The Class Placodermi are believed to be either a sister-group to the chondrichthyans or, alternatively, a sister-group to all other gnathostome groups:
Interrelations of the Gnathostomata showing the two views illustrated in Janvier (1996). A. shows the placoderms as the sister-group to all other gnathostomes. B. shows the placoderms and the chondrichthyans as sister-groups.
The theory that the Placodermi form the sister-group to all other gnathostomes is based on the implication that there are unique characters shared by osteichthyans and chondrichthyans. The most persuasive of these features is that the extrinsic eye musculature of placoderms is closer to that of lampreys than to that of osteichthyans and chondrichthyans. However, this theory is still rather poorly supported.
The other theory that the Placodermi and the Chondrichthyes are sister-groups within the gnathostome clade is based on the assumption that there more unique characteristics shared by placoderms and chondrichthyans than either share with the osteichthyans. These shared features are principally the presence of an eye stalk uniting the eyeball to the braincase and the presence of pelvic claspers in males derived from the pelvic metapterygium. This theory is currently better supported than the placoderm-gnathostome sister-group relationship and the name Elasmobranchiomorphi has been coined to cover both the chondrichthyans and placoderms.
The evolution of early and mid Palaeozoic fish showing the distribution of the placoderms (middle) in time and their changing diversity. Taken from Benton (2000).
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