Patterns of bilateral asymmetry and allometry in Late Devonian Polygnathus conodonts

Conodont animals were jawless vertebrates equipped with a feeding
apparatus composed of several tooth-like elements. The P1 elements, at the
rear of the apparatus, are well preserved and abundant in the sediments,
and their temporal evolution constitutes a tracer of ecological
interactions and/or abiotic changes in the water column. However,
occlusion occurred in these animals between paired right and left
elements, occasioning bilateral asymmetry, which, together with allometric
growth, may obliterate the temporal differences. This study aimed at
disentangling these different components of morphological variation in
Late Devonian Polygnathus P1 elements. An extensive 2D geometric
morphometric analysis of the platform shape was performed and completed by
a 3D study on a subset of elements. The 2D and 3D morphometric
quantifications provided highly congruent results, showing that the 2D
shape provides a good approximation of the element geometry. The 3D
analysis, however, provides a more complete description of the geometrical
changes involved in growth and bilateral asymmetry, providing insights on
the constraints related to occlusion. In complement, the 2D analysis
allowed a quantitative assessment of the variation between and within
species, showing that allometry and bilateral asymmetry were differently
expressed depending on the species considered. This suggests that
constraints imposed on pairing by the morphology of the elements, and the
pattern of bilateral asymmetry, varied even among related species. The
within-species variation was so important that it obliterated temporal
trends; a relationship of Polygnathus shape and conodont biofacies
variations through the Famennian suggested an evolution driven by
ecological interactions between conodonts.