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Species Pachydiscus (Pachydiscus) juv. Ifrim 2004

[1]
[2]
[1] text-figs. 8i-j Ifrim et al. (2004) [2] text-figs. 10a-e Ifrim et al. (2004)

Alternative name:
Pachydiscus juv. Ifrim 2004
Diagnosis / Definition:
Ifrim et al. (2004):
Description: The test is moderately involute (U/D is c. 24 per cent of the shell diameter) and discoidal. Whorls are slightly compressed (WB/WH is c. 0·85). Maximum whorl breadth is situated half-way between the umbilicus and middle of the flank. Umbilical walls are overhanging, and bending is moderately narrow into weakly convex flanks that converge to a widely rounded venter. Internal moulds are smooth. The largest specimen is poorly preserved; in consequence it is not possible to recognize probable juvenile ornamentation. The suture line is moderately incised. The first saddle (E/L) is considerably larger tban the second (L/U2) and extended towards tbe venter. L is trifid. The auxiliary lobes are strongly retracted, and tbeir number increases with growth and the grade of sutural incision.
Discussion / Comments:
Ifrim et al. (2004):
Remarks: The juvenile specimens of the Campanian P. (P.) travisi (Adkins 1929 loc. cit. Cobban and Kennedy 1993b) display whorl dimensions similar to OUf specimens, but are ornamented at diameters up to 25 mm by 'delicate concave ribs on the outer flank' (Cobban and Kennedy 1993b, p. D3). Almost smooth internal moulds and whorl dimensions clearly relate the Cerralvo species with the group of P. (P.) neubergicus (von Hauer, 1858). The species P. (P.) neubergicus sensu stricto displays smooth early growth stages up to a diameter of about 25 mm (Henderson and McNamara 1985) and at larger diameters. Its whorl seetion is similar to that of the Cerralvo specimens, and faint umbilical ribs are visible on some of OUf specimens (e.g. Text-fig. 1OA,D). Pachydiscus (P.) gollevillensis (d'Orbigny, 1850), a species closely related to P. (P.) neubergicus, is more compressed at a comparable state of growth than our specimens. The same is true for closely related P. (P.) armenicus Atabekian and Akopian, 1969, but the early juvenile growth stages of these species are very similar, and the determination of Pachydiscus species is based on the adult ornamentation, which is not developed on OUf small specimens. Although OUf individuals show close affinities towards the group of P. (P.) neubergicus, they are too small and immature to allow a reliable determination of the species.
Systematics:

36
 Ordo Ammonoidea
  Subordo Ammonitina
   Superfamilia Desmocerataceae
    Familia Pachydiscidae
     Genus Pachydiscus
      Subgenus Pachydiscus (Pachydiscus)
       Species Pachydiscus (Pachydiscus) juv.
Synonym list:
Ifrim et al. (2004):
2004 Pachydiscus (Pachydiscus) sp. juv. Ifrim. - Ifrim et al. : 20, 25 text-figs. 8i-j, 10a-e
References:

Ifrim,C..; Stinnesbeck,W.. and López-Oliva,J.G.. (2004):
Maastrichtian cephalopods from Cerralvo, north-eastern Mexico . Palaeontology Vol. 47(6) p. 1575–1627
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