Tracks in the Sand

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Science  17 Apr 2009:
Vol. 324, Issue 5925, pp. 315
DOI: 10.1126/science.324.5925.315a

Animals evolved the ability to breathe air and then colonized the land in the Silurian period about 430 million years ago; however, curious tracks preserved by Cambrian microbial mats sitting atop mudflats and dunes seem to imply that arthropods or other animals ventured ashore before then. Hagadorn and Seilacher provide considered thoughts about who made these tracks and how. The tracks show a distinctive tail trace and other marks suggesting that the animals scurried along via the synchronous action of pairs of legs. The tracks, and in particular the tail trace, imply that the animal carried only a small shell, in contrast to, for example, the capacious shell used by a modern-day hermit crab. The authors suggest that an early chelicerate—possibly a eurypterid (sea scorpion)—used a mollusk shell to protect its gills and to keep them hydrated; a larger shell would not have been necessary as there were no other land predators yet. — BH

Geology 37, 295 (2009).

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