Over 99% of all species that ever existed are now extinct, and the 99%, the fossil record must becritically interrogated to bring palaeontology to the forefront of a modern synthesis on evolutionary studies. The animalfossil record holds a staggering wealth of information on the radiations and extinctions that shaped the morphologicaldiversity of modern ecosystems. Two such radiations, the Cambrian Explosion and the Ordovician Radiation, occurredat the early stages of animal evolution, and established the overall organisation of animal body plans that persist intothe modern day. Yet the interplay between these two events is the subject of debate, with no clear answer as to howthey are related, and why there are such large differences in the type, diversity and abundances of taxa between them.Using exceptionally preserved arthropod fossils of the Fezouata Biota, this project starts to address the fundamental question:What evolutionary dynamics govern major faunal turnovers, such as that seen in the transition from theCambrian Explosion to the Ordovician Radiation? The Fezouata Biota is a recently discovered locality in Morocco that produces exceptionally preserved, openmarine, soft-bodied fossils and mineralised taxa. Its early Ordovician age places it exactly between the CambrianExplosion and Ordovician Radiation evolutionary events, and it is unique in the world for preserving the typicalelements of both animal communities together. It is ideally suited to provide the crucial data needed to understand thedynamics of faunal turnovers, and this project interrogates the Fezouata Biota fossil record by addressing three mainresearch objectives: (1) What arthropod taxa characterise this fossil locality? New taxa of horseshoe crabs, trilobites,radiodontans and bivalved arthropods are described using standard paleontological techniques (e.g. photography,camera lucida drawings) and through the development of imaging and analysis methodologies for enhancing contrast between anatomical features, such as multispectral macroimaging, elemental mapping, and CT-scanning. (2) What are the affinities and interrelationships of the new arthropods? The new fossil taxa are coded into a morphologicalcharacter matrix and subjected to phylogenetic analyses in order to determine their affinities and produce robust trees of arthropod relationships, including both stem-lineage and crown-group taxa. Paleobiogeoraphic studies also trace the evolutionary history of certain arthropod groups. (3) What are the effects of taphonomy (e.g. fossil preservation processes) on understanding the Fezouata Shale community? Two approaches are explored in this project. One is a new probabilistic modeling technique that interrogates fossil lagerstätten using the distribution and combination of anatomical structures amongst the animal taxa found there to gain information on the preservation conditions of each fossil locality. The second is an experimental taphonomy approach that using decay experimentation to understand preservational pathways of fossil lagerstätten. By providing a more accurate picture of the arthropod community present in the Fezouata Shale, this project starts to reveal the evolutionary dynamics that govern the faunal turnover between the Cambrian Explosion and Ordovician Radiation. This project also establishes new methodologies for imaging fossils and evaluating the preservation of extinct arthropods specifically, and of animal fossil lagerstätten more broadly. Results A major result of the project includes the descriptions of arthropod taxa from the Fezouata Shale, specifically radiodonts, trilobites, a nektaspid, bivalved arthropods, and chelicerates, including new and previously known taxa. These descriptions combine photography and drawings of specimens with analytical techniques such as multispectral imaging, synchrotron elemental mapping, and CT-scanning, to extract as much information as possible from the fossils. Phylogenetic analyses largely focus on the detailed inter-relationships of the trilobite subfamilies Cheirurinae and Deiphoninae, as well as on the basal relationships of Chelicerata. During the project, team members were also involved in developing a new probabilistic modeling approach to examine taphonomic effects at the Fezouata Shale, and compare them to Cambrian fossil lagerstätten. This work also incorporates aspects of their paleoecology and life history strategies. Preservation was further explored using experimental taphonomy approaches, mainly arthropod decay experimentation that follows changes in anatomy under different environmental conditions, while also studying what happens with oxygen levels and bacterial communities surrounding the carcass. Taken together, the results of this project reveal the diversity of arthropods that were present at the Fezouata Shale, and how they were preserved. Through comparisons with younger and older arthropod taxa at other localities, evolutionary relationships are clarified and trends in anatomical change, geographic distributions, and paleoecology are revealed. Additional geographical information This study mainly examines fossil material from the Fezouata Biota of Morocco. Specimens derive from outcrops located near Zagora, in the Anti-Atlas region of Morocco. The fossil collection is now housed in Lausanne, Switzerland, where the majority of the descriptive studies and analyses took place.

For the publication: Laibl L, Gueriau P, Saleh F, Pérez-Peris F, Lustri L, Drage HB, Bath Enright OG, Potin GJ-M and Daley AC (2023) Early developmental stages of a Lower Ordovician marrellid from Morocco suggest simple ontogenetic niche differentiation in early euarthropods. Front. Ecol. Evol. 11:1232612. doi: 10.3389/fevo.2023.1232612 High resolution image files, photographs of specimens showing the early developmental stages of the Fezouata Shale marvelled. Photos taken by Lukas Laibl. Specimens were mechanically prepared with a Micro-Jack 4 pneumatic air scribe equipped with a pointed stylus. Photographs were taken using a digital camera Canon EOS 800D coupled with a Canon MP-E 65-mm 1:2.8 1-5X macro lens. A polarizing filter was attached to the lens and the light source to reduce reflections and enhance the contrast between the rock and the specimen. To further increase the contrast, all specimens were immersed in diluted ethanol. The immature specimens were measured using the optical image analyzer TpsDig2 2.31. Eight dimensions of the immature specimens were measured, namely the maximal length and width of the cephalic shield (both including and excluding the cephalic spines), the lengths of the anterolateral, mediolateral, and posterolateral cephalic spines, and the observed length of the trunk. Three folders of images are provided: Raw image files with scale bars that were used for making the measurements (raw_images_measurements); Raw image files of the photographs used in the plates (raw_images_plates); Image files cropped and adjusted for the size in the plates with contrast/brightness adjustments (upgraded_images_plates). In addition, an excel spreadsheets contains the raw data of the measurements.