Temperature-driven heterochrony as a main evolutionary response to climate changes in conodonts

Abstract

Can we predict the evolutionary response of organisms to climate changes? The direction of greatest intraspecific phenotypic variance is thought to correspond to an ‘ evolutionary line of least resistance ’, i.e. a taxon's phenotype is expected to evolve along that general direction, if not constrained otherwise. In particular, heterochrony, whereby the timing or rate of developmental processes are modified, has often been invoked to describe evolutionary trajectories and it may be advantageous to organisms when rapid adaptation is critical. Yet, to date, little is known empirically as to which covariation patterns, whether static allometry, as measured in adult forms only, or ontogenetic allometry, the basis for heterochrony, may be prevalent in what circumstances. Here, we quantify the morphology of segminiplanate conodont elements during two distinct time intervals separated by more than 130 Myr: the Devonian-Carboniferous boundary and the Carnian-Norian boundary (Late Triassic). We evidence that the corresponding species share similar patterns of intraspecific static allometry. Yet, during both crises, conodont evolution was decoupled from this common evolutionary line of least resistance. Instead, it followed heterochrony-like trajectories that furthermore appear as driven by ocean temperature. This may have implications for our interpretation of conodonts' and past marine ecosystems’ response to environmental perturbations.

Potential of closed contour analysis for species differentiation and holotype designation: a case study on lower Norian (Upper Triassic) conodonts

“…Souquet et al . (2022) arrived at a similar conclusion in the case of both intervals as Guenser et al . (2019) and assumed that the observed morphological evolution was driven by environmental perturbations related to changes in ocean temperature.…”

“…(2019, 2022) and Souquet et al . (2022). The material originates from various localities of the western Tethys and western North America.…”

“…(2019) and Souquet et al . (2022). Norian biostratigraphy is hampered by several taxonomical issues that mainly derive from the overly general original diagnoses and/or the sparse illustration of the species characteristic for this age (Karádi 2018 a ).…”

“…A rather recent article by Souquet et al . (2022), that was published after the first version of the current paper was submitted, used a landmark‐based approach to quantify the morphology of P 1 elements (including holotypes) deriving from two different time periods: the Devonian–Carboniferous boundary and the Late Triassic Carnian–Norian boundary. For this latter interval, they used exactly the same species from the same section as Guenser et al .…”