Phylogeny and macroevolution of a “dead clade walking”: a systematic revision of the Paragaricocrinidae (Crinoidea)
by Richard G. Keyes, David F. Wright, and William I. Ausich
Summarized by Alberto Farfan, Alberto lives in the Bronx, New York, and is a student at Binghamton University who is seeking a B.S. in Biology. He is also the president of the Binghamton University Fencing Club. His dream is to pursue infectious disease research to help others dealing with sickness and disease.
What data were used? The scientists collected data about features of the body from previously discovered crinoids (sea creatures with a stem and many arms, within the group that includes sea stars), as well as a new fossil that they discovered. The new fossil was discovered in the Tuscumbia Limestone on the East Warrior Platform in Northeastern Alabama, USA. The limestone in this region was formed over 336 to 340 million years ago during the Middle Mississippian Period. The new fossil was found in three disarticulated pieces in a quarry in Madison, Alabama, which is a part of the Tuscumbia Limestone. The feature that sets this fossil apart from other discovered fossilsis that it has spikes covering its anus.
What was the goal of the paper? Initially, the goal was to analyze and place the new crinoid fossil into the currently existing crinoid family tree. But once they saw the results of their data, it caused them to change their goal. The goal then shifted to examining the entire crinoid group to reestablish the evolutionary relationships of the entire family that the new fossil is a member of.
Methods: The scientists first collected the recently discovered fossil and removed it from the limestone. Because of some of the preserved features, they suspected it to belong to a group of crinoids (a family) called the Paragaricocrinidae. Due to this, they compared the fossil to four species in the Paragaricocrinidae family from the middle to late Paleozoic Era (~340 million years ago). They then compiled every characteristic visible from the fossils’ bodies together and used it to create an evolutionary tree..The scientists then used two different methods to determine how every species is related to each other. They first used parsimony analysis to determine the relation based on their inherent features. This is done by arranging the relationships of the fossils in the tree in a way that uses the fewest number of changes in the characteristics. The scientists also used Bayesian statistics to infer an evolutionary tree, as well, which uses probability to arrange the relationships of the included species. For example, if you flip a coin, you have a 50% (0.5) chance of getting heads. If you flip it twice, the probability of it being heads twice is 0.5*0.5 (25%). Bayesian statistics uses probability to understand the likelihood of how the species evolved and calculates that probability from the ancestors to the descendents of the tree.This allowed them to get a full analysis of the family to determine where each member of the family fits in an evolutionary tree.
Results: The researchers determined that a reorganization of the Paragaricocrinidae family tree was needed due to the differences observed in species. The scientists determined that, of the fossils they studied, four new genera, four new species, and one preexisting species had to be created or moved to a pre-existing taxon. Some uncertainty remains, so some species were not yet moved to different taxonomic groups and will need to be restudied. Previously, scientists thought that the family was very diverse with six different species in the entire family but underwent an unexplained decline (Fig 1) around 300 million years ago in the Late Carboniferous. After the steep decline, the species still clung on with one to two species leading into the Middle Permian. It was originally believed that their sudden decline was a result of a mass extinction event and so the family was given the term “dead clade walking”. A DCW is defined as a group of species that barely survive a mass extinction event with the majority of its members dying except for a few. But the issue with this is that there has been no documented mass extinction event during the time of their decline. However, now that scientists have a better understanding of the true diversity in this group, these scientists no longer think a “dead clade walking” is an appropriate term, nor do they think this was due to the result of a mass extinction. Instead, they called the group a “dead clade staggering” since they believe that their decline is due to another outside factor not related to a mass extinction and more inline with a slower, less extreme background extinction, possibly due to ecological changes.
Why is this study important?: This study is important because it helped the scientists better understand a more accurate picture of the family Paragaricocrinidae as a whole. It then became a question of what caused the biodiversity in the Paragaricocrinidae clade to decline so abruptly. It was shown that this pattern was not as a result of mass extinction but potentially as a result of possible ecological changes. Understanding the history of Paragaricocrinidae helps to give us insight into how crinoids respond to ecological pressure (more limited resources, competition, etc). This shows that species can lose biodiversity not just as a result of a major mass extinction-level event (asteroids, volcanoes, etc), but also as a result of environmental factors such as potential competition or small natural changes in the environment.
Broader implications beyond this study: A better handle on the taxonomic diversity of the fossil record can change how we identify extinction events in the fossil record. The authors revised the understanding of Paragaricocrinidae from a “dead clade walking” to a “dead clade staggering” since the family went extinct less rapidly and less dramatically than thought as a result of small environmental factors such as competition or a lack of resources. This can aid scientists in better understanding the amount of ecological changes that are needed to bring a species this close to extinction.
Citation: Keyes, R. G., Wright, D. F., & Ausich, W. I. (2025). Phylogeny and macroevolution of a “dead clade walking”: a systematic revision of the Paragaricocrinidae (Crinoidea). Journal of Paleontology, 99(1), 144–162. doi:10.1017/jpa.2024.70, https://doi.org/10.1017/jpa.2024.70