A new nektaspid euarthropod from the Lower Ordovician strata of Morocco

International audienceNektaspids are Palaeozoic non-biomineralized euarthropods that were at the peak of their diversity during the Cambrian Period. Post-Cambrian nektaspids are a low-diversity group with only a few species described so far. Here we describe Tariccoia tazagurtensis , a new species of small-bodied nektaspid from the Lower Ordovician Fezouata Shale of Morocco. The new species differs from the type (and only other known) species from the Ordovician strata of Sardinia (Italy), Tariccoia arrusensis , in possessing more pointed genal angles, a cephalon with marginal rim, a pygidium with anterior margin curved forwards, a rounded posterior margin, and longer and more curved thoracic tergites. The two specimens of T. tazagurtensis sp. nov. show remains of digestive glands that are comparable to those seen in the Cambrian nektaspid Naraoia . The rare occurrence of T. tazagurtensis sp. nov. in the Fezouata Shale and the distribution of other liwiids suggest that these liwiids were originally minor members of open-marine communities during the Cambrian Period, and migrated into colder brackish or restricted seas during the Ordovician Period

Trace elements discriminate between tissues in highly weathered fossils

Palaeontologists assess the affinities of fossils using either morphology-based phylogenetic analyses, possibly enhanced by the use of advanced imaging techniques, or the identification of remnants or derivatives of fossil organic molecules with high taxonomic specificity (“biomarkers”). However, these approaches are often of little use for the majority of fossils whose original morphology and chemistry have been severely altered or completely lost during decay, diagenesis and modern weathering. Here we show that the inorganic incorporation of trace elements during fossilization and diagenesis can be used to assess the affinity of highly altered fossils, constituting a powerful tool overlooked so far. This is illustrated by the study of a wide range of animals from the Early Ordovician Fezouata Shale (Tremadocian, Morocco) using synchrotron X-ray fluorescence major-to-trace elemental mapping. Although all fossils studied here have turned into iron oxides, spectral analyses reveal that their different tissue types (i.e. biomineralised, sclerotised, cuticularised, and internal tissues) can be distinguished on the basis of their trace element inventories. The resulting elemental classes and distributions allowed us to identify an enigmatic, highly weathered organism as a new stem euarthropod preserving remains of its nervous system

Large trilobites in a stress-free Early Ordovician environment

International audienceUnderstanding variations in body-size is essential for deciphering the response of an organism to its surrounding environmental conditions and its ecological adaptations. In modern environments, large marine animals are mostly found in cold waters. However, numerous parameters can influence body size variations other than temperatures, such as oxygenation, nutrient availability, predation, or physical disturbances by storms. Here, we investigate trilobite size variations in the Lower Ordovician Fezouata Shale deposited in a cold water environment. Trilobite assemblages dominated by small-to normal-sized specimens that are few cm in length are found in proximal and intermediate settings, while those comprising larger taxa more than 20cm in length are found in the most distal environment of the Fezouata Shale. Drill core material from distal settings shows that sedimentary rocks hosting large trilobites preserved in-situ are extensively bioturbated with a high diversity of trace fossils, indicating that oxygen and nutrients were available in this environment. In intermediate and shallow settings, bioturbation is less extensive and shallower in depth. The rarity of storm events (minimal physical disturbance) and the lack of predators in deep environments in comparison to shallower settings would have also helped trilobites attain larger body sizes. This highly resolved spatial study investigating the effects of numerous biotic and abiotic parameters on body size has wider implications for the understanding of size fluctuations over geological time

Lower Ordovician synziphosurine reveals early euchelicerate diversity and evolution

Abstract Euchelicerata is a clade of arthropods comprising horseshoe crabs, scorpions, spiders, mites and ticks, as well as the extinct eurypterids (sea scorpions) and chasmataspidids. The understanding of the ground plans and relationships between these crown-group euchelicerates has benefited from the discovery of numerous fossils. However, little is known regarding the origin and early evolution of the euchelicerate body plan because the relationships between their Cambrian sister taxa and synziphosurines, a group of Silurian to Carboniferous stem euchelicerates with chelicerae and an unfused opisthosoma, remain poorly understood owing to the scarce fossil record of appendages. Here we describe a synziphosurine from the Lower Ordovician (ca. 478 Ma) Fezouata Shale of Morocco. This species possesses five biramous appendages with stenopodous exopods bearing setae in the prosoma and a fully expressed first tergite in the opisthosoma illuminating the ancestral anatomy of the group. Phylogenetic analyses recover this fossil as a member of the stem euchelicerate family Offacolidae, which is characterized by biramous prosomal appendages. Moreover, it also shares anatomical features with the Cambrian euarthropod Habelia optata, filling the anatomical gap between euchelicerates and Cambrian stem taxa, while also contributing to our understanding of the evolution of euchelicerate uniramous prosomal appendages and tagmosis

A revision of Prolimulus woodwardi Fritsch, 1899 with comparison to other highly paedomorphic belinurids

Xiphosurida is an ingroup of marine Euchelicerata often referred to as “living fossils”. However, this oxymoronic term is inapplicable for Paleozoic and early Mesozoic forms, as during these periods the group experienced notable evolutionary radiations; particularly the diverse late Palaeozoic clade Belinurina. Despite the iconic nature of the group, select species in this clade have been left undescribed in the light of recent geometric morphometric and phylogenetic considerations and methodologies. To this end, we re-describe Prolimulus woodwardi Fritsch, 1899 using new and type specimens to reveal more details on appendage anatomy and possible ecology. Furthermore, we present geometric morphometric and phylogenetic analyses that uncover relationships between P. woodwardi and other belinurids without genal spines. Both approaches highlight that a clade containing Prolimulus Fritsch, 1899, Liomesaspis Raymond, 1944, Alanops Racheboeuf, Vannier & Anderson, 2002 and Stilpnocephalus Selden, Simonetto & Marsiglio, 2019 may exist. While we do not erect a new group to contain these genera, we note that these genera exemplify the extreme limits of the Belinurina radiation and a peak in horseshoe crab diversity and disparity. This evidence also illustrates how changes in heterochronic timing are a key evolutionary phenomenon that can drive radiations among animals

Revision of “Bellinurus” carteri (Chelicerata: Xiphosura) from the Late Devonian of Pennsylvania, USA

Horseshoe crabs are an iconic group of marine chelicerates that have an impressive fossil record extending back to at least the Lower Ordovician. Despite their long fossil record and associated palaeontological interest, a range of fossil horseshoe crab taxa erected in the 19th and 20th centuries have remained understudied. Recent phylogenetic hypotheses have led to improvements in the understanding of xiphosuran origins and evolutionary history; however, the resolution among the basal-most Devonian-aged members remains poor. Here, the type specimen of “Bellinurus” carteri Eller, 1940 from the Late Devonian of Pennsylvania is reconsidered. Based on a revised morphological description and comparison, we conclude that the species is not referable to the genus Bellinurus and erected a new genus: Pickettia gen. nov. A phylogenetic analysis resolves Pickettia carteri within a polytomy containing taxa previously considered to comprise the group Kasibelinuridae, but which is currently a paraphyletic assemblage. We discuss P. carteri within the context of other stem xiphosurids and conclude that the diversity of this assemblage has been overstated. The redescription of P. carteri highlights the need for more inclusive studies to resolve the evolutionary relationships of stem xiphosurids

Taphonomic bias in exceptionally preserved biotas

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Soft tissue associations reveal the influence of preservation on biodiversity in Burgess Shale-type fossil deposits

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