SYSTEMATIC PALEONTOLOGY

Type species. Caprinuloidea perfecta Palmer, 1928, by original designation.

v. 1928 Caprinuloidea perfecta Palmer, p. 59-60, pl. VIII, fig. 8; pl. IX, figs. 1, 2; text-fig. 6.

v. 1991 Caprinuloidea perfecta Palmer, 1928; Scott and González-León, p. 62; figs. 7C-F; cites previous references.

Discussion. The caprinid genus, Caprinuloidea Palmer, occurs in Albian rocks in Mexico, Southwestern USA and the Caribbean (Alencáster et al. 1999; Coogan 1973; Payne et al. 2004). Major characteristics of this genus include two teeth in the left-free valve (LV-FV) and one tooth in the right-attached valve (RV-AV), body cavity, accessory cavity, two types of pallial canals, pyriform and polygonal, around both valve margins but not in internal structures, invaginated ligament marked by external groove and muscle attachment sites (myophores) (Skelton and Masse 1998). The two valves are highly unequal in size and have quite different shapes. The RV-AV is long and curved with a slight rotational twist. The LV-FV is trochospirally coiled with one whorl. The cross-sections of both valves are approximately quadrilateral.

Silicified specimens of Caprinuloidea perfecta are quite common in the Edwards Formation of central Texas. Specimen TMM NPL4387 is deposited in the Non-vertebrate Paleontology Laboratory (NPL) of the Texas Natural Science Center at The University of Texas at Austin (its exact location is not recorded). Ann Molineux of NPL made the specimen available to The University of Texas High-Resolution X-ray CT Facility for scanning in order to identify internal structures (Figure 1, Figure 3). The disarticulated RV-AV is well preserved and illustrates diagnostic internal structures. Other caprinid specimens are found in the Lower Albian Glen Rose Formation in Texas.

At least two species of Caprinuloidea are recognized in the Caribbean Province and the Gulf Coast: C. perfecta Palmer, 1928 and C. multitubifera Palmer, 1928 (Scott 2002). Both species range from lowermost Albian to the basal part of the Upper Albian. The two species are differentiated by the number of rows of inner polygonal canals; C. perfecta has two, and C. multitubifera has four or more (Coogan 1977). This specimen from the Edwards Formation has only two rows on its ventral and anterior margins (Figure 2). Although Palmer (1928) defined six species of Caprinuloidea, only two can be differentiated (Coogan 1977; Scott 2002). Tabulae are present in all of Palmer's species.

The shell structure includes ventrally trifurcating marginal plates cut by radial plates to form two rows of polygonal canals (coronal slice 127) (Figure 1.3). The body cavity is slightly off center, with anterior and posterior tooth sockets separated by the ligamentary infolding on the dorsal side. The ventral side is the thinner side of the skeleton and perhaps the recumbent side or attached side (coronal slice 1205, see Figure 4). The external ligamentary groove marks the trace of invagination of the ligament.

A series of coronal slices from near the apex of the juvenile portion to the commissure shows that the body cavity, accessory cavity and anterior tooth socket are developed early and simply enlarge during growth (Figure 3). The posterior pallial canals, however, are inserted at a stage about 1.5 cm from the apex. Note that the protoconch cannot be observed in this specimen. Although somewhat obscured by silicification, it appears that the pyriform pallial canals develop first, and at the 1.5 cm stage the polygonal canals begin to appear. A separate study in progress is examining additional specimens to clarify the details of these observations.