Macaronichnus, Planolites and Palaeophycos

Macaronichnus, Planolites and Palaeophycus are very similar burrows that are often confused with each other. All three are non-branching, curvilinear, horizontal tubes.

Macaronichnus is a horizontal, active fill, unlined burrow tube made by a deposit feeder in which the fill is identical to the enclosing matrix, but has a mantle of grains surrounding the burrow that were selected out and rejected during the feeding process. The grains in this mantle are often dark minerals, such as micas, or they may be grains that are nearly indistinguishable from the matrix grains. Most likely, the burrow organism fed on bacteria that were counting the grain surfaces, and certain grains, such as the darker ones, were less conducive to bacterial growth. Thus, these grains were pushed off to the side. Macaronichnus is generally found as a dense mass of burrows that weathers to resemble a mass of pellets. It was probably excavated by some sort of marine worm, and it is found primarily in shallow-marine, sand-rich rocks that represent low-energy, estuary-type environments.

Palaeophycus is a horizontal, passive fill, walled burrow tube probably made by a suspension feeder, detritus eater or carnivore. The burrow tubes are curvilinear to straight, and do not cross. The fill is identical to the surrounding matrix, and the burrow walls are stabilized by some sort of lining that allowed the organism to dwell in the burrow. Palaeophycus is typically found as separate burrows, and not as masses of burrows. It is occurs in both shallow-marine and terrestial settings, with worms of some sort being the likely burrow makers in marine rocks, and insects being the likely burrow makers in non-marine rocks.

Planolites is a horizontal, active fill, walled burrow tube made by a deposit feeder, in which the fill differs from the surrounding matrix. The burrow tubes are curvilinear to straight, and do not cross. Like Palaeophycus it is typically found as separate burrows and not as masses of burrows. The primary purpose of the burrow was mobility, less so feeding, whereas Macaronichnus can be characterized as primarily a feeding trace. It is generally found in shallow-marine rocks.