Written in the sand

An article in yesterday's New York Times about the interpretation of trace fossils found in a layer of the micritic limestones of the Green River Formation. This famous site is where I get most of the fossil fish I sell, so it caught my eye.

Strange patterns in a layer of the rock had led to some interest. Anthony Martin of Emory University identified fin and mouth marks associated with graze feeding on the substrate, and due to the estimated size of the fish responsible, suggests Notogoneus osculus. The significance of the find is that it had previously been thought the oxygen levels would have been too low at this depth of 35 to 50 feet to allow fish to swim along the bottom of the lake. This would also help account for the unusually large number of fossils from the site - a de-oxygenated layer greatly boosts the chances of fossilisation. The fact that this Notogoneus had been grubbing around the lake bottom shows that - at least for some time - there was enough oxygen down there for fish to feed.