Rhamphidoceras saxatilis n. gen. and sp., a micromorph ammonite from the lower Turonian of Trans-Pecos Texas

Abstract

Over the past few years, the authors, along with colleagues, have documented the late Cenomanian and early Turonian ammonite faunas of the Western Interior region of the United States and its extension into Texas (Cobban, 1987, 1988a, 1988b; Cobban and Hook, 1983; Cobban et al., 1989; Kennedy, 1988; Kennedy and Cobban, 1988a, 1988b; Kennedy et al., 1989; Kennedy et al., 1987; Kennedy and Cobban, 1989, 1990). In these works the authors have recognized a series of micromorph genera and species, taxa that are genuinely diminutive and adult at diameters of 6.5–38 mm. These taxa show characteristic features indicating them to be adult, such as crowding of septa and reduction or loss of ornament at or near the aperture. Where large numbers of specimens are available, dimorphism can be demonstrated (as in Nannometoicoceras Kennedy, 1988, p. 63, with Metoicoceras acceleratum Hyatt, 1903, p. 127, PI. 14, figs. 11–14, as type species) so that they are indeed micromorphs, not simply juveniles or microconchs of “normal-sized” taxa. Such micromorphs are known from other parts of the world (e.g., Protacanthoceras Spath, 1923; see Wright and Kennedy, 1980, 1987; Kennedy and Wright, 1985; Lymaniceras Matsumoto, 1965, and Haboroceras Toshimitsu, 1988). Not uncommonly, these micromorphs are found associated with “normal-sized” taxa that have similar early ontogenetic stages to the co-occurring micromorph. There is little doubt that the evolutionary origin of these small forms was through progenesis and precocious sexual maturation in which the early developmental stage of the ancestral taxon is preserved to a much later developmental stage of the descendant. The latter matured at much smaller size, and new characters are present in the latest phragmocone and body chamber, or sometimes only in the latter. These micromorphs are not unique to the Western Interior seaway, but are better represented there than elsewhere. This may simply be due to the remarkable preservation in concretions in the mudrocks of the Interior sequence, or it may reflect a repeated evolutionary strategy to deal with the stresses of the atypical environments developed there (Hattin, 1986). It also perhaps removed some of the taxa concerned from competition for trophic resources with their “normal-sized” contemporaries, although it could equally be argued that it turned them into suitably sized prey.