Spreite

Spreite (or spreiten in German) are tabular, curved laminae that are characteristic of certain trace fossils. They are formed by invertebrate organisms tunneling back and forth through sediment in search of food. The organism moves just enough on each back-and-forth pass so that it avoids reworking a previously tunneled area, thereby ensuring that it only makes feeding passes through fresh, unworked sediment.

Two types of spreite are generally recognized. Protrusive spreite result from movement of the organism away from its burrow entrance (i.e., a downward movement in vertical burrows), whereas retrusive spreite result from movement towards the burrow entrance (i.e., an upward movement in vertical burrows). Vertical burrows with retrusive spreite are also referred to as "escape burrows", as they represent attempts by the organism during periods of high sedimentation to prevent the burrow entrance from being buried, and at the same time keep the bottom of the burrow at the same depth from the sediment-water interface. In essence, the organism "escapes" from being buried too deeply by progressively tunneling upward, thereby leaving behind it a stacked vertical succession of concave-up spreite. Diplocraterion is the classic example of a vertical burrow with spreite, whereas Rhizocorallium is an example of a horizontal burrow with spreite.

Spreite differ from menisca, which are flat to slightly curved, back-filling structures found in some tubular burrows. Menisca are formed when the organism packs sediment, and sometimes fecal material as well, in behind itself as it moves through the burrow. Because spreite and menisci represent different behaviors, the distinction between spreiten and meniscate burrows is an important criterion for the identification of certain ichnotaxa. For example, generally parallel-sided, elongate, horizontal burrows with menisci are typically assigned to the Muensteria ichnospecies, whereas similar burrows with spreite will generally be assigned to the Rhizocorallium ichnogenus.

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