Y-Shaped Rhabdopleurids Have Lived on Shells of Dead Clams at Bottom of the Ocean for Over 500 Million Years

Posted on September 22, 2012

Rhabdopleurids are tiny sea creatures that build homes of collagen on the shells of dead clams at the bottom of the ocean. They have lived this way for over 500 million years. The individual creatures are a millimeter long and shaped like a Y. A Rhabdopleura compacta zooid is pictured above. The creatures have tentacle-like arms they use to filter food from the water. The little creatures build colonies that contain tubes resembling tiny elephant trunks. A colony of Rhabdopleura compacta is pictured below.

A new study reported in the journal Lethaia says the creatures have outlived more elaborate species that descended from a common ancestor. The paper also identifies rhabdopleurids as a predecessor to ancient zooplankton - known as pelagic graptolites -- that went extinct about 350 million years ago.

Charles Mitchell, lead author of the study, said in an announcement that the finding shows newer isn't always better. Mitchell says, "We think that change is always going to lead us to a better place, that evolution is always going to lead to something better. But all this progress in making all these wonderful pelagic graptolites didn't lead them to take over the world. They didn't survive, but these simple dudes, these bottom-dwelling creatures, did."

The researchers say the rhabdopleurids have remained pretty much the same, while their zooplankton relatives evolved rapidly, splitting into many new species and evolving many new traits. As the zooplankton developed ways to live closer to the ocean's surface, the rhabdopleurids continued dwelling on the ocean floor. While the zooplankton became important players in their new ecosystem, the rhabdopleurids remained inconspicuous at the bottom of the ocean. Ultimately, the conservative approach won out for the rhabdopleurids. They survived and are still around today, while the zooplankton graptolites went extinct.

Mitchell says, "High speciation rates generally go hand in hand with high extinction rates, and likewise low with low. Conservative lineages may weather the storms of climate change and other events, but do not become big parts of the ecosystem, whereas the major players are impressive but often brought low by mass extinction and other 'slings and arrows of outrageous fortune.'"

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