Early fossil record of Euarthropoda and the Cambrian Explosion

Euarthropoda is one of the best-preserved fossil animal groups and has been the most diverse animal phylum for over 500 million years. Fossil Konservat-Lagerstätten, such as Burgess Shale-type deposits (BSTs), show the evolution of the euarthropod stem lineage during the Cambrian from 518 million years ago (Ma). The stem lineage includes nonbiomineralized groups, such as Radiodonta (e.g., Anomalocaris) that provide insight into the step-by-step construction of euarthropod morphology, including the exoskeleton, biramous limbs, segmentation, and cephalic structures. Trilobites are crown group euarthropods that appear in the fossil record at 521 Ma, before the stem lineage fossils, implying a ghost lineage that needs to be constrained. These constraints come from the trace fossil record, which show the first evidence for total group Euarthropoda (e.g., Cruziana, Rusophycus) at around 537 Ma. A deep Precambrian root to the euarthropod evolutionary lineage is disproven by a comparison of Ediacaran and Cambrian lagerstätten. BSTs from the latest Ediacaran Period (e.g., Miaohe biota, 550 Ma) are abundantly fossiliferous with algae but completely lack animals, which are also missing from other Ediacaran windows, such as phosphate deposits (e.g., Doushantuo, 560 Ma). This constrains the appearance of the euarthropod stem lineage to no older than 550 Ma. While each of the major types of fossil evidence (BSTs, trace fossils, and biomineralized preservation) have their limitations and are incomplete in different ways, when taken together they allow a coherent picture to emerge of the origin and subsequent radiation of total group Euarthropoda during the Cambrian

Sequence of post-moult exoskeleton hardening preserved in a trilobite mass moult assemblage from the Lower Ordovician Fezouata Konservat-Lagerstätte, Morocco

Euarthropods have a tough exoskeleton that provides crucial protection from predation and parasitism. However, this is restrictive to growth and must be periodically moulted. The moulting sequence is well-known from extant arthropods, consisting of: (i) the long inter-moult stage, in which no changes occur to the hardened exoskeleton; (ii) the pre-moult stage where the old exoskeleton is detached and the new one secreted; (iii) exuviation, when the old exoskeleton is moulted; and (iv) the post-moult stage during which the new exoskeleton starts as soft, thin, and partially compressed and gradually hardens to the robust exoskeleton of the inter-moult stage. Trilobite fossils typically consist of inter-moult carcasses or moulted exuviae, but specimens preserving the post-moult stage are rare. Here we describe nine specimens assigned to Symphysurus ebbestadi representing the first group of contemporaneous fossils collected that preserve all key stages of the moulting process in one taxon, including the post-moult stage. They were collected from a single lens in the Tremadocian part of the Fezouata Shale Formation, Morocco. Based on cephalic displacement and comparison to other trilobite moults, one specimen appears to represent a moulted exoskeleton. Four specimens are typical inter-moult carcasses. Four others are wrinkled and flattened, with thin exoskeletons compared to inter-moult specimens, and are considered post-moult individuals. These S. ebbestadi specimens illuminate the preservation and morphology of the post-moulting stage, characterised by strong anterior-posterior exoskeleton wrinkling, as well as overall body flattening and reduced visibility of thoracic articulations. Being found in the same lens, these specimens likely represent the first preserved in-the-act mass moulting event. The displayed sequence of moulting suggests the moulting process in trilobites was comparable to modern arthropods, and conserved within euarthropod evolutionary history

An exceptional record of Cambrian trilobite moulting behaviour preserved in the Emu Bay Shale, South Australia

Trilobites dominate the Emu Bay Shale (EBS) assemblage (Cambrian Series 2, Stage 4, South Australia) in terms of numbers, with Estaingia bilobata Pocock 1964 being extremely abundant, and the larger Redlichia takooensis Lu 1950 being common. Many specimens within the EBS represent complete moulted exoskeletons, which is unusual for Cambrian fossil deposits. The abundance of complete moults provides an excellent record that has allowed the recognition of various recurrent moult configurations for both species, enabling the inference of movement sequences required to produce such arrangements. Moult configurations of E. bilobata are characterised by slight displacement of the joined rostral plate and librigenae, often accompanied by detachment of the cranidium, suggesting ecdysis was achieved by anterior withdrawal via opening of the cephalic sutures. Moulting in R. takooensis often followed the same method, but configurations show greater displacement of cephalic sclerites, suggesting more vigorous movement by the animal during moulting. Both species also show rare examples of Salter’s configuration, with the entire cephalon anteriorly inverted, and several other unusual configurations. These results indicate that moulting in trilobites was a more variable process than originally thought. In contrast, other Cambrian Konservat-Lagerstätten with an abundance of trilobites (e.g., Wheeler Shale, USA, and Mount Stephen Trilobite Beds, Canada)show larger numbers of ‘axial shields’ and isolated sclerites, often interpreted as disarticulated exuviae. This points to a higher level of disturbance from factors such as animal activity, depositional processes, or water movement, compared to that of the EBS, where quiescent conditions and intermittent seafloor anoxia contributed to an unparalleled trilobite moulting record

