In the southeastern part of the San Juan basin (north-central New Mexico), the upper Morrison, Dakota, and lower Mancos stratigraphic interval represents a transgressive nonmarine to marine, sedimentary sequence. Trace fossils, although quite

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sparse in the nonmarine sediments of the upper Morrison, are common in the marginal-marine deposits of the lower Dakota and are extremely abundant and taxonomically diverse in the nearshore-marine lithofacies of the upper Dakota and lower Mancos. The distribution of most trace fossils reflects strong control by substrate and environmental facies; however, several taxonomic and ethological variants of a few ichnogenera demonstrate a lack of such control.

The "Jackpile Sandstone" of the upper part of the Morrison Formation contains only a few trace-fossil structures; many of these can be ascribed to the ichnogenus Planolites, the only trace-fossil type present in all stratigraphic units investigated in this study. The paludal and strand-line deposits of the lower Dakota contain abundant Skolithos and Planolites, and Ophiomorpha, which displays transition in form to a small variety of Thalassinoides, "Reed(?) molds" also are common in the lower Dakota sandstones.

Shallow-water marine sandstones of the upper Dakota are intertongued with the lower Mancos shales and are characterized by an abundance of trace fossils. Well-developed Ophiomorpha and Teichichnus structures show transition in form to a small variety of Thalassinoides and are probably ecovariant forms made by the same organism. Additional ichnogenera studied include large polygonal Thalassinoides, Asterosoma, Arenicolites, Zoophycos, Chondrites, Crossopodia, Gyrochorte, Pelecypodichnus, Planolites, and Skolithos. The presence of Zoophycos is of particular interest because of its usual association with deep-water deposits. The large Thalassinoides and Asterosoma structures appear to be restricted to the deeper water sandstone deposits of the upper Dakota.

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