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Encrinurids are common in Ordovician and Silurian strata but whether they survived into the Early Devonian is still controversial. This paper documents the encrinurid Batocara sp. near the Silurian–Devonian boundary in western Junggar, Xinjiang. The highest horizon of Batocara sp. is located above the first appearance datum of the Devonian conodont Caudicriodus, confirming that encrinurids may cross the Silurian–Devonian boundary. The presence of Caudicriodus angustoides bidentatus, Zieglerodina planilingu and plate-type loboliths of scyphocrinoids above the highest horizon of Batocara sp. indicates that encrinurids here extend only into the lower part of the first conodont zone of the Lochkovian (i.e., Caudicriodus hesperius Biozone). Encrinurids are widely distributed and easily recognized, and unlike graptolites and conodonts are not controlled by lithofacies. Therefore, it might be possible to use the highest horizon of encrinurids as indicator fossils to identify the approximate position of the Silurian–Devonian boundary in areas or sections where graptolites and conodonts are not present, and at least in northwest China.

Phacopid trilobites are well documented during the Paleozoic. Nevertheless, while 2D quantitative analyses have advanced our understanding of the morphological relationships among trilobites, the quantification of their morphological traits in 3D remains rarely documented. Based on two sets of morphological data (head and tail), 2D versus 3D shape quantification approaches were used to explore shape allometries as well as to explore how the shape variations can be explained by the phylogenetic relationships among phacopid trilobite species for the first time. We demonstrate that (1) there are similar patterns of morphological variability across taxa in 3D and 2D; (2) there are rather congruent results between 3D and 2D to discriminate taxa; (3) 2D and 3D landmarks capture different levels of detail, and the third dimension in 3D is very important for making taxonomic distinctions at the genus level; (4) there is congruity between 2D and 3D datasets for allometric patterns with results showing similar allometric slopes among species exhibiting a glabellar length decrease during growth leading to wider cephala; (5) the phylomorphospaces show tree branches that do not intersect, suggesting possible phylogenetic constraints on morphospace occupation for each species and supporting the idea that the Austerops and Morocops groups are sister clades that experienced different modes of morphological evolution; and (6) the morphological descriptors in morphometric analyses in 2D and 3D throughout phacopid evolution are effective.

The Asteropyginae Delo, 1935 is a group of phacopid trilobites in the family Acastidae Delo, 1935 that has served as the focus for several studies due to their distinctive morphologies and diversity. However, despite an interest in these characteristic morphologies, there have been no studies that have examined this group using morphometric techniques. Our investigation utilized both geometric morphometric and elliptical Fourier methods to quantify the morphology of cephalic sclerites of asteropyginid specimens representing wide taxonomic sampling of the clade. We constructed a phylomorphospace that shows temporal and spatial patterns of phenotypic evolution within the framework of a novel tip-dated phylogenetic tree generated using Bayesian inference. We recovered similar patterns in disparity regardless of the morphometric approach. Both analyses illustrated a marked expansion into morphospace throughout the temporal range of the clade, peaking in disparity in the Emsian and with European taxa exhibiting the highest disparity in glabellar morphospace. Additionally, glabellar shape showed low phylogenetic signal and no major patterns in phylomorphospace. This study highlights the utility of employing different methodologies to quantitatively explore the disparity of fossil taxa. It also illustrates some of the patterns of morphological change occurring during one of the final and major evolutionary radiations within Phacopida.

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.

