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An Additional Pleistocene Occurrence of the Murre, Uria Aalge

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Among some vertebrate fossils recently found in a Pleistocene horizon at Mussel Rock, northwestern San Mateo County, California, is the distal end of a humerus of a murre (U. C. Mus. Paleo. No. 36056). The highly distinctive shape of this part of the alcid skeleton leaves no question concerning the family affinities of the fossil. Except for parts of the surfaces of the condyles, the fossil is well preserved, showing tendinous and ligamentous scars clearly. In every detail it corresponds with the modern Uris aa&e. The breadth across the shaft just proximal to the ectepicondylar crest is greater than in any of the other larger living alcids such as Lunda and Cerorlzirsca, and the crest is less divergent from the shaft at its proximal end. Although we are not able to find differences in configuration of the distal part of the humerus which would separate U. aalge and U. Zomvin, the latter averages larger, at least in the Pacific Basin. The fossil is nearly as small as the smallest humerus in a series of forty skeletons of the modem Uris aolge califomica. Furthermore, on geographic grounds, it is most unlikely that the northern U. Zomvia would occur on the coast of California even in the Pleistocene. We may conclude then that the fossil belongs to the species aalge.

Uria aalge has been known previously as a fossil only from the Upper San Pedro Pleistocene, near Playa de1 Rey, Los Angeles County (Howard, Condor, 38, 1936:212).

The fossil was found by the junior author on the east side of a road cut on State Highway No. 1 where the latter skirts the ocean shore at the 200-foot contour due east of Mussel Rock (U.S. G. S. map, San Mateo Quadrangle; U. C. Mus. Paleo. lot. V-4018). The Pleistocene exposed here is of fresh water origin and lies unconformably between the marine Merced Pliocene and late Quaternary red sands, also marine. The Pleistocene beds dip to the northeast, away from the present shore line. Probably they represent an area which was part of a small stream drainage system close to the shore and which subsequently through diastrophism has been elevated and tilted inland. The associated mammalian fossils include bison, horse, sloth and mammoth. Somewhat puzzling is the presence of such a strictly maritime species as the murre in anything but a salt water deposit. It would have been possible, however, for a dead or incapacitated murre on a beach to have been taken a short distance inland along a stream course by some carnivore or scavenger.

Museum of Vertebrate Zoology, Berkeley, California, December 3, 1940;

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