Growth patterns in teeth and jaws of the Middle Jurassic sphenodontian Cynosphenodon huizachalensis are compared to those of the extant Sphenodon, showing extreme similarity. Both taxa share the unique presence of a hatchling tooth series divided into anterior uniform size and posterior alternating size tooth regions by the eruption of successional teeth early in life, an ever-growing second-generation successional tooth that becomes a caniniform tooth in adults, and a symphysial region that grows anteriorly but with a differential faster rate in the dorsal margin, enlarging considerably the length between the caniniform tooth and the anterior tip of the dentary, and increasing the angle of the anterior margin of the symphysis. The sister-group relationships of Sphenodon and Cynosphenodon have been supported by the simple presence of a single caniniform tooth, but this tooth is shown to be a complex structure involving several other ontogenetic related synapomorphies as follows: (i) it is a second-generation successional tooth that replaces a first-generation successional tooth; (ii) it emerges between the anterior hatchling teeth and the alternating size hatchling series, replacing some teeth of the latter; (iii) it grows continuously in height and width during life at a rate faster than that of tooth wear; (iv) it denotes regionalization in the symphysis, marking previously undetected growing points of the dentary; and (v) it shows the posterior limit of the edentulous anterior margin diagnostic to Sphenodontinae. New developmental information about the caniniform tooth strongly supports the structure as a homologous character, endorsing the sister-group relationships of Cynosphenodon and Sphenodon. Developmental comparisons can provide good evidence concerning the homology of a similar character present in two or more taxa. These studies are particularly useful if a clade is weakly supported by a single character.

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