Giant Ocean Arthropod Rivals Largest in History

Giant Ocean Arthropod Rivals Largest in History
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A trio of paleontologists has announced the discovery of a fossil belonging to a new species of ancient arthropod that rivals the largest ever found. They detail their finding in Wednesday's publication of the journal Nature.

Hundreds of millions of years ago, arthropods, which include modern-day spiders, insects, and crustaceans, were much larger, and we're not talking the size of a small dog. An extinct millipede called Arthropleura reached up to 8.5 feet in length, making it the largest land invertebrate ever known to exist. Jaekelopterus rhenaniae, which extended 8.2 feet, dwelled in the water (pictured right).

At over seven feet in length, the new species, Aegirocassis benmoulae, fits in just behind them. Discovered in Morocco, and dated back to the Ordovician period roughly 443 to 485 million years ago, A. benmoulae may have been the "sperm whale" of the ancient oceans. The comparison comes not from how it looks -- a bit like a serrated feather duster -- but from how it ate. Notice the net-like appendage jutting out in front of the animal? The two spines on either side sandwich brush-like setae which are used to catch microscopic plankton. A. benmoulae was a filter feeder, which set it apart from many other early arthropods, especially larger ones.

"The huge size of A. benmoulae represents a much earlier example of a filter-feeding lifestyle correlating to gigantism," the authors write.

That such a filter-feeding animal could exist 450 million years ago also reveals something about Earth's oceans at the time, the authors say.

"The abundance of gigantic anomalocaridid filter-feeders... points to a complex planktic ecosystem."

Ancient arthropods were able to grow to ginormous sizes for two main reasons. First, oxygen was much more concentrated in the atmosphere 400 to 500 million years ago, making up 30% of Earth's atmosphere instead of 21% today. Arthropods rely on air flowing through their bodies in order to take in oxygen; they don't have lungs like us. The extra oxygen basically served as added fuel for growth. Second, and more simply, there weren't really any larger predators around to cull their numbers. Call it evolutionary luck.

Source: Peter Van Roy, Allison C. Daley, & Derek E. G. Briggs. "Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps." Nature. 2015. doi:10.1038/nature14256

(Images: Marianne Collins, ArtofFact & ДиБгд)

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