Choctawites, Titus & Korn & Harrell & Lambert, 2015

Titus, A. L., Korn, D., Harrell, J. E. & Lambert, L. L., 2015, Late Viséan (late Mississippian) ammonoids from the Barnett Shale, Sierra Diablo Escarpment, Culberson County, Texas, USA, Fossil Record 18 (2), pp. 81-104 : 94-95

Choctawites n. gen.

Type species: Goniatites choctawensis Shumard, 1863 .

Genus diagnosis: Dombaritinae with subevolute to evolute inner whorls; the umbilicus becomes closed in early ontogeny. Whorl profile with parallel or subparallel flanks, conch in juveniles widest at some distance from the umbilicus. Externally expressed constrictions extremely rare or absent. Growth lines slightly biconvex and rectiradiate; external sinus shallow. External lobe Y-shaped, very narrow or narrow, usually with slightly curved flanks.

Included species:

choctawensis : Goniatites choctawensis Shumard, 1863, p. 109 ; Oklahoma.

cumminsi : Glyphioceras cumminsi Hyatt, 1893, p. 467 ; Texas.

kentuckiensis : Goniatites kentuckiensis Miller, 1889, p. 439 ; Kentucky.

Discussion: All Late Viséan–early Serpukhovian goniatitid ammonoids with prominent adult spiral ornament found in North America can be readily divided into two major groups ( Gordon, 1965), (1) those with external shell constrictions and (2) those in which constrictions are developed as internal shell thickenings and thus only visible on the internal mould. This subdivision largely parallels the whorl cross sections, of which the first group possesses more or less trapezoidal early whorl profiles (in which the conch is widest at some distance from the umbilicus), and the second group shows “normal” whorl profiles with flanks converging from the umbilical wall towards the venter.

Examinations of collections from the Moorefield Formation, Caney Shale, and Barnett Shale show that the two morphological groups have very little morphological or ornamental overlap. Trapezoidal forms are consistently crenistriate or cross lirate to about 15 mm diameter. In contrast, the normal forms develop strong spiral ornament already by 10 mm conch diameter and can exhibit sub-triangular coiling. Furthermore, at any given locality or horizon in the eastern USA, either trapezoidal or rounded forms can totally dominate a sample.

Three possible hypotheses could explain this dimorphic pattern: (1) broad intraspecific variation; (2) sexual dimorphism, and (3) two different taxonomic groups are present in North America. Given the lack of morphological intermediates between trapezoidal and normal forms, we reject the first hypothesis. Regarding the second hypothesis, the stratigraphically lower normal forms associated with Sulcogirtyoceras limatum ( Miller and Faber, 1892) are readily referable to Lusitanoceras because they possess the characteristic early ontogeny, sutural shape, and ornament (particularly with granulated spiral lines) of that genus. In fact, these forms are so close to the European species of Lusitanoceras that they have been referred to as “ Goniatites granosus Portlock, 1843 ” since the early 1960s ( Gordon, 1965). While we doubt the species assignment, the referral of the North American material to the genus Lusitanoceras is correct. If the trapezoidal forms are simply sexual dimorphs of Lusitanoceras , it would be expected that this same dichotomy would exist in the European and North American occurrences. Instead, the trapezoidal forms are found almost exclusively in Marathon–Ouachita–Appalachian foreland basin system, being unknown from Europe and extremely rare in the Antler Foreland Basin. This pushes us to accept that the trapezoidal forms are a distinct taxonomic entity endemic to Laurentia (North America). Since they cannot be accommodated in any existing genus, we erect the new genus Choctawites for them.