Australasian Science: Australia's authority on science since 1938

The Placoderm Renaissance

By John Long

Placoderm fish were once thought to be an evolutionary dead end, but new evidence is rewriting their importance to the ancestry of all jawed animals – including humans.

Back in the 1960s, most of us had little interest in dinosaurs. They were thought to be a “dead end” lineage with no direct relationship to anything. Bob Bakker, a student at the time, said they were seen as “symbols of obsolescence and hulking inefficiency”. They were often depicted on the silver screen by using close-ups of living lizards bearing sails on their backs or with horns attached to their heads.

All this changed with just one man, Dr John Ostrom from Yale University. His discovery of the fleet-footed little predator Deinonychus, kin to the now infamous Velociraptor in Jurassic Park. His 1969 landmark paper on Deinonychus heralded what Bakker called the beginning of the Dinosaur Renaissance.

By comparing Deinonychus with the earliest known bird, Archaeopteryx, Ostrom pointed out some 20 anatomical similarities between living birds and dinosaurs. He proposed that birds evolved from dinosaurs and thus dinosaurs should no longer be seen as an evolutionary dead-end. Instead they gave insights into the origins and early adaptations of birds. It meant we could reinterpret dinosaurs in the light of bird behaviour.

Evidence emerged in many forms to support this theory. Dinosaurs were found brooding their young in nests, and caring for young before they could walk. The most convincing evidence comes from the recent discoveries of many fine complete specimens of dinosaurs with feathers (e.g. Sinosauropteryx, Caudipteryx) from the Jehol Biota sites in Liaoning, northern China.

Today birds are widely regarded by most scientists as the direct descendents of theropod dinosaurs, so dinosaurs are technically not all extinct – only some lineages died out 65 million years ago. With computer phylogenetic analysis, the anatomical features of birds and dinosaurs can now be statistically verified as a true kin relationship. Most scientists accept modern birds as dinosaurs, descended from an unbroken line of small predatory feathered theropod dinosaurs originating 170–160 million years ago.

Yet right now another major revolution of the same nature is occurring in palaeontology: the placoderm renaissance. For the past century there has been little scientific interest in this group of early armoured jawed fishes, which seemingly died out around 360 million years ago at the end of the Devonian period. They were thought to be a dead end, like the dinosaurs, with no relation to any living fishes, although some argued they were possibly related to sharks.

Yet in just the past 5 years spectacular new discoveries coming out of China, Australia and Arctic Canada have turned our ideas about placoderms completely around. The discovery of the oldest articulated placoderms alongside the world’s oldest known bony fishes (Osteichthyes) from Yunnan in China has opened up a wealth of new anatomical data to test phylogenetic analyses.

Placoderms are clearly embedded at the basal node of all jawed animals – gnathostome fish, amphibians, reptiles, dinosaurs/birds and mammals. This means our own ancestry is rooted in placoderm origins. We humans have many features that first appeared as novelties in the group some 430 or so million years back: jaws, teeth, paired skull bones, paired front and hind limbs (arms and legs), shoulder girdles, hip bones, vertebrae, and even well-developed sexual organs for copulation. Placoderms were the first fishes to give birth to live young. Their muscles, now revealed by synchrotron scanning of 3D specimens, surprised us all in 2013 when they revealed advanced muscular anatomy.

The 2013 discovery of Entelognathus, a 425-million-year-old placoderm from China with osteichthyan jaw bones and gular plates, suggests that placoderms could be much closer to the first bony fishes than we previously thought. Entelognathus filled the largest gap in vertebrate evolution since the discovery of Archaeoepteryx, the first fossil bird that gave Ostrom his radical ideas back in the mid-1960s. The discovery of Entelognathus holds much bigger implications for how the beginning of the line leading to humans has been dramatically rerouted by its discovery.

It’s only early days, and debates are still raging, but these discoveries have injected real passion into early vertebrate studies once more. Right now placoderms are very hot. Leading journals like Nature and Science are publishing series of papers on them.

The Placoderm Renaissance has officially begun. It is allowing us to completely rewrite the early history of vertebrates and think of humans in an entirely new historical and metaphysical framework.

John Long is Strategic Professor in Palaeontology at Flinders University.