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ABSTRACT A comprehensive review of a full developmental sequence of eodiscinid trilobites reported in recent decades from Cambrian Series 2 and 3 strata is presented. These mostly articulated specimens exhibit detailed morphologies with preservation of even delicate structures at different growth stages, such as the bacculae, axial pores, spine pores, pygidial marginal spines and line coaptative structures. Their trunk segmentation schedules displayed a consistent developmental mode in segment generation and liberation, that is tagmosis and somitogenesis occurred heterochronously after each moulting event, providing clues regarding the potential developmental strategy in isopygous and even macropygous trilobites. The fact that the rate of segmentation obviously exceeds that of articulation results in a seemingly prolonged process of the formation of thoracic segments, which might explain why eodiscinid trilobites have a fixed and limited number of thoracic segments. In addition, the relationship between enrollment mechanism and trunk segmentation during eodiscinid ontogeny confirms this highly unusual growth pattern among the Trilobita, revealing why these early‐diverging trilobites controlled the rate of segment increase and release during their life cycles, and is thus of interest with regard to the evolution of arthropod body patterning. [ABSTRACT FROM AUTHOR]

Head segments play a critical role in revealing the relationships within major arthropod groups but the accurate determination of their number and relationship is challenging because of the decoupling of dorsal tergites and ventral appendages. In recent decades, trilobites have been considered to have four pairs of cephalic appendages and, commonly, five dorsal segments, which has served as the basis for understanding euarthropod cephalic evolution. Based on multiple analytical techniques applied to well‐preserved soft‐bodied specimens of the Upper Ordovician olenid Triarthrus eatoni and middle Cambrian corynexochid Olenoides serratus, we argue that an additional pair of cephalic biramous appendages occurred just behind the antennae, indicating that trilobites had five pairs of cephalic appendages and six dorsally expressed segments. We propose that the mismatch between the four dorsal furrows and five ventral appendages in Olenoides may be reconciled by considering the morphology of the hypostome that marks the position of the antenna and the bifurcation of the first glabellar furrow that has two muscle attachment sites fused. The fusion of clustered attachment sites of the cephalic biramous appendages may have aided feeding ability by gathering the limb bases close to the mouth. [ABSTRACT FROM AUTHOR]

Documentation of cryptic trilobite behavior has presented important insights into the paleoecology of this fully extinct arthropod group. One such example is the preservation of trilobites inside the remains of larger animals. To date, evidence for trilobites within cephalopods, gastropods, hyoliths, and other trilobites has been presented. Importantly, most of these interactions show trilobite molts, suggesting that trilobites used larger animals for protection during molting. To expand the record of molted trilobites within cephalopods, we present a unique case of a Toxochasmops vormsiensis trilobite within the body chamber of a Gorbyoceras textumaraneum nautiloid from the Upper Ordovician Kõrgessaare Formation of Estonia. By examining this material, we present new insights into the ecology of pterygometopid trilobites, highlighting how these forms used large cephalopods as areas to successfully molt. [ABSTRACT FROM AUTHOR]

Problems surrounding the identification and systematics of taxa belonging to the trilobite family Ellipsocephalidae have been discussed for several decades. The ellipsocephalid genus Strenuaeva is revised herein on the basis of material from Spain, including the type material of the identified species together with newly collected specimens from the Ossa-Morena Zone and the Iberian Chains. Two species are recognized as valid for these regions: S. sampelayoi and S. incondita. The species ' S. melendezi ' and ' Ellipsostrenua alanisiana ' from Spain, as well as ' S. marocana ' from Morocco, are considered junior synonyms of S. sampelayoi. Previous assignment of the Spanish species to Issafeniella is rejected. The abundant available material of S. sampelayoi made it possible to evaluate the taphonomical role in the preservation of some characters and to recognize intraspecific variability similar to that described in S. inflata from Baltica, reinforcing its assignment to Strenuaeva. Biostratigraphically, Strenuaeva ranges from uppermost Cambrian Stage 3 to uppermost Cambrian Stage 4. In Iberia, it is restricted to the middle Marianian in the Ossa-Morena Zone, whereas it is known from the lowermost middle Marianian to the lowermost upper Marianian in the Iberian Chains. Strenuaeva is known from Baltica (Scandinavia and Holy Cross Mountains, Poland), Iberia (Spain), Morocco and, possibly, western Avalonia (Newfoundland), hence being a characteristic genus of the Acadobaltic faunal Province. [ABSTRACT FROM AUTHOR]

The accurate interpretation of trilobite moulting behaviour relies on a comprehensive understanding of their moult configurations, yet the focus has commonly been limited to a brief description of the exuviae, and how differences in moulting behaviour further impact the preservation of exuviae is often ignored. This study investigates the configuration, style, and process of moulting in Oryctocephalus indicus through analysis of 88 exuviae collected from the Kaili Formation (Cambrian, Wuliuan) in Guizhou Province, South China. The moult configurations of O. indicus are typically characterised by the lower cephalic unit (LCU), which comprises the librigenae and rostral-hypostomal plate connected as a whole, detached from the cephalon and positioned anterior to the thoracopygon, while the cranidium is mostly absent. From detailed observation and description of the available material, we believe that O. indicus completes its moult through an exuvial gape formed by disarticulation of the facial sutures, rostral sutures and/or sutures of the cephalothoracic joints. Although many exuviae exhibited an opening at the cephalothoracic joint--disjunction of which is usually accompanied by disarticulation of both the facial and rostral sutures--the Salter's configuration produced by the 'Salterian' mode of moulting was not observed. Additionally, the structural characteristics of Henningsmoen's configuration, Harrington's configuration, and Somersault's configuration are discussed based on the exuviae of O. indicus, and Henningsmoen's configuration has been categorised into three types according to the different states of fossil preservation. In this article, apart from promoting further research on moulting behaviour in O. indicus, we also provide a supplement for moult configuration based on the exuviae, which offers new materials for studying moulting behaviour in oryctocephalid trilobites. [ABSTRACT FROM AUTHOR]

