Anagaudryceras seymouriense Macellari, 1986

Raffi, María E., Olivero, Eduardo B. & Milanese, Florencia N., 2019, The gaudryceratid ammonoids from the Upper Cretaceous of the James Ross Basin, Antarctica, Acta Palaeontologica Polonica 64 (3), pp. 523-542 : 531-532

Anagaudryceras seymouriense Macellari, 1986

2015 Gaudryceras cf. seymouriense ( Macellari, 1986) ; Shigeta et al. 2015: 115, fig. 8.

For a complete synonymy list see Klein et al. (2009).

Holotype: OSU 38333 , complete phragmocone of 236 mm of D, preserving patches of the original shell, designated by Macellari (1986: figs. 9.1–9.2).

Type locality: Seymour Island, Antarctica.

Type horizon: Late Maastrichtian (cretaceous), Unit 8, López de Bertodano Formation.

Material. — One almost complete specimen with complete phragmocone and incomplete body chamber ( CADIC PI 446 ), three phragmocones ( CADIC PI 447–449 ) and three fragments of body chamber ( CADIC PI 450–452 ). From the type locality and horizon .

Description.—Early growth stage (D up to 40 mm): The coiling is evolute with fairly depressed (Wb/Wh ~1.13–1.28) whorl section and greatest width occurring slightly below mid flank. The umbilicus is wide, with a slightly vertical wall and rounded umbilical shoulder. The flanks converge to a rounded ventrolateral shoulder and slightly rounded venter. The ornamentation consists of very fine flexous lirae that arise straight from the umbilical seam, bend forward to the umbilical shoulder and bends backward at the ventrolateral shoulder. In addition, there are intercalate lirae that reach to mid flank. Collars with superimposed lirae are occasionally observed. The neanoconch is smooth.

Mid growth stage (D 40–90 mm): As the diameter increases, the coiling becomes less evolute and the whorl section becomes compressed (Wb/Wh ~0.91). Subparallel flanks that converge to a slightly rounded ventrolateral shoulder and arched venter. In addition to the lirae are collars (constrictions of Macellari 1986).

Later growth stage (D more than 90 mm): As diameter increases the coiling becomes more involute with a narrower umbilicus ( Table 4), and the ornamentation more delicate. The very fine lirae arise almost rectiradiate from the umbilical seam and prorsiradiate beyond the umbilical shoulder. Huge specimens, diameter larger than 600 mm, preserving the shell show wide flat-top ribs at the body chamber (see SOM: fig. 4).

Remarks. — Macellari (1986) mentioned that A. seymouriense has intermediate characters between Gaudryceras and Anagaudryceras , but places it within the genus Anagaudryceras by its weak ornamentation and change in whorlshape (from a depressed evolute to a compressed and involute shell), although it presents prominent constrictions at huge diameters. Shigeta et al. (2015) placed A. seymouriense within the genus Gaudryceras because of its ornamentation style. However, the Macellari’s (1986) species perfectly match all the Anagaudryceras features: early rounded to depressed whorl section, which may become compressed in later growth stages; early and middle growth stages with very fine lirae and periodic collar ribs and frequent ornamentation changes on the body chamber ( Kennedy and Klinger 1979) and thus, we kept it in Anagaudryceras . Furthermore, our material preserves the original aragonite shell, and the ornamentation in all growth stages does not resemble any known species of Gaudryceras .

Stratigraphic and geographic range. —Late Maastrichtian of the López de Bertodano Formation, Ammonite Assemblages 12 to 14, Seymour Island, Antarctica, Maastrichtian of Hobetsu area, Japan and Maastrichtian of Makarov area and Naiba area, Russia.