Résumé
The morphology of feeding structures is driven by the nature of the food
source, itself influenced by environmental factors, resulting in an
indirect relationship between the form of feeding structures and the
surrounding environment. However, food sources may change along growth,
leading to potentially complex interactions between ontogenetic
trajectories and environmental variation. The present study investigates
these interactions by analysing the platform shape of P1 elements in
unornamented Polygnathus from a latest Devonian - early Carboniferous
succession from the Montagne Noire, France, deposited after the Hangenberg
Event mass extinction. Allometric trajectories were found to vary along
the record, starting from different small-sized shapes but converging
towards a common adult morphology. To assess variation of environmental
forcing along ontogeny, the specimens from each stratigraphic level were
split into three size classes. The shape of the small size class varied
primarily with conodont biofacies, the medium size class with sedimentary
microfacies, while the large size class followed a random walk unrelated
to environmental variation. These results agree with a change of diet
along ontogeny, and with an increase of functional constraints related to
occlusion in large-sized elements, leading to their convergent adult
morphology. Biofacies and microfacies trace different signatures of
bathymetry and distance from the coast, suggesting that post-Hangenberg
unornamented Polygnathus varied in their exploitation of the water column
along ontogeny. Finally, the high disparity of small-sized elements
suggests a relaxation of selective constraints related to occlusion,
and/or different foraging strategies, making them more susceptible to
environmental influences than the large-sized elements.