Post-Ordovician Trilobite Diversity and Evolutionary Faunas

Bault, V., Balseiro, D., Monnet, C., and Crônier, C.

Summarized by Alexa Milcetic, a senior at the University of South Florida studying geology, with minors in astronomy and geographic information systems (GIS). She plans on furthering her education by obtaining a master’s degree in planetary geology. After she earns her degree, she plans to work for the National Aeronautics and Space Administration (NASA). When she isn’t studying geology, she loves to listen to music, watch movies, and read.

What was the hypothesis being tested? The hypothesis being tested required an investigation of the evolutionary history of trilobites, marine fossils, after the Late Ordovician Mass Extinction (LOME). Scientists wanted to evaluate the amount of diversity of trilobites after the LOME and understand the shifts in the broad groupings of diversity, called ‘evolutionary faunas’. Researchers asked what kind of environment did these different groups of trilobites live in? How did their differing regions they were acclimated to help or harm them during extinction events that happened after the LOME. 

What data were used? Trilobite data was downloaded from the Paleobiology Database which spanned 23 families. Where they were found, specifically the rock they were found in (lithology), can be used to estimate where they lived and where in the ocean they were found. For this, the Paleobiology Database was primarily used. This database was used to determine the post-Ordovician biodiversity of trilobites. Essentially, researchers put the data into the database, and these scientists then downloaded the information. This was then represented through the creation of a plotted graph (Figure 1), showing the diversity of trilobites throughout this range in geologic time. Using this same database, habitat conditions and the location of where these organisms lived was also collected. The lithology (the geology of their habitat) data became separated into the categories of carbonate, siliciclastic, or mixed. The preference for sediment that these trilobites had changed throughout time, and scientists wanted to see if there was any correlation with this preference and how certain groups became extinct. Bathymetrical data also became separated into categories of shallower or deeper, showing if a certain group preferred living in a certain region of the water where another group would not be able to survive. The latitude locations of their habitats also became separated into categories of low latitude, middle latitude (equatorial), and high latitude.

Methods: These trilobites within this period of the Post-Ordovician were in four distinct groups according to their similar characteristics, environments, and time they lived in. These were then used as variables to evaluate the evolutionary faunas. What the fauna was made of was determined by the latitude, lithology, and overall environment of the habitat they lived in. They looked at who was present, and the features of their environments, to better understand if things like lithology, etc. might explain more of their survival. They took all the data and ran tests to determine if there were patterns driving the biodiversity in these trilobites and tested the results to see if they were statistically significant. 

Results: Four groupings of trilobites showed up in four different chunks of time from the Silurian until the Permian–Triassic extinction (444 to 252 million years ago), where trilobites ultimately went extinct. Throughout the Silurian period, trilobites were highly diverse (i.e., many genera were present; more genera means more diversity). These are called the Silurian fauna. During this same period, these trilobites still maintained a high diversity even at higher latitudes and in richer siliciclastic (i.e., more sand and mud instead of limestone) environments, especially when compared to the more recent faunas. Next, there is the Devonian fauna, where especially in the Early Devonian there was the highest post-Ordovician diversity found within this study. In the Middle Devonian, there was a large reduction in trilobite diversity. This was likely due to the decrease in the amount of atmospheric oxygen, as well as with changes in sea level. The evolutionary fauna that developed here, within the Devonian, occurred during environmental changes, like an increased greenhouse environment, more carbonate environments, and high sea levels, indicating  a warmer climate. In the Late Devonian, diversity within trilobites was still low and the fauna is called the Kellerwasser Fauna.This was still due to the abrupt environmental changes that occurred during the Middle Devonian, that decimated previous evolutionary faunas. After this, there was the Hangenberg Event, known as the end-Devonian Extinction, which affected all existing trilobite groups. The survivors of this are called the Late Paleozoic Fauna (Figure 2). Since there was a decrease in diversity during the Mississippian (Early Carboniferous), there were only a select few faunas able to survive until the Permo-Triassic extinction. 

Why is this study important? In the trilobites, the diversity ranged vastly across different geologic times, which allowed them to make it through multiple extinction events. With this, we can begin to study who survived and who didn’t, and the common characteristics they shared or did not share with each other, such as: what made them more likely to live, and what characteristics made it more likely for them to die. This study is important because trilobites were an extremely common part of the early Paleozoic and why they went extinct in the pattern that they did (across multiple mass extinctions) isn’t well understood. The variables that likely controlled this include climate change and the environment each of these distinct trilobite groups lived in. While they never recovered from the Late Ordovician mass extinction, there were slight increases in diversity in the Early Devonian, possibly caused by warmer climates and large inland seas. 

Broader Implications beyond this study: These trilobites left us a blueprint. Since something with so much diversity has died out, it is important to find out what could have caused this. Their extinction was heavily affected by high greenhouse gasses. It is important to use this information in the past to decide how to best mitigate and protect the organisms we have today, as human activity is releasing high greenhouse gasses today. Understanding how trilobites responded to these mass extinctions can help us understand how other animals did too. We can use this information to see how current and future trends in climate will affect organisms today.  

Citation: Bault, V., Balseiro, D., Monnet, C., & Crônier, C. (2022). Post-Ordovician trilobite diversity and evolutionary faunas. Earth-Science Reviews, 230. https://doi.org/10.1016/j.earscirev.2022.104035