fossils

Dickinsonia is an animal

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Pete
Buchholz

Senior Writer
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Franz
Anthony

Editor and Artist
Researchers examining the pattern and rate of growth in the mysterious “quilted” organism Dickinsonia have shown that new segments appear to grow from a wedge-shaped section at the blunt end. This adds to a growing body of data showing that Dickinsonia, and possibly other “quilted” Ediacaran organisms were among the earliest animals.

The Cambrian Period is famous for the explosive emergence of animal groups in a short period of time. Less well-known to the public is the earlier Ediacaran Period, home to some of the earliest organisms visible to the naked eye. They come in a variety of shapes and sizes, but many are flat and appear to be made of repeated units, looking like weird air-mattresses or quilts.


Just what those Ediacaran organisms are related to has been the focus of much debate since their discovery. Possible relationships included representing living animal groups, lichens, fungi, a completely extinct multicellular kingdom, or gigantic single-celled organisms. Paleontologists Renee Hoekzema and colleagues have conducted research showing that at least one of these strange organisms, Dickinsonia, is an animal.


Dickinsonia is one of the largest and most iconic Ediacaran organisms, being roughly oval-shaped with the largest specimens nearly 1.5 meters (5 feet) long, but only a few centimeters thick. Its body is made up of numerous inflated segments or “units” reflected across the central axis, and one large, wedge-shaped segment at the blunt end, called the “deltoidal region.” The smallest and presumably youngest specimens of Dickinsonia have proportionally larger deltoidal regions and fewer units, while the largest specimens have smaller deltoidal regions and many more units.



Fossil of Dickinsonia costata and schematic showing the central axis, deltoidal region (top), anti-deltoidal region (bottom), and individual units. Image by Hoekzema et al.


Hoekzema and colleagues sought to determine where new units came from as Dickinsonia grew. Did they emerge from the deltoidal region, or from the other end, which they call the anti-deltoidal region? To do this, they examined 20 specimens of a variety of different sizes and measured the dimensions of the individual units and the wedge-shaped deltoidal region.


They found that it was much more likely new units split off from the deltoidal region and that the smaller units at the anti-deltoidal end are the oldest parts of the organism. This is supported by the deltoidal regions in some specimens, which appear to show new units partially attached. Additionally, once units split from the deltoidal region, they grew longer until they reach the middle third of the body, making the whole organism widest just behind the deltoidal region.


This is different from most previous ideas about growth in Dickinsonia, which considered the units at the anti-deltoidal end to be the newest simply because they are smallest.


The authors note that growth by adding tissue near (but not at) one end is consistent with what’s seen in some animals, but not seen in other organisms. Hoekzema and colleagues also point to trace fossils showing Dickinsonia moving and changing shape, behavior only seen in animals. In animals that grow like Dickinsonia, specifically bilateral animals with left and right sides, the tissue is normally added at the rear end. This would mean that the deltoidal region is at the back end and the anti-deltoidal region would be the front end.



Dickinsonia is one of many strange Ediacaran organisms with “quilted” bodies. Image by Franz Anthony.


This brings the origin of bilateral animals back to the Ediacaran, 560 million years ago, and suggests that many other Ediacaran organisms with similar quilted anatomies may also be early bilateral animals. The researchers could not assign Dickinsonia to any modern group of animals, and it may instead be part of an extinct early lineage. Further research on other Ediacaran organisms can determine if they grew in the same way as Dickinsonia and if they were also animals.


Read the original research in Proceedings of the Royal Society B.

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Pete
Buchholz

Senior Writer


http://sulc.us/gcx1c
http://www.eartharchives.org/articles/dickinsonia-is-an-animal/