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Paleontologists Unearth Ancient Sea Predator Fossil

May 26, 2011

Ancient sea creatures, that were the largest predators for millions of years, grew even larger and survived much longer than previously thought, according to paleontologists who discovered well-preserved fossils in Morocco.

The creatures, known as anomalocaridids, ranged in size from 2 to as much as 6 feet long. They had soft-jointed bodies and toothy maws with spiny limbs in front to catch their prey, scientists described in a paper published by the journal Nature.

“They were really at the top of the food chain,” researcher Peter Van Roy, a paleobiologist at Ghent University in Belgium, and formerly at Yale, told LiveScience. “The uncontested top predators of their time.”

Anomalocaridids are believed to have been apex predators during the “Cambrian Explosion,” 540 to 500 million years ago, when biodiversity increased significantly.

“The anomalocaridids are one of the most iconic groups of Cambrian animals,” said researcher Derek Briggs, director of the Yale Peabody Museum of Natural History, in a statement. “These giant invertebrate predators and scavengers have come to symbolize the unfamiliar morphologies displayed by organisms that branched off early from lineages leading to modern marine animals and then went extinct.”

Just as surprising, said the paper, is the discovery that these particular anomalocaridids lived not in the Cambrian age but in the Ordovician age (488 to 472 mya) which followed. The findings show that these animals existed far longer than previously believed — by 30 million years.

“The Moroccan specimens are the largest anomalocaridids known to date “” they are about double the size of their Cambrian counterparts,” Van Roy told LiveScience. “There have been suggestions of Cambrian anomalocaridids of over 6 feet in length, but these estimates are extrapolations from very fragmentary material, and hence not too reliable.”

Van Roy said the anomalocaridids lived on the muddy seafloor about 330 feet below the surface. “The seafloor near which the animals lived would have been teeming with bottom life,” said Van Roy. The habitat would have been covered in forests of fan-shaped colonies of creatures known as graptolites, dense populations of sponges, and loads of other creatures scurrying around.

These discoveries are just a small part of a new collection of fossils that includes thousands of specimens of soft-bodied animals. Soft tissues tend not to fossilize as easily as hard bones and shells, leading to incomplete and biased views of marine life that existed in the Ordovician stage before the new findings were made.

The animals in this discovery lived in deep waters and were trapped by sediment clouds that buried the creatures, preserving them.

Many of the creatures appeared to be oversized. “The large size of the Moroccan animals may be due to abundant food supplies,” said van Roy. “Also, at the time, the area where the animals lived was almost right at the South Pole, and organisms in high polar latitudes often tend to grow to larger sizes “” this is something that can also be witnessed in current-day faunas.”

The new findings question the long theorized belief that animals of the Cambrian age were quickly replaced during the Ordovician age by more advanced animals — the so-called Ordovician Bio-diversification Event.

“The Cambrian faunas persisted for much longer, and replacement was a much more gradual and protracted affair, than the incomplete shell-biased fossil record would suggest,” said van Roy. “The fact that anomalocaridids persisted for so long shows that they remained well-adapted and highly successful predators long after the Middle Cambrian.”

While it is not clear why anomalocaridids disappeared, two other large predators came into existence during the Ordovician period, either equal to or larger in size than anomalocaridids.

“It seems likely that anomalocaridids were out-competed by these more advanced and better adapted predators,” said Van Roy. “While anomalocaridids in essence are soft-bodied, eurypterids had a strong exoskeleton, and nautiloids had a sturdy shell and a powerful beak. It seems likely that, when anomalocaridids had to compete over food with these more advanced animals, they would likely lose out.” 

Image 1: Anomalocaridids had long, spiny head limbs presumably used to snag prey, and a series of blade-like filaments in segments across the animal’s back, which scientists think might have functioned as gills. Credit: Esben Horn

Image 2: The anomalocaridid fossil shows this ancient sea predator existed for much longer and grew to much larger sizes than previously thought. Credit: Peter Van Roy

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Paleontologists Unearth Ancient Sea Predator Fossil