The suborder Phacopina, characterized by schizochroal eyes, is among the most common groups of trilobites in Devonian strata. The marine sediments of the Famennian in western Junggar, Xinjiang, contain abundant low-disparity phacopids, which have previously been designated to Omegops accipitrinus mobilis, Phacops circumspectans tuberculosus, and Omegops cornelius on the basis of small numbers of poorly preserved specimens. In this study, these phacopids were identified as two new species of Omegops, O. honggulelengensis n. sp. and O. xiangi n. sp., on the basis of nearly 200 well-preserved specimens. The intraspecific variations of eye lenses of these specimens were quantitatively analyzed. On the basis of differences in the total number, number of dorsoventral files, and arrangement of the eye lenses, the absence of lenses in the middle part of the visual surface, and asymmetry of the number and/or arrangement of lenses in the two eyes, it was concluded that the reasons for intraspecific variation in eye lenses of Late Devonian Omegops from western Junggar were different from previously described factors but were likely genetic or embryological malfunctions or abnormalities caused by pathological conditions. Diversity of lenses in the schizochroal eyes shows that the number and arrangement of eye lenses was not stable in Phacopina. Therefore, many specimens are needed for quantitative study to determine the true characteristics of the number or arrangement of eye lenses when these features are used in the systematic taxonomy of Phacopina. UUID: http://zoobank.org/6bd14390-05fe-45ad-8eeb-3bf397a46a68

In 1912, the Field Museum purchased Henry Pratten's collection of fossils. One of the specimens was a dalmanitid trilobite with large eyes (Fig. 1) that Pratten collected from the Devil’s Backbone. In 1933, this specimen was described as a new species of trilobite, Dalmanites pratteni (P 16704) (Roy 1933). Delo (1935) described a new genus of trilobite, Dalmanitoides and assigned it to a new family, Synphorinae. He noted Dalmanites pratteni has similar ornamentation to this genus. Delo (1940) later places D. pratteni in the genus Odontochile . Other authors refer to it as Dalmanites pratteni (Clarkson and Levi-Setti 1975;Levi-Setti 1975; Frankie et al. 2008). Holloway and De Carvalho (2009) refer to it as ‘Dalmanites’ pratteni and state that this species “probably belongs to the Synphoriinae”. The stratigraphic position of this specimen was not recorded. Seid et al. (2009) published a detailed geologic map of the Devil’s Backbone. Most of the ridge is Middle Devonian Grand Tower and St. Laurent formations. However, the southern end of the ridge is faulted, and the Lower Devonian Backbone Limestone and Crystal Creek Chert (Emsian) is exposed. Frankie et al. (2008) and Devera et al. (2020) report Dalmanites pratteni occurring from the Crystal Creek Chert, indicating that the original specimen may have been collected here. One of the most remarkable features of this fossil is the record number of lenses for a Phacopida. Roy (1933) reports 42 rows of eyes, however: “The exact number of lenses cannot be determined, as not all of them are preserved, nor are the areas which they occupied distinctly outlined. It is, however, certain that there are not less than 770 lenses on each eye.” (Roy 1933). I used images of the eyes and illustration software to estimate the number of lenses by filling in the geometric pattern they exhibit. I counted 730 lenses on the right eye and 712 lenses on left eye, which is missing a sizable piece (Fig. 2). The structure of some individual lenses in this specimen can be seen due to differential weathering. Clarkson and Levi-Setti (1975) reported the large round lenses were composed of a doublet structure that followed the principles established by Descartes (1637) and Huygens (1690) to eliminate spherical distortion. This doublet structure allowed the lens to gather more light and focus it on a single point, and strongly indicates that these trilobites had better vision then previously established (Clarkson and Levi-Setti 1975).

