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Masters Thesis

The nonmarine mollusks of Pit 91, Rancho La Brea, Southern California and their paleoecologic and biogeographic implications

Pit 91 has yielded a fauna of 31 taxa of nonmarine mollusks from member C of the late Pleistocene Palos Verdes Sand. The fauna includes 5 freshwater bivalves (Anodonta californiensis, Pisidium casertanum, ~. compressum, Musculium lacustre, and M. cf. M. partumeium), 15 freshwater gastropods (Valvata humeralis, Pyrgulopsis cf. P. californiensis, Fossaria modicella, F. parva, F. bulimoides, F. cockerelli, F. cubensis, F. sonomaensis, Stagnicola elodes, ~. proxima, Physella concolor, Gyraulus circumstriatus, Q. parvus, Planorbella tenuis, and Menetus opercularis), and 11 terrestrial gastropods (Pupilla hebes, Vertigo occidentalis, Vallonia cyclophorella, Punctum californicum, Discus whitneyi, Succineidae genus and species indeterminate, Euconulus fulvus, Zonitoides arboreus, Striatura pugetensis, Deroceras laeve, and Helminthoglypta sp.). New records of taxa include 24 for the Palos Verdes Sand, 16 for the Pleistocene of California, and 4 new to the fossil record. Systematic descriptions include illustrations, synonomies, local presences, geological age ranges by epoch and land mammal age, stratigraphic and modern distributions, habitats, and remarks for each taxon, The sediments of Pit 91 were deposited 25-32 Ka in channel and lateral bars of anastamozing, streams upper and lower point bars of meandering streams, oxbow lakes, levees, and flood basins and correlate to submembers band c of member C of the San Pedro Sand. Mollusk fossils were common in deposits of all environments except upper point bars and flood basins. The aquatic mollusks lived in shallow, permanent but seasonally varying, sluggish streams with scattered deeper pools and vegetation rooted in mud bottoms and banks. The terrestrial environment included riparian forests or woodlands, upland woodlands, chaparral, and coastal sage scrub. The Pleistocene climate was cooler, milder, and moister than today. Summer maxima did not exceed 26.7�C, winter minima did not fall below 2.8�C, and annual precipitation as at least 522 mm. More moisture was available during the summers, either as rainfall or fog drip.

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