As the most recent time in Earth history when global temperatures were warmer than at present, the peak of the last interglacial (Marine Isotope Substage [MIS] 5e; ~120,000 years ago) can serve as a pre-anthropogenic baseline for a warmer near-future world. Here we use a new compilation of 22 fossil localities in California that have been reliably dated to Marine Isotope Stage (MIS) 5e to establish baseline expectations for contemporary bivalve species movements by identifying and analyzing bivalve species with “extralimital” ranges, i.e. species that occupied the California region during MIS 5e but are now restricted to adjacent regions. We find that 15% of species (n = 142) found in MIS 5e localities have extralimital ranges and currently occupy warmer waters to the south of the California region. The majority of extralimital occurrences occur in paleo-embayments, suggesting that these sheltered habitats were more suitable habitats for warm-water species than exposed coasts during the MIS 5e. We further find that extralimital species now tend to occur in cooler, more seasonally productive coastal waters and to occupy more offshore islands when compared to the broader species pool immediately south of California. These findings suggest that high dispersal potential and pre-existing tolerances to environmental conditions similar to California’s comparatively cool and seasonally productive environments may have enabled extralimital bivalves to colonize the California region during MIS 5e.

Appendices_1-3.xlsx

Data appendices formatted for readability and reference. Sheets 1-6 contain data, data sources, and descriptions for each file included as headings. All methods to generate appendices are described in Orzechowski and Finnegan 2020.

SI_analyses.R

R code to reproduce manuscript analyses, tables, and figures. Data necessary for reproduction are: "SI_coasts.csv", "SI_fauna_range_limits.csv", and "SI_fauna_range_through.csv". These files are described in "SI_files_description.txt."

SI_coasts.csv

Fossil localities classified as (paleo) embayment or open coast; presence or absence of extralimitals at each locality indicated.

SI_fauna_range_limits.csv

Range-limiting environmental variables for species analyzed.

SI_fauna_range_through.csv

Range-through environmental variables for species analyzed.

SI_files_description.txt

Description of "SI" files, with key for each column.