Moroccan deposits from the Ordovician era look like a sort of Lost World. Species thought to have died out 20 million years earlier in the Cambrian era sit next to others that were not believed to have evolved by the time the deposits were laid down.
The Cambrian era marks one of the most important points in the development of life, with an explosion of multicellular life forms. However, by 485 million years ago, many of these species had died out, or at least so we thought. A 477-million-year-old formation known as the Lower Ordovician Fezouata demonstrates that hundreds survived for far longer than we thought.
A marrellomorph arthropod from the upper Fezouata deposit. The body and appendages are concealed in its shell. Credit: Van Roy et al/Journal of the Geological Society.
The Fezouata is rich in creatures not seen elsewhere, such as a two-meter-long creature resembling a shrimp that was a giant of the day. However, in recent years the formation has also yielded many species that look very familiar to scientists who are used to studying Cambrian lifeforms. “A number of the Fezouata animals would not look out of place in Walcott's Burgess Shale quarry,” a paper in the Journal of the Geological Society notes. The Burgess Shale is famous for the extraordinary Cambrian animals seen there but thought to have gone extinct soon after.
“The Fezouata is extraordinarily significant,” says Yale's Professor Derek Briggs, one of the authors. “Animals typical of the Cambrian are still present in rocks 20 million years younger, which means there must be a cryptic record in between, which is not preserved.” The absence of intervening examples may be a result, the paper says, of “the general paucity of outer-shelf muddy environments favorable to Burgess Shale-type preservation.” However, the ancient microcontinent of Avalonia was an exception, with extended continental shelves. Traces of Avalonian strata survive in Morocco, Spain and northern Europe.
More than 160 genera of animals have been found in the Fezouata, and there is plenty of digging left to do. Examples of species that look Cambrian include Lobopodians, worms with legs and spines, and anomalocaridids, thought to have been the apex predators of the Cambrian era.
On the other hand, some species have proven older than we suspected. “Horseshoe crabs, for example, turn out to be at least 20 million years older than we thought. The formation demonstrates how important exceptionally preserved fossils are to our understanding of major evolutionary events in deep time” says first author Peter Van Roy, also of Yale.
Animals thought to have evolved after the Fezouata was laid down have also been found, such as a cheloniellid arthropod (a), a horseshoe crab (b) and Plumulites bengtsoni, a machaeridian (c). Credit: Van Roy et al/Journal of the Geological Society.
“Some of the organisms are enormous – several meters in length,” says Briggs. “With such exceptional preservation, in a fully marine exposure, we can develop a reasonably full picture of what marine life looked like in the Ordovician.”
The site of the Fezouata deposits in Morocco with the lower Ordovician outcropping in the foreground. Credit: Van Roy et al/Journal of the Geological Society.
The findings challenge the traditional timeline, which suggests an explosion of diversity in the Cambrian, followed by widespread extinctions that in turn led to the Great Ordovician Biodiversification Event. Instead there may have been unrecognized continuity between the two eras.
The paper is the first in the Journal of the Geological Society's open access series exploring extraordinary fossil sites and their contexts.
This giant filter-feeding Aegirocassis benmoulai was preserved in three dimensions. Credit: Van Roy et al/Journal of the Geological Society.