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Romundina (placoderm) and evolutionary origin of teeth (free pdf)

Ben Creisler

A new paper in open access:

Martin Rücklin & Philip C. J. Donoghue (2015)
Romundina and the evolutionary origin of teeth.
Biology Letters (advance online publication)
DOI: 10.1098/rsbl.2015.0326

Theories on the origin of vertebrate teeth have long focused on
chondrichthyans as reflecting a primitive condition—but this is better
informed by the extinct placoderms, which constitute a sister clade or
grade to the living gnathostomes. Here, we show that ‘supragnathal’
toothplates from the acanthothoracid placoderm Romundina stellina
comprise multi-cuspid teeth, each composed of an enameloid cap and
core of dentine. These were added sequentially, approximately
circumferentially, about a pioneer tooth. Teeth are bound to a bony
plate that grew with the addition of marginal teeth. Homologous
toothplates in arthrodire placoderms exhibit a more ordered
arrangement of teeth that lack enameloid, but their organization into
a gnathal, bound by layers of cellular bone associated with the
addition of each successional tooth, is the same. The presence of
enameloid in the teeth of Romundina suggests that it has been lost in
other placoderms. Its covariation in the teeth and dermal skeleton of
placoderms suggests a lack of independence early in the evolution of
jawed vertebrates. It also appears that the dentition—manifest as
discrete gnathal ossifications—was developmentally discrete from the
jaws during this formative episode of vertebrate evolution.