Examination of abnormal trilobites across the Paleozoic in China

Abnormalities in early Paleozoic trilobites from central and eastern China

By: Rui-Wen Zong

Summarized by: Matthew Ray, a geology major at The University of South Florida. Currently, he is a senior. He plans to attend graduate school following one to two years of internships in order to further his understanding of Florida geology and desire to make connections. When he is not actively studying geology, he is a fan of sketching and tennis.

What data were used? Researchers used ten trilobite samples with abnormal features and deformations that were located in the eastern and central locations of China dating back to the Cambrian, Silurian, and Ordovician periods. This is notably the first recorded evidence of trilobite deformations belonging to the Ordovician Period that has been documented in China.

Methods: Each of these samples were examined for abnormalities within the pygidia and thoraxes region, otherwise known as the back end and middle portions, respectively. After documenting variables such as rib overlap, breakage shape, and rib retention of these specimens, researchers hypothesized that these features were the results of: genetic malformations,  predatory attacks, or damage self-inflicted through the molting process.

Results: Trilobites are among the world’s most ancient arthropods and are a staple of the Paleozoic Era (542-241 million years ago), during which they flourished. These organisms have three main components: their head, in which most of the essential organs were kept, a mid-torso, and tail piece, named the cephalon, thorax, and pygidium, respectively. Abnormalities are common for trilobites, as seen in many fossilized specimens. Out of the samples discussed in this article, the three most notable abnormalities were: an absence of organs on the exoskeleton, morphological alterations in which ribs either overlapped, fused together, or were stretched, and the presence of breakage patterns in a ‘U’ or ‘V’ shape, suggestive of a predation event. Out of the ten trilobites, six displayed breakages on the outside of the thorax and/or pygidia which are reminiscent of markings inflicted by additional arthropod and cephalopod predation. These attacks are noted to have been non-lethal due to signs of regeneration and shell (see Figure 1). Two of the trilobites are hypothesized to have had genetic deformations, as evident by a stunted rib formation and the unusual arrangement of the surrounding ribs due to overlapping and/or fusion. The remaining two trilobites display issues with these ribs as well; however, these exhibit a large amount of rib retention on their back end (i.e., pygidium) which is considered to be a sign of breakage by tearing during the molting process. That is, their tail pieces were severed from the rest of the body in an attempt to rid itself of the old shell. These results allow a glimpse into the frequency of predation survival in addition to evidence pertaining to preexisting genetic developments and molting “errors” that result in a trilobite’s altered appearance.

Figure 1: 3 Well-preserved Cambrian trilobites (A, D, and G) that have show predation events. Boxes B, E, and G display close-up predation marks made by other organisms leaving a ‘U’, ‘V’, and ‘V’-shaped abnormality respectively. Box C displays a computer-generated model of the ‘U’- bite dimensions and layout. These marks can be used to establish exact predators if bite patterns are recorded and hypothesize community interactions. The black arrow on H depicts evidence of healing.

Why is this study important? This study succeeds in not only showcasing how trilobites can be born with abnormalities, but how some alterations can be formed due to a failure to properly molt or surviving predation events from other organisms. This in turn gives vital information concerning the trilobite’s lifestyle and overall survival rate, as these organisms were able to survive despite these morphological variations within specific regions of the body. The importance of maintaining more crucial body parts is here reinforced as trilobites with abnormalities on the pygidia and thorax were more likely to heal and prosper longer than trilobites that sustained injuries or had genetic deformations to their head, in which their more vital organs are held. Community relationships can be established, too, as the breakage patterns that were able to heal can be compared to those of organisms within the region at that geologic time, thus forming an understanding of feeding habits for others.

The big picture: Overall, this study adds to the information that researchers have about how organisms were able to survive predation attacks and how common genetic mutations occurred in organisms in the past (and how many of these were survivable). While trilobite knowledge across all of the documented times is enhanced, global data referring to the Cambrian Period was a crucial find as these samples were so well preserved, a trend which is unfortunately difficult to see around the world at this time.

Citation: Rui-Wen Zong , Abnormalities in early Paleozoic trilobites from central and eastern China, Palaeoworld (2020), doi: https://doi.org/10.1016/j.palwor.2020.07.003