During the Ordovician, trilobites of the order Harpetida and the superfamily Trinucleioidea evolved unusual cephalic brims, quite unlike any structure known in modern arthropods. Brimmed trilobites were diverse and widespread, but we still do not fully understand why the harpiform brim evolved or what role it played in their success. Many authors have speculated about the brim's function, generating many untested hypotheses. We tested the hypothesis that the brim evolved to prevent sinking in soft sediments by calculating the depth to which brimless trilobites would sink and showed this explanation to be untenable. We then three-dimensionally printed model cephala with various brim shapes and moved them through natural sediments, testing the hypothesis that the brim evolved as a sediment plough. We found that increased horizontal brim width allowed a cephalon to efficiently displace more sediment, while increased brim height merely impeded the cephalon's progress. However, there is no clear evolutionary trend among harpetids or trinucleids towards wider or flatter brims, indicating that ploughing did not exert a strong selective pressure on brimmed trilobites. This work also serves as a case study in researching taxa without modern biomechanical analogues, demonstrating that many functional hypotheses can be adequately tested by straightforward experimental methods.

Harpetid and trinucleid trilobites share a similar and unusual morphology, the most striking feature of which is a wide, flattened cephalic brim with many pits or holes. This similarity was once interpreted as a sign that these two groups of trilobites were closely related, but in recent years it has instead been assumed that the 'harpiform' brim arose in both groups independently. However, relatedness and similarity can be difficult to disentangle in fossil taxa without close living relatives, and this assumption about the harpiform brim has never been explicitly tested. Our study re-evaluates the relationship between Harpetida and Trinucleioidea in order to test a longstanding assumption about trilobite relationships and as a case study in evaluating different kinds of morphological similarity in extinct groups. We inferred a new phylogenetic tree using parsimony methods and discrete morphological character data from a broad sampling of harpetids, trinucleids, and their relatives. Despite their gross morphological similarities, we found that harpetids and trinucleids were readily distinguished in our analyses, a result consistent with a hypothesis of multiple origins for the harpiform brim. By mapping brim-related characters across our new phylogeny, we identified a sequence of morphological innovations that arose in parallel in both groups and led ultimately in each case to the evolution of the harpiform brim. These results indicate that harpiform brims are a prime example of parallel evolution-the similar development of a morphological trait in distantly related taxa that nevertheless share a similar original morphology. In addition, our phylogeny supports the idea that trinucleids are specialized, harpiform asaphids, rather than an independent order of trilobites. We also provide new information on the relationships of the putative 'basal-most' members of Trinucleioidea, the Liostracinidae, and confirm recent assessments that this family is more distantly related to trinucleids.

Mass extinctions have fundamentally altered the structure of the biosphere throughout Earth's history. The ecological severity of mass extinctions is well studied in marine ecosystems by categorizing marine taxa into functional groups based on ‘ecospace’ approaches, but the ecological response of terrestrial ecosystems to mass extinctions is less well understood due to the lack of a comparable methodology. Here, we present a new terrestrial ecospace framework that categorizes fauna into functional groups as defined by tiering, motility and feeding traits. We applied the new terrestrial and traditional marine ecospace analyses to data from the Paleobiology Database across the end-Triassic mass extinction—a time of catastrophic global warming—to compare changes between the marine and terrestrial biospheres. We found that terrestrial functional groups experienced higher extinction severity, that taxonomic and functional richness are more tightly coupled in the terrestrial, and that the terrestrial realm continued to experience high ecological dissimilarity in the wake of the extinction. Although signals of extinction severity and ecological turnover are sensitive to the quality of the terrestrial fossil record, our findings suggest greater ecological pressure from the end-Triassic mass extinction on terrestrial ecosystems than marine ecosystems, contributing to more prolonged terrestrial ecological flux.

The trilobite order Harpetida has long been easily recognized but many unanswered evolutionary questions about the group remain. This work explores the phylogenetic relationships within Harpetida and studies the harpetid response to the Late Ordovician mass extinction to better understand the relationship between extinction events and disparity. A discrete morphological character matrix was assembled from published descriptions and refined through first‐hand observations. This matrix is the first attempt of its kind to characterize the overall morphology of Harpetida, rather than focusing on individual harpetid genera. Phylogenetic analyses under both maximum parsimony and Bayesian inference optimality criteria retrieve tree topologies that support harpetid monophyly but throw doubt onto previous hypotheses of the internal relationships of the order. Harpetid disparity proves remarkably stable over time. A modest peak in the Ordovician is followed by a slow decline throughout the Silurian and Devonian. After the Ordovician period, harpetids demonstrate little or no ability to colonize new areas of morphospace. This may represent a fundamental failure to recover, in which the lasting impacts of Late Ordovician mass extinction continue to suppress morphological innovation. These findings demonstrate that mass extinction events may have complex impacts that play out over many millions of years.

The trilobite order Harpetida has long been easily recognized but poorly understood. This study seeks to better understand the phylogenetic relationships within Harpetida, with a view towards using this group to explore the relationship between extinction intensity and disparity. The harpetid response to the Late Ordovician mass extinction is of particular interest. A discrete morphological character matrix was created from the formal descriptions of harpetids in the published trilobite literature, and refined using first-hand observations of harpetid fossils. The final matrix consists of 76 discrete characters, including 69 cephalic characters, three thoracic characters, and four pygidial characters. This matrix is the first attempt of its kind to characterize the morphology of Harpetida as a whole, rather than focusing on individual harpetid genera. Exemplar species from a broad selection of harpetid genera, along with ptychopariid and redlichiid out groups, were included in the matrix. These taxa were coded from published figures and from direct observation of specimens held in the collections of the Yale Peabody Museum of Natural History. From the matrix, a hypothetical tree of harpetid phylogenetic relationships was generated. The topology of this tree indicates support for harpetid monophyly but throws doubt onto the previous hypotheses of the internal relationships of the group. Disparity analysis of Harpetida reveals a decline in morphological diversity following Late Ordovician, with slow or nonexistent recovery.