New trilobites from the Ophir Formation of the western Uinta Mountains in northern Utah represent some of the first identifiable and biostratigraphically useful fossils in the range, outside of Dinosaur National Monument. The fossils indicate that the Ophir Formation is similar in age to the Lodore Formation.

Fossils are rare in Cambrian strata of the Uinta Mountains of northeastern Utah, and are important because they can help integrate our understanding of laterally adjacent but discontiguous rock units, e. g., the Tintic Quartzite of Utah and the Lodore Formation of Utah-Colorado. New body fossils from strata previously mapped as Tintic or Cambrian Undifferentiated, but here interpreted as the Ophir Formation, include indeterminate hyoliths and hyolithids, brachiopods including a linguloid, and the trilobites Trachycheilus Resser, 1945 and Elrathiella Poulsen, 1927. The last two assign these strata to the Ehmaniella Biozone (uppermost Wuliuan Stage; Miaolingian Series) or traditional Laurentian middle Cambrian. These data, together with fossil occurrences elsewhere in Utah, require that the Tintic Quartzite was deposited prior to and/or during the early Wuliuan, and suggest that the unit could be correlative to much of the Lodore Formation of Utah and Colorado.

Regarded as a low-diversity, Burgess Shale-type Lagerstätte deposit, the Eager Formation (Cambrian Series 2, Stage 4) of the Cranbrook area, British Columbia, contains abundant, sometimes beautifully preserved, trilobites and rare non-biomineralized taxa. Although trilobites from this area were first reported just over a century ago, they have received little study. This paper describes the trilobites of the Cranbrook Lagerstätte. A minimum of eight trilobite species are recognized: four species of olenelloids (two of which are new) are by far the most abundant elements of the fauna, and at least two species of dorypygids and two species of “ptychoparioids” are also present. Trilobite diversity in the Cranbrook Lagerstätte is comparable to that within other Lagerstätten from Cambrian Stage 4 of Laurentia. Preservational attributes of the trilobites and sedimentological data suggest that the assemblage experienced minimal transportation prior to fossilization, and that the local environment was at least occasionally able to support a “typical” seafloor trilobite community. The Cranbrook Lagerstätte is demonstrated to lie within the middle Dyeran Stage of Laurentia, within what was a substantial stratigraphic gap in the distribution of Burgess Shale-type Lagerstätten.

The Eager Formation (Cambrian Stage 4) of the Cranbrook area, British Columbia, contains abundant, sometimes beautifully preserved, trilobites and rare non-biomineralized taxa. Trilobites were first reported just over a century ago but have received little research attention, resulting in uncertainty in the number and identity of species within the assemblage and ambiguity in the age of the fauna. The trilobites of the Eager Formation in the Cranbrook area are described herein based largely upon material collected in 2015. A minimum of eight (and perhaps up to 11) trilobite species are recognized. The four species of olenelloids (Olenellus santuccii Webster n. sp., Olenellus? schofieldi, Mesonacis eagerensis, and Wanneria cranbrookense Webster n. sp.) are by far the most abundant elements of the fauna. At least two, and perhaps as many as five, species of dorypygid are present, as are two species of “ptychoparioids”. Paucity and poor preservational quality of specimens mean that the various dorypygid and “ptychoparioid” species are left in open nomenclature. Trilobite diversity in the Cranbrook Lagerstätte is comparable to that within other Lagerstätten from Cambrian Stage 4 (Series 2) of Laurentia. The diversity and abundance of trilobites, combined with biostratinomic and trace fossil data, suggest that the assemblage is autochthonous and/or parautochthonous, and that the local environment was at least periodically able to support a “typical” benthic trilobite community. The age of the Cranbrook Lagerstätte is constrained to lie within the middle Dyeran Stage of Laurentia, within what was a substantial stratigraphic gap in the distribution of Burgess Shale-type Lagerstätten.

