Onset of Late Cretaceous diversification in Europe’s freshwater gastropod fauna links to global climatic and biotic events

Summarized by Jordan Orton. Jordan Orton is a geology major at the University of South Florida. Currently, he is a senior. He plans to work for the water management district to help protect and preserve the aquifer system to ensure we have plenty of safe water to drink and use. When he’s not studying geology, he loves to watch movies, garden, play board games, and go on little adventures. 

What was the hypothesis being tested? The hypothesis of this paper is to determine which factors influenced the rate of speciation (the rate that new distinct species evolve from a common ancestor) to increase so rapidly. Was it because of annual precipitation, average temperature, geographic distance, or continental area (which is determined by the sea level)?

What data were used? Data for this study was collected previously by the same authors and includes estimates of temperature, precipitation, and other variables by geographic location. Their data set also included taxonomic records of 3,122 species of snails represented in this fossil record.

Methods: They compared the results of a birth-death model (a statistical model of how the population of a species changes over time) and a multivariate birth-death analysis (a more complex statistical model to estimate population changes over time that takes into account several contributing factors) with shifts from a 10-million-year timeframe before the peak in speciation of snails to a 10-million-year timeframe after the peak in speciation of snails in order to determine which of the four variables in the hypothesis most affected the rate of speciation.

Results: The researchers analyzed four factors that may have contributed the most to this increase in diversity of snails: annual precipitation, mean annual temperature, geographic distance, and continental area (which is a function of sea level rise and fall). According to the results of the analyses and models, the factors that had the most influence for so many species of snails evolving was a reduction in continental area (i.e., sea level rise) and an increase in geographic distance. 

A reduction in continental area means that sea levels rose and flooded parts of the continent, creating new niche habitats that are brackish (slightly saline) to freshwater. This allows species that can tolerate the less salty waters have a place to flourish and escape predation from marine organisms. This coincided with the Cretaceous Terrestrial Revolution, a bloom in diversity of flowering plant life, and high global water surface temperatures, which also increased marine animal diversity. The creation of these new habitats allowed multiple species to develop and feed from varying types of new flowering plants that were also diversifying in the new habitats. Over time, these organisms evolved into new species that specialized which plants they consumed (similar to the example of Darwin’s finches). The secondary factor that allowed for this increase in diversity of snails is that there was a greater distance between continents, so there was more habitable area for the snails to spread out into. Because the snails venture out further apart, they don’t have the opportunity to intermingle with each other as much, which causes more species to develop. 

The rate of extinction of snails was consistent through each of these 10-million-year windows, so the rate of extinction wasn’t particularly affected by these four factors. It is likely that there was a decline in the rate of speciation from 85 Mya to 80 Mya due to interspecies competition. Interspecies competition is when there are too many different species competing for a limited resource, so there is a decline in population.

Why is this study important? This study is important because there hasn’t been much research into the factors that drove the diversity of species of European freshwater snails; the marine and terrestrial snails are more studied and better understood.

Broader Implications beyond this study: Sea levels and average annual temperature are rising today. If we want to understand what sort of impact human activity is having on the increasing and decreasing rates of speciation of snails, we need to understand how they were affected by the paleoclimate (historical climate). We need to see how snails reacted to these conditions in the past to have a baseline that we can compare to how they react now to the same conditions (which are now being driven by humans).  Scientists can then determine how human activity (habitat destruction, nutrification, etc.) is affecting their rates of speciation. 

Citation: Neubauer, T. A., & Harzhauser, M. (2022). Onset of late cretaceous diversification in Europe’s freshwater gastropod fauna links to global climatic and biotic events. Scientific Reports, 12, 1–6.