Growth patterns or models of early Cambrian trilobites

Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis

By: Dai, T., Hughes, N., Zhang, X., & Fusco, G.

Summarized by Jared Duncan. Jared Duncan is a senior at University of South Florida working to be a geologist. He plans to work as a hydrogeologist after graduation. 

What data were used? The data used in this research came from approximately 1700 specimen of the trilobite species Oryctocarella duyunensis. From this collection, usable specimens were sorted out, as some were were too damaged or too incomplete to be able to recognize the key features of the fossils. The most important key feature the researchers were looking at in this analysis was measurements of a feature called the trunk (Figure 1). Along the trunks are numerous small ridges; the researchers recorded the distances between each ridge. 

Methods: The method for recording the sizes of these specific parts of the trilobites was through the use of calipers, a kind of ruler that gives precise measurements. Researchers also used high quality photographs of the specimens that were then measured using computer programs for precise measurements. 

Figure 1 The specimen looks like this and the trunks are the ridges with the pink squares located down the center of the fossil.

Results: From the data, they were able to find potential growth stages of the trunks. What this means is that as the trilobite grew, it kept growing more ridges to the trunk until a specific age was reached, where it stopped adding them. They found ample amounts of information regarding the actual growth patterns of the trunks. The growth between stages was not constant, but researchers were able to identify some patterns. The first 4 stages showed increasing growth rate, then there was a significant stall in growth for the fifth stage. From there, the growth was relatively constant, but slower than the initial growth stages. Last, the growth slowed down through the final stages. Usually, crustaceans and other organisms like the one studied here (trilobites belong to Arthropoda, the same group crustaceans belong to) experience indeterminate growth. This means that the creature grows until it dies. Arthropods in particular grow by molting (i.e., shedding their exoskeleton) as well, so trilobites usually grow using both of these methods. Indeterminate growth was not found in the species they researched. That lead them to conclude that it is plausible that the species had a single holaspid stage, meaning the time after the thorax (the middle section) was fully grown. It was found that, instead, they experienced determinate growth, which means the species had a molt at a specific time then stopped molting and growing larger from that point. 

Why is this important? This research is important because it came with some new discoveries on the growth of Oryctocarella duyunensis. They were able to figure out that the species showed determinant growth pattern, alongside with getting extensive data on the specific stage growths of the trunks. 

The Big Picture: Having access to this big of a collection of a type of fossil is important. Having loads of data helps make conclusions about the biology and the growth of fossil species more concrete. It further expands our knowledge on not only this extinct creature, but also gives a way for us to make comparisons with other closely related creatures. By knowing how this one kind of trilobite grows, we can make connections to other trilobites and expand on that knowledge. 

Citation: Dai, T., Hughes, N., Zhang, X., & Fusco, G. (2021). Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis. Paleobiology, 1-16. doi:10.1017/pab.2020.63