A 520-year-old fossil from the anti-atlas mountains in Morocco belongs to a newly discovered species that is helping scientists better understand echinoderm evolution and diversity.
Echinoderms are marine animals with radial symmetry. If you were to draw a line across an echinoderm’s body, it would produce identical pieces, like a pie or a daisy. Most bare pentaradial symmetry—there are five planes which produce symmetry. Echinoderms first appeared in the Lower Cambrian—the geological time period lasting from 542 to 511 million years ago—and have survived to this day with over 7000 extant species, including starfish and sand dollars.
The new species, Helicocystis moroccoensis, reported on in the journal Proceedings of the Royal Society B, is lending new information about the evolution of the pentaradial body plan. Andrew Smith from The Natural History Museum in London, and Samuel Zamora from the Smithsonian Institution in Washington, D.C., say it is the oldest echinoderm discovered to date that has pentaradial symmetry.
Helicocystis lived on the ancient supercontinent Gondwana during the Cambrian Explosion—a period of about 70 million years in which a colossal amount of animal phyla appeared on Earth, and existing phyla diversified. But although the Cambrian Explosion did reveal many new types of animals, often body types among phyla were relatively consistent. H. moroccoensis is unusual in that it had a novel body plan for echinoderms at the time.
Helicocystis had an body made of three parts: a stem, a cup, and a spiral-plated part that included the mouth and was the largest part of the body. It measured between 5 and 20 mm in length. The creature, which was shaped somewhat like an upside-down teardrop, had a mouth at the tip of its body consisting of seven oral plates and five grooves that opened and closed when food was floating in the vicinity.
The new find is offering unprecedented vision into the mystery of the echinoderm body plan. Paleontologists have long wondered how the two major clades of echinoderms, pelmatozoans and eleutherozoans, evolved and differentiated. The H. moroccoensis fossil specimen exposes the commonality in body plans between these two clades and shows that the divergence in the two clades occurred most likely due to varying growth rates and patterns on different regions of the body in different types of echinoderms.
Find the complete study in the journal Proceedings of the Royal Society B; June 25, 2013.
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