Fishes by default, the placoderms appeared low in the Devonian, underwent a remarkable diversification into 40 families and disappeared entirely before the Carboniferous started

A 25 cm long specimen of the placoderm Bothriolepis from the Upper Devonian cliffs at Miguasha, Gaspé Peninsula. Miguasha Museum Collections. (Photo by BDEC (c))

Bothriolepis is the most common fish fossil in the shales and sandstones of the Escuminac Formation (Late Devonian, 380 Ma) on the south shore of the Gaspé Peninsula at Miguasha. Abraham Gesner (1797-1864), the provincial geologist of New Brunswick who discovered the site in 1842, referred to this fossil as "a small species of tortoise with foot-marks". The polygonal plates are, indeed, reminiscent of the scutes of a turtle carapace. A well-preserved specimen looks like a finely embossed jewelry box.

Bothriolepis is a placoderm with a heavily armoured head fused with the thoracic shield. Instead of typical fish-like pectoral fins, it bears a pair of rigid arms that are joined at two points -- one where the arm leaves the trunk and one a little more than half way along. These arms, like the limbs of an arthropod, are articulated by interior muscles. Bothriolepis does have a slender fish-like tail that extends behind the heavily armoured portion but, because it is almost naked with few scales, it is rarely preserved. There are two openings through its solidly armoured head -- a keyhole opening along the midline on the upper side for the eyes and nostrils and a mouth on the lower side near the front. It has a peculiar spiral, sediment-filled gut and it appears to have been a mud grubber. Well-preserved specimens reveal the presence of gills in addition to a pair of pouches off the esophagus that may have functioned as lungs. This dorsally compressed fish might have walked on its peculiar arms -- in the shallow brackish water and possibly onto the land.