Images and measurements of early developmental stages of trilobites from Fezouata Shale

Description

Abstract

Over 99% of all species that ever existed are now extinct, and the <1% of species alive today are the result of hundreds
of millions of years of evolution. Many fields in evolutionary biology, developmental biology, ecology, and molecular
phylogenetics have made great advances examining evolutionary processes using extant taxa, and yet this work
necessarily focuses on the <1% represented by living animals. To access the other >99%, the fossil record must be
critically interrogated to bring palaeontology to the forefront of a modern synthesis on evolutionary studies. The animal
fossil record holds a staggering wealth of information on the radiations and extinctions that shaped the morphological
diversity of modern ecosystems. Two such radiations, the Cambrian Explosion and the Ordovician Radiation, occurred
at the early stages of animal evolution, and established the overall organisation of animal body plans that persist into
the modern day. Yet the interplay between these two events is the subject of debate, with no clear answer as to how
they 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 the
Cambrian Explosion to the Ordovician Radiation?

The Fezouata Biota is a recently discovered locality in Morocco that produces exceptionally preserved, open
marine, soft-bodied fossils and mineralised taxa. Its early Ordovician age places it exactly between the Cambrian
Explosion and Ordovician Radiation evolutionary events, and it is unique in the world for preserving the typical
elements of both animal communities together. It is ideally suited to provide the crucial data needed to understand the
dynamics of faunal turnovers, and this project interrogates the Fezouata Biota fossil record by addressing three main
research 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 morphological
character 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.