Skolithos

Skolithos ranges in age from early Cambrian and is found throughout the world. They occur in sediments and sedimentary rocks, primarily sands and sandstones. They are typically marine in origin, and are commonly associated with high-energy environments close to the shoreline. This periodic water fluctuation corresponds to tidal activity in shallow marine environments, but also occurs over longer time intervals in alluvial deposits. == Classification and history ==

The ichnogenus Skolithos was first described as a subgenus of the supposed seaweed Fucoides in 1840, by Samuel Stehman Haldeman, a renowned Pennsylvania naturalist in the early 19th century, who labeled the structure as the “oldest fossil in the state”. He named the trace fossil Skolithos, meaning “worm-stone”, suggesting its morphologic similarity to a worm. Trypanites is superficially similar in form but is a boring excavated in hard substrates, and lacks the diagnostic lining of Skolithos. == Structure and use as a strain marker ==

Skolithos structure The structure of the trace fossil is cylindrical and elongated in shape, usually at a perpendicular angle to the surface where it has been deposited. They can reach lengths of up to about and diameters of up to about . Funnel-shaped apertures of Skolithos reflect the filter- and suspension-feeding habits of burrowing genera. The high intensity of bioturbation of these organisms indicate the shallow water paleoenvironment in which the Skolithos burrows formed shortly after the deposition of the bed. Example of strain analysis using Skolithos The famous "Pipe Rock" of northwest Scotland is a well-known example of Skolithos. The 'pipes' that give the rock its name are closely packed straight Skolithos tubes that were presumably made by a worm-like organism. The Pipe Rock can be found in the Stack of Glencoul region beneath the Moine Thrust Belt, Scotland. This area which has a history of thrust faulting activity is a highly deformed mylonite zone with a quartzite protolith where many structural geologists have used microstructures such as the Skolithos borings in conjunction with other strain markers, such as quartz vein recrystallization, in order to approximate strain in the region. While it is common for Skolithos burrows to form normal to the deposition plane, this is not always true, in which case, the ideal, undeformed state can no longer be used as a reference orientation. Since the rheological properties between the structure and the host rock are usually very similar, observations of the fossils are conducted with the assumption that they have deformed homogeneously, where the deformation forces are distributed evenly along the entire deformation zone. This is directly contradicted by the presence of folding and varying elongation measurements of the fossil at different locations in the same deformation zone. Deformation mechanisms are difficult to distinguish using this strain marker, as the thinning and flattening of the highly deformed rocks where they are found, cannot necessarily be attributed to pure shear since the planes may have simply rotated near parallel to the shear plane. It is therefore only possible to make accurate strain determinations of the host rock provided the correct assumption of the deformation mechanism and original measurements. == References ==