Abstract
The vertebrate dermal skeleton has long been interpreted to have evolved from a primitive condition exemplified by chondrichthyans. However, chondrichthyans and osteichthyans evolved from an ancestral gnathostome stem‐lineage in which the dermal skeleton was more extensively developed. To elucidate the histology and skeletal structure of the gnathostome crown‐ancestor we conducted a histological survey of the diversity of the dermal skeleton among the placoderms, a diverse clade or grade of early jawed vertebrates. The dermal skeleton of all placoderms is composed largely of a cancellar architecture of cellular dermal bone, surmounted by dermal tubercles in the most ancestral clades, including antiarchs. Acanthothoracids retain an ancestral condition for the dermal skeleton, and we record its secondary reduction in antiarchs. We also find that mechanisms for remodeling bone and facilitating different growth rates between adjoining plates are widespread throughout the placoderms. J. Morphol., 2013. © 2013 Wiley Periodicals, Inc.
Highlights
INTRODUCTIONOnly the gnathostomes (the clade of living jawed vertebrates) possess a mineralized skeleton
Among living vertebrates, only the gnathostomes possess a mineralized skeleton
This expectation is true of the chondrichthyan skeleton, and most especially of the micromeric dermal skeleton, which is reduced with respect to the primitive gnathostome condition, as evidenced by successive extinct sister lineages to the chondrichthyans, osteichthyans and, to crown gnathostomes (Donoghue and Sansom, 2002)
Summary
Only the gnathostomes (the clade of living jawed vertebrates) possess a mineralized skeleton. Placoderms remain integral to understanding the histological structure of the dermal skeleton in the ancestral crown gnathostome, the condition from which both chondrichthyans and osteichthyans are derived. The dermal skeleton is perceived generally to consist of a superficial compact lamellar layer (referred to as the ‘‘Oberflachenschicht’’ by Heintz, 1929, or, within phyllolepids, as the ‘‘Skulpturschicht’’ by Gross, 1934), a cancellous spongiosa, and a compact lamellar base. It has been speculated that semidentine is present in ancestral antiarchs (Young, 2008), but this has not been substantiated Both the superficial and middle layers contain evidence of extensive resorption and secondary bone growth (Downs and Donoghue, 2009). Gross (1973) describes three layers of tissue: an ‘‘isopedinous’’ basal layer with possible evidence of bone reworking, a middle layer of vascular canals and previous generations of tubercles, and a ‘‘Skulpturschicht’’ of orthodentinelike semidentine
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