Adult and juvenile specimens of two species of trilobites, Sahtuia carcajouensis and Mackenzieaspis parallelispinosa, were collected from the Mount Cap Formation of the eastern Mackenzie Mountains. This formation was deposited within a semi-enclosed, epicontinental sea during the early and middle Cambrian (approximately 530–500 million years ago). These two species are endemic to one section at Carcajou Falls in strata of the Glossopleura walcotti Zone, where they also are prolific, suggesting they had some type of affinity to the locality. These species are characterized by their unusually low number of thoracic segments and high number of pygidial segments. We describe and quantify their growth and development here and compare them to those of other species from their families, Dolichometopidae and Zacanthoididae, respectively. The results indicate that S. carcajouensis and M. parallelispinosa underwent relatively normal growth and development of their cranidia and developed a typical number of segments in their trunks (the combined thorax and pygidium). However, both species underwent fewer occurrences of segment release, when segments are transferred from the pygidium to the thorax, than was typical for dolichometopids and zacanthoidids. We hypothesize here that these species are not closely related, and instead that their unique development was brought about by changes to the developmental timing of two probable ancestor taxa by local environmental conditions.

The ontogeny of two species of corynexochid trilobites from the middle Cambrian Mount Cap Formation of the eastern Mackenzie Mountains, northern Canada, is documented. Sahtuia carcajouensis (Dolichometopidae) and Mackenzieaspis parallelispinosa (Zacanthoididae) are both endemic to this formation and only known from one locality. They, along with several other corynexochid taxa, occur in a succession of mudstone with scattered carbonate interbeds, deposited in a weakly storm-agitated setting near the flank of a semi-enclosed basin. The ontogeny of both species is characterized by mainly normal cranidial development, but a unique distribution of segments in their thoraxes and pygidia. The number of trunk segments was typical for their respective families, whereas the final number of segments released into the thorax was reduced. This occurred in both species through timing modifications to segment release, indicating heterochrony. Sahtuia carcajouensis and Mackenzieaspis parallelispinosa are likely derived from two separate clades, and heterochrony probably arose separately but synchronously. The endemicity of both species probably reflects unique paleoecological conditions in this part of the basin. Preliminary results indicate that the fossil-bearing mudstone was deposited under well-oxygenated conditions that underwent high nutrient flux and possibly experienced varying salinity. These factors may have affected the organisms' physiology, or perhaps provoked an adaptation to achieve early maturation.

Distinctive changes in carbon isotope curves are used extensively alongside trilobite faunal turnover in the international correlation of Cambrian strata. One such isotopic signature, called “SPICE” (Steptoean Positive Isotope Carbon Excursion), is widely used, but in North America, the co-occurring trilobite fossils have never been illustrated. We here describe, discuss, and illustrate the 34 trilobite species (two new) that occur below, within, and above the SPICE from the same section where the carbon isotope data were collected in Utah. The illustration of the specimens, rather than just listing taxa, allows other scientists to evaluate the conclusions made here: the SPICE began in the Aphelaspis Biochron and ended within the Elvinia Biochron.

The trilobite faunas that occur with the Steptoean Positive Isotope Carbon Excursion (SPICE) at Smithfield Canyon, Utah, have been reported, but not illustrated. Given the importance of the SPICE at this section for international correlations, the trilobites from new collections from the upper Nounan Dolomite to lower St. Charles Formation at Smithfield Canyon are reported herein and integrated with the previously reported taxa. Trilobite assemblages indicate that the upper Cedaria to the Ellipsocephaloides biozones (Miaolingian Series, Guzhangian Stage to Furongian Series, Jiangshanian Stage) are present stratigraphically below or above the SPICE.

Some of the taxa reported herein may represent new species, but they are not represented by well-enough preserved specimens and are left in open nomenclature. However, Kingstonia smithfieldensis n. sp. and Bromella utahensis n. sp. are named on the basis of common and well-preserved specimens.

New carbon isotope data from Smithfield Canyon from an overlapping section of the lower St. Charles Formation, that add to the overall shape of the SPICE curve, are presented. The new δ¹³C values above the Elvinia Biozone range from –0.36‰ to +1.5‰, confirming that the SPICE concludes within the Elvinia Biozone.

