The preorbital depression and recess of antiarch placoderms (jawed stem-gnathostomes) revisited from an ontogenetic (saltatory) point of view

Abstract

During fish growth, short periods of rapid changes (morphological, behavioral, physiological, ecological), or thresholds, are interspersed with longer periods of slower development, the steps. This growth pattern is known as saltatory ontogeny. Thresholds delimit main developmental stages (embryo, larva, juvenile, adult) and are periods where modifications can lead to new life-histories, and ultimately to evolutionary novelties. We sought to determine whether saltatory ontogeny could be recognized in a jawed stem-gnathostome, the Late Devonian antiarch placoderm Bothriolepis canadensis, using an extensive size series (220 specimens: 5–220 mm in armor length). The small specimens of this series reveal a previously undocumented immature feature, a preorbital depression on the premedian and lateral plates of the dermal headshield. This depression is a plesiomorphic condition reputed to be absent in highly nested antiarchs such as B. canadensis, which possess instead a preorbital recess. Our objectives were: to describe the ontogenetic morphological changes of the preorbital area in B. canadensis, including the timing of disappearance of the depression using binomial logistic regressions, and to quantify growth allometries of the premedian and lateral plates, with linear and segmented regressions. We found significant segmented allometric patterns in the premedian plate, suggesting saltatory ontogeny in B. canadensis. Moreover, segmented patterns were congruent with the ontogenetic loss of the preorbital depression. An ecomorphological hypothesis is proposed to explain these simultaneous morphological and morphometric changes in B. canadensis. A similar hypothesis is extrapolated to interpret the innovation of the preorbital recess and loss of the preorbital depression during the antiarch phylogeny.