Trilobites from the middle Koumenzi Formation (Katian, Upper Ordovician) of the North Qilian Mountains, Menyuan, northeastern Qinghai Province are systematically documented for the first time. The fauna consists of five families, seven genera and seven species, one of which is new (Remopleurides zhangi), showing a close relationship to those of the Kazakh terranes (such as the Chu‐Ili terrane, Chingiz‐Tarbagatai area and the Karatau‐Naryn and North‐Tien Shan microcontinents), and the North China and Laurentia palaeoplates during the Katian (Late Ordovician). The cluster and non‐metric multidimensional scaling analyses of the Middle–Late Ordovician (late Darriwilian–Katian) trilobite faunas with 299 genera or subgenera from 46 horizons of 37 areas, provide valuable information for the palaeogeographical reconstruction of the Proto‐Tethys Archipelagic Ocean (PTAO) of this interval. The Qilian terrane and adjacent areas are essential components of the PTAO, some of which include the Qilian terrane, the North Qilian Mountains area, the Altun faulted terrane, the Hexi Corridor area and the East Qinling terrane. Their relative positions in the PTAO are inferred by the palaeobiogeography of trilobite faunas. Furthermore, based on discussions of the spatiotemporal distribution of those faunas, the Pliomerina and/or Sinocybele Province of the Middle–Late Ordovician (late Darriwilian–Katian) age is defined as a trilobite faunal province of the PTAO. Moreover, a distinct faunal subprovince, essentially comprising the South China Palaeoplate and its neighbours (e.g. Tarim, Annamia, Sibuma, East Qinling, Turkestan‐Alai and probably Talesh), might be surrounded by the equatorial cold‐water tongue. [ABSTRACT FROM AUTHOR]

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Trilobites were notably flexible in the moulting behaviours they employed, producing a variety of moult configurations preserved in the fossil record. Investigations seeking to explain this moulting variability and its potential impacts are few, despite abundant material being available for study. We present the first quantitative study on moulting in a single trilobite species using a dataset of almost 500 moult specimens of Estaingia bilobata from the Cambrian (Series 2, Stage 4) Emu Bay Shale, South Australia. Specimens were categorized by moulting mode (Salter's or Sutural Gape) and their associated configurations, and their body proportions measured from both a museum collection (including a bycatch sample) and a randomly‐collected field sample. This enabled analysis of the proportion of E. bilobata specimens displaying the Sutural Gape and Salter's modes of moulting and their different configurations, and tests for association between moulting behaviour and body proportions. The results show a wide range of E. bilobata moulting configurations in all samples, suggesting that configurations represent definable instances in a largely continuous spectrum of variation. Analyses comparing body proportions of specimens showing the two modes of moulting were non‐significant, suggesting there is no true association between moulting behaviour and body proportion, except for a single significant result for body length. All results were relatively consistent between the museum and field samples. However, removing accessioned specimens from the museum sample brought results even further in line with the field sample, supporting the need for consideration of museum collection bias in palaeontological analyses. [ABSTRACT FROM AUTHOR]

The evolution process can be reconstructed by tracking the changes in the dynamic characters of life cycles. A number of related trilobites from the Cambrian of South China provide additional information for the study of trilobite evolutionary patterns, which has been hampered by previous incomplete fossil record though. Here, Balangia and Duyunaspis represent related Cambrian oryctocephalid trilobites from South China, are comprehensively discussed over the ontogeny, and the results show that, from B. balangensis via D. duyunensis to D. jianheensis, their exoskeletal morphology shows a directional evolution. Based on the direction of evolutionary changes in the development of Balangia and Duyunaspis, we speculate that Duyunaspis likely evolved from Balangia instead of Balangia evolved from Duyunaspis, as was previously assumed. This inference is also supported by the phylogenetic tree. This research provides not only a better understanding of the mechanisms of evolution in trilobites, but also new insights for the relationship between developmental evolutionary changes and phylogeny in trilobites. [ABSTRACT FROM AUTHOR]

The outcrop of San José de Gracia yields an agnostid assemblage that has been assigned to the middle Cambrian. Although agnostid trilobites are important biostratigraphic markers, the mosaic evolution of this group makes species-level determination difficult, and thus biostratigraphic correlations become cumbersome. The assemblage is described here using morphogroups of the 'peronopsid'-like agnostids, and specimen-level phylogenetics is used to assess the statistical significance of assigning different ages to the final topology (stratigraphic fit), including the morphogroups as operational taxonomic units. A proper phylogenetic analysis requires considering modularity and ontogenetic development in agnostids, but the overall topology can respond to different stratigraphic calibrations even if the individual relationships are not resolved. Using the distribution of several stratigraphic fit metrics, namely SCI, GER and MSM*, we find that the biozone Plagiura-Poliella (509–598.4 Mya) is the one that offers the best stratigraphic fit for the outcrop of San José de Gracia, and it is consistent with the current biostratigraphic framework that has been built for the middle Cambrian of Sonora. [ABSTRACT FROM AUTHOR]

Trilobites are an abundant group of Palaeozoic marine euarthropods that appear abruptly in the fossil record c. 521 million years ago. Quantifying the development of morphological variation (or 'disparity') through time in fossil groups like trilobites is critical in understanding evolutionary radiations such as the Cambrian 'explosion'. Here, I use geometric morphometrics to quantify 'cumulative disparity' in functionally‐important structures within the trilobite cephalon across their initial radiation during Cambrian Series 2. Overall cephalic disparity increased rapidly and attained a maximum within several million years. This pattern is dominated by the cephalic outline (in particular the genal spines), reflecting rapid, convergent expansion to the extremes of morphospace in a few early families. In contrast, removing the outline and focusing on structures such as the glabella and eye ridges (associated with feeding and vision, respectively) showed a more gradual increase in disparity, closer in line with taxonomic diversity and supporting the hypothesis of a relatively accurate trilobite fossil record. These contrasting patterns suggest that disparity in different structures was constrained in different ways, with extrinsic (ecological) factors probably having the major impact on overall disparity. It also implies that patterns of disparity in isolated substructures cannot necessarily be taken individually as representative of overall morphologies. [ABSTRACT FROM AUTHOR]

The developmental mode of four redlichiid trilobites is summarized, based upon exceptionally well‐preserved, articulated specimens from Cambrian Series 2 (stages 3 and 4) strata in southwestern China and South Australia. These relatively complete developmental sequences indicate a balanced rate in segment increase and addition to the thorax during the meraspid phase, which might explain why most redlichiids possess micropygous body patterning during ontogeny. In addition, an analysis of the size distribution, developmental strategy, and distribution of specimen numbers at different growth stages reveals a distinct developmental strategy during the redlichiid life cycle. A relatively short pre‐holaspid and a prolonged holaspid phase in these redlichiid taxa offers insight into the developmental control and life strategy in these primitive arthropods. [ABSTRACT FROM AUTHOR]