In northern Iran, the late Cambrian trilobites have been documented largely from Furongian deposits exposed at Mila-Kuh, Shahmirzad, and some sections in western Alborz, whereas the available information on this group of fossils from the sections located in eastern Alborz is sparse. The major objective of the paper is a small trilobite assemblage, assignable to the Parabolina Fauna, which has been recovered for the first time from the Furongian (Cambrian Stage 10) Sah Member of the Mila Formation in the Tuyeh–Darvar section, eastern Alborz, northern Iran. This assemblage includes eight genera and species; two of them, Niobella darvarensis n. sp. and Macropyge (Promacropyge) sahensis n. sp., are new to science. The incursion of this fauna into Alborz probably occurred due to a significant drowning event and associated dark-gray shale deposition. While the generic composition of the assemblage is mostly cosmopolitan, it also contains a local endemic Alborsella; although some taxa exhibit a distinct sign of faunal links with South China and Tarim. On the other hand, Parabolina (Neoparabolina) frequens, which is common in olenid trilobite associations, is widespread mainly offshore in deposits from temperate Gondwana (Armorican terrane assemblage, Argentina) and Baltica of about that age.

A small trilobite assemblage, including Parabolina (Neoparabolina) frequens, assignable to the Parabolina Fauna, has been recovered from the Furongian (Cambrian Stage 10) Sah Member of the Mila Formation in the Tuyeh–-Darvar section, the eastern Alborz Mountains, north Iran. The assemblage includes eight genera and species; two of them, Niobella darvarensis n. sp. and Macropyge (Promacropyge) sahensis n. sp., are new to science. The incursion of a Parabolina fauna into Alborz is confined to a significant drowning event with associated dark-gray shale deposition, which most probably occurred in the lower part of the Cordylodus proavus conodont Zone. While the generic composition of the assemblage is mostly cosmopolitan with the exception of the endemic Alborsella, the occurrence of Indiligens, Macropyge (Promacropyge) sahensis n. sp., Agnostotes sp. aff. A. sulcatus, and Leiagnostus bexelli indicates faunal links with South China and Tarim. Parabolina (Neoparabolina) frequens is widespread mainly in offshore deposits from temperate Gondwana (Armorican terrane assemblage, Argentina) and Baltica of about that age.

Sites in Nevada and Oklahoma preserve a record of trilobite extinction in what were shallow- and deep-marine environments during the Cambrian Period, about 492 million years ago. Faunal changes include losses of some trilobite genera and changes in abundances and distributions of others. The North American shelf seas became more ecologically homogenous, with fewer trilobite communities that were spread over a broader range of environments. One unusual but poorly understood feature of the extinction interval is the brief appearance of abundant brachiopods in shallow-water Cambrian sites like those in Oklahoma. Evidence from sedimentary rocks, including geochemical data, show that extinctions were associated with a deepening in the outer part of the continental shelf in Nevada, possibly with upwelling of less-oxygenated waters, but there is little evidence for physical environmental change in interior sites like Oklahoma. This suggests that regional environmental change may have acted as a catalyst for the extinction by promoting ecological effects associated with immigration and changing geographic distributions.

Successions in Oklahoma and Nevada record trilobite extinction and replacement near the Steptoean–Sunwaptan boundary in inner-shelf and outer-shelf settings, respectively. Prior to the extinctions, different trilobite biofacies occupied these environments, but faunas became similar in composition across the environmental gradient in the overlying I. “major” and Taenicephalus zones. Faunal changes in the outer shelf at the I. “major” Zone begin at a drowning unconformity that brought dark, laminated calcisiltite and silty lime mudstone above a subtidal carbonate succession. In contrast, Oklahoma shows facies continuity in a succession of tidally influenced bioclastic carbonates. Loss of genera and a dramatic abundance “spike” of Irvingella are features of the I. “major” Zone in both regions. Turnover of biofacies occurred in the succeeding Taenicephalus Zone, with both the inner and outer shelf dominated by Orygmaspis (Parabolinoides). Blooms of orthid brachiopods in shallow water settings are underappreciated signals of faunal change in the extinction interval. Although absent from the outer shelf in Nevada, orthids became abundant enough in Oklahoma to form shell beds in the lower Taenicephalus Zone, but became rare in overlying strata. Carbon isotope stratigraphy includes a modest positive δ¹³C excursion that peaks in the extinction interval at 1.4‰ (Oklahoma) and 2.2‰ (Nevada), which is congruent with previous reports from Utah and Wyoming. Although consistent with regional upwelling of dysoxic waters, the absence of sedimentary evidence for significant environmental change over much of the shelf is problematic. This suggests that physical environmental change acted primarily as a catalyst for cascading ecological and biogeographic effects.

