If the placoderm was not our ancestor, what was?

Flinders University researchers have questioned the traditionally held belief that we vertebrates descended from the archaic armoured jawed fish known as placoderms.

Working in conjunction with colleagues from the Chinese Academy of Sciences in Beijing, the local scientists have reanalysed existing data and made use of recent technological leaps to challenge the orthodoxy and suggest that our path from sea-dwelling protovertebrates to land-dwelling primates might not be explained in such a simplistic fashion.

The path to this new finding began with Flinders University palaeontology researcher Benedict King, who asked the question: “So if placoderms were not our ancestors, what was?” The answer is as muddy as the primordial soup our ancestors emerged from.

“Our study suggests that no particular group of known jawed vertebrates is ancestral to the others. Rather, the true jawed vertebrate ancestor probably combined features of bony fish (such as carp), cartilaginous fish (sharks and rays) and placoderms — in much the same way that the common ancestor of humans and chimpanzees was neither human nor chimp but a unique amalgam of both.”

The placoderms were once the dominant animal group on earth, but an abrupt extinction occurred about 360 million years ago. This allowed for the expansion of carp, sharks and other modern groups, but recent fossil discoveries in China combine features common to both placoderms and bony fish.

The Flinders team have examined these new fossil discoveries, and they believe that the ongoing debate within the palaeontological community over the presumed timeline from placoderm to vertebrates needs re-examination.

Dr Martin Brazeau of Imperial College London believes that “placoderms may be what we call a grade: an array of groups or species that form a succession of ever-closer relatives of modern jawed vertebrates. This is an ideal case for palaeontologists, because it helps us reconstruct detailed sequences of character evolution.

“However, the work by King and colleagues attacks this problem from a new angle, bringing detailed reinvestigations of existing data and — most significantly — new, cutting-edge methods. The combined effect of their work has upset the ‘new orthodoxy’ on placoderms.”

Partly funded by the Australian Research Council, ‘Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates’ is published in the Oxford University Press journal Systematic Biology. Contributors include Strategic Professor John Long of Flinders University, Adelaide; Professor Mike Lee of the South Australian Museum; and Dr Tuo Qiao and Professor Min Zhu of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences in Beijing.

Placoderm image courtesy of Víctor Alejandro Correa Rueda under CC BY 2.0