Fossil tracks are rare and no burrows under the sea-floor have so far been found in the Burgess Shale. These absences have been used to support the idea that the water near the sea-floor was anoxic. However it is possible that the water just above the sea-floor was oxygenated while the water in the sediment below it was anoxic, and also possible that there simply were no deep-burrowing animals in the Burgess Shale.[1] Some fossils, such as Marrella, are almost always the right way up, which suggests they were not transported far if at all.