Abstract

The trace fossil, Ophiomorpha nodosa Lundgren, reported in rocks from Pleistocene to Cretaceous in age, is considered by a number of workers to be a burrow analogous to that constructed by the decapod crustaceans (callianassids) found in Holocene marine sands of the littoral zone. In a road outcrop 48.1 mi south of Starkville, Mississippi, well-preserved, cemented, typical Ophiomorpha nodosa and Ophiomorpha-like burrows were found in fine-to coarse-grained, cross-bedded, nonlithified sand of Eocene age.

The burrows consist of (1) individual nodose cylindrical forms, 15 to 25 mm in diameter and as much as 1 m in length, oriented normal to and parallel with bedding and (2) morphologically similar vertically oriented compound blade-shaped forms with the long axes horizontal. X-radiographs of serial sections show that the blade forms consist of distinct burrow tubes stacked in a vertical array. Burrow remnants appear as Spreiten in vertical section and suggest that the organism worked upward through the sediment, constructing “permanent” domiciles.

Petrographic studies of thin sections and bulk mineral analyses by x-ray diffraction show that the burrow cement, the dwelling burrow lining, and the burrow filling consist of opaline silica. As the enclosing sediment contains no opaline silica matrix and only the burrows are cemented, the burrow-building organisms could have extracted silica from the silica-rich waters of the littoral environment and concentrated it in their burrows

Blade-like burrow composites of this general type have been described as Teichichnus. However, the tubular and nodose character of the outer wall, the presence of burrow linings, the abnormal abundance of opaline silica matrix, the occurrence of sand-grain aggregates (pellets) throughout the burrow walls, the presence of burrows that range in form from simple tube to multiple-stacked blade forms, and the morphological similarity to and connection with individual cylindrical Ophiomorpha burrows, all strongly suggest that the two types of burrows were constructed by the same organism. The informal name Ophiomorpha nodosa var. spatha is suggested for this blade-shaped burrow.

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