58-8 Latitudinal Trends in Morphological Complexity: Gradients, Filters, and Discordance with Species Richness

Session: Future Leaders in Paleontology

Presenting Author:

Authors:

Abstract:

Biodiversity is distributed highly unevenly across space. The latitudinal diversity gradient, perhaps the most pervasive pattern, is observed across marine and terrestrial groups, but the underlying mechanisms remain contested, and the trends often diverge when measured in different currencies, e.g. taxonomy, morphology, and ecology. Here, we investigate the latitudinal distribution of ornamentation complexity in West Atlantic marine bivalves in the Northern Hemisphere (701 species, 67 families) and its underlying taxonomic and ecological structure.

Bivalve ornamentation serves multiple functions, from biomechanical efficiency to anti-predation. We developed a feature-explicit, quantitative model for this trait, defining complexity as the weighted sum of pattern-complexity scores from distinct classes of morphological features while accounting for the interaction among classes. The complexity score aims to reflect the developmental information inherent to each morphology.

Along the West Atlantic, species richness peaks within 15-25°N and drops off both towards the equator and the pole, perhaps reflecting adverse conditions on the Amazon coast and increasing seasonality at high latitudes. By contrast, morphological diversity peaks and remains stable throughout the tropics and sharply declines around 35°N, indicating that tropical species contribute disproportionately to morphological disparity. Notably, complexity levels remain stable within a 10° window after taxonomic diversity starts to decline, corresponding to the set of “bridge species” that maintain tropical levels of ecological diversity into the warm-temperate zones. Almost no species with moderate-to-high complexity occur above 40°N, likely reflecting both decreased predation pressure and increased energetic cost of elaborate ornamentation at higher latitudes.

Regarding latitudinal range size, narrow-ranged species show both low and high complexity, but latitudinal span significantly declines with complexity. Since wide geographic ranges often buffer against extinction, this constraint may represent a “cost of complexity”. Thus, tropical species may experience greater extinction and faster turnover, concordant with the tropics’ macroevolutionary role as both a cradle and a museum. 

Bivalve families differ markedly in the shape of their individual morphological latitudinal gradients, reflecting their differential contribution to morphological complexity across latitudes. Crucially, the shape of morphological latitudinal gradients is poorly predicted by species richness – even the 10 most species-rich families exhibit a wide range of morphological gradients. This shows a pronounced decoupling between taxonomic and phenotypic diversity and suggests a substantial phylogenetic influence on sculptural complexity.

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

doi: 10.1130/abs/2025AM-7590

© Copyright 2025 The Geological Society of America (GSA), all rights reserved.