22-10 Developmental components of the molar tooth crown show stronger potential for recovering microevolutionary signal in the fossil record

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives

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Abstract:

The fossil record has unique potential for directly testing hypotheses of long-term microevolution. One major challenge to this line of inquiry is that informativeness of these investigations relies on trait choice. We review the reasons why this choice is both critical and difficult: there are nearly infinite options for trait choice in metazoan bodies, but these options are not interchangeable, have different relationships to the external environment, have variably strong connections to theory (affecting interpretability), are variably robust to taphonomic filters, and their identification relies fundamentally on the perceptive abilities of human investigators. Put another way, analysis of one trait may not give the same answer as another trait, but it is not initially clear which trait to measure, especially in the geometrically complex structures common to the fossil record. Geometric morphometrics (GMM), a multivariate approach to shape measurement that can measure many traits simultaneously, should be able to aid in this choice. However, we illustrate ways in which it does not fulfill this potential, particularly at the microevolutionary level.

Instead, we propose two modifications to a morphometric approach that better addresses this challenge of trait choice. First, we propose that investigations of evolutionary signal use knowledge of growth and development to inform the units of analysis because development filters underlying genetic variation into modular units. Second, we repurpose new advances in geometric morphometric data collection methods to measure characteristics of these units using approximately the same amount of time and effort required for a GMM analysis.

We illustrate this approach with first lower molars of the Northern Grasshopper Mouse Onychomys leucogaster, analyzing modern variation to generate predictions for the fossil record. Prior GMM analysis detected no significant pattern of phenotypic differentiation between genetically and geographically separated populations. Here, the developmentally based unit of analysis is the tooth crown and each component cusp. Analysis of only three classes of developmentally important traits replicated GMM-based major patterns of phenotypic variation, indicating that both approaches capture a shared biological reality. However, in contrast to GMM, this analysis also recovered significant secondary signal of trait differentiation between genetic groups and environments, changing our predictions of likely patterns in the Pleistocene fossil record. Overall, our initial exploration of this new approach shows a promising addition to the morphometric toolkit.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8083

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