Parvamussium holzense Sundberg, 1989
SUNDBERG, F. A. 1989. Propeamussium Species (Bivalvia: Propeamussiidae) from the Upper Cretaceous of Southern California. Journal of Paleontology, 63 (1): 53-63, figs. 1-9. [p. 54, figs. 3, 4]
1989 Propeamussium (Parvamussium) holzense Sundberg, 1989
F. A. Sundberg, 1989, figure 3.
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«Diagnosis.— Propeamussium (Parvamussium) with a posteriorly ovate to subcircular valve outline; well-developed to moderately well-developed byssal notch on right anterior auricle; 6-9 radial costellae on right anterior auricle; 11-15 internal ribs on right valve (10-15 primary and 0-3 secondary); 9-15 on left valve (9-15 primary and 0-3 secondary); hinge line straight
Description.— Valves posteriorly ovate to subcircular in outline, approximately as wide as high (height/width = 1.07); auricles relatively large, equal; average umbonal angle 111°; average maximum internal rib height 75 percent of the left valve height; secondary internal ribs present in some right and left valves.
Exterior of right valves covered with several fine submarginal lirae of moderate relief; ventral margin fragile beyond distal ends of internal ribs typically not preserved; posterior auricles with fine growth lines acute to obtuse auricle angles; anterior auricles with fine growth lines, 6-9 radial costellae, well-developed to moderately well-developed byssal notches, acute to 90° auricle angles; interior of right valves with 12-15 ribs (10-15 primary and 0-3 secondary); internal ribs straight anteriorly and curved towards margin posteriorly, lateral internal ribs slightly swollen at termination. Exterior of left valves covered with several fine radial costae; radial costae variable in height, typically with one or two weaker costae between two stronger costae; radial costae sharp in cross section in valves less than 20 mm in height, rounded and less pronounced at shell heights greater than 20 mm; growth lines faint to raised, fine submarginal lirae; grid pattern formed by radial costae and submarginal lirae; posterior auricles with faint growth lines, rare radial costellae (1-5 when present), slightly acute to obtuse auricle angles; anterior auricles with faint growth lines, common radial costellae (5-8 when present), 90° to slightly obtuse auricle angles; interior of left valves with 9-15 ribs (9-15 primary and 0-3 secondary); central internal ribs straight, lateral internal ribs curved toward margin anteriorly and pos- teriorly; lateral internal ribs swollen at termination. Measurements.— See Table 1.
Discussion.— Propeamussium (Parvamussium) holzense n. sp. is variable in the development of the auricles and valve outline. The byssal notch on the right valve ranges from relatively deep to only moderately well-developed (Figure 4). Posterior auricle angles of right valves range from acute to obtuse (Figure 4). Valve outlines (Figure 5) range from posteriorly ovate (AW/PW < 1.00) to slightly anteriorly ovate (AW/PW > 1.00). The distribution of data points in the scatter plot (Figure 5) shows that the posteriorly ovate specimens are part of a continuum of forms ranging from posteriorly ovate to subcircular to slightly anteriorly ovate. Some valves may have been compressed through compaction, thus changing their outlines, but the predominance of posteriorly ovate forms suggests that it was originally present.
Specimens of Propeamussium (Parvamussium) holzense with posteriorly ovate outlines are unlike other Mesozoic species of Parvamussium from the circum-Pacific area. Tamura (1973) refered to P. (Parvamussium) hinagense as circular, although some of his figured specimens appear to be posteriorly ovate. The variability in the outline of P. (Parvamussium) hinagense may be the result of structural deformation. If this outline variation is not the result of deformation, then the two species still can be separated on the basis of P. (Parvamussium) hinagense having more internal ribs, radial costae on the right valve exterior, and coarser radial costae on the left valve exterior.
Specimens of P. (Parvamussium) holzense with semicircular outlines are similar to other Cretaceous Parvamussium species from Japan and Borneo. Propeamussium (Parvamussium) hinagense (Tamura, 1973) and P. (P.) sp. cf. hinagense (Tamura, 1973) are similar to P. (P.) holzense but differ in having fewer and stronger radial costae on the exterior of the left valve, 17 internal ribs (P. hinagense), and radial costae on lateral and ventral areas on the right valve. Propeamussium (Parvamussium) yubarense (Yabe and Nagao, 1928) differs from P. (P.) holzense in having faint radial costae on the right valve, of a well-developed byssal notch (except for a specimen figured by Tamura (1976, Pl. 1, figs. 10, 11) that has a well-developed byssal notch and is probably a new species), fewer internal ribs (typically 8-9), unequal auricles, and different auricle angles (Yabe and Nagao, 1928; Tashiro, 1976). Propeamussium (Parvamussium?) awajense Ichikawa and Maeda (1958) differs from P. (P.) holzense in having coarse submarginal lirae and fine radial costae on the left valve (if Ichikawa and Maeda are correct in identifying the valves as left valves, no right valves are known) and a smaller umbonal angle (90°). Propeamussium (Parvamussium) kimurai (Hayami, 1965) differs from P. (P.) holzense in having unequal auricles, smaller umbonal angle (100?), fewer ribs (8-9), different auricle angles, absence of radial costellae on right valve auricles, and coarser radial costae on left valve. Propeamussium (Parvamussium) tosaense (Tashiro and Matsuda, 1986) differs from P. (P.) holzense in having unequal auricles, posterior auricle on right valve with a strongly acute angle, and smaller umbonal angle (90°).
Etymology.— The species is named after the Holz Shale, Ladd Formation, in which it is found.
Material.— Holotype, LACMIP 7242, right valve from UCLA loc. 7074; paratypes, LACMIP 7212, 7223-7226 from UCLA loc. 7057; LACMIP 7214, 7215, 7227-7230 from UCLA loc. 7058; LACMIP 7218-7221, 7231-7236 from UCLA loc. 7059; LACMIP 7216 and 7217 from UCLA loc. 7061; LACMIP 7207 from UCLA loc. 7062; LACMIP 7241-7244 from UCLA loc. 7074; LACMIP 7238-7240 from LACMIP loc. 8087.
Age.— Late medial Santonian, Bostrychoceras elongatum Biozone.
Occurrence.— Holz Shale Member of the Ladd Formation at the Robinson Ranch, on Plano Trabuco (UCLA locs. 7057, 7058, 7059, 7061, and 7074); and in Silverado Canyon (LACMIP loc. 8087), Santa Ana Mountains, Orange County, California.
FREDERICK A. SUNDBERG, 1989
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