A research team from the University of Rennes, led by Professor of Paleontology Didier Néraudeau, and bringing together master’s students from Rennes and doctoral students from Canadian universities in Alberta and Saskatchewan, have re-studyed Breton fossils known from the XIX century. In fact, the first discoveries were made by Paul Lebesconte, a Rennes pharmacist and geologist at the time. The rocks containing the fossils were formed during the Ediacaran and Cambrian geological periods, more than 530 million years (Ma) ago. These are fine slates, or slate schists, where fossils are preserved horizontally on their surface. This rediscovery and the new studies that derive from it allow a better understanding of the date, the environment of the time and the oldest fauna currently known in France.
Brittany was in the tropics
Imagine the world more than 530 Ma ago: days of only 22 hours, a generally warmer climate, unrecognizable continents and, above all, the absence of life on land! However, in the oceans everything is different: life has been diversifying and abounding for several million years. Present-day Britain was then… submerged and located near the present position of Australia! The study of the sediments associated with the fossils also informs us about the conditions of this aquatic environment. It makes us dive into relatively shallow seabeds, subject to the influence of tides and sometimes storms.
A seabed teeming with life: the proof in the footprints
There are no large fossils like in museum galleries, but traces discovered on blackboards that show the passage of animals. These fossil traces -called ichnofossils- reveal, depending on their arrangement and organization, a great deal of information about the behavior of an animal: did it move? I was resting? How did you eat? They can also inform us about the morphology of the author of the footprints.
A research team from the University of Rennes, led by Professor of Palaeontology Didier Néraudeau, and bringing together master’s students from Rennes and doctoral students from Canadian universities in Alberta and Saskatchewan, have re-studyed known Breton fossils since 19the century. In fact, the first discoveries were made by Paul Lebesconte, a Rennes pharmacist and geologist at the time. The rocks containing the fossils were formed during the Ediacaran and Cambrian geological periods, more than 530 million years (Ma) ago. These are fine slates, or slate schists, where fossils are preserved horizontally on their surface. This rediscovery and the new studies that derive from it allow a better understanding of the date, the environment of the time and the oldest fauna currently known in France.
Brittany was in the tropics
Imagine the world more than 530 Ma ago: days of only 22 hours, a generally warmer climate, unrecognizable continents and, above all, the absence of life on land! However, in the oceans everything is different: life has been diversifying and abounding for several million years. Present-day Britain was then… submerged and located near the present position of Australia! The study of the sediments associated with the fossils also informs us about the conditions of this aquatic environment. It makes us dive into relatively shallow seabeds, subject to the influence of tides and sometimes storms.
A seabed teeming with life: the proof in the footprints
There are no large fossils like in museum galleries, but traces discovered on blackboards that show the passage of animals. These fossil traces -called ichnofossils- reveal, depending on their arrangement and organization, a great deal of information about the behavior of an animal: did it move? I was resting? How did you eat? They can also inform us about the morphology of the author of the footprints.
Rocks sampled from some Breton sites, recently dated to an interval of 550 Ma to 530 Ma, revealed the great diversity of these trace fossils. These findings published in 2018 Y 2021 they testify to a proliferation of life at the bottom of the sea at that time. They allow various traces to be identified according to their curvature: straight to slightly curved, meandering, irregularly curved or even spiral. In addition to all this, microbial mats were discovered, surfaces where microbes proliferated, forming wrinkles and pustules on the slates.
Photo of a slate with trace fossils and microbial mats from the Ediacaran/Cambrian of Brittany. Microbial mats are noticeable by the wrinkles they form. Credits: Didier Néraudeau
However, the study of these traces remains limited as they offer few remarkable features. Therefore, it is difficult to say for sure who could have produced them. It will be noted that most of these prints are generally between 1 and 2 millimeters. It is simply possible to imagine that animals similar to small marine worms are found at the origin.
The latest study published in August 2022 in the journal Annals of Paleontology used a new method that consists of quantifying the density of fossil footprints on the slates using software (ARC-GIS). This allowed us to observe that the slates without an apparent microbial mat reveal only a few tracks, most of the time long and straight. In contrast, the density of trace fossils increases where fossilized microbial mats are found. In these areas, the footprints are not only more numerous, but also smaller, intersecting and overlapping. In short, the tracks associated with microbial mats are more complex and suggest that animals living on the seafloor “grazed” them. In fact, the microbial mats provided a food source, hence the high concentration and multiple changes in direction observed on the surface of the shales.
Photo of trace fossils in Ediacaran/Cambrian slate from Brittany. Note the earlier removal of the board with the old label pasted to the bottom right. Credits: Didier Néraudeau
Future studies for a better context
About 530 Ma ago, the Breton seafloor was populated by animals that looked like small sea worms. They left many traces when colonizing the seabed. But since their bodies have not been fossilized, it is difficult to identify and date them precisely. Other studies are being carried out to enrich the knowledge about these ancient traces of life. This will allow the diversification of this fauna to be better contextualized in relation to the evolution of living beings. Especially in these Ediacaran and Cambrian periods more than 530 million years ago, a crucial moment in which many groups of animals appear.