Placodermi (from the Greek πλάξ = plate and δέρμα = skin, literally "plate-skinned") is an
extinct class of armoured
prehistoric fish, known from
fossils, which lived from the
Silurian to the end of the
Devonian Period. Their
head and
thorax were covered by articulated armoured plates and the rest of the body was
scaled or naked, depending on the
species. Placoderms were among the first
jawed fish; their
jaws likely evolved from the first of their
gill arches. Placoderms are
paraphyletic, and comprise several distinct
outgroups or
sister taxa to all living jawed
vertebrates, which originated among their ranks. This is illustrated by a 419-million-year-old fossil,
Entelognathus, from
China, which is the only known placoderm with a type of
bony jaw like that found in modern
bony fishes. This includes a
dentary bone, which is found in humans and other
tetrapods,. The jaws in other placoderms were simplified and consisted of a single
bone. Placoderms were also the first fish to develop
pelvic fins, the precursor to
hindlimbs in tetrapods, as well as true
teeth. 380-million-year-old fossils of three other genera,
Incistoscutum,
Materpiscis and
Austroptyctodus, represent the oldest known example of
live birth.