Walcottaspis vanhornei (Walcott, 1914) is a large, late Cambrian trilobite with a unique pygidial morphology known only from a narrow outcrop belt of the St. Lawrence Formation in the Upper Mississippi Valley. Found in carbonate-rich layers within heterolithic facies that represent the toesets of a prograding shoreface, it is restricted to a single or small number of parasequences. Only four specimens of any of its sclerites have been illustrated previously. Here holaspid examples of all its biomineralized sclerites are described and illustrated, along with a morphometric analysis of cranidial landmarks and landmarks plus semilandmarks for the pygidium. Ontogenetic allometry accounts for 29% of the variance among holaspid cranidia and includes a relative shortening and narrowing of the palpebral lobe and a reduction in the relative length and width of the frontal area. Notable pygidial phenotypic variation occurs in the extent of the postaxial region and in the proportion of the structure occupied by the axis. Phylogenetic analysis suggests that W. vanhornei is sister taxon to Dikelocephalus minnesotensis Owen, 1852, which also occurs in the St. Lawrence Formation and has a broadly similar cephalon but distinctive trunk. The holaspid pygidium of W. vanhornei is uniquely characterized by the interpleural furrows of the first two segments becoming abruptly obsolete on approaching the axial furrow. Some pygidia show narrow, shallow, flat-bottomed grooves etched into the internal sides of the dorsal surface or doublure that apparently represent infestation of the live trilobite.

Some trilobite species lived in deeper-water environments that are not well preserved in the fossil record. There are also intervals of time that are not well represented in some areas. This is the case for the earliest part of the Ordovician in Morocco. Because of this, any well-preserved trilobite specimens from these rocks are important for understanding how they relate to other parts of the world. They are also important for our understanding of migration and evolution of trilobites during this time. Here we present the first well-preserved trilobites of the family Olenidae from Morocco. Species from this family were often restricted to deeper or low-oxygen environments. This is the first discovery of olenid trilobites from Africa that are well preserved enough to be able to identify the species.

Here we describe the first articulated olenid trilobite specimens recovered from the lowermost Fezouata Shale Formation (lower Tremadocian, Ordovician) of Morocco. Prior to the discovery of this sample, only two partial olenid trilobite specimens had been found from this part of the rock record. The specimens are well preserved enough to confidently identify as Leptoplastides salteri (Callaway, 1877), extending the species geographic range from Avalonia into Gondwana. We argue that the Moroccan occurrences formerly referred to as “Beltella sp.” in the literature are likely to be those of L. salteri. This species is the only olenid trilobite known from African Gondwana.

During the Early Ordovician, much of the Laurentian paleocontinent was flooded by shallow, warm seas in which a great variety of endemic trilobites evolved, particularly those belonging to the family Bathyuridae. These have been collected and well studied from many localities across the Great Basin and adjacent areas. Deeper-water trilobites belonging to biofacies in peripheral sites around the carbonate-rich platform have a much poorer fossil record, largely because later tectonics have erased the appropriate sites. We describe here a rare example of a relatively low-diversity trilobite fauna of this kind recovered from the Al Rose Formation in the Inyo Mountains, California. Trilobites of the families Metagnostidae, Olenidae, and Raphiophoridae are well represented in this fauna, although they are rare or unknown from contemporary platform carbonates.

The Lower Ordovician (Floian) Al Rose Formation from the Inyo Mountains, California, is a deeper-water, graptolitic equivalent of the well-known and richly fossiliferous successions described from Utah and Nevada. It is considered to have been originally marginal to the Laurentian paleocontinent. It has yielded a low-diversity trilobite fauna that differs strikingly from contemporary faunas to the east in its abundance of raphiophorid, nileid, olenid, and agnostoid trilobites, resembling that of the Nileid Biofacies known from scattered locations marginal to Laurentia. Two new trilobite species are described: Globampyx sexsegmentatus (Raphiophoridae) and Protopresbynileus divergens (Nileidae). Carolinites genacinaca Ross, 1951 is a link with the Great Basin. Other trilobites include the olenid Cloacaspis cf. C. ceryx anataphra Fortey, 1974, metagnostid Geragnostus cf. G. (Novoagnostus) longicollis Raymond, 1925, and pliomerid Hintzeia sp.

