Anapachydiscus terminus from: Ward, P.D..Kennedy, W.J.. (1993): Maastrichtian Ammonites from the Biscay Region (France, Spain) . Memoir (The Paleontological Society), Journal of Paleontology34 (Supplement to Vol. 67)(5) p. 1-58
|Notice: This catalogue page may contain unedited data.
Species Anapachydiscus terminus
|Diagnosis / Definition:
Ward & Kennedy (1993):
A large species of Anapachydiscus in which early
and middle growth stages have 60 or more crowded flexuous
prorsiradiate ribs per whorl.
All specimens crushed, so that original whorl
section cannot be reconstructed. To diameter of 120 mm, perhaps
more, coiling very involute, with umbilicus comprising 20
percent of diameter, with 70 percent of previous whorl being
covered. Ten to 12 primary ribs per half whorl arise at umbilical
seam. Some strengthen into flat-topped bulla that is septate base
of an umbilical spine to give 7-8 tubercles per half whorl. Tubercles
give rise to pairs of ribs, while nontuberculate ribs that
extend to umbilical shoulder may be single or branch into pairs.
Shorter ribs intercalate both high and low on flank, to give total
of 60 or more ribs per whorl. Ribs prorsiradiate, straight to
feebly flexuous on inner to mid-flank, flexing forwards across
ventrolateral shoulder where they are concave, crossing venter
in broad convexity. Ribs increase in strength progressively across
flanks, and are at maximum development on ventrolateral
shoulders and venter, where they are narrow and rounded, and
separated by somewhat wider interspaces. Umbilical bullae
weaken from around 70 mm diameter, rib density increases
with consequent crowding, and ribs may become flexuous.From
120mm diameter onwards, flank ribbing declines progressively,
and early part of adult whorl ornamented by closely spaced
ventrolateral and ventral ribs only. This ornament appears to
weaken towards adult aperture in largest specimens, which are
up to 350 mm in diameter. Sutures not seen.
|Discussion / Comments:
Ward & Kennedy (1993):
Holotype is UWBM74501(Figure 39.2) from Member
IV, Zumaya, Spain. Paratype UWBM76079 (Figure 40.9)
is from Member IV, Bidart, France. Paratype UWBM76096
(Figure 41) is from Member IV, Hendaye, France.
Terminus (Latin), the Roman deity who presided
over boundaries or landmarks.
Anapachydiscus terminus n. sp. differs from A.
fresvillensis in the much higher rib density in early and middle
growth stages (more than 60 vs. 31-32, increasingto 40). Later
stages are very similar in that the massive involute shell of both
species has ventral ribbing only, but flank ribs persist in A.
terminus at a stage where they are effacing in A. fresvillensis
(compare Figure 40.9 and Figure 35.6). The high rib density
immediately distinguishes A. terminus from other European
Campanian Anapachydiscus, such as A. wittekindi (Schlüter,
1872) (see Blaszkiewicz, 1980, p. 50, Pl. 42, figs. 1, 2, Pl. 43,
fig. 2, Pls. 44-47, Pl. 48, figs. 3, 4, Pl. 49, figs. 1, 3, Pl. 50, figs.
2, 3, Pls. 51-53) and A. vistulensis Blaszkiewicz, 1980 (p. 48,
Pl.42, figs. 3,4, Pl. 43, figs. 1, 3, P1.48, figs. 1, 2). The massive
involute shell,higher whorls and crowded ribs immediately distinguish
A. terminus from all other pachydiscids in the Biscay
faunas. Pachydiscus (Pachydiscus)sersensis Atabekian and Akopian,
1969 (p. 17, Pl. 8, fig. 2, Pl. 10, fig. 1, Pl. 11, fig. 1) from
the Maastrichtian sensu lato of Armenia is also said to occur in the upper upper Maastrichtian of the Vistula Valley, Poland
(Machalski and Walaszczyk, 1988). The holotype is a composite
internal mold of a phragmocone 237 mm in diameter, a cast of
which is before us. There are an estimated 50-60 ribs at 200
mm diameter, but the inner whorls lack bullae or spines, ribbing
is persistent, and much finer at large diameters than in A. terminus.
In the Biscay sections this taxon is short ranging,
having been recovered from the top of Member IV (upper
Maastrichtian) to the top of Member V (upper Maastrichtian).
The species also occurs in the upper upper Maastrichtian of
Denmark and in the Maastrichtian of Azerbaijan.
|Ward & Kennedy (1993):
|Stratigraphy - relative ages:
|upper Maastrichtian: Ward & Kennedy (1993)
The last Maastrichian ammonites.
In: Cretaceous-Tertiary Boundary, The Maastrichtian and Danian of Denmark Eds: Events Symposium, 1.. p. 210 pp.
Alizade,A..; Aliev,G.A..; Aliev,M.M..; Aliyulla,K.. and Khaliev,A.G.. (1988):
Cretaceous faunas of Azerbaijan . Akademiya Nauk Azerbaidzhanskoi SSR [in Russian] p. 1-454
Ward,P.D.. and Kennedy,W.J.. (1993):
Maastrichtian Ammonites from the Biscay Region (France, Spain) . Memoir (The Paleontological Society), Journal of Paleontology34 (Supplement to Vol. 67)(5) p. 1-58
Ammonites from the Maastrichtian White Chalk in Denmark . Bulletin of the Geological Society of Denmark Vol. 40 p. 33-81
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