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Did prehistoric shrimp rule ancient oceans?

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By Earl Filskov
April 17, 2015

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Butterflies, cockroaches, spiders, horseshoe crabs and lobsters are known as arthropods, and they are all distant cousins, according to a recent discovery in Morocco.

Arthropods possess a hard exoskeleton, jointed bodies and jointed legs. Evolution has proven that for some land and sea species share traits, so learning that lobsters and butterflies have should not sound farfetched, but finding out that both of them, as well as several other unlikely cousins, share in common a 480 million-year-old, seven-foot (2 meters) shrimp, now that’s weird. However, a fossil discovered in the Lower Fezouata Formation in Morocco by Mohamed Ben Moula and kept in a depository at Yale Peabody Museum of Natural History at Yale University has shown just that. A recent study of these fossils produced a near-perfect three-dimensional fossil of an anomalocaridids (strange shrimp), a long extinct branch of arthropods. Peter Van Roy, Allison C. Daley and Derek E. G. Briggs wrote the results of the study in Nature.

Artist’s rendition of the giant filter-feeding anomalocaridid Aegirocassis benmoulae feeding on a plankton cloud. (Photo credit: Marianne Collins, ArtofFact, licensed under CC BY-NC-ND)
Artist’s rendition of the giant filter-feeding anomalocaridid Aegirocassis benmoulae feeding on a plankton cloud. (Photo credit: Marianne Collins, ArtofFact, licensed under CC BY-NC-ND)

The newly discovered species, Aegirocassis benmoulae (Aegir is the god of the sea in Norse mythology, cassis is the Latin word for helmet, and benmoulae honors the Moroccan collector who first discovered fossils of the creature) lived in the early Ordovician period and was believed to have been a filter feeder. If correct, that would make the Aegirocassis benmoulae the earliest giant filter feeder discovered to date. With filter-like appendages on its head outside the mouth, the giant arthropod sifted very small creatures like krill and mini-crustaceans into its mouth; much like today’s largest animal on earth, the blue whale. The A. benmoulae had 11 body segments, each with a pair of flippers on the sides possibly used for swimming but in an undulating fashion much like today’s cuttlefish.

“This would have been one of the largest animals alive at the time,” said study co-author Allison Daley of Oxford University.

Finding this gentle giant of the ancient seas is exciting considering anomalocaridids are in the fossil record as far as 520 million years ago and were all sharp-toothed apex predators. However, the discovery does leave a large gap in evolution. The blue whale and other filter feeding leviathans of today’s seas are believed to have entered the fossil record about 60 to 70 million years ago. The discovery of A. benmoulae at 480 million years ago leaves a huge period without any giant filter feeders.

“Filter feeding and gigantism are associated, which is a pattern we see in different groups of animals across the tree of life,” said Gregory Edgecombe, a paleobiologist at the Natural History Museum in London, who wasn’t involved in the research.

This would also mean that there are some big fossils yet to be discovered.

Lateral view of a complete specimen of Aegirocassis benmoulae. Note the presence of two sets of lateral flaps, providing critical new insights into the origins of modern arthropod limbs. Photograph by Peter Van Roy, Yale University. (Photo credit: drawing by Allison C. Daley, University of Oxford., is licensed under CC BY-NC-ND)
Lateral view of a complete specimen of Aegirocassis benmoulae. Note the presence of two sets of lateral flaps, providing critical new insights into the origins of modern arthropod limbs. Photograph by Peter Van Roy, Yale University. (Photo credit: drawing by Allison C. Daley, University of Oxford., is licensed under CC BY-NC-ND)

The ‘strange shrimp’ does provide evolutionary context for many of today’s water-born arthropods. From as far back as trilobites to today’s shrimps and crabs, arthropods only have a two-segmented exoskeleton; one for bearing their weight and the other for gills. As the arthropods evolved the need for all the body segments disappeared.

“This species is a very important intermediate, a transitional form,” says Javier Ortega-Hernández, a paleobiologist at the University of Cambridge in the United Kingdom, who wasn’t involved in the research.

SOURCES:
Van Roy, Peter; Daley, Allison C.; Briggs, Derek E.G., Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps, Nature, Published March 11, 2015, Web Accessed April 09, 2015, 

Frankensquid creature roamed seas 480m years ago (w/ Video), Phys.org, March 11, 2015, Web Accessed April 10, 2015

Perkins, Sid, Newly discovered sea creature was once the largest animal on Earth. OmniFeed. March 11, 2015. Web Accessed April 13, 2015.

 
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