Diversification and disparity in a major Palaeozoic clade of Brachiopoda: the rise and fall of the Plectambonitoidea
By: Yves Candela, Zhen Guo, and David A.T. Harper
Summarized by Logan Mullin, an undergraduate student studying biological sciences at Binghamton University. Outside of school, Logan enjoys going to the gym, staying active outdoors, and spending time with friends and family.
Purpose of Study: The purpose of this paper was to study the changes over time in population and characteristic traits of an extinct group of shelled organisms (brachiopods) called the Plectambonitoidea.. The Plectambonitoidea lived across the ocean floors more than 400 million years ago, and, at their peak, were one of the most common groups of brachiopods in the world. Scientists study Plectambonitoidea because of its rich fossil record, which can allow us to understand how marine organisms and oceans as a whole responded to major geological events. Using their collected data described below, the authors connected the rise of the Plectambonitoidea to major changes in the fossil record, which we call the the Great Ordovician Biodiversification Event and their extinction to the Late Ordovician Mass Extinction. These findings also help us understand how other ancient marine species reacted in times of global environmental change. This makes Plectambonitoidea a great example for broader debates in paleobiology, such as the Court Jester versus the Red Queen debate, which states that environmental events and interactions mold biodiversity over time.
Data Used: The authors created a large dataset of 4,586 individual fossil occurrences of 123 genera of Plectambonitoidea from the Paleobiology Database, a public, online website where scientists across the world can view and add to fossil records from all time periods. Their dataset was global, with many different fossil occurrences from all across different continents and regions. These occurrences were from the Early Ordovician Tremadocian(485 million years ago) to the Middle Devonian Eifelian(393 million years ago). In addition to their occurrence data, the authors created a table of 43 different physical characteristic traits for each genus. Some traits include shell shape, hinge structure, and muscle scar patterns. To create this table, the authors drew on previously published articles, scientific illustrations, and direct examinations in museums to identify the different traits. The authors organized each trait into categories to compare the different genera fairly, with unknown or missing data marked with a “?” to preserve accuracy. This table allowed the authors to see how the Plectambonitoidea body types changed through time and in different geological environments.
Methods: After completing their fossil dataset, the authors checked how complete their fossil record was with a statistical analysis called Good’s u, which estimates if the dataset being studied is represented by enough samples. Then, the authors plotted fossil occurrences by time and location to demonstrate how widely the distribution Plectambonitoidea fossils varied over time. After that, with the use of a computer application called PyRate, the origination, extinction, and diversity of Plectambonitoidea throughout time were estimated. PyRate also accounts for biases as well, such as preservation bias by estimating how often fossils were likely to be preserved in each time interval.. Lastly, for the morphological analysis, the authors used their table of 43 morphological traits to show how the variety of Plectambonitoidea body forms diversified over time. The authors compared how similar or different each genus was to the others and used a visualization tool called a Principal Coordinate Analysis to reconstruct the groups’ morphospace. A morphospace is the full range of shapes of an organism over time. These approaches allowed the authors to identify when the group expanded, declined, and how it changed over time during major geological events
Results: The authors found that the rise and fall of the Plectambonitoidea superfamily had a connection to major geological events. During the Great Ordovician Biodiversification Event, a time when ocean life rapidly expanded and evolved, the Plectambonitoidea reached their highest levels in diversity. More specifically, it was during the Middle Ordovician about 460 million years ago. Morphological diversity and the number of organisms were the greatest at this time. After the Great Ordovician Biodiversification Event, the diversity began to decline. During the Late Ordovician Mass Extinction, an event caused by major climate and sea level changes, extinction rates started to surpass origination rates causing a sharp decline in population for most lineages. Although some lineages survived into the Silurian (444-419 million years ago), the superfamily was never able to recover and ultimately went extinct in the Middle Devonian (393-382 million years ago). Figure One supports the results by displaying statistics showing a shrinkage of morphospace occupied after the Late Ordovician Mass Extinction.
Why is this study important? This study is important because it demonstrates how the Plectambonitoidea’s success and decline were shaped by major changes on Earth. By tracking when new genera appeared, changes in body shapes, and when extinction rates rose, the authors were able to reveal how Plectambonitoidea expanded during times such as the Great Ordovician Biodiversification Event and struggled during major environmental disruptions such as the Late Ordovician Mass Extinction. This study’s findings also contribute to larger debates in paleobiology.
Broader Implications beyond this study: This study has many broader implications. It demonstrates how mass extinctions like the LOME can completely change evolutionary trends. The elimination of some clades opens more space in the environment for other clades to thrive. This study also discusses major debates in paleobiology, such as the Court Jester versus the Red Queen debate, which states that environmental events and interactions mold biodiversity over time. Lastly, this study provides information to better understand long term diversification and extinction patterns for other fossils.
Citation: Candela, Y., Guo, Z., & Harper, D. A. T. (2025). Diversification and disparity in a major Palaeozoic clade of Brachiopoda: the rise and fall of the Plectambonitoidea. Palaeontology, 68: e70010. https://doi.org/10.1111/pala.70010