Silurian trilobites: a breakdown of gregarious behavior as it relates to predation

Summarized by David Elias (he/him). David Elias is an undergraduate student at USF expecting graduation in Summer 2021. An avid rock climber, it was only a matter of time before David found he would have a passion for studying the very rocks he climbed on. After taking a gap year, David will attend graduate school for either geophysics or structural geology. Upon graduation, David will have completed a B.Sc. in Geology with a minor in Geographical Information Systems, with undergraduate research in geophysics having contributed to the NASA TubeX project. Beyond academics, David enjoys the outdoors and stewarding it for others that are interested in getting outside. Other interests of his include spending time loved ones, coffee, and most furry animals (sorry, spiders).

What data were used? A cluster of 18 Arctinurus boltoni trilobites were extracted from the famous Caleb Quarry of Orleans County, New York. Notable for its astounding preservation potential, the locality sits within the Silurian-age Rochester Shale, placing the specimens’ age at approximately 425 million years old. Many of these trilobite fossils had evidence of injuries on their bodies. Injuries were defined by location and description. Twelve measurements were made on each specimen’s body for analysis of how similar morphology was within the collected cluster.

Methods: After collection from the Caleb Quarry in 2006, this cluster of trilobite specimens underwent preparation by minor reconstruction to fuse pieces of the specimens collected, without reducing fossil quality in visible light. After preparation, this slab of trilobites called the American Museum of Natural History (New York City, USA) home. Areas that underwent reconstruction were visible under UV light (Fig. 1) but weren’t easy to pick out under the fluorescent lights of a lab. Injury sites of each specimen were assessed under both visible and UV light in order to differentiate between injuries and reconstruction sites. The authors ran analyses to determine if there were any tangible differences in body shape between injured and uninjured trilobite specimens by employing morphometrics. Morphometrics, simply put, is analysis of body proportions of an individual specimen to other specimens. Twelve defined measurements, or landmarks, were made and compared to one another for each specimen to aid in morphometrics analysis. One specimen was left out since it had another fossilized organism covering the body. A Principle Component Analysis (PCA) was employed for morphometric analysis. A PCA essentially boils down all the different measurements made in an analysis into two variables plotted against one another to more easily understand how similar the specimens measured are. Scientists then used a statistical test, called Procrustes ANOVA, to determine significant differences in body shapes of the 18 Arctinurus boltoni trilobites. 

Results: Of the 18 Arctinurus boltoni specimens, 44% of them have injuries. Injuries to the right side comprise 87.5% of the injured specimens. All injuries found were located primarily on the pygidium (the rear end) of the trilobites, but none on the cephalon (the head). Types of injuries described were W-shaped, U-shaped, and V-shaped, as well as single spinal injuries (SSIs). It is unclear exactly which ancient predators were responsible for these injuries, though ancient mollusks or arthropods were likely suspects for the time period. It should be noted that injuries to the specimens were likely not a result of transport, as there is evidence for rapid burial in place. The depositional environment of the Rochester Shale bed was periodically tumbled by storm events, which would have suddenly buried these organisms alive. Each specimen in the cluster was also lying in similar, outstretched position, a sign that they likely aggregated during life.

An example of injuries sustained to specimen AMNH-FI-101520. Figure A points to injuries to the thorax (middle) and pygidium (end towards the left of the photo) under plain light. Figure B shows the same area under UV light. It is easy to see areas of reconstruction under UV light that might otherwise look like an injury. Scale = 1 mm.

Why is this study important?: A big question paleontologists ask is “How did critters interact with one another in the past?”. This may be a harder question to answer than you might think. Preservation of predator-prey interactions are rare in the fossil record, leaving large holes in the understanding of past ecosystems, also known as paleoecology. In order to try and fill in the blanks in this area, this cluster of 18 Arctinurus boltoni trilobites were put under the microscope, so to speak. One technique to better our understanding of paleoecology has been to use extant species as analogues for the past, and that’s just what Bicknell et al. did. Animal behavior today gave researchers a window into the past, gaining better understanding of how these trilobites received their injuries. Documenting this cluster of Arctinurus boltoni shed light on whether the cluster shows signs of predation, and if so, did the predator or prey have a strategy against one another? Were the specimens clustered together before or after death? Findings by the authors suggest the cluster had been predated on, predator-prey strategies were indeed at play, and that these individuals had aggregated before death. 

The big picture: Bicknell et al. determined that there may have been multiple predator groups that caused a range of injuries in the cluster of trilobites. Based on the preference for injuries to be on the posterior right side of the injured trilobites, either the trilobites had a lateralized defense or predators had a lateralized attack strategy. These deductions painted a picture where these trilobites were likely gathered in a group to provide protection from predators on a more open sea floor when they were buried by a storm event. This behavior has been observed in animals alive today. A dilution effect is created, making it harder for one individual to be picked off from the group. Knowing these extinct organisms engaged in gregarious behaviors as a defense mechanism from multiple predator groups gives future paleontologists a better idea of overall paleoecology in times that may otherwise seem otherworldly and perplexing.

Citation: Russell D.C. BicknellJohn R. Paterson, and Melanie J. Hopkins “A Trilobite Cluster from the Silurian Rochester Shale of New York: Predation Patterns and Possible Defensive Behavior,” American Museum Novitates 2019(3937), 1-16, (9 September 2019).