The trilobite head served multiple functions and was composed of several fused segments. Yet, the underlying organization of the trilobite head, and whether patterns are conserved across trilobites, remains unclear. Modeling the head as being composed of modules, or subunits that vary and thus have the potential to evolve semi‐independently can reveal underlying patterns of organization. Hypotheses of modular organization based on the comparative developmental biology of arthropods were evaluated using geometric morphometrics. Two‐dimensional (semi)landmark datasets collected from the cranidia of two Ordovician trilobite species, Calyptaulax annulata (Phacopida) and Cloacaspis senilis (Olenida sensu Adrain, 2011) were analyzed. The degree and pattern of modularity were assessed using the covariance ratio (CR), which compares the covariation within putative modules to the covariation between them, and the fit of different models was compared using an effect size measure derived from the CR. When treating the eyes as a distinct module, the best modular hypothesis identified for C. annulata shows the eyes and anteriormost region of the head integrated as a single module. The best modular hypotheses for C. senilis are more complex but the eyes still covary mostly strongly with the anterior part of the head. The latter is also the case for all other well‐supported models for both species. These results can be interpreted as a developmental signal corresponding to the anteriormost ocular segment of early arthropods that is retained throughout development, despite any likely selective pressures related to functional needs. Research highlights: Degree and structure of modularity are identified in two trilobite species.In both species, the eyes and anteriormost head highly covary.Interpreted as a developmental signal corresponding to the ocular segment of early arthropods. [ABSTRACT FROM AUTHOR]

The ontogenetic sequence of an early Cambrian redlichiid trilobite Bathynotus kueichouensis is presented on the basis of numerous articulated specimens from Guizhou, China. The body regionalization of B. kueichouensis witnessed a wide variation in the morphology of trunk segments between different thoracic regions, referred to as the ‘thoracic region 1-4’. A greater size variation of these long pleural spines is recognized from the earlier meraspid degrees in a gradual appearance or disappearance until the holaspid period. The thoroughly inverse variation of these pleural spines in size and form is seen for the first time during the process of trilobite segment formation, which offers an opportunity to trace the evolutionary path of trilobite body structures, associated with their functional morphology and behavior. The ontogenetic strategy of B. kueichouensis is broadly similar to those of other redlichioid representatives where the full ontogenetic sequence exhibited a balanced rate in segment expression and liberation. Such a developmental mode may be the reason why most redlichiids possess a micropygous body patterning during ontogeny and provides insights into the controls of early arthropod developmental segmentation during evolutionary process. [ABSTRACT FROM AUTHOR]

This paper takes as its starting point an important conversation held between Thomas Hardy and Leslie Stephen in 1875 in which they discussed the "unreality of time" (Maitland 1906, 203). This idea informs Hardy's (1873) novel, A Pair of Blue Eyes, in which the amateur geologist Henry Knight hangs from a cliff and before him flashes a vision of deep time, which culminates in an eye-to-eye encounter with a trilobite. Several critics have argued for Leslie Stephen's "A Bad Five Minutes in the Alps" (1872), published just weeks before, as a source of inspiration for Hardy's scene. I investigate these claims, and argue that Gideon Mantell's Wonders of Geology (1848) was a greater influence. Drawing on the work of Patricia Ingham and Adelene Buckland, I consider the differences between these texts, and explore the implications of Hardy's removing the gaze of Mantell's "higher intelligence" and replacing his teleological view of time with a backwards slide down the evolutionary scale. This prefigured Thomas Henry Huxley's fear that with the cooling of the sun, humankind would eventually devolve into "low" and "simple" organisms "such as the Diatom of the arctic and antarctic ice" (1894, 191). This paper also draws on Hardy's copy of The Wonders of Geology, now in the Beinecke, and contested questions around its provenance. Finally, I conclude that this scene raises questions of scale that are never fully resolved and that these questions haunt the rest of Hardy's oeuvre. [ABSTRACT FROM AUTHOR]

The Devonian was a time of drastic environmental change that shaped the morphology of trilobites. This study aims to investigate their morphological evolution and to show the influence of some abiotic and biotic factors (bathymetry, feeding habits and visual abilities) on their shape. A dataset was compiled to investigate the shape of three structures (cephalon as a whole, central part of the cephalon, and pygidium) of Devonian trilobites from North Africa, using a geometric morphometric approach. Based on empirical morphospaces, the morphological changes were quantified through the Devonian stages. The results reveal important variations in the morphological disparity of the glabella shape, the facial suture location, the pygidial length and the presence of spines. In the Lochkovian, morphological disparity was low, subsequently increased in the Pragian with numerous innovations, and reached a maximum in the Emsian. If the morphospace occupancy remained until the Eifelian, a severe loss of disparity occurred in the Givetian, a time known for important environmental changes. Disparity then remained low in the Late Devonian. The shapes inherited from the Silurian persisted throughout the Devonian whereas Pragian novelties were most affected by losses. These persistent shapes were more versatile for environmental adaptation, helping those trilobites to survive the environmental events. Similarly, the trilobite orders that survived Devonian events had a wide morphological spectrum and were better adapted to withstand environmental change. [ABSTRACT FROM AUTHOR]

