Scott and Claggett—Albian pholadomyid bivalves Texas
Description.—Shell globose to inflated, heart-shaped, anterior margin truncated, posterior margin subrounded, inflated, orna- mented by concentric irregular striae and growth rings; umbo prominent, enlarged, prosogyrate (Roemer, 1852, p. 45, trans- lated from the Latin). Valve length nearly the same as height. Valve shape triangular, angle between anterior margin and
dorsal margin about 65°, posterior margin short, straight, inclined about 145° from dorsal margin; posterior-ventral corner arcuate merging with broadly curved ventral margin that curves broadly into nearly straight anteriormargin,wide ovate gape.Umbo broadly curved prosogyrate. Escutcheon narrow, elongate, bordered by low ridges on either valve. Ornamented by low, rounded, concentric growth rugae wider than interspaces, topped by minute, granule- like nodes aligned forming weak indistinct lineations. Roemer (1852) listed the length of H. alta as 1″ 9‴, height
1″ 9‴, width 1″ 5‴, which are translated as 1.9 inch and 1.5 inch. Converted to metric units: L=48.26mm; H=48.26mm; W=38.10mm.
Remarks.—Ayoub-Hannaa et al. (2015) designated Homomya alta Roemer (1852) the type species of their new subgenus, Sergipemya. Their diagnosis of Sergipemya includes “posterior gape lacking; numerous fine commarginal ribs covering the valves and crossed by faint radial tubercles”; however, they state in their description of Liopistha (Sergipemya) alta that “radial ornament is absent” (Ayoub-Hannaa et al., 2015, p. 58). They also say that a posterior gape is absent. The type specimen of H. alta is ornamented by irregular concentric ridges and grooves (Roemer, 1852); minute, closely spaced nodes top the concentric ridges and are vertically aligned in faint radial rows (Fig. 11.5–11.7), which is consistent with the subgeneric diagnosis. However, the diagnosis should be modified to include the presence of a posterior gape. Roemer’s species is middle to upper Albian, and Ayoub-Hannaa et al. (2015) extend the range into the Cenomanian. Roemer (1852) compared the overall form of H. alta to
Pholadomya ligeriensis d’Orbigny (1843), which Ayoub- Hannaa et al. (2015, p. 50) reassigned to Pleuromya. The triangular shell morphology of H. alta is very similar to that of Pholadomya (Bucardiomya) Rollier in Cossman (1912), which, however, has distinct, even, wide radial costae. A species very similar to H. alta is Pholadomya (Bucardiomya) gigantea Jaitly (2013) in the Middle Jurassic of western India. This species is about 60cm tall and has faint indistinct umbonal radial ribs. A note to clarify the stratigraphic position of Liopistha
[Cardium] elegantulum (Roemer, 1852), the type species of Liopistha: Roemer collected his specimen at the waterfall in the Guadalupe River beyond (“unterhalb” was Roemer’s word; Roemer, 1852, p. 49) New Braunfels, Texas. In this area, either the lower Cenomanian Buda Limestone or the middle to upper Albian Edwards Limestone is exposed, so the age of L. elegantulum is not Santonian as suggested by Dhondt and Jagt (1988) but Albian or lower Cenomanian.
Order Pholadomyida Newell, 1965 Superfamily Pholadomyoidea King, 1844
The superfamily Pholadomyoidea is composed of four phylogenetically related families: Pholadomyidae, Arenigo-
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myidae, Margaritariidae, and Ucumariidae (Carter et al., 2006; Carter et al., 2011).
Family Pholadomyidae King, 1844
Remarks.—The family Pholadomyidae is composed of the subfamilies Pholadomyinae and Chaenomyinae (Carter et al., 2011).
Description.—Small- to large-sized, equivalved, very inequi- lateral, oblong, ovate or subtrigonal, moderately to very inflated bivalves; anterior margin evenly curved, ventral margin gently curved to nearly straight, some with shallow sulcus; ventral- posterior margin curved to truncate; dorsal-posterior margin straight, inclined ventrally or dorsally. Posterior gape narrow to wide, anterior gape where present is narrow. Umbos rounded to subangular, anterior of midline; posterior umbonal carina weak or absent; lunule absent or shallow in most species; escutcheon, if present, shallow, bordered by low ridges. Ligament external, opisthodetic, quasi-parivincular,
attached to low, rounded pseudonymph in escutcheon (Carter et al., 2012, p. 148); cardinal plate extends from umbo posteriorly, with a resilium of low wave-like swellings and grooves; hinge edentulous, narrow, and straight in most, some with one or more small knob-like projections on hinge. Pallial sinus depth variable among genera; adductor and pedal muscle scars present. Radial ornament is absent or low, radial folds or ribs, some with tubercles, some with fine radial lines or radial rows of pustules, most with distinct to weak concentric growth rugae. Shell composed of aragonitic, thin, nacreous ostracum but rarely preserved (personal communication, J.G. Carter, 2016); Early Triassic to Holocene.
Subfamily Pholadomyinae King, 1844
Remarks.—Subfamily comprises 23 genera (Newell, 1969), including Homomya and Pachymya.
Genus Homomya Agassiz, 1843
Type species.—Mactra gibbosa J. Sowerby, 1813 in Sowerby and Sowerby, 1812–1846, p. 91; Herrmannsen, 1847, p. 541.
Diagnosis.—Equivalved, elongate to ovate, moderately pro- truding umbos situated anterior of valve midline; dorsoposterior margin straight, downwardly sloping, or slightly upturned; edentulous, opisthodetic ligament attached to pseudonymph in escutcheon; deep pallial sinus; ornamented by concentric growth rugae; radial ribs may be present only in juvenile stages.
Occurrence.—Early Triassic to middle Eocene.
Homomya knowltoni (Hill, 1893) Figure 9.1–9.8
1893 Pholadomya knowltoni Hill, p. 30, pl. 2, figs. 1, 2. 1901 Pholadomya knowltoni; Hill, p. 232. 1940 Pholadomya knowltoni; Smith, p. 611. 1982 Pholadomya knowltoni; Offeman et al., p. 79, fig. 103. 1896 Pleuromya knowltoni; Stanton and Vaughan, p. 23.
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