A large sample of postembryonic specimens of Dalmanitina proaeva elfrida and D. socialis from the Upper Ordovician (Sandbian to Katian) Prague Basin allows for the first reasonably complete ontogenetic sequence of Dalmanitoidea (Phacopina). The material provides an abundance of morphological information, including well-preserved marginal spines in protaspides and meraspides, and hypostome external surfaces throughout. The development of D. proaeva elfrida is unusual due to variability in timing of the first trunk articulation. This broadens our developmental understanding of Phacopina, a diverse group of phacopid trilobites, and also allows us to study the evolution of their specializations in exoskeletal molting behavior. Adult phacopines, unlike most other trilobites, had fused facial sutures. This means that rather than molting through the sutural gape mode, characterized by opening of the facial sutures and separation of the librigenae, they disarticulated the entire cephalon in Salter’s mode of molting. For other phacopine clades (Phacopoidea) the transition to Salter’s mode occurs during the meraspid period or at the onset of holaspis, and its developmental timing is intraspecifically fixed. However, owing to the large sample size, we can see that facial suture fusion likely occurred later in Dalmanitina, usually during the holaspid period, and was intraspecifically variable with holaspides of varying sizes showing unfused sutures. Further, D. proaeva elfrida specimens showed an initial librigenal–rostral plate fusion event, where the librigenae began as separate entities but appear fused with the rostral plate as one structure (the “lower cephalic unit”) from M1, and are discarded as such during molting. Dalmanitoidea is considered to represent the first phacopine divergence, occurring earliest in the fossil record. This material therefore provides insight into how linked morphologies and behaviors evolved, potentially suggesting the timing of facial suture fusion in Phacopina moved earlier during development and became more intraspecifically fixed over geological time.

Major transitions in trilobite ontogeny have historically been defined based on the number and distribution of trunk segments, and articulation between the trunk and cephalon. This study documents additional morphological change across the meraspid-holaspid transition on the Ordovician phacopid trilobite Calyptaulax strasburgensis. An extensive dataset of silicified cranidia and pygidia collected from the mid-Ordovician Edinburg Formation of Virginia was subjected to a series of multivariate analyses, with a primary focus on the intersections and termini of furrows. Multivariate regression of partial warp scores demonstrates statistically significant change in allometric growth patterns over the course of development. These changes are concentrated in earlier instars, but are coincident in cranidia and pygidia. This sharp decrease in the rate of allometry, present in both tagmata, is expressed as significant breakpoints derived from a segmented regression, with the largest portion of allometric change found in the pre-breakpoint individuals. The term holeidos is proposed to describe the completion of form during trilobite development, independent of the completion of the thorax. The most dramatic change in shape during this period of ontogeny includes lateral glabellar expansion through deflection of the axial and palpebral furrows, possibly reflecting a change in the feeding habit during later development. Other morphological changes include the development of a more angular appearance to the anterior portion of the glabella, and anterior migration of the pygidial anterior margin. The appearance of these growth patterns in Calyptaulax extends the temporal range of these changes, some of which have only been documented in Devonian phacopids.

Trilobites are important elements of the Devonian macrobenthos; some of them were collected in the Chefar el Ahmar Formation, from two sections located near Beni Abbes in the Saoura Valley (Ougarta Basin, Saharan Algeria). This formation is characterized by alternations of claystones and limestones, and it is considered to be late Emsian to early Frasnian in age. Only the lower part of this formation has yielded trilobites so far; their presence has been known for a long time. Phacopines clearly dominate the trilobite assemblages, with Austerops, Barrandeops, Chotecops and Phacops s.l. as the main genera. Two new species are described (Austerops salamandaroides sp. nov. and Phacops ouarouroutensis sp. nov.), while some other taxa are presented in open nomenclature. Comparisons are made with closely allied species. These new trilobite occurrences have been analysed in terms of their intra- and interspecific variability and biodiversity. The occurrence of Struveaspis maroccanica, previously known from the Saoura Valley, provides an early Eifelian age, which is also confirmed by the presence of trilobites Thysanopeltis and Koneprusites, and ostracods Bairdiocypris devonica and Bufina ?subovalis.

Trilobites are widespread in Lower Devonian deposits of north Gondwana, and some have been collected from two known sections of the Saoura Valley in SW Algeria, from the ‘Chefar el Ahmar’ Formation. This formation is considered to be from late Emsian to Frasnian in age, but only the lower parts of this formation have yielded trilobites. Nevertheless, no detailed studies have focused on their biodiversity and their morphological variability. New occurrences of phacopids including Barrandeops chattertoni sp. nov., Geesops fabrei sp. nov., Austerops legrandi sp. nov. and Phacops boudjemaai sp. nov. are described from this area and comparisons are made with closely allied species. These new occurrences have been integrated into analyses of intra- and inter-specific variability and biodiversity.