The trouble with trilobites: classification, phylogeny and the cryptogenesis problem

Abstract

Trilobites are an iconic group of extinct arthropods that lived in Palaeozoic oceans for c. 270 Ma, before their demise at the end of the Permian Period. Despite their considerable diversity (> 22 000 species) and abundance, particularly in Cambrian and Ordovician rocks, as well as intensive study for well over 200 years, trilobite systematics remains in a state of flux. In this contribution, the complex history of trilobite classification over the last century is briefly reviewed, including the seminal scheme published by Henry Swinnerton in 1915. The cryptogenesis problem, which relates to the supposedly obscure phylogenetic links between major post-Cambrian trilobite clades and their Cambrian sister taxa, is also discussed. Previous studies have suggested that the cryptogenesis problem is largely a taxonomic artefact, but the Cambrian origins of some post-Cambrian groups, such as the orders Proetida and Odontopleurida, are still unclear. Future directions for research on trilobite systematics are outlined, from taxonomic studies involving comprehensive documentation and extensive illustration of morphology at the species level, through to broad-scale phylogenetic analyses that initiate or test hypotheses about relationships across the major groups. Other ongoing issues to be addressed include identifying the sister group of Trilobita, and determining whether certain taxa, such as the suborder Agnostina and Cambrian family Burlingiidae, represent trilobites.

Reassessing a cryptic history of early trilobite evolution

“…Trilobites are a clade of total-group euarthropods whose first appearance datum (FAD) marks the boundary between the Terreneuvian and provisional Cambrian Series 2 (currently dated to c. 521 Ma) 12 , 13 . They are one of the largest and most successful Paleozoic groups, persisting for some 270 million years, and represented by over 22,000 described species 14 , 15 . This excellent fossil record—a result of their easily-preserved, biomineralized exoskeleton that was molted many times during life—can be used to address important questions concerning early animal evolution 16 , 17 .…”

Long-term clade-wide shifts in trilobite segment number and allocation during the Palaeozoic

“…We tabulated the number of segments in the trunk for 1589 trilobite species from images sourced from primary and secondary literature and museum catalogues (electronic supplementary material, Dataset S1). We limited our search to complete, well-preserved, articulated specimens, necessarily excluding the vast majority of named trilobite species ( N > 22 000, [ 12 ]) for which the exoskeleton is known only from disarticulated sclerites. We also focused our search on specimens with stratigraphic age information precise enough to place within global geological series.…”

New species of Liostracina Monke, 1903 (Trilobita, Cambrian) from Yunnan, China: complete holaspid exoskeleton and implications for higher level classification

“…Most of the families assigned to an uncertain order were previously included in the order Ptychopariida. Lieberman and Karim (2010) and Paterson (2020) likewise questioned the practicality of recognizing the order Ptychopariida. Alternatively, Yuan et al (2012Yuan et al ( , 2016, Laibl et al (2014), Esteve (2015), Krylov (2018), andBignon et al (2020), among others, recently have argued for continued recognition of the order Ptychopariida.…”

“…Historically, more than one-third of trilobite families have been assigned to the order Ptychopariida, which raises considerable issues related to our understanding of phylogenetic relationships. The Ptychopariida has undergone several revisions since its original conception, and is now commonly considered to be a paraphyletic or polyphyletic group (Babcock, 1994;Cotton, 2001;Lieberman and Karim, 2010;Adrain, 2011;Bignon et al, 2020;Paterson, 2020). A thorough restudy of the 'ptychopariids' is long overdue, and it will undoubtedly lead to significant revision of the concept of the group.…”