The arthrodires are the most specious group of placoderms with nearly 200 known genra and range in size from a few centimetres to six meters (over twenty metres according to Benton, 2000) in some of the Late Devonian forms such as Dunkleosteus and Titanichthys.
The arthrodires are divided into three major groups:
The last two posessing an articulating hinge between the head and thoracic armour and are therefore thought to be more closely related.
The gape on Dunkleosteous (a brachythoracid) allowed by the upward movement of the cranium, facillitated by the nuchal gap and cranio-thoracic articulation. Also shown are the anterior and posterior supergnathals that make up the cutting blade on the upper jaw and the infrognathal of the lower jaw with its anterior biting part and posterior strengthening for the underlying Meckelian cartilage (dotted line). Modified from Janvier (1996).
The jaws are armed with tuberculated gnathal plates, these tubecules may have been true teeth that were independently derived from those of the other gnathostomes, altough this is controversial. In any case the main functioning elements were the gnathal plates which were worn into an effective shearing structure.
The exoskeleton of the arthrodires consists of thick layers of cellular bone covered by dermal tubercles, which in turn had outer layers composed of semidentine. In many brachythoracids, however, this external ornamentation was lost and the dermal armour was presumably covered by skin in life.
The Actinolepidoids were dorsoventrally flattened and lacked the cranio-thoracic articulation that is present in the other arthrodires. They also characteristically lack posterior lateral and dorsolateral plates in their thoracic armour and the anterior supergnathal. The dermal armour is ornamented with concentric rings of bone, derived from concentric rows of large tubercles. The tail is rather long and the pelvic fins are well developed and may have functioned in a similar way to the claspers of male sharks today.
The Actinolepidoids Phyllolepis (a) and Austrophyllolepis (b) showing the characteristic features of the group. Note the absence of a nuchal gap, in this group there is a simple sliding contact between the skull-roof and the thoracic armour. [Figure a is modified from Benton (2000), while figure b is modified from Janvier (1996).
The Phlyctaenii generally have an a long and narrow median dorsal plate and one family the, Groenlandaspididae, the thoracic armour is shortenedand the median dorsal plate is drawn up into a high dorsal crest.
Tiaraspis, a member of the Family Groenlandaspididae, showing their enlarged median dorsal plate, which forms the dorsal crest. Taken from Janvier (1996).
The Brachythoraci are recognised by their thick dermal bones with over-lapping or sutured margins, an inferognathal comprising of a biting part and a large posterior non-biting blade and a well-developed ventral keel on the median dorsal plate. The largest placoderms, the dinichthyids, belong to this group.
The cranial and thoracic dermal armour of the Brachythoraci Compagopiscis preserved in three-diamensions showing some of the characteristics of the group plus the position of the brachial opening. Note in life the main structural element of the lower jaw was the Meckelian cartilage which carried the inferognathial. Modified from Benton (2000).
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