Parvamussium stanfordense (Arnold, 1906)
ARNOLD, R. 1906. The Tertiary and Quaternary pectens of California. United States Geological Survey Professional Paper, 47: 1-264, pls. 1-53. [p. 91, pl. 23, fig. 4]
1906 Pecten (Propeamusium) [sic] stanfordensis Arnold, 1906
R. Arnold, 1906, plate 23.
|
«Description.— Shell averaging about 9 millimeters in altitude, subcircular, equivalve, equilateral, thin, only slightly ventricose, and with smooth margins. Right valve with the exterior surface ornamented by numerous fine concentric undulations and, in some cases, microscopic radiating strire; interior with 10 to 12 prominent, rounded, equidistant ribs extending from the umbo to or near to the periphery of the disk, those ribs nearest the center of the shell being straight, while those near the sides are somewhat arcuate toward the central rib; hinge line slightly longer than one-half the length of the disk; ears equal in length, and ornamented by the fine concentric undulations; anterior ear with a small byssal notch. Left valve similar to right, except that its anterior ear lacks the byssal notch .
Dimensions.— Alt. 7 mm.; long. 7.2 mm.; hinge line 4 mm.; diameter 1.2 mm. In the specimens showing the original shell material the internal ribs sometimes break out and leave corresponding grooves in the disk, thus showing that they are of a rather secondary growth compared to that of the disk. This species is somewhat similar to P. interradiatus of the Tejon (Eocene), but may be distinguished from it by having 10 to 12 instead of 8 ribs, being smaller, and having the ribs reach to or nearly to the periphery rather than abruptly terminating near the middle of the disk. The type specimen, which is the one figured, is now in the collection of Delos Arnold. It is from the buff-colored Miocene shale in a small ravine on the Burke ranch, one-third mile south of Los Trancos Creek, near Stanford University, San Mateo County. This species is named in honor of Leland Stanford Junior University. So far this species has been found only in the type locality, where it is associated with Pecten pedroanus (?) and several small undetermined pelecypods. Its range, as determined stratigraphically, is probably middle Miocene or Monterey. Its associated fauna also adds weight to this theory. RANGE
Miocene (middle). Burke ranch, 3 miles south of Stanford University (Branner, J. P. Smith, R . Arnold).»
RALPH ARNOLD, 1906
|
«Discussion: Propeamussium stanfordensis is reconsidered here in light of the revision of its stratigraphic occurrence – from middle Miocene to upper Eocene – and the availability of additional material from near the type locality for figuring. One of the specimens figured herein, a virtual topotype (Figure 5) has a much better preserved external surface showing relatively strong, regularly spaced concentric undulations.
ARNOLD (1906, p. 92) originally classified the stratigraphic occurrence of this species as middle Miocene, apparently believing the light-colored shale exposure from which it was collected to be part of the Monterey Shale. Consequently, subsequent treatments of Tertiary Propeamussium (SLODKEWITSCH, 1938; WEAVER, 1942) have considered species, and phylogenies, in the context of a Neogene stratigraphic occurrence of this taxon. A doubtful identification of this species from lower Tertiary rocks in the Aleutian Islands (SCHOLL et al., 1969) is now believed to represent a different species, described herein as P. leohertleini. The occurrence of a similar species in lower Tertiary strata led to re-examination of the type locality of P. stanfordensis near Stanford University in central California. It was found that the type locality is in an area of upper Eocene rather than middle Miocene rocks, according to recent geologic mapping and related micropaleontologic studies. Two of the specimens of Propeamussium stanfordensis figured in this report (Figure 5) can be regarded as topotypes. They are from a locality on the north wall of the same drainage from which the holotype was collected by Ralph Arnold, possibly on strike from his locality. They occur with a diverse foraminiferal assemblage of late Eocene Narizian age according to M. C. Israelsky (in written communication to E. H. Pampeyan, April 16, 1963; USGS microfossil locality Mf707). The occurrence is about 45 m stratigraphically below the base of middle Miocene rocks in a unit mapped by DIBBLEE (1966) and by PAMPEYAN (1969) as Butano(?) Sandstone and by PAGE & TABOR (1967) as unnamed Eocene rocks. Study of the recently collected specimens from USGS Cenozoic loc. M1452 (Figure 5) indicates that the anterior ear of the right valve has a rather strong byssal notch that is reflected in the curvature of the growth lines. Moreover, the internal ribs extend almost to the margins of the valves. Accordingly, placement in the subgenus Parvamussium is clearly indicated. One of the early Eocene specimens of Propeamussium mideocenicum VOKES, 1939, UCMP 15585 from the Arroyo Hondo Shale Member of the Lodo Formation in the San Joaquin basin, California, might be P. stanfordensis. The ears, number and configuration of internal ribs, and size of this syntype are similar to P. stanfordensis. The other syntypes (both left valves) are quite distinct; they probably represent an entirely different species than P. stanfordensis judging by the occurrence of external ribbing and by the fact that these ribs are much more numerous than on the right valve – 12 instead of 9. It seems doubtful that Propeamussium stanfordensis represents young specimens of the well-known Eocene species P. interradiatum (GABB) as was suggested by GRANT & GALE (1931). The shorter, consistently fewer internal ribs – 8 rather than 10 or more – and the weaker, much finer concentric sculpture of P. interradiatum (Figures 6, 11) permit differentiation. The ears are incompletely preserved on the lectotype of P. interradiatum (STEWART, 1930, p. 123). Whereas the ears and the length of the internal ribs on one of the original hand-drawn illustrations representing a left valve of P. interradiatum (GABB, 1869, plt. 33, fig. 98) indicate reference to Propeamussium (Propeamussium), the other illustration of a portion of the dorsal margin of a right valve (figure 98a) indicates a deep byssal notch and an extremely narrow right anterior ear. These features are not characteristic of Propeamussium; they suggest that the figures are of entirely different taxa. It is noteworthy that specimens in the collection from USGS loc. 5742 (basal part of the Kreyenhagen Shale near Cantua Creek, Fresno County, California) are here regarded as conspecific with P. interradiatum, compare closely with STEWART's lectotype (1930, plt. 8, fig. 10), and have a very shallow byssal notch and a broad anterior ear indicative of assignment to Propeamussium (Propeamussium). Occurrences: Late Eocene: Butano(?) Sandstone of DIBBLEE (1966), Palo Alto quadrangle, California (unnumbered Stanford Univ. locality, USGS loc. M1492); near the base of the Kreyenhagen Shale, Cantua Creek area, Fresno County, California (USGS loc. 5749); near Idria, San Benito County, California (USGS loc. 5743); Aldwell? Formation, Clallam County, Washington (USGS loc. M4149).»
ADDICOTT, W. O. 1971. Some Paleogene mud pectens of the genus Propeamussium from Alaska and California. The Veliger, 13 (3): 226-230, figs. 1-13. [p. 228, 229]
|
Propeamussium (Parvamussium) stanfordensis (Arnold); W. O. Addicott, 1971, Some Paleogene mud pectens of the genus Propeamussium from Alaska and California, figures 3, 5, 8, 10.
|