616


Journal of Paleontology 92(4):611–633


Figure 6. Histograms show the difference in shell height of five cylindrical homomyid species in stratigraphic succession. STDEV.P is Excel standard deviation function.


Figure 5. Histograms show the difference in shell height of five streamlined homomyid species in stratigraphic succession. STDEV.P is Excel standard deviation function.


Comanchean shelf in early Albian in the middle part of the Glen Rose Formation. These species occupied shallow shelf marls and successive species became larger. The cylindrical forms, H. budaensis, H. auroraensis, H. bravoensis Böse, 1910, H. vulgaris, and H. washitae, first appeared in the late Albian in the Del Norte Formation, southwest Texas. These large species occupied both calcareous shale and muddy carbonate substrates. Future cladistics analysis of all Mesozoic Pholadomyidae is recommended.


Materials and methods


Holotypes and other specimens have been reexamined and measured with calipers and selected specimens have been


photographed (Table 1); measurements of all studied specimens are available as supplemental data. Additional new specimens have been collected during numerous field seasons in Texas Comanchean Cretaceous strata since the 1970s and have been measured and used for the analyses. Most species are uncom- mon to rare at any one outcrop, and some are incompletely preserved so that some species are represented by a small number of specimens. The taxonomy of Carter et al. (2011) is followed. The suprafamilial classification and thus phylogeny of


Bivalvia has advanced significantly since the application of molecular phylogenetics (Harper et al., 2006; Bieler et al., 2010, 2014; Combosch et al., 2017). The resulting classification dif- fers from that based on “all available sources of phylogenetic information, including molecular, anatomical, shell morpho- logical, shell microstructural, bio- and paleobiogeographic as well as stratigraphic, have been integrated into the classifica- tion” (Carter et al., 2011, p. 1). The two classification positions construct different suprafamilial schemes, but the family-level schemes are similar. Analysis and critique of these two positions are beyond the scope of this paper. Because fossil bivalves in this study are preserved as composite casts, for which only rudimentary morphological features are preserved, the Carter classification is used. Collections used in this study include the following: NMNH=National Museum of Natural History, Smithsonian


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