Following closely on discovery in 1 Ga old sediments of the earliest evidence for eukaryote life in continental environments (see Eukaryote conquest of the continents posted June 11, 2011) it seems that metazoan animals colonised non-marine environments earlier than had previously been thought. Up to now most palaeontologists believed that there was a lag of at least 80 Ma between the emergence of marine bilaterian metazoans and their expansion into freshwater, due to a number of physiological hurdles that had to be overcome, such as regulation of trace element chemistry within their cells and bodily fluids. It has been know for more than a century that the first signs of sturdy animals in the marine realm are burrows in tidal sediments that formed more or less at the Cambrian-Precambrian boundary; the earlier sac-like Ediacaran fauna seemed ill-suited to a burrowing or infaunal habitat. A considerable thickness of clastic sediments occur in the Cambrian of eastern California, USA. The earliest are clearly shallow-marine and contain abundant evidence of burrowing. Succeeding them are intensively studied fluviatile sands and silts that have been used a model for sedimentation in the absence of the stabilising influence of land plants. What has been overlooked until recently is evidence for colonisation of the river-laid deposits by burrowing animals (Kennedy, M.J. & Droser, M.L. 2011. Early Cambrian metazoans in fluvial environments, evidence of the non-marine Cambrian radiation. Geology, v. 39, p. 583-586).
The burrows include the vertical U-shaped forms given the name Arenicolites, which is the most common trace fossil, simple vertical tubes (Skolithus) and horizontal, meandering tubes with furrowed sides (Psammichnites). Anyone who has seen the Early Cambrian Pipe Rock of NW Scotland will also have seen these trace fossils, yet the Pipe Rock shows evidence of tidal deposition and is shallow marine. Their non-marine equivalents in California are coeval with the earliest known trilobites in the Cambrian marine sequence. It seems that whatever the burrowing animals were, they easily overcame any physiological or environmental barriers to adopting a life in freshwater, encouraged by the ready sustenance that terrestrially adapted acritarchs and cyanobacteria had provided for half a billion years previously.