Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts

Abstract

Conodont animals were early jawless vertebrates equipped with a feeding apparatus composed of several tooth‐like elements. The P1 elements, at the rear of the apparatus, were characterized by a robust shape and rapid morphological evolution. Occlusion occurred between paired right and left P1 elements, occasioning some bilateral asymmetry, which, together with allometric growth, may partially obliterate the temporal differences. The present study aims to disentangle these different components of morphological variation in Late Devonian Polygnathus P1 conodont elements. An extensive 2D geometric morphometric analysis of the platform shape was performed through the Famennian record of two outcrops. This analysis was completed by a 3D study on a subset of conodont elements. The 2D and 3D morphometric quantifications provided highly congruent results, showing that the 2D shape constitutes a good approximation of the element geometry. The 3D analysis delivered further insights into the relationship between the geometry of the elements and the constraints related to occlusion. The 2D analysis allowed a quantitative assessment of the variation among species and through time. Allometry and bilateral asymmetry were differently expressed depending on the species considered, suggesting that constraints imposed on pairing by the morphology of the elements varied even among related species. The within‐species variation was so important that it largely obliterated temporal trends; a relationship of Polygnathus shape and conodont biofacies variations through the Famennian nevertheless suggested an evolution driven by ecological interactions between conodont genera.

Morphometric variance, evolutionary constraints and their change through time in Late Devonian Palmatolepis conodonts

“…Note that asymmetry was never found important in Palmatolepis elements, being one order of magnitude less than what has been found in Polygnathus (Renaud et al. 2021). Asymmetry was less pronounced in the subgenera with a large platform, corroborating that the opposite platforms had a few functional interactions in these morphologies.…”

“…The 2D projection of the platform shape provided a good approximation of 3D morphological changes in a related taxon, Polygnathus (Renaud et al. 2021), belonging as Palmatolepis to the Ozarkodinida order (Donoghue et al. 2008).…”

“…The 2D projection of the platform shape provided a good approximation of 3D morphological changes in a related taxon, Polygnathus (Renaud et al 2021), belonging as Palmatolepis to the Ozarkodinida order (Donoghue et al 2008). Such 2D simplification allows an extensive sampling, and thus, the quantification of variance patterns through time; it however, provides only indirect information about the three-dimensional geometry of the elements (Fig.…”

“…Platform elements P1 (Supporting information Fig. S1), located at the rear of the conodont mouth, functioned with an occlusion between right and left elements, sometimes leading to a pronounced asymmetry (Donoghue and Purnell 1999;Martínez-Pérez et al 2016;Renaud et al 2021). Conodont elements grew by the incremental addition of lamellae.…”

3D models related to the publication: Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts

“…The 3D surface of these elements were used in a geometric morphometric analysis complementing an extensive 2D quantitative study (Renaud et al 2021). Based on a set of sliding semi-landmarks located at the edge of the platform and on the carina, this study aimed at disentangling the different components of the morphological variation in Late Devonian Polygnathus P1 elements.…”