2-meter-long arthropods dominated the seas 470 million years ago

"The Fezouata Biota keeps surprising us with new, unexpected discoveries.”
Nergis Firtina
MARRELLOMORPHA.jpg
Life reconstruction of T. dimitrii, a marrellomorph arthropod from southern Ontario; art by Christian McCall.

Journal of Paleontology 

According to a recent discovery, ancestors of contemporary species, such as shrimps, insects, and spiders, may have dominated the oceans 470 million years ago. Moreover, some of them were approximately 2 meters long.

Led by international researchers, the study also demonstrates that fossil records are very different from other species which were described beforehand. The research was conducted at the paleontological excavation site Taichoute, which was home to the ocean millions of years ago but is now a desert. The fossils discovered here differed from those excavated and studied in the past 80 kilometers away.

The study was published in Scientific Reports on December 13.

“Everything is new about this locality – its sedimentology, paleontology, and even the preservation of fossils – further highlighting the importance of the Fezouata Biota in completing our understanding of past life on Earth,” said lead author Dr. Farid Saleh, from the University of Lausanne and Yunnan University.

2-meter-long arthropods dominated the seas 470 million years ago
The newly discovered site from the Fezouata Shale

“While the giant arthropods we discovered have not yet been fully identified, some may belong to previously described species of the Fezouata Biota, and some will certainly be new species," said Dr. Xiaoya Ma, from the University of Exeter and Yunnan University.

“Nevertheless, their large size and free-swimming lifestyle suggest they played a unique role in these ecosystems.”

Understanding the evolution

As stated, the Fezouata Shale is ultimately crucial for understanding the evolution of the Early Ordovician period. Thus, it was chosen as one of the 100 most important geological sites worldwide by International Commission on Geoheritage.

Animals of the Fezouata Shale, in the Zagora region of Morocco, lived in a shallow sea that frequently underwent storm and wave activity. The animal populations were buried as a result, and they are now preserved in situ as remarkable fossils.

2-meter-long arthropods dominated the seas 470 million years ago
Fossils from the Fezouata Shale. From left to right, a non-mineralized arthropod (Marrellomorpha), a palaeoscolecid worm and a trilobites

“Carcasses were transported to a relatively deep marine environment by underwater landslides, which contrasts with previous discoveries of carcass preservation in shallower settings, which were buried in place by storm deposits,” said Dr. Romain Vaucher, from the University of Lausanne.

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Dr. Bertrand Lefebvre, from the University of Lyon, who is the senior author of the paper, and who has been working on the Fezouata Biota for the past two decades, concluded: “The Fezouata Biota keeps surprising us with new, unexpected discoveries.”

More about Fezouata Shale

The Early Ordovician Burgess shale-type deposits known as the Upper and Lower Fezouata Shale of Morocco fill an important preservational window between the widespread Cambrian Lagerstätten and the Late Ordovician Soom Shale. Numerous animals that were assumed to have disappeared after the mid-Cambrian have been discovered as petrified fauna. According to a study published in 2010, along with a less common shelly fauna, the strata have yielded around 1,500 non-mineralized specimens, representing 50 unique taxa, with a composition similar to earlier Burgess Shale-type biotas.

Study abstract:

The Fezouata Biota (Morocco) is a unique Early Ordovician fossil assemblage. The discovery of this biota revolutionized our understanding of Earth’s early animal diversifications—the Cambrian Explosion and the Ordovician Radiation—by suggesting an evolutionary continuum between both events. Herein, we describe Taichoute, a new fossil locality from the Fezouata Shale. This locality extends the temporal distribution of fossil preservation from this formation into the upper Floian, while also expanding the range of depositional environments to more distal parts of the shelf. In Taichoute, most animals were transported by density flows, unlike the in-situ preservation of animals recovered in previously investigated Fezouata sites. Taichoute is dominated by three-dimensionally preserved, and heavily sclerotized fragments of large euarthropods—possibly representing nektobenthic/nektic bivalved taxa and/or hurdiid radiodonts. Resolving whether this dominance reflects a legitimate aspect of the original ecosystem or a preservational bias requires an in-depth assessment of the environmental conditions at this site. Nevertheless, Taichoute provides novel preservational and palaeontological insights during a key evolutionary transition in the history of life on Earth.