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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). They are easily distinguished however by their paired fins and presence of jaws, an adaptation that gave them a tremendous advantage over their earlier agnathan cousins, for it enabled them to bite solid food (in some cases, including their earlier agnathan cousins) rather than simply suck up organic particles from the mud. Yet still these creatures were very primitive compared to other fish. The fourth pair of gill arches had not yet become supports for the jaws, and the backbone consisted of a notochord (an embryonic feature in higher vertebrates) that persisted throughout life and vertebra consisting only of Y-shaped spines above and below it. These were often cartilaginous, like the skeleton of sharks (Chondrichthyes), a group with which the Placoderms are considered to share a common ancestry. Placoderms also lacked teeth, but biting or grinding structures are often be found in the dermal bones lining their mouths. The head and trunk shields of most placoderms were articulated by bony joints, allowing the forward part of the skull to tilt up, increasing the gape and allowing them to take in larger food
Teeth dermal bone, not replaced; jaw muscles medial to palatoquadrate; nostrils anteroventral; nasal capsules not fused to braincase; stalked eyeballs (as chondrichthians); sclerotic plates of 1-4 pieces; spiracle absent; probably 5 branchial arches; single branchial opening between cephalic & thoracic armor, unique neck joint; head and (variably) body covered with dermal bone plates; pattern of plates characteristic of each placoderm order; neural arches present and, in the tail region, hemal arch (no ossified centra); heterocercal tail; single dorsal fin; anal fin unknown or absent; internal skeleton]] of cartilage and perichondral bone; well-developed and unique pectoral fins; primitively, presence of semi-dentine with tear-drop shaped cavities for encased odontocytes; body often flattened (benthic).
The body and head armor are primitively not articulated, with several independent lines developing articulated armor.
Palaeos com - Placodermi - Characters
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