Scott and Claggett—Albian pholadomyid bivalves Texas
2002 Homomya budaensis; Akers and Akers, p. 369, 468, fig. 352.
1961 Homomya auroraensis Perkins, p. 81, pl. 24, figs. 1–3. 2002 Homomya auroraensis; Akers and Akers, p. 468.
Holotype.—Homomya auroraensis holotype UMMP 32853. Reference specimens in the Non-vertebrate Paleontology Laboratory: UT 32300, UT 30019, UT 32298, UT 32294.
Lectotype.—Homomya budaensis lectotype UT 32300; topo- types: UT 30019, UT 32298, UT 32294.
Diagnosis.—Medium to large, 75–100mm long, equivalved, greatly inflated shell; prosogyrate beaks nearly even with anterior margin, anterior margin truncated, ventral margin con- cave, posterior margin expanded, dorsal margin concave, dorsal- posterior corner inclined upward, higher than anterior margin.
Occurrence.—Whitney’s specimens are from the upper 2m of the Buda Limestone at Shoal Creek, Austin, Travis County, Texas. Perkins’s specimens of H. budaensis are from seven sites in the upper Aurora Limestone in the P. kellumi zone, which correlates with the Washita Group of north Texas (Scott et al., 2003). The holotype of H. auroraensis co-occurs with H. budaensis at Perkins’s site 18 in the H. kellumi zone of the upper Albian Aurora Limestone, Ojo de Agua, Sierra de Tlahualilo, Coahuila, Mexico. The total known range of H. budaensis is middle Albian to lower Cenomanian.
Description.—“Shell large, globose, equivalved, inequilateral, greatly produced and broadened posteriorly; anterior margin short, curved; ventral margin deeply curved; posterior margin broadly rounded; dorsal margin curved, subparallel with ventral; umbos globose, approximate, elevated, curved inward and slightly forward, situated anteriorly; shell closed anteriorly, gaping posteriorly. Surface marked by lines of growth, which vary considerably in size” (Whitney, 1911, p. 15–16). The valves gape posteriorly; beaks are low, large, subterminal in position; surface ornament of irregular, coarse, concentric growth lines (Perkins, 1961, p. 81–82). H. budaensis: length= ~110mm; height=60mm; width=
61mm (Buda Formation; Whitney, 1911, p. 16); mean length=88.37mm; mean height=48.0mm; mean width= 52.1mm; mean beak distance to anterior margin 10.8mm (Aurora Limestone). H. auroraensis: length=85.8mm; height=46.9mm; width=44mm (Table 1).
Remarks.—Whitney distinguished H. budaensis from H. vulgaris by its overall shape, the more curved dorsal and ventral margins, the more expanded, flaring posterior margin, the greater width, and the variable growth rings. The dorsal- posterior corner flares up and the posterior margin is straight and tilted upward. Perkins (1961, p. 81) distinguished H. auroraensis from H. budaensis by its “more concave dorsal border, a greater height in relation to the thickness of the shell and in not having the broad posterior, vertical expansion.” Perkins also noted that the growth rings of H. auroraensis seemed less coarse. The height-to-width ratio of the holotype of H. auroraensis is 1.2 and that of eight specimens of
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H. budaensis is 0.923 ± 0.066 (Fig. 6). Furthermore, the shapes and dimensions are very similar except that the beak-to-anterior- margin distance is greater in H. auroraensis, 19mm compared to 10.8mm in budaensis. Because these two species are morphologically similar and because H. auroraensis occurs together with H. budaensis in the H. kellumi zone, Aurora Limestone, the species are synonymized. Perkins (1961) identified four specimens as Homomya
budaensis from the Texigryphaea mucronata Zone in the lower part of the upper member of the Aurora Limestone, Coahuila, Mexico: UMMP 32972, 32974, 32976, and 32977. Two relatively complete specimens are indistinguishable from the holotype. This extends the range of H. budaensis from the upper part of the Washita Group–equivalent Aurora Limestone into the lower part of the Fredericksburg Group– equivalent.
Homomya cymbiformis Perkins, 1961 Figure 10.6, 10.8, 10.9
1961 Homomya cymbiformis Perkins, p. 83, pl. 26, figs. 1, 2, 5.
2002 Homomya cymbiformis; Akers and Akers, p. 367, 468, fig. 350.
Holotype.—Homomya cymbiformis holotypeUMMP 33012, an external cast; paratype: UMMP 33013, cast of incomplete specimen.
Diagnosis.—“A Homomya distinguished by the strong central concavity of the dorsal border and its cymbiform profile” (Perkins, 1961, p. 83).
Occurrence.—The type locality of H. cymbiformis is the lower part of the Marys Creek Member of the Goodland Formation in Fort Worth, Tarrant County, Texas, Fredericks- burg Group, middle Albian. An additional specimen in the Non-vertebrate Paleontological Laboratory, University of Texas, is from the Weno Formation, Washita Group on Old Mansfield Road, Tarrant County, Texas (UT 35247), which extends the species range into the middle part of the upper AlbianWashita.
Description.—A medium-sized, 60–70mm long, equivalved, elongate, compressed shell characterized by a concave dorsal margin and dorsally arched, expanded posterior margin result- ing in the so-called cymbiform margin. Prosogyrate umbos close to the anterior margin, distance about 25% of length. Ornament of simple concentric, co-marginal, rounded growth rugae.
Remarks.—H. cymbiformis is similar in outline to H. budaensis and their ranges overlap from uppermost middle Albian to middle upper Albian. The valves of H. cymbiformis are more equidimensional than H. budaensis; the length-to-height ratio is 1.48 versus 1.69. H. cymbiformis is higher than wide, 1.25 versus 1.08, and its volume is less than half of the volume of H. budaensis: 73,419mm3 versus 164,760mm3.
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