I do not know about fossil traces attributable to land snails, but traces of marine radula scratches are known starting in the late Precambrian (mostly in Seilacher's work; I think it is Radulichnus but am rather unsure on spelling).
Dr. David Campbell
"Old Seashells"
Biology Department
Saint Mary's College of Maryland
18952 E. Fisher Road
St. Mary's City, MD 20686-3001 USA
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"Mollusks murmured 'Morning!'. And salmon chanted 'Evening!'."-Frank Muir, Oh My Word!