The trace fossils Palaeophycus, ?Scoyenia, ?Scolicia, and ?Chondrites occur in lacustrine and fluvial deposits of the Miocene-Pliocene Ridge basin of southern California. Palaeophycus is the most common ichnofossil in the sequence. Palaeophycus is a curved, cylindrical burrow that rarely branches. This ichnofossil is divided by diameter into two types: type A and type B. Type A averages 2 to 6 mm and type B averages 8 to 18 mm in diameter. Subaerial burrowing by insects probably formed these traces, hence they possibly can be used as evidence of subaerial exposure of the sediment. ?Scoyenia is morphologically similar to Palaeophycus except ?Scoyenia contains meniscate packing indicating active filling of the burrow, whereas Palaeophycus was passively filled. ?Scoyenia was probably formed by deposit-feeding insects. Both Palaeophycus and ?Scoyenia occur in fluvial and shallow freshwater lacustrine facies.

?Scolicia was formed by grazing gastropods. These ichnofossils are ribbonlike depressions which occur both as simple, sinuous furrows and as intertwined paths. ?Scolicia is found in delta-front and shallow nearshore lacustrine environments in the Ridge basin. The presence of this trace fossil indicates that water depths were less than a few tens of meters.

?Chondrites is found in prodelta, delta-front, and deep brackish-water lacustrine deposits. It appears as small (averaging 1.75 mm), curved, commonly branching burrows which locally are replaced by pyrite. Aquatic worms constructed this trace fossil. In the deep, brackish-water deposits, zones of unbioturbated sediment are interbedded with zones of sediment which are partially or totally bioturbated by ?Chondrites. This probably is the result of alternating oxic and anoxic conditions in the lake's bottom waters.

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