The study of the Ordovician of Paraná Basin culminated on the three-fold lithostratigraphic subdivision of the Rio Ivaí Group as follows: Alto Garças, Iapó and Vila Maria formations. The history of deposition of these rocks is linked to the transition from a marine fluvial environment into the glacial diamictites and shales with dropstones, overlain by post-glacial transgressive shales, siltstones and sandstones. The OrdovicianâSilurian transition is marked by a glacial and an extinction event that impacted the marine diversity of life and the permanence of the first land plants. At least three sections, designated as the sections 1, 2 and 3 below, had their sedimentary facies, taphonomy, organic carbon content and thermal maturation analysed as well as their macro- and microfossil assemblages recognized. All studied sections were productive for macro- and microfossils, although the section 1 has limited occurrence and lower preservation of palynomorphs. The greatest fossil diversity was recovered from the section 2. To date, the diversity recovered from the OrdovicianâSilurian of the Paraná Basin comprises 12 fossil groups, namely ostracods, brachiopods, bivalves, gastropods, cryptospores, fungi, acritarchs, chitinozoans, prasinophyte algae, scole­codonts, a possible graptolite and, more recently, a trilobite of the order Dalmanitidae. 51 species of palynomorphs of terrestrial and marine origin were recognized. This is the highest diversity reported from the glacialâpostglacial transition in the OrdovicianâSilurian boundary interval of Brazil. 18 species of cryptospores, acritarchs and fungi occur in the basal diamictites (the Iapó Formation) as well as the discinoid Kosoidea australis. In the upper part of these diamictites, the palynomorph assemblage comprises 26 taxa, most of which persist also in the postglacial shales. Still, in the shales with dropstones of the Iapó Formation, brachiopods (K. australis, infaunal lingulids, ?Palaeoglossa and rhynchonelliformeans), endemic ostracods such as Satiellina paranaensis and pyritized specimens of a widely common Hirnantian index species Harpabollia harparum occur together with indeterminable ostracod species. At least two different species of bivalve mollusks were also found as well as a gastropod species (Bucanellasp.). Observing the palynomorph assemblage, it was possible to record also chitinozoans restricted to the lowermost portion of the Vila Maria Formation. This part of the formation was observed in the outcrops 2 and 3 and contains postglacial chitinozoan assemblages that are not younger than the earliest Rhuddanian. Some centimeters above this interval but still in the lower part of the Vila Maria Formation, the occurrence of Spinachitina debbajae followed by Spinachitina silurica refer to the Silurian in the Paraná Basin. In the section 1, the recovery of a trilobite thorax configures the oldest record of this group in Brazil and shows that this ancient sea was also thriving with life even after the glaciation-related Hirnantian extinction event.

Trilobites are organized in packages; the adult trilobite body plan is composed of a cephalon (head), thorax (midsection), and pygidium (tail). These packages, or modules, are composed of traits that evolve semi-independently such that change in one module does not necessarily beget change in another module. At the macroevolutionary level, this decoupling and relaxation of evolutionary constraints is thought to promote evolvability. Thus, it is thought that modularity facilitates rapid diversification in diverse evolutionary directions, the hallmark of an adaptive radiation, as evolutionary rates among modules can vary along phylogenetic branches. Trilobites provide an unmatched fossil record, due to their biomineralized exoskeleton, to examine the long-term relationship between macroevolutionary diversification and modularity. However, the lack of a phylogenetic framework for major trilobite groups hampers the study of macroevolutionary questions. For instance, the trilobite family Pterygometopidae diversified during the Middle and Late Ordovician in Baltica, Avalonia, Laurentia, and Siberia. This group traditionally comprises four subfamilies with strong biogeographic signal including Pterygometopinae, Chasmopinae, Eomonorachinae, and Monokainae. However, relationships between and within subfamilies remain unresolved. Further, relationships with other families in the suborder Phacopina, especially with Phacopidae, remain unclear. To even begin tackling the relationship between macroevolution and modularity, phylogenetic relationships must be resolved in this trilobite group. To do so, we constructed a comprehensive character matrix comprising >240 characters including discrete, meristic, and continuous characters. Analyses include taxa from all 36 genera assigned to Pterygometopidae and include Ordovician exemplars from the trilobite families Diaphanometopidae, Phacopidae, Dalmanitidae, and Acastidae. We ran Bayesian phylogenetic analyses to produce trees that would co-estimate topology and evolutionary rates using the birth-death model. Further, we quantified the 3D morphology of the trilobite head using high-density geometric morphometrics for exemplar taxa within Pterygometopidae to identify the structure and degree of modularity of the trilobite head in this group. Future work will assess evolutionary rates for the trilobite head and, importantly, evolutionary rates of individual modules over the Ordovician Radiation to determine an increase or decrease in modularity over this diversification event.

A recent paper by Suárez & Esteve (2021) concludes that taxa with enrolment types not associated with coadaptive structures were less constrained in the shape of the cephalon and were thus able to achieve greater disparity. The analysis presented in their paper is limited to a single disparity index, the sum of variances. Consideration of this index in the context of changes in sample size and sum of ranges, a disparity index which reflects the overall breadth of morphospace occupation, refutes this interpretation. [ABSTRACT FROM AUTHOR]

Three new Lower Devonian dalmanitid trilobite taxa are recognized from the Talacasto Formation in the Precordillera Basin, Argentina, which includes two monospecific genera (Ivanites leonorae n. gen., n. sp. and Aguaditaspis mediaspina n. gen., n. sp.), and one additional new genus and species left in open nomenclature. These taxa combine characters of the subfamilies Dalmanitinae Vogdes, 1890 and Synphoriinae Delo, 1935, challenging their taxonomic distinction. Forty percent of the dalmanitids from high paleolatitudinal basins from southwestern Gondwana (Malvinokaffric) exhibit non-homologous dorsal spinosity. Aguaditaspis mediaspina n. gen., n. sp. shares similar spines with Trypaulites calypso (Hall, 1861). The unnamed new genus resembles the spinosity of endemic Malvinokaffric dalmanitids, like Dalmanitoides Delo, 1935 and Fenestraspis Braniša & Vaněk, 1973. Considered defensive, the recorded spinosity along with putative sublethal-attack marks, suggest higher predation pressure than previously thought at high paleolatitudes. This evidence is in accordance with some hypothesis on rising predation pressure in the mid-Paleozoic marine ecosystems. [ABSTRACT FROM AUTHOR]

