern gnathostomes are clearly separated from what now appear to be the most primitive vertebrates with jaws: a collection of armored fishes called placoderms.
“There appears to be a fundamental distinction between the placoderms and all other vertebrates with jaws,” Finarelli said.
This new revision of the lineage of early jawed vertebrates will allow paleontologists to dig into deeper mysteries, including how the body plan of these ancient species transformed over the transition from jawless to jawed fishes. …..
Tags: Acanthodes, Acanthodes bronni, Common descent, Evolution, spiny sharks
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