The benthic, free-living oyster Texigryphaea was the dominant constituent of many late Albian marine communities in the Texas and southern Western Interior regions. Large topotypic assemblages of three common lower–middle Washita Group species (T. navia and T. pitcheri in Oklahoma and T. tucumcarii in New Mexico) each display considerable morphological variation in valve shape and the proportions and expression of various features. Variation within an assemblage is partly due to ontogenetic changes but is mainly ecophenotypic, with local variation in nature of substrate, water turbulence, length of attachment time, and other factors influencing the final morphology of the mature shell. The T. navia assemblage is distinct in several important morphological characters from the other species, and the differences become more pronounced with growth. Texigryphaea navia appears to have been adapted to relatively firm substrates in moderately agitated conditions, in contrast to the other species, which occupied softer substrates in quieter environments. The essentially contemporaneous T. pitcheri and T. tucumcarii assemblages display much overlap in all measured dimensions of the left valve and in the range of intergrading morphs that compose each assemblage. Accordingly, T. tucumcarii is considered a synonym of T. pitcheri, representing populations of that species that lived in the West Texas-New Mexico area and developed only minor differences from the eastern populations. Within the T. navia topotypic assemblage are specimens intermediate between T. navia and T. pitcheri, and the eastern and western T. pitcheri assemblages contain forms apparently transitional to two other species, T. washitaensis and T. belviderensis. Ecophenotypic variation in the T. pitcheri assemblages appears to be greater than that in European Jurassic Gryphaea species and mirrors to some extent phyletic variation in European Jurassic Gryphaea lineages.
Plant Distribution Surrounding Rocky Mountain Pinyon Pine and Oneseed Juniper in South-Central New Mexico
