New records of injured Cambrian and Ordovician trilobites
Summarized by Matthew Gaborik, an undergraduate student studying geology at the University of South Florida. He will be graduating with the class of 2023. After his undergraduate program, he plans to gain some experience and return to school for a Master’s program. When he’s not studying geology, he likes to play mechanic, kayak, and hike.
What was the point of the paper? The point of the paper was to present new findings on select abnormal (injured or malformed) trilobite fossils in order to expand the record of abnormal trilobite fossils and obtain a clearer understanding of trilobite predation.
Data used: Seven abnormal trilobite fossils, originally housed in the Australian Museum, the Utah Field House of Natural History State Park Museum (U.S.), and the Museums of Western Colorado (U.S.), were gathered for this study because they portray damage to the exoskeleton. These abnormal trilobite fossils were: Lyriaspis sigillum, from the Beetle Creek Formation in Australia, Zacanthoides, from the Half Moon Mine, which is part of the Chisholm Formation in Nevada (U.S.), Asaphiscus wheeleri (two specimens) and Elrathia kingii (two specimens), both of which are from the Wheeler Formation in Utah (U.S.), and Ogyogiocarella debuchii from a quarry in Wales. All formations from which these fossils were sourced are aged to around the middle Cambrian (~510 million years ago), except for O. debuchii, which is from the Middle Ordovician (~450 million years ago).
Method: Fossils were treated with magnesium oxide (which highlights details on the specimen for photography), photographed, and examined for abnormalities. Additionally, a computer program, ImageJ, was used to measure the dimensions of the specimens and their abnormalities.
Results: L. sigillum specimen was found to have a U-shaped ident on the upper left side of the body. The Zacanthoides specimen was found to have a U-shaped indent on the lower left side of the body. The first A. wheeleri specimen was found to have an L-shaped indent on the lower left side of the body, and a U-shaped indent on the upper left side of the body. The second A. wheeleri specimen was found to have a small injury in the middle of the right-side of the body. The first E. kingii specimen was found to have a W-shaped indent along most of the left-side of the body. The second E. kingii specimen was found to have a V-shaped indent in the middle of the right-side of the body. The O. debuchii specimen was found to have a W-shaped indent towards the very bottom of the body. None of the specimens possess abnormalities that indicate damage due to genetic malformations or sickness. Therefore, it is likely that the abnormalities on these fossils are from injuries. Previous studies have shown that these types of indentations are usually a result of failed predation. Therefore, these abnormalities in the specimens described above (i.e., the indentations; Fig. 1) are concluded to be evidence of failed predation.
Why is this study important? This study is important because it provides insight into the environment from which these trilobites come from and how the predators in this environment would have operated. For example, this specimen of L. sigillum is the first known case of an injured trilobite from the Beetle Creek Formation, and only the second case of predation from the Beetle Creek Formation (middle Cambrian). Additionally, the abnormalities (injuries) on the L. sigillum indicate that durophages, which are predatory animals that consume organisms with harder exteriors, like trilobite exoskeletons, were likely present in the environment. Furthermore, the A. wheeleri described in this study is the first documented injury on this genus and species of trilobite, which indicates that A. wheeleri may have experienced higher rates of predation than previously believed.
Broader implications beyond this paper: This study is a prime example of how past environments can become clearer with closer examination of fossils. Fossils are one of our best available methods of piecing together the puzzles of the past. As stated before, the injuries on the L. sigillum indicate that durophages might have been present in the environment, which tells us more about how trilobites functioned as prey in the middle Cambrian. Predation rates in the middle Cambrian are not currently well understood, so this evidence adds more information to what is currently known.
Citation: Bicknell, R., Smith, P., Howells, T., & Foster, J. (2022). New records of injured Cambrian and Ordovician trilobites. Journal of Paleontology, 96(4), 921–929. doi:10.1017/jpa.2022.14