Was it possible for trilobites to live in brackish water?

Were all trilobites fully marine? Trilobite expansion into brackish water during the early Palaeozoic

By: M. Gabriela Mángano , Luis A. Buatois, Beatriz G. Waisfeld, Diego F. Muñoz, N. Emilio Vaccari and Ricardo A. Astini

Summarized by: Abby McAleer, a senior at the University of South Florida.  She is majoring in geology with a minor in geographic informational systems. After graduation she plans to get a job in conservation or become an elementary school science teacher. In her free time, Abby loves to hike and travel with her friends. 

What data were used? Trilobite trace fossils (meaning, the marks left behind by an organism, separate from a body fossil) from the early Paleozoic were used along with stratigraphic sections from four ancient estuary (an area where fresh and sea water mix) sites. The specimens were found in sediment structures located in Northwest Argentina.  

Methods: The methods used in this study were a combination of ancient estuary outcrop identification, analysis of the different sediment types from these outcrops, and an analysis of the tracks, burrows, and trace fossils of the trilobites to compare fully marine trilobite fossils to fossils of trilobites found in brackish waters. The ancient estuary sediments were identified by dividing the valley systems of the Paleozoic Northwest Argentina Basin into 3 estuary zones; inner fluvio-estuarine (closer to the river), middle estuarine, and outer estuarine (closer to the ocean). The ancient estuary sediments were examined in a stratigraphic log, which describes the vertical changes of sediments from bottom to top in a particular area. Additionally, an analysis of how the fossil record of trilobites was altered by sedimentary processes was preformed to create a connection between paleobiology and the stratigraphic layers of the outcrops. Lastly, body fossil analysis was preformed on the trilobites to compare characteristics of offshore and onshore assemblages (deeper and shallower water, respectively). 

Results: The presence of trilobite fossils in ancient estuary environments supports the hypothesis that trilobites could handle a change of salinity and still survive. Although the presence of these fossils in the ancient estuary fossil record does not mean that the trilobites permanently inhabited these regions, it leads us to believe that trilobites migrated to these areas for food and possibly a safe place to nest and spawn. It is likely that the realization that tidal influenced estuaries were not fully marine environments helped us come to this conclusion. Figure 1 illustrates the 4 stratigraphic logs that were taken from the ancient estuaries. In this figure, we can see the expansion of the trilobites from marine to brackish water. 

Figure 1. This diagram represents the four stratigraphic logs of the ancient estuaries. In this figure, we can see the expansion of the trilobites from marine to brackish water through the column.

Why is this study important? This study helps us understand that trilobites made evolutionary changes to be able to handle the salinity change to survive, unlike other strictly marine invertebrates, like echinoderms. We can use the findings of this study to better understand the lifestyles of other marine organisms that lived during this time. 

The big picture: Previously, it was believed that trilobites could not handle salinity changes.  After this study, it has been indicated that trilobites were able to migrate to areas with fluctuating salinity for evolutionary advantages. This has helped scientist understand that assumptions of an organism’s tolerance for salinity may need to be reevaluated to limit future biases in paleontological studies.  

Citation: Mángano MG, Buatois LA, Waisfeld BG, Muñoz DF, Vaccari NE, Astini RA. 2021 Were all trilobites fully marine? Trilobite expansion into brackish water during the early Palaeozoic. Proc. R. Soc. B 288: 20202263. https://doi.org/10.1098/rspb.2020.2263