Later Cambrian and earliest Ordovician trilobites and brachiopods spanning eight horizons from five localities within the Sông Mã, Hàm Rồng and Đông Sơn formations of the Thanh Hóa province of Việt Nam, constrain the age and faunal affinities of rocks within the Sông Đà terrane, one of several suture/fault-bounded units situated between South China to the north and Indochina to the south. 'Ghost-like' preservation in dolomite coupled with tectonic deformation leaves many of the fossils poorly preserved, and poor exposure precludes collecting within continuously exposed stratigraphic successions. Cambrian carbonate facies pass conformably into Lower Ordovician carbonate-rich strata that also include minor siliciclastic facies, and the recovered fauna spans several uppermost Cambrian and Lower Ordovician biozones. The fauna is of equatorial Gondwanan affinity, and comparable to that from South China, North China, Sibumasu and Australia. A new species of Miaolingian 'ptychopariid' trilobite, Kaotaia xuanensis, is described. Detrital zircon samples from Cambrian–Ordovician rocks of the North Việt Nam and Sông Đà terranes, and from Palaeozoic samples from the Trường Sơn sector of Indochina immediately to the south, contain a predominance of ages spanning the Neoproterozoic period and have a typical equatorial Gondwanan signature. We associate the Cambrian and Tremadocian of the Sông Đà terrane with areas immediately to the north of it, including the North Việt Nam terrane and the southern parts of Yunnan and Guangxi provinces of China. [ABSTRACT FROM AUTHOR]

A new olenid trilobite species, Peltura undulata n. sp., is described from the Furongian (upper Cambrian) Parabolina lobata Zone, Peltura Superzone, of the Scandinavian Alum Shale Formation (Cambrian provisional Stage 10). The new species is recorded from the Oslo Region, southern Norway, Västergötland and Öland, south-central Sweden, as well as from ice-rafted boulders found in Germany. Like most other representatives of Peltura, the new taxon is characterised by pygidial features. It thus has a pygidium with three pairs of swellings along the posterolateral margin instead of spines, and the margin outline is undulating to some degree. In some cases the swellings are developed as broad-based semitriangular projections. The new species, which is comparatively small, is restricted to the Pa. lobata Zone and accounts for up to 1 / 3 of the pelturid fauna, typically less. Remarks are given on the stratigraphic range of the co-occurring agnostoid Trilobagnostus holmi; it is inferred to be restricted to the Pa. lobata Zone. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Moulting behaviors in trilobites are a crucial strategy during development. Previous studies have demonstrated inter-and intraspecific variability of moulting behavior in trilobites. Currently, ecdysial motifs for trilobites are considered not stable even within species and fewer detailed studies dealt with moulting behaviors in a single species of trilobite during development. Here a large sample of meraspid to holaspid exuviae of Arthricocephalites xinzhaiheensis (131 specimens) from the Cambrian Balang Formation of South China has allowed description of the reasonably complete ontogenic moulting sequence. Both ontogenetic stage and body size reveal gradual transition of configuration from Somersault configuration to Henningsmoen’s configuration during development. Somersault configuration is exclusive till meraspid degree five and exists in subsequent growth stages. This suggests that opening of the facial and rostral sutures allowing the emergence forward of the post-ecdysial trilobite was prevalent in early growth stages. In later development, Henningsmoen’s configuration (showing disarticulation of the cranidium) became more dominant. This study indicates that gradual transition of ontogenetic moulting behavior occurred in oryctocephalid trilobites in the early Cambrian.

Queuing behaviour has been documented in marine arthropods from Cambrian to modern oceans. It was previously hypothesized that this behaviour provided energy savings through hydrodynamic drafting, with trilobites in following positions hypothesized to produce less drag than those leading. In this study, we evaluate the hydrodynamics of queuing behaviour in the Devonian trilobite Trimerocephalus chopini using computational fluid dynamics. The results show that the drag forces of the trilobites following in the queue were substantially lower than those produced by the leader (˜65–79% lower at velocities of 0.5–2 cm s−1). Drag reduction was positively correlated with the movement speed of the trilobites, but decreased with increasing distance from the leader, and there was essentially no drag reduction at all for the first following trilobite when the following distance was greater than about six times its body length. This agrees with fossil evidence preserving trilobites in queues in close proximity to each other. The results also show that drag reduction was still significant (˜86.8% at 2 cm s−1) even for the longest queues preserved in the fossil record. Our findings support the hypothesis that the queuing behaviour of trilobites was an adaptation for reducing hydrodynamic drag. This drag reduction effect compensated for the energy cost of movement, which would have been particularly advantageous during migration. [ABSTRACT FROM AUTHOR]

A new species of late middle Cambrian (Guzhangian: Miaolingian) ptychoparioid trilobite is described from the Devoncourt Limestone just west of the Burke River crossing on the Duchess to Cloncurry Road. This is an addition to the extensive study of the fossil fauna of that district in the Burke River Structural Belt and provides another link to the Georgina Basin fauna, albeit younger by one zone, about 200 km west on Mungerebar Station. Generic assignment of the new species is discussed in relation to a worldwide complex of similar solenopleurid genera with small eyes situated well forward on the cephalon. Some comments are made in relation to family placement of the species, but the question of family concepts relies heavily on revision of an extensive northern hemisphere literature on these standard trilobites; so, although placement in the Solenopleuridae is favoured, no formal family assignment is made until family groups in this large complex of genera are better defined. [ABSTRACT FROM AUTHOR]

This study presents a case in trunk regionalization and functional morphology of redlichiid trilobites, based on a globally distributed Bathynotus, collected from the base of the Kaili Formation in the late Cambrian Series 2 of China. This micropygous trilobite processed a body pattern with four types of thoracic tergites within two batches at maturity: a homonymous morphotype with similar thoracic tergite structure throughout the anterior portion of the thorax, and a heteronomous morphotype in the posterior portion of the thorax, with a pair of macropleurae in the 11th thoracic tergite and two pairs of tiny pleurae (micropleurae) in the 12th and 13th thoracic tergites. The unique morphology by regionalization in Bathynotusis considered closely related to its articulation structures and the ability to perform encapsulated enrolment, permitting the posterior trunk tergites with long pleural spines to fully cover the ventral cephalic shield during dorsal flexure. The regional integration of trunk tergites into structural regions provides the opportunity to trace the origin of their functional morphology and behavior in the evolutionary process of trilobite caudalization. [ABSTRACT FROM AUTHOR]

