Examining the types of Devonian trilobites in North Africa

Trilobite biodiversity trends in the Devonian of North Africa

Bault, V., Crônier, C., Allaire, N., & Monnet, C.

Summarized by Tom Shea, a fourth-year geology major at the University of South Florida. After completing his degree, he plans on going to graduate school to study seismology. Outside of class, Tom likes to go to USF games and spend time at the beach. 

What data were used?  1,171 trilobite fossils of 556 different species found in 168 locations throughout Northern Africa.

Methods:  This study ranked the trilobite samples by taxonomic rank (i.e., species, genus, family, etc.) to find ranks which were unique to Northern Africa and then used that information, called taxonomic richness, to show the biodiversity trends over the course of the early Paleozoic. This experiment focused heavily on trilobite genera (plural of genus).

Results: The data show that the biodiversity of trilobites varied significantly from the late Silurian Period through the Devonian Period. During the late Silurian, specifically during the Ludfordian and Přídolí Epochs (~427-423 MA and ~423-419 MA, respectively), biodiversity of trilobites declined steeply until there were eventually zero trilobites in the entire region (figure 1; also refer to figure 1 for the geologic time scale terminology used throughout this summary). The biodiversity of trilobites started rising again once the Devonian Period began.

Figure: A graph showing the biodiversity of trilobite genera in the late Silurian Period and the entire Devonian Period. The x-axis represents time, running from the Ludfordian Epoch to the end of the Devonian Period. The y-axis represents the number of genera, running from 0 to 60. Four lines are plotted on the graph: one for shareholder quorum sampling for 70 samples, one for sampled in bin index, one for range-through diversity, and one for boundary-crosser diversity. Each of the four plots show that the population of trilobites in this area started at nearly none at the end of the Silurian, but then exploded during the first few epochs of the Devonian before crashing back down near zero once again in the mid to late Devonian.
Figure 1: This graph shows that the number of different trilobite genera peaked in the Emsian Epoch of the Devonian Period after a few million years of rapid growth, followed by the population crashing right back down to near zero. The different curves indicate different measures of diversity, but all follow the same overall trends of the highs and lows of diversity.

During the early to mid-Devonian, and particularly in the mid to late Pragian Epoch within the mid-Devonian (~410-407 MA), trilobite diversity rose rapidly, from around 10 different genera in an interval to around 40 in only 1.5 million years. Once the Pragian gave way to the Emsian Epoch (~407-393 MA) though, the trilobite numbers in North Africa began sharply decreasing once again. This decrease continued until around halfway through the epoch, when the trilobite biodiversity suddenly and rapidly rose once again. The trilobites in North Africa eventually peaked with roughly 60 different genera sampled in a single interval near the end of the Emsian. After the peak, the number of genera began rapidly falling, a fall which became even steeper when the Emsian became the Eifelian Epoch. The event became known as the Choteč Event, which was relatively minor to most creatures for an extinction event but was absolutely devastating to trilobites in North Africa; this was caused by water becoming much deeper and causing extinction in shallower water trilobites. Trilobite numbers continued falling until there was not a single genus of trilobite living in this part of North Africa, which happened shortly after the Frasnian Epoch of the Devonian (~383-372 MA) began. Some genera of trilobites would return to North Africa later in the Frasnian, and these trilobites in the area fared better than many other creatures would in the Kellwasser mass extinction (~372 MA, the boundary of the Frasnian and Fammenian epochs of the late Devonian), which was one of the largest mass extinction events in history. Three of the five remaining orders of trilobites went extinct due to the Kellwasser event, severely limiting the chance of trilobite biodiversity in North Africa being anywhere near what existed prior to the Choteč Event.

Why is this study important? This study is important because it revealed a lot about the history of the past ocean, now part of Morocco, and the chronostratigraphy of that area, meaning the types of rocks in relation to the time period. This study ties sea level change and hypoxia (i.e., low levels of oxygen) to biodiversity in trilobites. For example, the sea level rise of the Choteč Event caused extinctions in shallow water trilobites; since trilobites were unusually hard-hit during the Choteč, this allowed scientists to see a different view of this geologic event. The Kellwasser extinction showed increasing hypoxia that led to extinction as well. With such a great fossil record like the trilobites seen here, we can get extremely detailed pictures of how certain groups responded to extinction events.

The big picture:  This study shows that by looking at the fossil record, you can tell a lot about the geologic history of an area. It is also important to note that this study helps us understand more about extinction events and biodiversity trends in the Paleozoic of Africa, as most paleontological studies have focused on data collection from Europe and North America that tend to bias our understanding of global events. 

Citation: Bault, V., Crônier, C., Allaire, N., & Monnet, C. (2021). Trilobite biodiversity trends in the Devonian of North Africa. Palaeogeography, Palaeoclimatology, Palaeoecology, 565, 110208–. https://doi.org/10.1016/j.palaeo.2020.110208