Understanding latitudinal variations in biodiversity is central for biogeography. Along the coasts of the Southeast Pacific, several taxa show inverse latitudinal patterns of biodiversity, i.e., increasing species numbers from lower to higher latitudes. A plausible explanation for these patterns is that fjords, formed during the Pleistocene glaciations, increased the diversity of available biotopes that allowed for higher diversity in high latitudes. Assessing this hypothesis requires us to analyze latitudinal patterns of functional diversity (which is intimately related to niche use) in the absence of fjords, i.e., earlier than their formation. Here we test if the fossil record before the generation of fjords shows higher functional diversity at lower than at higher latitudes (a “regular” functional diversity gradient). We analyzed four components of functional diversity (functional richness, functional divergence, functional dispersion and functional evenness) for a fossil marine mollusk fauna from the lower Miocene (~18 million years ago) across four regions spanning more than 10 latitudinal degrees of the Chilean coast (between ~34°S and 45°S). The functional richness of gastropods and bivalves decreased non-linearly from low to high latitudes. Contrarily, the functional evenness of gastropods remained relatively constant across the region and that of bivalves peaked at 45°S. Both taxonomic groups’ functional divergence and functional dispersion did not exhibit clear patterns of latitudinal variation. The multivariate analysis uncovered non-linear latitudinal patterns in the abundance of functional groups (gastropods and bivalves), with the largest abundances towards lower latitudes. These results suggest that the niche breadth of mollusk assemblages might have decreased with latitude in the lower Miocene. This work can shed light on the mechanisms underlying the latitudinal variation of diversity observed today, offering a better understanding of how these patterns changed over time.

The dataset was collected at four regions along the Chilean Pacific Coast. The regions are Navidad South of San Antonio, 34°S) with 14 localities, Ranquil (Arauco peninsula, South of Concepción, 37°S) with 7 localities, Lacui (Pacific coast of Chiloé island, ca. 42°S) with 11 localities, and Los Chonos Archipelago (South of Chiloé Island, ca. 45°S) with 28 localities. These localities were selected, because they present a similar age and depositional environment (Kiel & Nielsen 2010, Encinas et al. 2018) of ca. 18Ma corresponding to the Early Miocene. Localities were sampled between the years 2000 and 2016, several of them repeatedly.

Specimens were collected from the surface of the rocky outcrops (superficial sampling) and by collecting bulk sediment samples (1.5kg to > 50kg due to the accessibility of localities and logistical difficulties). The area of each locality ranged from various decimeters to tens of meters, normally within one rock layer. Many localities were sampled several times over the years, but, because of difficult access, not all of them.

Each specimen was identified to the lowest possible taxonomic level, usually species level (see also Kiel & Nielsen 2010). The database contains a total of 355 species (93 bivalves and 262 gastropods) and 36,420 individuals (7,009 bivalves and 29,411 gastropods). Refer to Kiel and Nielsen (2010) for a detailed description of the dataset, although a taxonomically updated version including additional material is used here.

The dataset has been collected using Microsoft Access and porcessed using the R programming environment version 3.6.3 with the packages 'FD', 'tidyverse', 'lme4', 'MuMin', and 'vegan'.