Hoplitoplacenticeras lasfresnayanum d'Orbigny 1841 (TaxonConcept

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Species Hoplitoplacenticeras lasfresnayanum d'Orbigny 1841

Diagnosis / Definition:

Kennedy (1986):
Description. The holetype and only known specimen is an internal mould with half a whorl of body-chamber. Coiling moderately involute on inner whorls, becoming more evolute on last half whorl, suggesting maturity. Umbilicus small (23,6% of diameter at maximum) and shallow, with flattened, outwards-sloping wall. Umbilical shoulder sharply and narrowly rounded. Whorl seetion compressed with greatest breadth at umbilical shoulder; whorl breadth to height ratio O• 76. Inner flanks rounded; outer flanks flattened, converging to narrow venter that is flattened intercostally with narrowly rounded ventrolateral shouIders. Sixteen smalI, sharp umbilical bullae on outer whor1. On phragmocone and early body-chamber these give rise to broad, flat-topped, rapidly expanding, flexuous, prorsiradiate primary ribs, each ofwhich is separated by a narrow, deep interspace from a similarly broad and flat-topped secondary rib, inserted weIl below mid-flank. Each rib terminates in small outer ventrolateral clavus, while corresponding'if'small ventral clavus is also present (PI. 9, fig. 6). Venter between smooth and sulcate with clavi on opposite sides slightly offset to opposite but not alternate. Ribbing irregular on last quarter whorl of body-chamber, with irregular non-bullate primaries and intercalated secondaries. Ventrolateral and ventral clavi have merged into a single clavus and there is irregular spacing and development of this tubercle suggesting minor pathological disturbance of ornament. Suture incompletely exposed (text-fig. lOB) with moderately subdivided lobes and saddles; L asymmetrically bifid to subtrifid; L/D2 asymmetrically bifid, U2 small.

Discussion / Comments:

Kennedy (1986):
Discussion. Ammonites lasfresnayanus is referred to H oplitoplacenticeras on the basis of the ribbing and presence of outer ventrolateral and ventral tuberc1es, which recall in several respects the style seen in the type species, H. plasticum Paulcke, 1906. It differs from all other Hoplitoplacenticeras in the very simple suture, without the multiplication of adventive and auxiliary elements so typical ofthe main stream of the genus. In terms of ornament it is closest to H. marroti (Coquand, 1859) (de Grossouvre 1894, p. 118, pl. 8, fig. 3; pI. 9, figs. 2 and 3) which has more numerous, narrower, rounded rather than flattened ribs and stronger tuberc1es, and H. dolbergensis (Schlüter, 1876) (p. 159, pI. 44, figs. 1-4) which has rather similar (if coarser) tuberc1es but ribs that loop between umbilical bullae and inner ventrolateral clavi (see also Giers 1964, pI. 6, figs. 2-7). The suture line is not typical of Hoplitoplacenticeras nor of the Placenticeratidae as a whole; its simplicity recalls that of ancestral Hoplitidae, This apart, I see no reason for not referring it to Hoplitoplacenticeras, which is otherwise restricted to the Upper Campanian (fide Howarth 1965).

Systematics:

39
Ordo Ammonoidea
  Subordo Ammonitina
   Superfamilia Hoplitaceae
    Familia Placenticeratidae
     Genus Hoplitoplacenticeras
      Species Hoplitoplacenticeras lasfresnayanum

Synonym list:

Kennedy (1986):

1840 Ammonites lasfresnayanus d'Orbigny. - Orbigny : p.326 pl. 97, figs. 3-5

1850 Ammonites lasfresnayanus d'Orbigny. - Orbigny : p.212

1894 Hoplites sp. lasfresnayi d'Orbigny. - Grossouvre : p.121 pl. 23, fig. 4

1925 Hoplites lasfresnayanus d'Orbigny. - Diener : p.176

1965 Hoplitoplacenticeras lasfresnayanum d'Orbigny. - Howarth : p.391

1986 Hoplitoplacenticeras lasfresnayanum d'Orbigny. - Kennedy : p.48 pl. 9, figs. 4-6; text-figs. 3-5

References:

Orbigny,A.D.. (1840):
1840-1842. Paleontologie francaise: Terrains cretaces. 1, Cephalopodes. p. 1-120 (1840),12

Orbigny,A.D.. (1850):
Prodrome de Paleontologie stratigraphique universelle des animaux mollusques et rayonnes. Vol. 2 p. 428 pp.

Grossouvre,d.A.. (1894):
Recherehes sur la craie superieure (misdated 1893). .
In: Paleontologie. Les ammonites de la craie superieure, Mem. Servo Carte geol. det. Fr. Vol. 2 p. 264 pp.

Diener,C.. (1925):
Ammonoidea neocretacea . Fossilium Cat, (1: Animalia) Vol. 29 p. 244 pp.

Howarth,M.K.. (1965):
Cretaceous ammonites and nautiloids from Angola . Bull. Br. Mus. nat. Hist. (Geol.) Vol. 10 p. 335-415

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

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