BIOGENIC STRUCTURES REVEAL IMPACT OF PERMIAN EXTINCTION ON STREAM INFAUNA AND HIGH PALEOLATITUDE EARLY TRIASSIC PRECIPITATION PATTERN

Analysis of the facies distribution and morphology of trace fossils in Permian and Triassic braided stream deposits in the Beardmore and Shackleton Glacier areas, Antarctica, indicates that the arthropod whose traces dominate the preserved record of the stream infauna was not affected by the Permian extinction. In addition, it demonstrates that the high latitude Early Triassic was characterized by depressed ground water levels and extended periods of low flow in stream channels. Horizontal and bilobed endostratal trails and vertical burrows are the dominant invertebrate biogenic structures both in the Permian (Buckley Fm.) and the Triassic (Fremouw Fm.) fluvial deposits, which lack invertebrate body fossils. Similar size distributions and change within individual specimens from one morphology to another imply that all of the biogenic structures were produced by a single type of animal; based on the presence of scratch marks, the animal was an arthropod, probably an insect. These traces are abundant in both Permian and Triassic deposits, demonstrating that the producer was unscathed by the end-Permian extinction. However, the traces occur only in Permian crevasse splay deposits and are absent from Permian channel deposits, whereas they occur in both Triassic channel and splay deposits. The facies distribution is interpreted as follows: in the Permian the arthropod, probably an immature insect, did not burrow in channel-bottom sands because consistently high water velocity precluded settlement through the water column, but periods of low flow in the Triassic allowed the insects to settle to the channel bottom to colonize the substrate. These periods of low water flow must have been sufficiently long and regularly occurring to provide the insects with dependable access to channel-bottom sands. Dry conditions, at least seasonally, during the Early Triassic are corroborated by the presence of burrows of therapsids (mammal-like reptiles) in floodplain/vegetated bar deposits; the burrows, some > 50 cm deep, must have been above the water table when inhabited. Absence of vertebrates from these high latitudes (70-80 degrees south) during the Permian as indicated by lack of body or trace fossils may reflect water table levels too high for therapsids to construct burrows that were safe from flooding and could have provided protection during the long, cold winters.