Scott and Claggett—Albian pholadomyid bivalves Texas
Occurrence.—Type specimens of H. kellumi and H. tlahualiloensis were collected from the upper part of the upper interval of the Aurora Limestone. Two specimens of H. kellumi were also collected from the Fort Worth Limestone near Fort Worth, Tarrant County, Texas (Perkins, 1961, p. 55). Specimens of Homomya aff. H. ligeriensis were collected at Cerro de Muleros (now Cristo de Rey), New Mexico, west of El Paso, Texas, in units 4–6with the upper Albian ammonite Mortoniceras equidistans (originally Schloenbachia trinodosa) (Cragin, 1893). This stratigraphic interval is now composed of the upper Albian Smeltertown, Muleros and Mesilla Valley formations (Lucas et al., 2010) and correlate with the Washita Group of North Texas.
Remarks.—Perkins (1961) characterized H. kellumi by “its high beaks and pronounced shoulder anterior to the beaks.” He con- trasted H. tlahualiloensis with H. kellumi by the “less pointed and less terminal beaks,” its greater width, and by the absence of an anterior shoulder (Perkins, 1961, p. 83). However, these characters intergrade; the distance between the anterior margin and beak of the former is 17.2 ± 3.14mm compared to 17.3 ± 2.03mm in the latter (data of Homomya species described here available as Supplementary Data). The mean widths of both species are within the standard deviation: mean width of H. tlahualiloensis is 33.7 ± 3.23mm; mean width of H. kellumi is 32.5 ± 3.921mm. These relative proportions, however, are gradational and an insufficient basis upon which to define species among this variable group of homomyids. The “pro- nounced shoulder anterior to the beaks” of P. kellumi is a subtle and impersistent feature subject to compaction and distortion. In addition, the species occur together at the same sites in the upper part of the Aurora Limestone at Sierra de Tlahualilo, thus have no stratigraphic or geographic significance. Therefore, they are here judged to be synonyms. Perkins compared H. kellumi to Homomya bravoensis,
H. ligeriensis d’Orbigny (1845), which has finer growth rugae and is less elongate than H. kellumi. Böse (1910) identified a species at Cerro de Muleros (now Cristo de Rey), New Mexico, west of El Paso,Texas,that he compared to Homomya ligeriensis:
Cenomanian Homomya austinensis Shattuck (1903, pl. 16, figs. 1–3), they are quite similar in form, proportions, and size (Table 2). The gape even extends anteriorly along the posterior hinge, which merges into a low ridge, the pseudonymph beneath the beaks. However, H. austinensis differs from H. kellumi by its more broadly rounded,wider posterior margin, which reflects a wider siphon. Perkins also compared H. kellumi to Homomya aff.
which has a lower height-to-thickness ratio (Fig. 4). Homomya bravoensis is an oblong species with a length-to-height ratio of 1.38; its anterior margin is nearly at a right angle to the dorsal margin; whereas H. kellumi is a low triangular form with a length-to-height ratio of 1.34 and apical angle of about 115°. Although Perkins did not compare H. kellumi to the lower
The average shell has an ovate form, transversally exten- ded, somewhat convex, posteriorly extended; anterior margin arcuate passing by a curve into the ventral margin; this margin is also somewhat arcuate; posterior margin is not preserved in any of the specimens but nevertheless the outline of lines for a superficial very regular curve; dorsal
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margin straight; umbos strong large, prominent, proso- gyrate; situated very near the anterior of the dorsal margin. The valves are gaping posteriorly. The ornament on the surface consists of very regular concentric costellae separated by relatively deep grooves.
Our species is very similar to Homomya ligerienses of the Cenomanian at Le Mans, the differences are quite small; in the Mexican form the umbos are smaller and lower and the outline is slightly different. We cannot compare our specimens with the French species because none of our examples are complete.
Number of specimens: 4.
Locality: Cerro Muleros close to the monument marking the Mexican-US border.
Horizon: Subdivisions 4, 5 and 6, horizon with Schloenbachia trinodosa, lower Cenomanian. (Böse, 1910, p. 137)
Adkins (1928, p. 142) described Homomya aff. H. ligeriensis as a “form subrectangular, beaks nearly terminal; prominent somewhat irregular growth lines.” The specimen illustrated by Akers and Akers (2002, fig. 352) is incompletely preserved and is within the size range of H. kellumi. Because the size, shape, and stratigraphic and geographic ranges of Homomya aff. H. ligeriensis are similar to those of H. kellumi, it is placed in synonymy with H. kellumi. Ayoub-Hannaa et al. (2015) reassigned the species
Pholadomya ligeriensis to Pleuromya. Pleuromya ligeriensis d’Orbigny (1845) is well known from Cenomanian-Turonian strata in Europe, North Africa, Nigeria, and Brazil (Ayoub- Hannaa et al., 2015). P. ligeriensis is about the same size as H. kellumi and H. tlahualiloensis but the length and height of P. ligeriensis are nearly the same, whereas the Comanchean species are longer than high. The valve outline of H. ligeriensis differs from the two Albian species in having a straighter anterior margin and a more steeply inclined posterior margin, and the angle between the dorsal and anterior margins is nearly 90° whereas the same angle of the Albian species is greater than 90°. P. ligeriensis differs from H. kellumi by its strongly prosogyrate beaks and by the right angle between dorsal and anterior margins (Ayoub-Hannaa et al., 2015).
Homomya washitae Cragin, 1894 Figure 10.1–10.3
1894 Homomya washita (sic) Cragin, p. 59. 1928 Homomya washita (sic); Adkins, p. 141. 2002 Homomya washita (sic); Akers and Akers, p. 469. 1961 Homomya washitae; Perkins, p. 40.
Holotype.—NMNH PAL 32689; topotype: WSA 10444, the label reads “Homomya washitaensis (sic), railroad cut, 0.5 mi SE of Union Depot, Denison.”
Diagnosis.—Shell elongate, broadly inflated, beak at anterior margin; escutcheon a narrow shallow depression merging posteriorly with a straight commissure line posterior to beaks; concentric rugae as flat ridges wider than the interspaces; ribs slope slightly ventrally and widen from beak to ventral margin.
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