Armored placoderms invented jaws, teeth, and pelvic fins early
Placoderms—“plate skins”—were Paleozoic jawed fishes of the Silurian and Devonian with cartilaginous endoskeletons under heavy armor. They were among the first jawed vertebrates, first with true teeth, and first fish clade to evolve pelvic fins, the forerunners of tetrapod hindlimbs.
The name joins Greek words for plate and skin. Placoderms were a class of prehistoric fish from the Silurian and Devonian periods whose skeletons were mostly cartilage, but whose heads and chests were covered by jointed armour plates; the rest of the body was scaly or bare depending on the species. They were among the first fish with jaws, probably derived from the first pair of gill arches, the first vertebrates with true teeth, and the first fish group with pelvic fins, the second pair of paired fins that later became tetrapod hindlimbs.
Fossils of Incisoscutum, Materpiscis and Austroptyctodus about 380 million years old are the oldest known evidence of live birth. A 2012 study of the arthrodire Compagopiscis confirmed that placoderm teeth had pulp cavities and were built of bone and dentine, though teeth and jaws developed less closely together than in modern jawed fish. How the group relates to living vertebrates is debated: most researchers see it as a series of side branches from which modern jawed vertebrates arose, while a 2016 analysis argued it formed one natural group.
Placoderms first appear in the late Llandovery of the early Silurian and went on to displace sea scorpions and straight-shelled cephalopods as dominant marine hunters. Many were bottom-dwellers, notably antiarchs with jointed bony pectoral fins; the antiarch Bothriolepis has over 100 valid species. Others, especially arthrodires, cruised the open water. Dunkleosteus terrelli, 3.5 to 4.1 metres long and found in Europe, North America and possibly Morocco, is regarded by some as the first vertebrate superpredator. Not all were hunters: Holonema grazed plants and the giant Titanichthys fed on plankton.
Diverse as they were in the Devonian, every placoderm died out in the Hangenberg event at the end of that period, 358.9 million years ago. Their disappearance left open niches that the bony fish and cartilaginous fish survivors filled as they diversified through the Carboniferous.
Source: Placoderm
