Provenance of rare Nevada shark fossil questioned
© 2009 Clarence D. Basso
The cast of the coiled jaw of an unusual extinct species of shark, Helicoprion nevadensis Wheeler, supposedly collected from the Koipato Group rhyolite of Rochester in the Humboldt Range twenty miles northeast of Lovelock, Nevada, in 1929 was not recovered in situ according to data from later studies. The Paleozoic age originally assigned to the formation on the basis of H. nevadensis Wheeler is inconsistent with lithology of the fossil’s matrix and that of the native formation.
Wheeler (1939) contended “The stratigraphic restriction of Helicoprion to the Uralian and Artinskian stages impels the assignment of these strata of Nevada and California to the Anthracolithic.” Later studies of the lithology of Helicoprion nevadensis Wheeler seriously challenge the specimen’s origin and consequently the formation age assignment based on it.Wheeler reported the matrix consisted of about 50 per cent quartz with occasional feldspar and muscovite fragments, fine fragmented glass, minute grains of pyrite and other fine-grained, carbonaceous material, but x-ray diffraction analysis of the matrix in1960 (Silberling,1973) showed that it contained more than 80 percent quartz and lesser amounts of mica and chlorite and no detectable feldspar, leading Silberling to conclude, “The presence of chlorite, and the absence of feldspar, make this rock [fossil matrix] unlike any of the large numberof rhyolitic volcanic, tuffaceous and clastic rocks analyzed from the Rochester Rhyolite… Thus, although it cannot be proved, the probability that this specimen did not originate from the Koipato [Rochester Rhyolite] is strong.” Other geologists also question the provenance of the Wheeler specimen. Johnson (1977) noted that fission-track (McKee and Burke, 1972) and lead-alpha (Wallace et al, 1960) median ages of the Rochester Rhyolite support a Triassic age assignment and Hanger (1998) concurred that the specimen has “no age control.”
The biostratigraphic significance of this exceptional specimen of Helicoprion has been greatly diminished by the circumstances of its collection and later comparative lithology. Further study of H. nevadensis has been compromised by the specimen’s undetermined disposition following renovation in the early1990s of the Mackay Museum at the University of Nevada, Reno, where the specimen had been housed as Paleontological Specimen No. 1001/28.
Johnson, M.G., 1977, “Geology and Mineral Deposits of Pershing County, Nevada.” Nevada Bureau of Mines and Geology Bulletin 89.
McKee, E.H. and Burke, D.B., 1972, “Fission-track Age Bearing on the Permian-Triassic Boundary and Time of the Sonoma Orogeny in North-centralNevada.” Geological Society of America Bulletin 83:7.
Silberling, N.J., 1973, “Geologic Events During Permian-Triassic Time Along The Pacific Margin of the United States” in Logan, A. and Hills, L.V.,ed.,”Permian and Triassic Systems and Their Mutual Boundary.” Canadian Society of Petroleum Geologists Memoir 2.
Wallace, R.E. et al, 1960, “Intrusive Rocks of Permian and Triassic Age in the Humboldt Range, Nevada.” U.S. Geological Survey Profession Paper 400B.
Wheeler, Harry E.,1939, “Helicoprion in the Anthracolithic (Late Paleozoic) of Nevada and California and Its Stratigraphic Significance.” Journal of Paleontology 13:1.
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