The Late Cretaceous oyster Cameleolopha bellaplicata (Shumard 1860), guide fossil to middle Turonian strata in New Mexico

Abstract

Cameleolopha bellaplicata (Shumard 1860) is an easily recognized fossil oyster that occurs abundantly in sandy strata in Arizona, Colorado, New Mexico, Utah, and Texas, where it is restricted to the middle Turonian ammonite zones of Prionocyclus hyatti and P. macombi. It is a distinctive, medium-sized, plano-convex, ribbed oyster that has an undulating or zigzag margin and a small to nonexistent attachment scar; it occurs in great numbers, usually as original shells. In central New Mexico, C. bellaplicata is an excellent guide fossil to the Fite Ranch Sandstone Member of the Tres Hermanos Formation, which was deposited as nearshore sands during the second major transgression of the Late Cretaceous seaway in the state. Since 1965 Cameleolopha bellaplicata has been split, erroneously, into two chronological subspecies: a supposedly older, more coarsely ribbed form, C. bellaplicata novamexicana (Kauffman), that was thought to be restricted to the middle part of the Prionocyclus hyatti Zone, and a younger, more delicately sculptured form, C. bellaplicata bellaplicata (Shumard), that ranges into the overlying P. macombi Zone. Although the two subspecies have type localities in Socorro County, New Mexico, and Grayson County, Texas, respectively, their supposed chronostratigraphic relationship could not be established at either type locality because the two supposed subspecies do not occur together at either type locality. Presumably, this chronostratigraphic relationship was established in Huerfano County, Colorado, where the ranges of the two morphotypes were interpreted to lie one above the other within the P. hyatti Zone and with no zone of overlap. However, the chronological separation of the morphotypes by ammonite zone was based on a misidentification of the prionocyclid ammonite that occurs with the holotype of Cameleolopha bellaplicata novamexicana. Initially, only fragments of large individual prionocyclids were found; they were identified as Prionocyclus hyatti. Recent collections of ammonites from the type area of C. b. novamexicana contain small diameter internal molds that are unequivocally the younger P. macombi, rather than the older P. hyatti; associated fauna includes Inoceramus dimidius White, which substantiates assignment of the holotype of C. b. novamexicana to the younger P. macombi Zone. With the index ammonite identified correctly, there is no chronostratigraphic basis for the subspecies separation. A reinterpretation of the original morphometric data shows that the presumed differences between the subspecies are subtle and represent normal species variation. Beginning in the late 1940s, strata that are now included in the Tres Hermanos Formation in New Mexico and Arizona were correlated with and included in the stratigraphically higher Gallup Sandstone. In the late 1970s, collections of Cameleolopha bellaplicata and its descendant C. lugubris from the Zuni and Acoma Basins in west and west-central New Mexico were instrumental in establishing that the Tres Hermanos Formation in central New Mexico is older than the oldest part of the Gallup Sandstone in its type area near Gallup, New Mexico. The upper part of the Tres Hermanos Formation was deposited during the transgressive phase (T-2) of the second major depositional cycle in the seaway, whereas the Gallup Sandstone was deposited during the regressive phase (R-2) of the same cycle. Introduction Late Cretaceous oysters of the Cameleolopha lugubris group (Fig. 1) are common, generally abundant, and easily identifiable members of middle through late Turonian faunas in the southern part of the Western Interior of the United States. This is especially true in New Mexico where the older and larger of the two named species of the group, C. bellaplicata (Shumard 1860), is a guide fossil to the FIGURE 1—The Cameleolopha lugubris group consists of two formally named species, C. lugubris (A, B, C, and E) and C. bellaplicata (D, F, G, H, I, and J). Cameleolopha lugubris: A—Lower (left) valve from New Mexico. B—Upper (right) valve from Huerfano Park, Colorado. C and E—External and internal views of lower valves from Mancos, Colorado; and C. bellaplicata: D and F—Lower valves from Huerfano Park, Colorado. G—Lower valve from near Pueblo, Colorado. H—Lower valve from near Sherman, Texas. I—Interior of lower valve from Huerfano Park, Colorado. J—Interior mold of an upper valve from Huerfano Park, Colorado. This diagram was used originally by Stanton (1893, pl. 4) to show his concept of the group as single species that varied from a small, dwarf form with a large attachment scar, Cameleolopha lugubris Conrad 1857 to a medium-sized form with a small to absent attachment scar, C. bellaplicata Shumard 1860. Cameleolopha lugubris became the namebearer for the species because it had date priority. One of Conrad’s (1857, pl. 10, fig. 5b) type specimens from east of the Red (now Canadian) River, Colfax County, New Mexico, is shown as specimen “A.” Specimens are at 90%. 10 mm D A