Parallel Evolution of Unusual ‘Harpiform’ Morphologies in Distantly Related Trilobites
By: James D. Beech, David J. Bottjer, Nathan D. Smith
Summarized by Erich Reineking: Erich lives in Binghamton, New York, but is from Oakdale, New York on Long Island. He is a student at Binghamton University who is pursuing a B.S. in Geological Sciences and a B.A. in Mathematical Sciences. During the summers, Erich is a lifeguard at the beach. In his free time, Erich likes to watch baseball, play golf, play video games, and hang out with his friends.
What data were used? Scientists collected data from around 114 different trilobite species. Out of the 114 trilobite species, 21 were from the one order and 56 from a different family. The remaining 38 species were a mix from different groups, including. The scientists constructed lists of characteristics that were visible across all of these different trilobite species. They identified 112 characteristics (or characters) to use in the experiment. They found out which trilobites had which characters by looking at digital and physical museum specimens, in addition to photographs found in published literature on trilobites.
What was the hypothesis being tested ? The goal of the paper was to figure out how two two groups of trilobites, the Harpetida order, and the Trinucleioidea family, evolved their ‘harpiform’ brim (Fig 1), which is the wide, flat part that sticks out around the head region, and has many holes in it. Scientists were examining if the two groups are closely related and gained their brim from common ancestry, or did both groups evolve the feature separately.
Methods: Scientists used the 112-character traits in order to perform an evolutionary analysis using parsimony. The parsimony analysis was performed in computer programs, which examines the possible evolutionary paths, and finds the one that has the fewest changes from the beginning to the end of the tree , meaning it is the most likely one to have occurred. The analysis took three main things into consideration, which are believed to be the characters most important in the evolution of the trilobites. The first being the “yoked condition of the libriganae”, which is the piece that makes the front of the head shield look fused together. The second was the placement of the facial suture, which runs along the bottom, around the edge of the trilobite brim. The third was either the appearance, or absence of a well-developed brim. These features can be seen in Fig. 1. The scientists also performed a constraint analysis, which restricted some of the created trees to certain parameters, and was then compared to the other created trees. In other words, the scientists forced specific groups to be more closely related in the analysis so they could compare the total number of changes in the trees of these forced results against the results where no such constraints were enforced.
Results: The evolutionary analysis inferred a number of most parsimonious trees (i.e., trees with the same number of changes). Scientists created a strict consensus of all of these trees, which only uses relationships found across all of the most parsimonious trees. The researchers found that the three characteristics mentioned in the above methods section, i.e., the head shield, the edge of the brim, and the development of the whole structure, all showed up many times, in separate and distinct ways between the harpetids and the trinucleids. These data suggest that the harpiform brim was a characteristic that evolved separately between the groups, and not something that made them closely related. It is kind of like how you wouldn’t say an eagle and a butterfly are closely related, just because they both have wings. Tracing both groups back to their last common ancestor shows that it did not have the harpiform brim, which is strong evidence that they arose on their own. These two groups can be referred to as “phylogenetically independent”.
Why is this study important? This study shows a great example of homoplasy, or convergent evolution (Fig. 2). Homoplasy means that similar traits that evolved separately, like the eagle and butterfly example explained above. Scientists studied two distantly related groups of trilobites that share a similar morphology. This study shows ways to use phylogenetic analytical methods to tell the differences between different groups. This helps us to reevaluate the relationships between species, so that scientists can learn how they evolved. Was it environmental, ecological, or other factors that impacted how they evolved over long time periods? Although this wasn’t what was tested, scientists still hypothesized that they could of used the brim as a sort of snowshoe, as the brim would have spread out its body density and not sink into soft sediment. It also could have been used as a body part that was good at picking up vibrations, letting the organisms know more about their surroundings. Due to the shape of the brim, it also could have been used to filter the food that it was eating. These are all great ideas, but it would take further studies to know the true reason.
Broader Implications beyond this study: By conducting this study, we have learned more about how different groups of trilobites are related. The researcher sexamined species that were thought to have been extremely closely related to each other because of their similar morphology. Often, original groupings of animals were based on what things looked like, and not the possibility that things can look the same but not be closely related–again, even though butterflies and eagles both have wings, it does not mean they are closely related. In this study’s case, their close relationship was believed to be because of their harpiform brims. But in reality, they are not as closely related as we once thought. This raises the question: how many other species could this misunderstanding have happened to? The methods that were used in this experiment can be adopted and used in other similar experiments. This experiment can open the door to the revaluation of many species, which will better help our understanding of the relationships between ancient species.
Citation: Beech, James D., David J. Bottjer, and Nathan D. Smith, ‘Parallel Evolution of Unusual “Harpiform” Morphologies in Distantly Related Trilobites’, Journal of Paleontology, 98 (2024), 732–43 <http://dx.doi.org.proxy.binghamton.edu/10.1017/jpa.2024.47>