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Placoderms: armored fish
c. 430–359 million years ago· Organism or group· Animals
Well establishedStrong fossil, genetic or geological evidence.
Jawed fish with bony armor over the head and chest, dominant in Devonian seas and rivers and extinct by 359 million years ago.
See it on the Life map →What happened
Placoderms ('plated skins') had a hinged armor shield over the head and trunk; instead of true teeth, many had sharp, self-sharpening bony jaw plates. They ranged from bottom-dwellers a few centimeters long to Dunkleosteus, a top predator up to roughly 4 m by recent estimates (older estimates were larger). Fossils from Gogo, Western Australia, include a 380-million-year-old mother with an embryo, the oldest known vertebrate live birth.
Why it matters
They were among the first great jawed predators, and their fossils show early versions of teeth, jaw bones, live birth and internal fertilization.
The evidence (fossils, genes, rocks)
Thousands of fossils worldwide, especially three-dimensional specimens from Gogo, Australia, and Dunkleosteus skulls from Ohio's Cleveland Shale.
Still debated
Whether placoderms form one natural group or a series of early branches leading to other jawed fish is still debated.
Connections (2)
▸Jawed fish include Placoderms: armored fishWell established
The connection
Placoderms were among the earliest jawed fish and dominated the Devonian. They had jaws and paired fins like all jawed fish, but covered the head and chest in bony plates. Fossils such as Entelognathus suggest the bones of our own jaw first appeared in placoderms.
When and how
Seas and rivers worldwide, about 430–359 million years ago; Entelognathus from Yunnan, China, described by Zhu Min and colleagues in 2013.
The evidence
Many well-preserved fossils, including three-dimensional specimens from Gogo, Australia, and family-tree analyses of jaw and skull bones.
Debates and open questions
Whether some placoderms had true teeth, as 2012 scans of fossil jaws suggested, or only toothlike bony points, is still debated.
▸Late Devonian extinction wiped out Placoderms: armored fishWell established
The connection
Placoderms were the most varied fish of the Devonian, but they suffered in the Kellwasser crisis about 372 million years ago and vanished in the Hangenberg event at the end of the Devonian, about 359 million. Sharks and ray-finned fish, which survived, then took over their roles.
When and how
Oceans and rivers worldwide, about 372–359 million years ago; the turnover was analyzed by Lauren Sallan and Michael Coates in 2010, among others.
The evidence
No placoderm fossils occur in rocks after the end of the Devonian despite good fish records, and fossil counts show the sharp turnover.
Debates and open questions
Why placoderms died out while sharks and ray-finned fish survived: size, habitat, slow reproduction or simple bad luck.