Cravenoceras fayettevillae Gordon

Cravenoceras fayettevillae Gordon, 1965, is a goniatite from the Upper Mississippian (Chesterian = early Namurian) of northwest Arkansas. The shell is relatively small, evolute, and globular, with a small umbilicus, thin, closely spaced ribs, and three or four constrictions per whorl (Fig. 9.4A). The body chamber is approximately 1.5 whorls long, and the suture is simple and very similar to that of Cravenoceras articum (Fig. 9.4B). Growth occurs in three stages (Fig. 9.5). This species is present only in the Fayetteville Shale (Fig. 9.3), although the genus has a cosmopolitan distribution (Eurasia, North Africa, and North America) during the Late Mississippian and Early Pennsylvanian.

The specimens we studied came from the lower part of the Fayetteville Shale along the White River near Durham, Arkansas (Fig. 9.2). Many of the specimens were obtained from carbonate concretion "halos" that occasionally surround the body chambers of large (up to 3 m in length) actinoceratids. The concretions were slabbed with a diamond saw to expose the goniatites (Mapes and Dalton, 2002). Most of the ammonoids appear to be Cravenoceras fayettevillae, although specimens of other taxa, such as Tumulites and Paracravenoceras, have been observed.

Fig. 9.4 A. Cravenoceras fayettevillae Gordon, 1965, UA 77-205-1, lower Fayetteville Formation, Upper Mississippian (Chesterian), Fayetteville, Arkansas. Apertural view and left side, maximum diameter = 24mm. From Saunders et al. (1977:129). B. Partial suture of a closely related species, Cravenoceras articum Librovich, 1938, Middle Carboniferous (Namurian), Novaya Zemlya, Russia. From Ruzhentsev (1962: 589).

Fig. 9.4 A. Cravenoceras fayettevillae Gordon, 1965, UA 77-205-1, lower Fayetteville Formation, Upper Mississippian (Chesterian), Fayetteville, Arkansas. Apertural view and left side, maximum diameter = 24mm. From Saunders et al. (1977:129). B. Partial suture of a closely related species, Cravenoceras articum Librovich, 1938, Middle Carboniferous (Namurian), Novaya Zemlya, Russia. From Ruzhentsev (1962: 589).

Fig. 9.5 Plot of septum number versus septal angle (angle between septa) in Cravenoceras fayet-tevillae Gordon, 1965, AMNH 51239, lower Fayetteville Formation, Durham, Arkansas. Vertical lines demarcate three growth stages as described by Bucher et al. (1996). The arrow indicates the point at which siphuncular membranes appear at about 3.5 whorls. Siphuncular membranes may have appeared even earlier, but this is impossible to determine as the siphuncle is missing between septa 30 and 40.

Fig. 9.5 Plot of septum number versus septal angle (angle between septa) in Cravenoceras fayet-tevillae Gordon, 1965, AMNH 51239, lower Fayetteville Formation, Durham, Arkansas. Vertical lines demarcate three growth stages as described by Bucher et al. (1996). The arrow indicates the point at which siphuncular membranes appear at about 3.5 whorls. Siphuncular membranes may have appeared even earlier, but this is impossible to determine as the siphuncle is missing between septa 30 and 40.

The Fayetteville Shale is a black, concretionary marine shale with a primarily molluscan fauna dominated by cephalopods (Gordon, 1965; Saunders et al., 1977). Nautiloids and ammonoids occur as pyritized casts, or alternatively, in carbonate concretions. The environment most likely was a deep, muddy shelf with anoxic bottom conditions. As with the specimens of Crimites from Nevada, such conditions were conducive to the preservation of soft tissue. The shells of Cravenoceras fayettevillae are calcitic. Rapid burial and an abundance of phosphorus resulted in the preservation of the originally organic membranes, through replacement by phosphate or through the development of phosphatic coatings on the surfaces.

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