Bathyurid trilobites are one of the most common shallow-marine groups of the Early Ordovician of Laurentia. During the Ordovician, Laurentian North America straddled the Equator, and the current western region formed the northern continental margin. Sampling of secondarily silicified trilobite faunas along this margin in the Great Basin has yielded a wealth of new information, including many dozens of new species of bathyurids. This new knowledge permits an assessment of currently understood natural evolutionary groups within the family. The genus Licnocephala, which in the past has been widely applied, has quite different morphology than has been attributed to it in previous literature. The type species is revised on the basis of new specimens and four new species (two well enough known to formally name) are illustrated. A related new genus, Ibexocephala, includes two new species with highly unusual cranidial morphology involving a sharp posterior change in angle in sagittal (midline) profile. All of the new and revised taxa are Tulean (late Tremadocian, Early Ordovician) in age. They were collected from the Fillmore Formation in western Utah and the Garden City Formation of southeastern Idaho.

Revision of the type species of the Early Ordovician (Tulean, late Tremadocian) bathyurid trilobite Licnocephala Ross, 1951 demonstrates that it has significantly different morphology than that ascribed to it in the earlier literature, which was based largely on species now assigned to a different genus. In addition to the type species, L. bicornuta Ross, 1951, which is fully revised on the basis of new material, four species, all apparently new, have been recovered, two of which, L. ngi n. sp. and L. bradleyi n. sp., are well enough known to formally name. The overall phylogenetic structure of bathyurids is yet to be determined, but several apparent clades can now be recognized and are discussed. Among these is what is termed the “Chapmanopyge group,” including Chapmanopyge Fortey and Bruton, 2013, Punka Fortey, 1979, Uromystrum Whittington, 1953, and Licnocephala. These genera are united in the occurrence of much of the anterior cephalic border on the librigenal anterior projection, with most of the anterior margin of the cranidium representing the suture, the possession of very short (exsag.) strap-like posterior cranidial projections, and extremely narrow visual surfaces. A fifth genus of the group, Ibexocephala n. gen., is represented by two new species, I. lossoae (type species) and I. dekosterae. The taxon features a remarkable cranidial morphology involving a strong deflection of the posteriormost part of the cranidium from the anterior part in sagittal profile.

The El Qaid Errami area in the Moroccan Anti-Atlas has become famous in the last 20 years for its spectacular fossil specimens from the Ordovician Period (ca. 485–443 million years ago). However, due to challenges of field access to the sites and the reluctance of Moroccan collectors to reveal precise locality information there is a mismatch between information provided informally through traders, social media, etc. and that available in the formal scientific literature. Verifying exact specimen provenance is particularly difficult, and all these issues hinder resolution of scientifically crucial information concerning phylogenetic lineages, the paleogeography, and faunal connections pertaining at the time of deposition. Here, we formally describe a famous trilobite association from this region of Morocco from the Upper Ordovician (ca. 450 Ma) that is dominated by a bizarre-looking trilobite, Declivolithus, which is the most conspicuous element of an assemblage occurring principally in the locality called Bofloss, in the Tizi n'Ounfite area. A decade of our work reveals a level of geological control impossible to obtain by studying materials collected by others, which allows us to properly geographically locate previous published data. We have thus increased the diversity of this assemblage from four to 11 species, including a new species: Ulugtella? biformis n. sp. A great opportunity offered by this work has been to study conspecific specimens preserved both in 3-D (in sandstones) and flattened (in mudstones), highlighting the importance of preservation of original morphology in taxonomic studies. With this revision, we not only clarify the identity of previously reported species of the Declivolithus Fauna from Morocco, but also increased diversity and demonstrated that the famous Declivolithus titan, well known among collectors, is a junior synonym of the type species D. alfredi erected in the Czech Republic. Links to the Czech assemblages remained strong during the Late Ordovician. These data help improve paleogeographical reconstructions of the Gondwana margin in this time period, as well as provide new information on several phylogenetic lineages endemic to the peri-Gondwana realm.