New, accurately located and well‐preserved agnostid trilobite material has been collected from the type locality of the Drumian (middle Cambrian, Miaolingian) Manuels River Formation, Newfoundland, Canada. The well‐exposed grey to black shales containing the fauna were deposited on the former microcontinent Avalonia. Four interval zones for the succession are proposed, namely, the Tomagnostus fissus, Hypagnostus parvifrons, Ptychagnostus atavus and Ptychagnostus punctuosus zones. The Tomagnostus fissus and Ptychagnostus atavus zones are distinct from each other, which leads to a significantly shorter and stratigraphically higher Ptychagnostus atavus Zone compared with other sections (Scandinavia, South China, Utah and Nevada, Greenland). Specimens of Ptychagnostus atavus, however, occur abundantly, with 107 specimens identified in a short interval of the succession. The Ptychagnostus punctuosus Zone can be correlated globally. The faunal assemblage is comparable to that of Scandinavia, Greenland and England. Ptychagnostus affinis is recorded for the first time from Avalonia. The 1408 collected specimens were assigned to the suborders Agnostina and Eodiscina and to the families Peronopsidae and Ptychagnostidae, and Condylopygidae and Eodiscidae, respectively, with the following species: Peronopsis fallax (Linnarsson), Peronopsis scutalis (Hicks), Hypagnostus parvifrons (Linnarsson), Ptychagnostus punctuosus (Angelin), Ptychagnostus affinis (Brøgger), Ptychagnostus atavus (Tullberg), Tomagnostus fissus (Lundgren), Tomagnostus perrugatus (Grönwall), Pleuroctenium granulatum (Barrande) and Eodiscus punctatus (Salter). [ABSTRACT FROM AUTHOR]

The Silurian–Devonian Boundary (SDB) was the first Global Stratotype Section and Point (GSSP) to be established. Extensive and well-preserved SDB strata are found in Western Europe and North America. Territory of China is composed of multiple plates and terranes, which were located in the equatorial and low-latitude regions of the northern hemisphere during the late Silurian to the Early Devonian. Due to the Kwangsian Orogeny, continuous Silurian-Devonian sequences are absent in the main parts of North China, Yangtze, and Tarim cratons in China. Accordingly, continuous stratal sections across the SDB have only been sporadically reported from the orogenic belts and southwestern margin of South China. The lack of continuous carbonate sediments during the Silurian-Devonian and scarcity of fossil markers obtained has long time limited for complex research on the SDB in China. However, the discovery of the conodont Caudicriodus woschmidti in several regions, and the widespread presence of encrinurid trilobites and crinoids, have allowed recognition of the SDB in China. Abundant volcanic material near the SDB facilitates high-precision chronological studies. Apart from the global δ 13C positive excursion and Icriodus Event above the SDB, most other events are only evident in certain regions or even in a single section. Therefore, there is an urgent need for more data and evidence from other areas and plates. This work summarizes research on the SDB in China from various tectonic settings, tectonic plates, and from different facies and regions, and provides a brief review of the research history, current state of art, achievements in study of the SDB in China, and outlines prospects for future research. [ABSTRACT FROM AUTHOR]

Arthropods (i.e. insects, spiders, crustaceans, myriapods and others), are the most successful Phanerozoic animals. The group is characterized by the possession of a segmented body, jointed limbs and a hard cuticle that is episodically moulted. One highly successful but now extinct group of arthropods is the trilobites. Trilobites underwent episodic moulting (ecdysis), and most trilobites possess facial sutures, lines of weakness in the cephalon, via which the exuviae is shed and the animal emerges. However, zones of weakness appear to represent a structural trade‐off or constraint, particularly during burrowing; sacrificing a consolidated head region useful in burrowing for the ability to moult. Here we reconcile this trade‐off by using biomechanical modelling to demonstrate that facial sutures exist in regions of low stress during the application of burrowing loads. Furthermore, facial sutures and the structure of the cephalon enable sutured trilobites to withstand greater stresses than their non‐suture counterparts. We suggest that this ability to withstand greater burrowing loads enabled trilobites to successfully invade bioturbated and more consolidated sediments of the Cambrian Sediment Revolution, thus facilitating their diversification in the Cambrian and Ordovician and contributing to the evolutionary success of this iconic arthropod group. [ABSTRACT FROM AUTHOR]

Trilobites offer almost unparalleled insight into the growth and development of fossil ecdysozoans. Here I use newly collected material of Elrathia kingii (Meek) to estimate growth rates and describe shape change over the ontogeny of E. kingii. Well‐preserved, articulated specimens from all post‐embryonic stages were collected from a 1.5 m interval of the upper Wheeler Formation (Miaolingian Series, Cambrian) in western Utah (USA), and size and landmark‐based shape data were digitized from photographs. Growth rates were estimated and compared with previously published data on the Silurian trilobite, Aulacopleura koninckii (Barrande). Like A. koninckii, the cephalic growth rate in E. kingii was constant and of similar magnitude to the minimum growth rate along the trunk, and growth rates in the trunk were lower during the holaspid ('adult') period than during the meraspid ('juvenile') period. However, body length at the onset of meraspis was smaller, the growth gradient along the trunk during meraspis was shallower, and the terminal number of thoracic tergites was smaller in E. kingii than in A. koninckii. Despite these differences, these two species had similar maximum body lengths, because higher overall growth rates in E. kingii compensated for other differences. The rate of cephalic shape change in E. kingii decreased at the transition from meraspis to holaspis, while the pygidium became more morphologically distinct from the thorax during holaspis. I also provide an emended diagnosis for E. kingii, descriptions of the ontogeny and ventral morphology, and evidence that E. kingii holaspids had an invariant number of tergites. [ABSTRACT FROM AUTHOR]

The biodiversity changes of trilobites in the middle Yangtze region of South China in the Great Ordovician Biodiversification Event and the end-Ordovician mass extinction have been analysed. The Ordovician trilobites in this region originated in the early Tremadocian and radiated from the late Floian to the early Katian. Two peaks of diversity appeared in the late Dapingian to the early Darriwilian and the early Katian. The extinction occurred in the late Katian, followed by a survival period in the Hirnantian. The Whiterock fauna began to surpass the Ibex fauna during the early Katian and were dominant by the late Katian to the Hirnantian, indicating that Cambrian Evolutionary Fauna was gradually replaced. The study once again confirms the view that the effects of factors such as sea level, climate, oxygen content, nutrient supply, and tectogenesis differ temporally and to different degrees in different regions, resulting in differences in the vertical sequence and horizontal combination of ecosystems, which is the reason for the different evolution patterns of Ordovician trilobites in different regions. [ABSTRACT FROM AUTHOR]

