Paleontologists working in Morocco have found a fossil of a bizarre sea creature that could grow up to seven feet in length and gathered plankton like a whale.
The newly discovered animal, dubbed Aegirocassis benmoulae, is an early member of the arthropod family tree, making it an ancient ancestor of cockroaches, butterflies and shrimp. It lived about 480 million years ago in a shallow sea that once covered part of the Sahara Desert.
“It is one of the biggest arthropods that ever existed, far bigger than any arthropod today,” said Peter Van Roy, a paleobiologist at Yale University who helped uncover some of specimens of the extinct animal.
A paper describing this strange early arthropod was published Wednesday in the journal Nature.
Aegirocassis benmoulae is part of an extinct group of animals known as anomalocaridid, which were already known to be the largest animals of the Cambrian time period 543 million to 490 million years ago. Aegirocassis is the most recent of these animals to be found and more than double the size of the biggest of its older known relatives.
It lived during the Great Ordovician Biodiversification Event, about 10 million years after the Cambrian period came to a close.
All of the anamolocardids were flat-bodied water dwellers, and all of them had two appendages in the front of their mouths. These structures were mostly used to grasp prey like worms and mollusks. However, Aegirocassis modified these appendages, creating an elaborate netlike structure of spikes that allowed it to catch the rapidly diversifying plankton found in the water column at the time.
Despite its large size, Aegirocassis appears to have been fairly common, Van Roy said. He and the Moroccan fossil collector Mohamed Ben Moula, who first discovered these large animals, have found dozens of preserved specimens in the Moroccan desert.
“They may have come together to molt, or they may have lived in a group,” he said. “It is interesting we found so many of them. It shows there was a well developed, rich plankton system in place 480 million years ago.”
Arthropods first showed up in the fossil record about 530 million years ago, and today the phylum consists of the most morphologically diverse group of animals on the planet.
However, early fossils of arthropods are hard to come by, which made this particular find so exciting.
Most fossils are the remains of the hard parts of organisms, like shells and bone, because soft tissues decay so rapidly after death, explained John Paterson, an associate professor at the University of New England in Armidale, Australia, who was not involved in the study.
“However, these extraordinary fossils are soft-bodied, which are typically very rare,” he said. “They require rapid burial and low oxygen conditions in order to be preserved in such exquisite detail."
According to Van Roy, giant storms occasionally caused large mud flows to wash into the ancient sea, smothering everything at the bottom and making it inaccessible for scavengers.
“If you have the right sediments and they react with the right decay products, then you can basically turn soft tissues to stone,” he said. “But a very specific set of criteria must be met, so sites like this are extremely rare.”
The fossils were so well preserved that they allowed researchers to see a previously hidden feature on the body of these early arthropods, helping them to solve a longstanding evolutionary mystery.
The success of arthropods is associated with the way they are constructed. Their bodies and legs are made of multiple segments that can be be modified for a variety of purposes, allowing them to adapt to a wide range of environments.
“Morphologically, they are incredibly plastic,” Van Roy said.
Today, the legs of most arthropods have two branches, but researchers were not sure how these double-branched limbs had evolved over time.
Paleontologists thought they could trace the origins of this double-branched limb to an arthropod ancestor from the early Cambrian. This ancient animal had a set of flaps that ran down the top side of its body for swimming, representing an upper limb, and a set of legs on its belly side, for walking.
And yet, previous studies of anamolocardids, a later arthropod, had found just one set of flaps on the lower side of the body. This led researchers to wonder where the upper limb had gone.
The new fossils described in the Nature paper suggest that the second limb did not go anywhere. They show that anamolocardids did have a second set of swimming flaps on the back side of their segments, but they have only been preserved in these particularly complete fossils.
“These flaps represent the precursors of more modern-style (two-branched) limbs that we see in other arthropods,” Paterson said via email.
Van Roy said there were still many more fossils to go through from the Moroccan site.
“We have something like 5,000 or 6,000 specimens, most of them unstudied,” he said.
Among the other animals they found are more advanced arthropods, a variety of horseshoe crabs, some which look remarkably like modern-day horseshoe crabs, jellyfish relatives, and early starfish and snails.
“There are a lot of really exciting things coming,” he said.