Futakami and Haggart—early Albian (Early Cretaceous) douvilleiceratid ammonites
Douvilleiceras aff. spiniferum (Whiteaves, 1876) Figure 12
Description.—Although the specimen has a secondarily deformed body-chamber shell, the ornamentation on the septate part of the whorl is well-preserved. The shell is of the middle growth stage, ca. 67.8mm in diameter, evolute and widely umbilicate. The whorl is depressed (B/H =1.34). The whorl section is polygonal along the costa, with maximum thickness at the mid-flank. Ribs on the outer whorl are rectiradiate across the middle and outer parts of the flank, but rursiradiate on the umbilical margin; they number twenty-two on the outer whorl, and are coarse and separated by interspaces wider than the ribs themselves. There are six rows of tubercles on the body chamber at about 67mm diameter. Three ventral tubercles are clavate, forming somewhat distinct spiral ridges. On the septate part of the whorl greater than 40mm diameter, the lateral and umbilical tubercles are faint or lacking, and the ribs on the inner half-whorl are fused to form a wing-like feature, rather than distinct tubercles (Fig. 12.3). The ventral sulcus is wide and deep, but not U-shaped in outline. The suture line is well-preserved on the septate portion of the whorl (Fig. 12.4). The external (E) and lateral (A) lobes are relatively narrow and deep, and the first lateral (E/A) and umbilical (A/U1) saddles are broad. The character of the suture is a typical douvilleiceratid.
Dimensions.—See Table 5.
Material.—A single specimen, GSC No. 10710, was collected by M. Futakami from a calcareous concretion at GSC Loc. C-300855.
Occurrence.—The specimen was obtained from the lower member of the Haida Formation. Its stratigraphic position is assigned to the upper part of the lower Albian, the mammillatum zone.
Remarks.—The specimen is characterized by an extended rib which forms a wing-like feature on the inner half-whorl of the middle growth stage. Due to the presence of this extended rib on the flanks, the umbilical tubercle has disappeared, and the lateral tubercle becomes faint or disappears altogether. The specimen is unique, particularly in the shape of its costal section, and differs from all other species of Douvilleiceras in this feature. The specimen is similar to D. spiniferum (Whiteaves) in the mode of the ventrolateral clavate tubercles, which exhibit the great range of variation described above. The specimen differs from typical D. spiniferum, however, in having the unique, extended rib on the inner half-whorl on the middle growth stage. In addition, the
Table 5. Dimensions of specimens of Douvilleiceras aff. spiniferum (Whiteaves) from Haida Gwaii, British Columbia
Specimen GSC No. 10710 (67.8) -
D H B B/H U U/D NR NT -
- 25.1 (0.37) 22 (6) GSC No. 10710 (-90°) 56.0 21.6 29.0 1.34 19.1 0.34 20 4
All measurements in millimeters. Measurements in parentheses are approximate.
55
Figure 11. Whorl sections of Douvilleiceras scabrosum Casey from Haida Gwaii. GSC No. 21241, GSC Loc. 7594, at approximately 75mm in shell diameter (1) and at approximately 127mm in shell diameter (2).
ribs on this species are sharper than those of D. spiniferum. This specimen may represent an extreme variant of D. spiniferum, possibly a malformed shell of that species, or possibly a new species altogether.
Discussion
Ammonites assigned to the genus Douvilleiceras are char- acterized by the presence of a ventral sulcus and multi- tuberculate ribs in the early and middle growth stages. The tuberculation consists of five to ten rows of tubercles on each rib in the middle growth stage, about 50 or 60mm to 100mm in shell diameter. The tubercles are variable in morphology and position; typically they consist of umbilical bullae or spinose nodes, lateral spinose nodes, and ventrolateral clavi, often with numerous intermediate tubercles. The umbilical and lateral tubercles form the bases of spines, which were probably broken off from the shell during post-mortem processes. The rather depressed whorl is polygonal in cross-section. In late growth stage, generally over 100 to ~150mm in
shell diameter, the ventral sulcus and ventrolateral tubercles become gradually fainter with growth and finally disappear altogether. The non-tuberculate ribs on the outer whorl of the full-grown shell are more numerous than the tuberculate ones in the middle growth stage. The whorl has a nearly circular cross- section, and maintains faint tubercles on the umbilical margin. The largest shells of the genus known to us include an example assigned to Douvilleiceras sp. cf. leightonense (Casey, 1962, p. 276, text-fig. 97) from southeast England, which is about 440mm in diameter (approximately 650mm in estimated shell diameter), and also another example assigned toD. clementinum (Courville and Lebrun, 2010, p. 27, fig. 12) from Troyes (Aube) in the Paris basin which is over 800mm in diameter. These specimens exhibit weak tuberculation only in the umbilical region and the ribs are also faint, weakening on the latest preserved part of the whorl to the extent the flanks are almost smooth. The simplified suture line is characterized by fairly deep and narrow external and lateral lobes, and a broad and somewhat rounded first lateral saddle. Previously-described species of Douvilleiceras exhibit
various morphological features in the middle growth stage of their ontogenetic sequences. We consider the mode of tuberculation and ribbing in the middle growth stage, as well as the general shell proportions, as the key morphological criteria
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