Novel marrellomorph moulting behaviour preserved in the Lower Ordovician Fezouata Shale, Morocco

Exoskeleton moulting is the process of shedding the old exoskeleton to enable growth, development and repair, representing a crucial recurrent event in the life histories of all euarthropods. The fossil record of moulting allows us to interpret the evolution of this important behaviour and its impact on the evolutionary trajectories of extinct and extant euarthropods. Current knowledge of Palaeozoic euarthropod moulting relates largely to trilobites, with fewer examples known for non-mineralised extinct taxa from early in euarthropod evolutionary history. We describe exuviae from a marrellid marrellomorph found abundantly in the Early Ordovician Fezouata Shale Lagerstätte of Morocco, which allow a novel reconstruction, the second ever, of marrellid moulting behaviours. We identify the moulting suture location, describe preserved moulting assemblages, and suggest how its moulting behaviours are adaptive to its morphology. Several specimens represent complete and nearly complete assemblages and additional disarticulated specimens confirm the suture line location. The suture line is located between the mediolateral and posterolateral spine pairs, dividing the cephalic shield into anterior and posterior parts. The Fezouata marrellid likely exited the exoskeleton during exuviation using posterior and upwards movements, analogous in terms of movement to lobster-like extant arthropods. The suture line is comparable in the closely related marrellid Mimetaster, and distinctive from that of another marrellid, Marrella splendens, which has an exuvial opening at the anterior of the cephalic shield and exited the exoskeleton anteriorly. This difference in moulting behaviour as compared to Marrella is likely adaptive to the greater complexity of the Fezouata marrellid, with upwards rather than forwards movement presumably providing a more favourable angle for the extraction of complex spines. This description of the moulting behaviours and related morphological features of marrellomorphs expands our understanding of this crucial characteristic in extinct euarthropods

Taxonbridge: an R package to create custom taxonomies based on the NCBI and GBIF taxonomies

Biological taxonomies establish conventions by which researchers can catalogue and systematically compare their work using nomenclature such as species binomial names and reference identifiers. The ideal taxonomy is unambiguous and exhaustive; however, no such single taxonomy exists, partly due to continuous changes and contributions made to existing taxonomies. The degree to which a taxonomy is useful furthermore depends on context provided by such variables as the taxonomic neighbourhood of a species (e.g., selecting arthropod or vertebrate species) or the geological time frame of the study (e.g., selecting extinct versus extant species). Collating the most relevant taxonomic information from multiple taxonomies is hampered by arbitrarily defined identifiers, ambiguity in scientific names, as well as duplicated and erroneous entries. The goal of taxonbridge is to provide tools for merging the Global Biodiversity Information Facility (GBIF) Backbone Taxonomy and the United States National Center for Biotechnology Information (NCBI) Taxonomy in order to create consistent, deduplicated and disambiguated custom taxonomies that reference both extant and extinct species

The evolution of exoskeleton moulting in Trilobita

Moulting is the process of replacing an exoskeleton. The exoskeleton provides protection but must be periodically moulted for growth, development, and repair. Moulting is therefore intrinsic to ontogeny, but is risky and energetically-expensive, meaning the morphology and behaviours of an animal are also important. Ecdysozoa (e.g. Euarthropoda, Nematoida) are united by moulting, but Euarthropoda (e.g. insects, crustaceans) have particularly tough, usually reinforced, exoskeletons. Correspondingly they have a high preservation potential and exceptional fossil record. Amongst arthropods, the extinct trilobites were extraordinarily abundant and diverse, with an unparalleled fossil record. Both trilobite carcasses and moults are abundant, and when correctly identified these moults inform on many aspects of evolutionary history. Trilobites were both extremely inter- and intraspecifically variable in their moulting compared to other euarthropods. By examining trilobite moults we can reconstruct and contrast moulting behaviours to other groups to explore the ways in which moulting has affected trilobite evolutionary history. This thesis illustrates the importance of moulting to the evolution of trilobite, and euarthropod, life history. First, I provide a review of published information on trilobite moulting, and discuss the importance of accurate moult identification. I then present a broad-scale database of trilobite behavioural and morphological data, which is used to quantitatively answer âhow variable was trilobite moulting behaviour, and is this linked to differences in morphology?â Case studies then explore the detailed moulting information available from Konservat-Lagerstätten (focusing on the early Cambrian Emu Bay Shale); the association between moulting and morphology through trilobite ontogenetic history (by describing the development of Dalmanitina); and show that the process of moulting has been conserved throughout euarthropod evolution (by describing a complete moulting sequence of Symphysurus). The manuscripts constituting this thesis demonstrate the trilobite fossil record of moulting is an important source of information that can inform on the evolutionary history of development, behaviour, ecology, and morphology in this influential group.</p

Distinct causes underlie double-peaked trilobite morphological disparity

Appendices (all raw data, principal component landmarks, and statistical test results) and R code needed to replicate the result of Drage &amp; Pates (2024)