Definition of placoderm in English:



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

    Class Placodermi: several orders

    ‘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.’
    • ‘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.’
    • ‘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).’
    • ‘However, they were later surpassed in this role by certain fish, notably the placoderms, a highly diverse group that sported interlocking plates of armor.’
    • ‘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 site has yielded numerous vertebrate fossils, particularly placoderms and early osteichthyans, such as the Cheirolepis shown here.’
    • ‘Jawless fishes and placoderms were also affected, although many placoderm lineages survived quite happily until the end Devonian.’
    • ‘This is characterized by a diversity of prehistoric armoured fish known as placoderms (principally, arthrodires)’
    • ‘Four hundred million years ago armor-plated fishes, the placoderms, swam along the sandy floor of oceans that once covered much of Australia.’
    • ‘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.’
    • ‘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.’
    • ‘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.’
    • ‘There is never a lack of unanswered questions in paleontology, but the placoderms do seem to have more than their fair share.’
    • ‘By the Mid-Devonian, placoderms, the first jawed fish, appear.’
    • ‘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.’
    • ‘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.’
    • ‘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.’
    • ‘Only fragmentary remains are known of animals that may represent early stages in the differentiation of placoderms, chondrichthyes, and osteichthyes.’
    • ‘They found that the primitive state for all chondrichthyans (and quite likely placoderms as well) is autodiastyly.’
    • ‘Many placoderms had one or both, and it is from this circumorbital series that the jugal seems to have derived.’



/ˈplakəˌdərm/ /ˈplækəˌdərm/


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