Intense commercial exploitation of fossils in the famous El Qaid Errami area in the last 20 years has led to the discovery of the interesting Declivolithus Fauna in the Moroccan Anti-Atlas. This unusually large trinucleid trilobite, described originally from the Czech Republic, is the most conspicuous element of an assemblage mainly occurring in the Bofloss locality, a local biofacies development of pelagic mudstones and sandstones cropping out in a structurally isolated place in the Tizi n'Ounfite area. Here we revise this Declivolithus Fauna trilobite assemblage from Morocco, increasing the known trilobite diversity from four to 11 species: Ulugtella? biformis n. sp., Selenopeltis cf. S. vultuosa, Phacopidina quadrata, Eudolatites cf. E. bondoni, Prionocheilus cf. P. verneuili, Nobiliasaphus cf. N. kumatox, Cyclopyge cf. C. rediviva, Symphysops stevaninae, Heterocyclopyge sp., Dionide sp., and Declivolithus alfredi. The new data and the very good preservation of specimens in sandstones, clarify the specific identity of previously reported taxa. Although the stratigraphical correlation of the fossiliferous levels remains problematic, it probably corresponds to the upper part of the Lower Ktaoua Formation or to the lower half of the Upper Tiouririne Formation. Most taxa support previous assignment of the Moroccan assemblage to the late Berounian (ca. early Katian, Ka2), although a middle Berounian (ca. Sa2–Ka1) age cannot be excluded. Most of the identified species are known from the Czech Republic (eight out of 11), showing that the strong faunal link between Morocco and the Czech Republic still existed during the Late Ordovician, being stronger than the link with the coeval Ibero-Armorican domain faunas.

After fossils form, their original shape can become distorted, which makes it hard for scientists to figure out what these ancient creatures really looked like in life. This problem affects many aspects of the study of ancient life forms, including understanding how such organisms moved or how they were related to each other. Scientists try to overcome these distortions by figuring out how the fossils got bent out of shape in the first place. Although we know a bit about how the Earth's tectonic movement can distort fossils, we are still learning about the details of how pressure affects the shape of organisms as they turn into fossils. In this study, we used three-dimensional computer simulations to see how fossils of ancient sea creatures called trilobites might have changed shape when they were buried and distorted over time. We found that trilobites on flat ground stayed more true to their original shape than those on uneven ground. Also, the way a trilobite was positioned when it was fossilized—whether it was right-side up or flipped over—made a big difference in how much its shape changed. This research helps us form a better picture of what trilobites and other ancient animals looked like in real life.

Shape deformation during fossilization can prevent accurate reconstruction of an organism's form during life, hampering areas of paleontology ranging from functional morphology to systematics. Retrodeformation attempts to restore the original shape of deformed fossil specimens and requires an adequate knowledge of the deformation process. Although tectonic processes and retrodeformation are relatively well understood, research on quantifying the effect of compressive deformation on fossil morphology is scant. Here we investigate the factors that can cause changes in the shape of fossil specimens during compressive deformation. Three-dimensional (3D) models of trilobite cranidia/cephala are subjected to simulated deposition and compaction using rigid body simulation and scaling features of the open-source 3D software Blender. The variation in pitch and roll angle is lowest on flat surfaces, intermediate on tilted surfaces, and highest on irregular surfaces. These trends are reflected in the morphological differences captured by principal component scores in geometric morphometric analyses using landmarks. In addition, the different shapes of trilobite cranidia/ cephala according to their systematic affinity influence the degree of angular variation, which in turn affects their posture —normal or inverted. Inverted cranidia/cephala show greater morphological variability than those with normal postures.

Harpetids and trinucleids are two different types of trilobite. They both shared an unusual body plan, with a wide, flat brim extending from the head. Scientists once thought this must mean they were closely related, but more recently they've instead assumed that these two groups evolved their matching brims independently. We wanted to find out which of these two ideas was correct and learn more about how those unusual brims actually evolved. To do this, we studied the fossils of harpetids, trinucleids, and their relatives, and built up a detailed family tree. Our tree showed that harpetids and trinucleids most likely evolved their brims separately. What's more, both seem to have evolved their brims in the same way, following the same steps in the same order. This makes these brims a perfect example of what's called “parallel evolution”. Our detailed family tree showed a few other interesting features as well. It suggested that trinucleids actually belong to a bigger group of trilobites called Asaphida, and that liostracinidid trilobites, which some people thought were an early kind of trinucleid, are actually more distantly related to their trinucleid cousins.

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.