Los Angeles Times

Cockroach ancestor grew up to 7 feet

Rare fossils of the sea creature, discovered in Morocco, solve a longtime mystery.

- By Deborah Netburn deborah.netburn @latimes.com Twitter: @DeborahNet­burn

Paleontolo­gists working in Morocco have found a fossil of a bizarre sea creature that could grow up to 7 feet in length and that gathered plankton like a whale does.

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.

“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 the specimens of the extinct animal.

A paper describing the 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 benmoulae 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 anomalocar­idids 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 such as worms and mollusks. However, Aegirocass­is benmoulae 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 benmoulae 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 deser t.

“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 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 long-standing 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 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 anomalocar­idids, a later arthropod, had found just one set of flaps on the lower side of the body. This led researcher­s to wonder what had happened to the upper limb.

The new fossils described in the Nature paper suggest that the second limb did not go anywhere. They show that anomalocar­idids did have a second set of swimming flaps on the back side of their segments, but they have been preserved only 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 from the Moroccan site to go through.

“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.

 ?? Illustrati­on by Marianne Collins AFP/Getty Images ?? PALEONTOLO­GISTS found complete fossils of a sea creature that ate plankton the way a whale does.
Illustrati­on by Marianne Collins AFP/Getty Images PALEONTOLO­GISTS found complete fossils of a sea creature that ate plankton the way a whale does.

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