The mass extinction characterizing the Permian/Triassic boundary (PTB; ~ 252 Ma) corresponds to a major faunal shift between the Palaeozoic and the Modern evolutionary fauna. The temporal, spatial, environmental, and ecological dynamics of the associated biotic recovery remain highly debated, partly due to the scarce, or poorly-known, Early Triassic fossil record. Recently, an exceptionally complex ecosystem dated from immediately after the Smithian/Spathian boundary (~ 3 myr after the PTB) was reported: the Paris Biota (Idaho, USA). However, the spatiotemporal representativeness of this unique assemblage remained questionable as it was hitherto only reported from a single site. Here we describe three new exceptionally diverse assemblages of the same age as the Paris Biota, and a fourth younger one. They are located in Idaho and Nevada, and are taxonomic subsets of the Paris Biota. We show that the latter covered a region-wide area and persisted at least partially throughout the Spathian. The presence of a well-established marine fauna such as the Paris Biota, as soon as the early Spathian, indicates that the post-PTB biotic recovery and the installation of complex ecosystems probably took place earlier than often assumed, at least at a regional scale.

No abstract

No abstract

Known from at least the Silurian to the Cretaceous, Thylacocephala is an enigmatic fossil euarthropod ingroup, often allied with Pancrustacea. Previous studies show that thylacocephalans are characterized by a folded protective shield, hypertrophied compound eyes, three pairs of raptorial appendages, a posterior trunk comprised of eight to 22 segments bearing appendages, and eight pairs of gills. Despite this knowledge of their anatomy, many questions remain, especially surrounding the anatomy of Paleozoic representatives.The Upper Devonian Woodford Shale (upper Famennian, Oklahoma, USA) has yielded several fossil euarthropods, including two species of Thylacocephala: Concavicaris elytroides and Concavicaris woodfordi. Here, we use micro-computed Xray tomography to re-explore the anatomy of the holotype of C. woodfordi, illustrating fine details of the shield structure, of the circulatory, digestive and reproductive systems, and of the appendages. A marginal fold of the shield as well as an inner layer are described for the first time in a thylacocephalan. Concavicaris woodfordi shares similarities with Concavicaris submarinus, another Famennian species, including the morphology of the shield and the internal anatomy. It also displays a similar organisation as Mesozoic taxa, such as Dollocaris ingens. All of this provides important information that will be crucial to reconstruct the evolution and the affinities of Thylacocephala.

Thylacocephala is an enigmatic ingroup of Euarthropoda. Thylacocephalans, only known from Palaeozoic and Mesozoic fossils, are characterized by a particular anatomy: a prominent folded shield enveloping most of the body, large compound eyes, three pairs of large, presumably raptorial appendages and a trunk with 8-22 stout segments bearing swimming appendages. However, lifestyle(s) and phylogenetic relationships of Thylacocephala are still largely unknown. This study is focused on thylacocephalans from the Mazon Creek Lagerstätte (c. 307 Ma, Middle Pennsylvanian, Carboniferous). A new species is described from a siderite concretion of Mazon Creek: Eodollocaris keithflinti n. gen., n. sp. The new species displays a particular mixture of characters typical for Palaeozoic species on the one hand and Mesozoic species on the other hand. Additionally, new details of the appendages and of the trunk are provided for already known species from the same locality, namely Concavicaris georgeorum Schram, 1990 and Convexicaris mazonensis Schram, 1990. These new informations are helpful to better understand the possible lifestyle of these representatives of Thylacocephala.