Background: The Svalbard Archipelago is commonly believed to have been located at comparable latitude and, possibly, to have been attached to Laurentia in the early Paleozoic (500-420 Ma) based on trilobite assemblage similarities. Trilobite assemblage differences and lack of mixing between Laurentia-Svalbard and Baltica were further used to propose that these continents were separated by the Iapetus Ocean at that time. However, recent structural correlation of Timanian (650-550 Ma) thrust systems throughout the Barents Sea show that Svalbard was already attached to Baltica in the latest Neoproterozoic and remained so during the Phanerozoic., Methods: The present study presents a new interpretation of seismic reflection data from the DISKOS database, which were tied to nearby exploration wells. The study uses recently acquired knowledge of the seismic facies of intensely deformed pre-Caledonian rocks and principles of seismic stratigraphy to interpret the data., Results: The present study reconciles the proximity of Svalbard and Laurentia with the early accretion of Svalbard to Baltica in the latest Neoproterozoic. It also describes the influence of Timanian thrust systems on paleoenvironments and possible effects on trilobite assemblages, e.g. , the lack of mixing between those of Laurentia-Svalbard and Baltica., Conclusions: The identification of elongate, emerged topographic highs in the Barents Sea and Svalbard in the late Neoproterozoic-early Paleozoic suggest that paleontological constraints should be considered with greater care when discussing continent separation since thrust systems may act as major faunal barriers within a single tectonic plate. Other factors to consider when discussing plate separation include paleoclimatic belts., Competing Interests: No competing interests were disclosed., (Copyright: © 2024 Koehl JBP.)

A trilobite pygidium, likely referable to the genus Bonnima, is the first evidence of a member of the Corynexochida reported from the Lower Cambrian (Dyeran Stage) Chambless Limestone of the southern Marble Mountains in the Mojave Desert of California. This specimen represents the first occurrence of the family Dorypygidae in the cratonic facies of the Lower Cambrian in the California-western Nevada region, as all of the few previous reports of the family (mostly Bonnia) have been from much thicker, more distal open-shelf deposits far to the northwest in the White-Inyo—Esmeralda County region of California and Nevada. Although still relatively rare, the occurrence of Dorypygidae across a range of environments biofacies realms in this area is typical of their distribution in other regions.

Ecdysis was a vital process during the lives of trilobites. In addition to preserving the morphological changes in trilobite ontogeny, the preservation of its action often captured interesting behavioral information. Abundant exuviae of Ovalocephalus tetrasulcatus are preserved in the Ordovician strata in central Hubei, China, and some of them are arranged with two or three together end to end or superimposed. The preserved patterns and burial conditions indicate that these specimens were caused by the active behavior of trilobites. It is speculated that these exuvial clusters were formed by two or three trilobites in line to molt; that is, after one trilobite finished molting, other trilobites molted in front of, behind, or overlying the previously molted shells. This ecdysis strategy is interpreted as related to the postulated herding behavior of some trilobites, representing a behavioral response of the trilobites to choose a nearby safe zone during some risky life activities.

The fossil record of arthropod compound eyes reflects different modes and occasions of eye reduction and blindness. In the best-studied fossil examples, the trilobites [trilobites: extinct arthropods, dominant during the Palaeozoic], which have an excellent geological record, eyes are primary structures, and in all known genera which lack them, eye-loss is always secondary. Once the eyes were lost, they never were never re-established. The most striking examples occur in the Upper Devonian, when two unrelated major groups of trilobites, with different kinds of eyes, underwent eye reduction and even total loss of the eyes over the same time period, undoubtedly due to long-term environmental change. One reason is that a mud blanket spread over a vast area, there was no firm substrate, and many trilobites became small and many became endobenthic, reducing or losing their eyes in the process. Toxic environmental conditions may also have had an effect. Certain coeval forms remained, however, which still possess perfectly good compound eyes. Either they found vacant refuges where they could survive, or alternatively their visual systems were elaborate enough to adapt to the changing conditions. Another inducement for evolving small, reduced compound eyes is to become a tiny organism oneself, with simply not enough space to establish a regular and functional compound eye, and in such minaturised eyes special adaptations for capturing enough photons are necessary. Thus very small compound eyes often establish wide acceptance angles of their ommatidia, collecting light over large angular ranges of space and it is beneficial to have a wide rhabdom provided that it is short, has a wide lens diameter, and perhaps even possess highly sensitive receptor cells. We find such a miniaturised system in the first recorded planktonic trilobite. Another kind of reduction of a compound eye, or parts of it, also occurs, if selective pressure claims for a high specialisation of eyes that results in several facets fusing into a single functional unit. This probably can be found in phacopid trilobites, ~400 million years old. Here the enlarged aperture of a resulting large lens may allow vision under dim light conditions such as at greater depth. The fossil record gives relatively little evidence about parasites, which often have reduced eyes. Agnostida are blind relatives of trilobites which lived during the Cambrian and Ordovician. An early suggestion was that some of these were parasitic, but this was never commonly adopted. Finally penstastomids (Crustacea), worm-like parasitic organisms, already have been blind from the Cambrian (~487Ma).

This report documents the new location of the representative Ordovician trilobite Asaphus expansus. This well-known trilobite was discovered in the "Storkowo" Mining Plant in the central part of the West Pomeranian Province in Poland. [ABSTRACT FROM AUTHOR]

Two research teams have different opinions regarding the number of paradoxidine species close to the base of the Miaolingian in Morocco and Spain. Geyer & Vincent (2015) recognized five species based on a mosaic of overlapping characters that include some relative length and width ratios, qualitative descriptions of cranidial and pygidial outlines, and relief observations. In contrast, Álvaro et al. (2018) conducted a morphometric analysis of all these specimens from the Cambropallas telesto acme level of the Assemame quarry and concluded that the Moroccan material had been taxonomically over-split and was represented by only two species, Acadoparadoxides mureroensis and A. nobilis. Subsequently, Geyer et al. (2019) commented that we had not analysed some diagnostic characters, and applied the stratigraphic setting of their morphospecies as a diagnostic character for taxonomic identification. After sampling the Cambropallas telesto acme level in the Taroucht quarry, where the other team purchased its collection, and re-analysing the diagnostic characters claimed by Geyer et al. (2019), our conclusions are maintained: (i) morphometric values of several paratype specimens fall outside the purported ranges of diagnostic characters; (ii) all 2D morphometric analyses incorporating the diagnostic characters of the morphospecies are unable to detect interspecific differences, except for A. nobilis; and (iii) the slight concavity displayed by some pygidia, strongly controlled by preservation, is in need of 3D biometrical analysis. Hence, our proposed synonymy should be maintained until 3D statistical analyses are available on material preserved in carbonate or concretions. [ABSTRACT FROM AUTHOR]

