What Is an Arthrodire? Armoured Fish With Shearing Plates for Jaws
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For a stretch of the Devonian period the largest predators in the sea were fish encased in bony plates, with a hinge between head and trunk and self-sharpening blades instead of teeth. They dominated and then disappeared entirely.
The armour and the hinge
These animals wore a two-part shield of interlocking bony plates, one covering the head and one the front of the trunk, joined at the sides by a genuine joint. That joint is the feature the group is named for, and it allowed the head shield to rotate upward while the lower jaw dropped, opening the mouth far wider than a simple hinged jaw permits and creating suction that drew prey inward. The rear of the body was unarmoured and is consequently known from far fewer specimens, which biased early reconstructions towards the armoured front end. The plates themselves are diagnostic and have been studied in enormous detail, since their arrangement varies between species and provides the basis for classifying the group, and an isolated plate is frequently identifiable where other material would not be.
What they had instead of teeth
The feeding apparatus was unlike anything alive and its features follow from the material:
- •Bony blades formed from the jaw bones themselves rather than separate teeth set into them
- •Self-sharpening edges, since the upper and lower blades sheared past each other and wore into a cutting edge with use
- •Bite forces estimated among the highest of any animal, from reconstructions of the jaw muscles and the mechanical arrangement
- •The ability to cut through armour, including the armour of other members of the group
- •No replacement, unlike the continuous tooth replacement of sharks, so the blades had to last
- •Wear patterns on fossil blades that record what an individual animal actually ate
The famous one
The best-known member reached lengths estimated between six and nine metres and was the dominant marine predator of its time, with the head shield alone exceeding a metre in some specimens. It is known almost entirely from that armoured front end, so body length estimates depend on scaling from relatives and have been revised downward as the reasoning improved, which is a useful reminder that the famous size figures for extinct animals frequently rest on assumptions rather than measurements. Bite force reconstructions place it among the most powerful known. Specimens show damage consistent with attacks by others of its kind. Regurgitated material attributed to it indicates a diet including other armoured fish and sharks. The animal has become the standard illustration for the Devonian seas and is notably more interesting than the popular treatment suggests, particularly for what it shows about the mechanics of a jaw built from plates.
What else was in that sea
The period is sometimes called the age of fishes and the label is earned, since nearly every major group of fish either originated or diversified within it. Jawless fish, already ancient, were still abundant and included heavily armoured forms. Early sharks and their relatives were present and increasing. Ray-finned fish, which would eventually become the overwhelming majority of fish species, were present and modest. Lobe-finned fish were diverse and included the lineages leading to land vertebrates. Enormous reef systems built by corals, sponges and calcifying algae covered vast areas and were more extensive than modern reefs. Land was being colonised simultaneously, with the first forests appearing and the first vertebrates moving into shallow water and beyond. The armoured fish sat at the top of that marine system for tens of millions of years, which makes their complete disappearance a genuinely striking fact rather than a footnote.
Why they vanished
The entire armoured fish group disappeared at the end of the Devonian, in an extinction event that removed a very large proportion of marine life, and none of its members survived into the following period. The causes of that event are debated and probably multiple, with reduced oxygen in the oceans, climate change and the effects of the newly spreading land plants on nutrient runoff all implicated. What is striking is the completeness of the loss, since the group had been dominant and diverse across a long interval and left no descendants at all, unlike most major extinctions where some lineage persists. The ecological space was taken by sharks and by ray-finned fish, both of which were present but subordinate beforehand and both of which expanded afterwards, which is the standard pattern of an incumbent group being removed and the survivors radiating into what it left.
The takeaway
A real joint between head and trunk shields let the head rotate upward, opening the mouth wide and creating suction, which is the feature the group is named for. Bone blades sheared past each other and self-sharpened with use, with no replacement as sharks have. The largest was the dominant marine predator of its time. The group vanished completely at the end of the Devonian, leaving no descendants.