Pachydiscus (Pachydiscus) armenicus Atabekian & Akopian 1969 (TaxonConcept

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Species Pachydiscus (Pachydiscus) armenicus Atabekian & Akopian 1969

Alternative name:

Pachydiscus armenicus Atabekian & Akopian 1969

Diagnosis / Definition:

Ward & Kennedy (1993):
Description. - Holotype (Figure 29.4-29.6) is composite mold 129 mm in diameter. Coiling moderately involute, comprising around 30 percent of diameter, increasing somewhat on last half whorl of body chamber, at end of which 45 percent of previous whorl is covered. Umbilical wall low, flattened and outward-inclined, umbilical shoulder narrowly rounded. Whorl section compressed, with whorl breadth to height ratio of around 0.63 intercostally, greatest breadth just outside umbilical shoulder, inner flanks broadly rounded, outer flanks flattened and convergent, ventrolateral shoulders broadly rounded, venter somewhat flattened. Last whorl of phragmocone has 16 prominent umbilical bullae per whorl, last whorl, most of which is body chamber, has 20-2 1. Bullae arise on umbilical wall, sweep back over umbilical shoulder and are markedly concave on inner flank. they give rise to narrow straight ribs, recti- to feebly prorsiradiate on phragmocone, but concave and initially rursiradiate on last part of body chamber. Ribs sweep forwards across mid-flank and bi- or trifurcate on outer flank, where additional intercalated ribs arise, all ribs strengthening and sweeping forwards over ventrolateral shoulder, where they are feebly concave, to cross venter transversely.

Discussion / Comments:

Ward & Kennedy (1993):
Type. - Holotype is the original of Atabekian and Akopian, 1969, Pl. 3, fig. 1, from the upper Maastrichtian of the Azizbek region of the Armenian SSR, illustrated here as Figure 29.4- 29.6. Discussion. - The Biscay material matches the Armenian types well. Pachydiscus (P.) armenicus combines numerous umbilical bullae (16 to 20 per whorl) with 62-73 ventral ribs per whorl, the primary ribs extending across the flanks and bi- or trifurcating on the outer flank. The style of flank ribbing recalls that of Pachydiscus (P.) neubergicus neubergicus, described above, from which the species is distinguished by its more numerous umbilical bullae (16-20 vs. 14-17) and fewer ventral ribs (62- 73 vs. 48-60). Pachydiscus (P.) gollevillensis has only 9-1 1 umbilical bullae and 80 ventral ribs. Pachydiscus (P.) armenicus thus occupies a morphologically intermediate position between P. (P.) neubergicus and P. (P.) gollevillensis. The high density of ribbing immediately distinguishes it from P. (P.) jacquoti jacquoti described above. Occurrence.- In the Biscay sections this taxon rapges from near the top of Member I (lower Maastrichtian) to the top of Member V (upper Maastrichtian). The species is otherwise known only from the upper Maastrichtian of the Armenian SSR.

Systematics:

38
Ordo Ammonoidea
  Subordo Ammonitina
   Superfamilia Desmocerataceae
    Familia Pachydiscidae
     Genus Pachydiscus
      Subgenus Pachydiscus
       Species Pachydiscus (Pachydiscus) armenicus

Synonym list:

Ward & Kennedy (1993):

1969 Pachydiscus gollevillensis armenicus Atabekian & Akopian. - ATABEKIAN & AKOPIAN : p.8 pl. 1, fig. 2; pl. 3, 1, 2

1986 Pachydiscus gollevillensis armenicus Atabekian & Akopian. - Kennedy : p.34

1987 Pachydiscus gollevillensis armenicus Atabekian & Akopian. - Kennedy : p.171

1993 Pachydiscus (Pachydiscus) armenicus Atabekian & Akopian. - Ward & Kennedy : p. 32, 35, 40 fig. 27.1, 27.2, 27.8-27.10, 29.5, 34.1-34.6

Stratigraphy - relative ages:

upper Maastrichtian - lower Maastrichtian: Ward & Kennedy (1993)

References:

ATABEKIAN,A.A.. and AKOPIAN,v.T.. (1969):
Late Cretaceous ammonites of the Armenian SSR (Pachydiscidae). . Proceedings of the Armenian Academy of Science, Earth Sciences. [In Georgian]. Vol. 6 p. 3-20

Kennedy,W.J.. (1986):
The ammonite fauna of the Calcaire à Baculites (Upper Maastrichtian) of the Cotentin Peninsula (Manche, France) . Palaeontology Vol. 29(1) p. 25-83

Kennedy,W.J.. (1987):
The ammonite faunas of the type Maastrichtian, with a revision of Ammonites colligatus Binkhorst, 1861 . Bulletin de l'Institut Royal des Sciences Naturelles de Belgique, Sciences de la Terre Vol. 56 p. 151-267

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

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