Recent reorganisation of the trilobite collections in the Natural History Museum, London UK, led to the discovery of two drawers of specimens from 'Arctic America'. These fragmentary specimens had remained unincorporated within the systematic trilobite collections. Original labels identified them as the H.W. Feilden Collection from the Arctic Expedition of 1875-6, led by Sir George Nares with HMS Alert and HMS Discovery. Some of the specimens were described but not illustrated by Robert Etheridge in 1878 but his determinations were approximate at best. The early Ordovician trilobites are an important historic collection, as the first collected from so far north. They are re-described and figured here. They have close affinities with species from East Greenland, Svalbard and western USA. A Silurian pygidium from western Greenland is identified as Ekwanoscutellum agmen. All indicate an origination in shallow carbonate facies of Laurentia. [ABSTRACT FROM AUTHOR]

The aim of this study is to analyze the assemblage of trilobites in the Silurian sequences of Iran by exploring stratigraphic section of the Silurian Niur Formation (Asadli section). Based on comprehensive field studies and laboratory investigations three Trilobite genera were identified in the study area including Calymene blumenbachii, Gravicalymene convolva, Dalmanites brevigladiolus and Calymene sp. according to this trilobite assemblage a Rhuddanian to Telychian stages (Llandovery) is suggested for deposition of the study strata. The recognized trilobite fauna will be used as a base for future geological studies on the Silurian strata in Kopeh-Dagh Basin. This is the first report of trilobites from the Silurian (Llandovery) Niur Formation in Asadli section, north- east, Iran. [ABSTRACT FROM AUTHOR]

The morphology and trunk development of the trilobite Arthricocephalus chauveaui described herein are based on 216 articulated specimens collected from the Lazizhai section of the Balang Formation (Stage 4, Series 2 of the Cambrian) in Jianhe County, Guizhou Province, South China. The new specimens exhibit a relatively complete ontogenetic sequence ranging from the protaspid to holaspid periods and reveal more morphological changes of the species in detail. For example, from degrees 0 to 4, the facial sutures are proparian and then gradually move backward from degree 5, until in the holaspid period, the suture becomes gonatoparian. Based on both linear measurements, we investigate here the morphological variation of some cranial traits throughout ontogeny. Our results demonstrate that the allometric growth pattern shows continuous variations, some of which are related to trunk development. By exploring the trunk segmentation in A.chauveaui, the developmental mode shows that somitogenesis and tagmosis may have developed independently at each meraspid degree beyond degree 0. The holaspid period is consistent with the beginning of the epimorphic phase. [ABSTRACT FROM AUTHOR]

Early to Middle Cambrian successions exposed at the Shirgesht area, Tabas Block of central Iran has yielded a high diversity and abundant ichnofossils with arthropod trackways. These trace fossils contain diverse ethological patterns, including furrowing or crawling (Cruziana), resting (Rusophycus), walking (Diplichnites), sideways grazing (Dimorphichnus) and swimming grazing (Monomorphichnus). The assemblage of trace fossils reported from the Early to Middle Cambrian of the Tabas Block, has a significant stratigraphic importance as they provide evidence regarding the developmental patterns during the early phase of life on earth. Stratigraphically, trace fossils studies is significant for defining the age of the unfossiliferous Cambrian successions of the Tabas Block from Central Iran where typical diagnostic trilobite fauna is absent. According to Cruziana ichnostratigraphy and comparsion the Lalun ichnofauna with other examples from similar age, the present assemblage could represent the late Early to Middle Cambrian. The distribution of trace fossils in the Early to Middle Cambrian successions reflects the paleoecological conditions including the availability of oxygen and nutrient influx. Hwever, the diversity and abundance of infauna, smaller trace fossil size, sporadic distributions of burrowing and lower intensities of bioturbation of trace fossil suite and the presences of evidence of predominance of trilobite trace fossils with morphologically simple represents a stressed expression of the ichnofacies and ichnologically stressed environment during deposition such as estuary.

Documentation of cryptic trilobite behavior has presented important insights into the paleoecology of this fully extinct arthropod group. One such example is the preservation of trilobites inside the remains of larger animals. To date, evidence for trilobites within cephalopods, gastropods, hyoliths, and other trilobites has been presented. Importantly, most of these interactions show trilobite molts, suggesting that trilobites used larger animals for protection during molting. To expand the record of molted trilobites within cephalopods, we present a unique case of a Toxochasmops vormsiensis trilobite within the body chamber of a Gorbyoceras textumaraneum nautiloid from the Upper Ordovician Kõrgessaare Formation of Estonia. By examining this material, we present new insights into the ecology of pterygometopid trilobites, highlighting how these forms used large cephalopods as areas to successfully molt., (© 2024. The Author(s).)

Modularity and integration are variational properties expressed at various levels of the biological hierarchy. Mismatches among these levels, for example developmental modules that are integrated in a functional unit, could be informative of how evolutionary processes and trade‐offs have shaped organismal morphologies as well as clade diversification. In the present study, we explored the full, integrated and modular spaces of two developmental modules in phacopid trilobites, the cephalon and the pygidium, and highlight some differences among them. Such contrasts reveal firstly that evolutionary processes operating in the modular spaces are stronger in the cephalon, probably due to a complex regime of selection related to the numerous functions ensured by this module. Secondly, we demonstrate that the same pattern of covariation is shared among species, which also differentiate along this common functional integration. This common pattern might be the result of stabilizing selection acting on the enrolment and implying a coordinate variation between the cephalon and the pygidium in a certain direction of the morphospace. Finally, we noticed that Austerops legrandi differs slightly from other species in that its integration is partly restructured in the way the two modules interact. Such a divergence can result from the involvement of the cephalon in several vital functions that may have constrained the response of the features involved in enrolment and reorganized the covariation of the pygidium with the cephalon. Therefore, it is possible that important evolutionary trade‐offs between enrolment and other functions on the cephalon might have partly shaped the diversification of trilobites. [ABSTRACT FROM AUTHOR]