Synchronized Shedders? Trilobites Molting Patterns and Implications on Defense Strategy

Synchronized Moulting Behavior in Trilobites from the Cambrian Series 2 of South China

Alejandro Corrales-García, Jorge Esteve, Yuanlong Zhao,  and Xinglian Yang

Summarized by Makayla Palm

What data were used? Slabs of trilobites found from Cambrian-age rocks in South China were discovered in large clusters of several hundred individuals. There were several species represented within these clusters. Were these full trilobites? These fossils did not have a cephalon, or a protective head “shield” that concealed sensory organs, indicating they were molts, or leftover exoskeletons that had been shed off after a molting cycle (much like modern lobsters and tarantulas, which belong to the same phylum as trilobites, Arthropoda). All of the trilobite specimens were measured; scientists planned to use this data to test the hypothesis that these specific taxa, or groups of trilobites, had the same molting patterns as other members of Arthropoda. 

Methods: Scientists recorded measurement data to estimate average specimen size for each species. Researchers performed other data analyses, as well, such as: if different species were clustered together (or not), the orientation of the trilobites, or the way they were facing (e.g., – dorsal, or back, up or down) to learn more about how they were buried, and how differently the exoskeletons had molted, by observing how they deviated from a typical, complete trilobite.

Results: The sizes for all the species were all relatively small, which is evidence to support the idea they had gathered to molt for protection. If they had clustered together for reproduction, various sizes would have been found together. The smaller sizes indicate these may have been juveniles that stuck together for strength in numbers, which is observed in modern-day arthropods. The researchers observed all of the previous molting patterns found in other trilobites in these four trilobite species, confirming a wide variety of species molted in similar ways. They also observed that each species was clustered together and they had not intermixed with one another. The fact that these species did not intermix implies group synchronization, which is found in extant species of arthropods as a defense mechanism. It is inferred that these trilobites coordinated their molts in order to protect themselves during the vulnerable process of molting, which leaves their softer insides more exposed to predation until their new exoskeleton hardens.

There are ten known ways of trilobite molting, with various parts of the body either missing or displaced, depending on the growth stage the trilobite was in or if the trilobite needed to replace any body parts.There are two rows of five configurations. All of these configurations are with a dorsal view. The first five configurations are where different parts of the body are omitted, but not disfigured or displaced. Configuration A is missing the top of the head that extends around and almost touches the side. Configuration B is missing the inner part of the head and retains the outer rim of the head. Configuration C is missing the segment that connects the head with the thorax. Configurations D and E are missing body segments in the thorax. Configuration F has the crown of the head displaced under the thorax. Configuration G is missing the crown of the head and the connection between the head and thorax. Configuration H has all parts, but they are disconnected. Configuration I has the head bent forward on top of the thorax. Configuration J has the crown facing down and behind the thorax.
There are ten different molting configurations found within the cluster of trilobites found in all species of the study. The molting patterns differ in where a segment of the exoskeleton is missing, a body part displaced, or a body part that has been shifted. For example, some of the head pieces have been removed or displaced to lay behind the rest of the body. There are pieces of the thorax missing in some, or shifted relative to the rest of the body. These different configurations represent the known molting patterns of trilobites and show clear similarities in molting patterns with extant arthropod species. The relatively small size of the trilobites indicates they may have banded together for protection against predators, and molted in groups for strength in numbers.

Why is this study important? Several different trilobite types in Cambrian strata were found clustered together, but the fossilized remains weren’t complete trilobites. These were molts or leftover exoskeletons they had outgrown and shed. Molting is a common behavior in living arthropods today, and there are certain ways these creatures can molt. Several of these molting patterns have been described and documented previous to this study in other trilobites, and this study expanded on knowledge of molting patterns. This study also shows evidence that trilobites may have worked together in synchronized molting as a protection mechanism.  

The big picture: Fossils like these preserved here, along with modern analogs, can help us understand more about the behavior of long-extinct organisms.  Evidence from extant species of arthropods today has shown groups of species molt together as a defense mechanism, and the hypothesis of this paper was that the four tested groups of trilobites did the same thing. By finding the different species separated in different groups with various molting patterns, the researchers were able to conclude these trilobites likely synchronized, or coordinated molting together in groups. 

Citation: Corrales-García, A., Esteve, J., Zhao, Y., & Yang, X. (2020). Synchronized moulting behaviour in trilobites from the Cambrian Series 2 of South China. Scientific reports, 10(1), 1-11.

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