Definition of placoderm in English:

placoderm

noun

  • An extinct fish of the Devonian period, having the front part of the body encased in broad flat bony plates.

    • ‘If large body lengths are primitive for jawed fishes, which is also true for many placoderms, then a marked reduction in body size may have occurred with the origin of ray-finned fishes.’
    • ‘In this capacity, it came to bear a very wide variety of cutting surfaces, from the inferognathals of arthrodire placoderms to the bizarre spiral symphysial tooth whorls of Helicoprion.’
    • ‘In addition, it has several features, such as the fin spines and characters of the shoulder plate, which are associated with placoderms, chondrichthyans, or acanthodians.’
    • ‘Only fragmentary remains are known of animals that may represent early stages in the differentiation of placoderms, chondrichthyes, and osteichthyes.’
    • ‘Four hundred million years ago armor-plated fishes, the placoderms, swam along the sandy floor of oceans that once covered much of Australia.’
    • ‘This site has yielded numerous vertebrate fossils, particularly placoderms and early osteichthyans, such as the Cheirolepis shown here.’
    • ‘There is never a lack of unanswered questions in paleontology, but the placoderms do seem to have more than their fair share.’
    • ‘It was also covered in bony overlapping plates, like all placoderms, but had weak jaws and a peculiar high-crested bone, somewhat like a dorsal fin, straddled its back.’
    • ‘This is characterized by a diversity of prehistoric armoured fish known as placoderms (principally, arthrodires)’
    • ‘The extinct armored fishes known as placoderms make up what is considered to be the earliest branch of the gnathostome family tree - the earliest branch of the jawed fishes.’
    • ‘Jawless fishes and placoderms were also affected, although many placoderm lineages survived quite happily until the end Devonian.’
    • ‘The placoderms and chondrichthyans both show at least some capsular protuberance of the braincase, but the braincase is a single, undivided mass, whether or not ossified.’
    • ‘By the Mid-Devonian, placoderms, the first jawed fish, appear.’
    • ‘Many placoderms had one or both, and it is from this circumorbital series that the jugal seems to have derived.’
    • ‘During this period the agnathan and gnathostome fishes, too, gave rise to many distinctive groups: the armoured ostracoderm and placoderms, the bony fishes, and elasmobranchs to populate marine and freshwater environments.’
    • ‘To take the chain of speculation one step further, this suggests that the maxilla itself was originally a flat dermal plate of some kind, possibly homologous to the superognathal of placoderms.’
    • ‘They found that the primitive state for all chondrichthyans (and quite likely placoderms as well) is autodiastyly.’
    • ‘Such giant cephalopods play or played a similar ecological role of top predator to that of Devonian arthrodire placoderms, Mesozoic pliosaurs and Cenozoic toothed whales.’
    • ‘However, they were later surpassed in this role by certain fish, notably the placoderms, a highly diverse group that sported interlocking plates of armor.’
    • ‘With their armored head shield and trunk shield composed of overlapping bony plates, the placoderms appear at first glance extremely similar to the ostracoderms (armoured jawless fish).’

Origin

Mid 19th century: from Greek plax, plak- flat plate + derma skin.

Pronunciation:

placoderm

/ˈplakədərm/