Hyperossification in the vertebral column of Devonian placoderm fishes (Arthrodira)
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ID: 00000C875
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Skeletal bone enlargement (hyperossification) is thought to only occur in endochondral and dermal bone (e.g., in the endoskeletons of marine tetrapods and the dermal bones of certain jawless vertebrates and placoderms, and teleost fishes). However, we demonstrate several types of bone enlargement in the vertebral endoskeleton of some arthrodiran placoderms (basal jawed vertebrates)—Millerosteus minor, Compagopiscis croucheri, and Eastmanosteus calliaspis. This is significant, because not only is placoderm endoskeletal bone thought to be thin-walled, it is thought to ossify only perichondrally (in the fibrous perichondrium layer surrounding a cartilage precursor), rather than endochondrally/dermally. Hyperossification differs considerably among these taxa, revealing a range of internal and external bone morphologies previously undescribed for the placoderm endoskeleton. For example, neural arches of Millerosteus minor are swollen in external appearance, in cross-section showing considerable deposition of layered, compact bone, by comparison, the neural spines are not swollen, but show considerable compact bone internally. In Compagopiscis croucheri, although the perichondral bone of both structures is thickened and compact as in Millerosteus minor, the spine and arch are unswollen externally. Similar to Compagopiscis croucheri, the spines and arches of Eastmanosteus calliaspis are unswollen externally, yet the neural spine cortex is composed of the thin perichondral bone expected for placoderms. In contrast, the arches comprise numerous layers of perichondral bone. The nature of vertebral hyperossification in these related taxa differs considerably in the degree and mode of cortical tissue thickening, via addition of tissue to the bone’s external (periosteal) and/or internal (endosteal) surfaces. Hyperossification in these arthrodires demonstrates that increases in endoskeletal bone mass are not restricted to crown group gnathostomes (Chondrichthyes + Actinopterygii), and may represent a first step in the evolution of this process, involving modifications to cortical bone layers.