Anagaudryceras subcompressum Raffi and Olivero, 2019

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 : 535-536

Anagaudryceras subcompressum Raffi and Olivero sp. nov.

Fig. 9B− F.

ZooBank LSID: urn:lsid:zoobank.org:act:81F09A97-D667-477F-A6EE-56FC0D1964CC

Etymology: From Latin sub, somewhat; meaning that the new species is somewhat similar to the type species of Anagaudryceras compressum .

Holotype: CADIC PI 506 , moderate shell (D max 79 mm) with complete phragmocone and incomplete body chamber (Fig. 9C).

Type locality: Sanctuary Cliffs Nunatak, southern Snow Hill Island, Antarctica.

Type horizon: Maastrichtian (Cretaceous), the Sanctuary Cliffs Member, Snow Hill Island Formation, Ammonite Assemblage 8.2.

Material. — 27 internal molds of phragmocone and body chamber, some of them preserve patches of the shell ( CADIC PI 506–532 ). From the type locality and horizon .

Diagnosis.—Moderate shell with coiling markedly serpenticone and depressed whorl section. Inner whorls ornamented by prorsiradiate lirae and up to six prosiradiate constrictions per whorl. Adult body chamber with slightly flexuous constrictions associated with fine lirae almost imperceptible to the naked eye.

Description.— Early growth stage (D up to 40 mm): The coiling is evolute and strongly serpenticone, with small and vertical umbilical wall, and rounded umbilical shoulder. The whorl section is fairly depressed (Wb/Wh ~1.1–1.2; Table 5) up to 30 mm of diameter becoming compressed to larger diameters. The umbilicus represents more than the 50% of the shell ( Table 5). The ornamentation consists of rectiradiate, fine and simple lirae that arise from the umbilical seam and become markedly prorsiradiate in the flank, with some intercalated lirae. Additional to the lirae are two or three constrictions per whorl and in the venter both lirae and constrictions project slightly adorally. The neanoconch is smooth. The internal mold is smooth with flat or even furrows representing the location of the constrictions in the shell. The suture is typical for the genus.

Mid growth stage (D up to 80 mm): The coiling becomes slightly less evolute and the umbilicus less wide (U ~35% of the shell at D ~ 80 mm; Table 5), with larger umbilical wall. The flanks converge to a gentle ventrolateral shoulder, mildly rounded and high venter. The whorl section becomes progressively more compressed (Wb/Wh ~0.92– 0.72; Table 5). As the diameter increases the ornamentation becomes weaker. The body chamber, with more than 360°, is ornamentated by slightly flexuous and prorsiradiate constrictions. In addition to the constrictions there are fine flexuous lirae almost imperceptible to the naked eye.

Remarks.— Anagaudryceras subcompressum Raffi and Olivero sp. nov. closely resembles A. compressum Shigeta and Nishimura, 2014 from the early Maastrichtian of Hokkaido, Japan. However, our new species differs in its less compressed whorl section ( A. compressum has a Wb/ Wh ~0.65, D 73 mm), degree of involution and in the absence of band-like ribs in the adult stage. The whorl section and broad venter of the juvenile shells of Anagaudryceras tennenti Henderson, 1970 , Haumurian age (the Haumurian is approximately equivalent to the Campanian–Maastrichtian; see Crampton et al. 2000) are very similar to those of A. subcompressum sp. nov., however, the Vertebrites -like ornament of the former is not present in A. subcompressum Raffi and Olivero sp. nov. In addition, A. tennenti has five collars per whorl in the adult stage and is more depressed (Wb/Wh ~1.10, D 48 mm).

Matsumoto in Matsumoto et al. (1985) described Anagaudryceras nanum from a juvenile specimen (D 24 mm) and Maeda et al. (2005) described new material from Hokkaido but did not give morphological measurements. However, these juveniles specimens with a very evolute shell present sinuous lirae and narrow constrictions followed by flares (or band-like ribs according to Maeda et al. 2005). Anagaudryceras mikobokense is ornamented by sinuous ribs without constrictions, also being more involute and less compressed at same diameters (Wb/Wh ~0.80−0.90, D 80 mm).

Besides, the whorl section and constrictions in the adult shell of Anagaudryceras yamashitai ( Yabe, 1903) , from the early Campanian of Hokkaido closely resemble A. subcompressum Raffi and Olivero sp. nov. However, the coiling is much more involute with an umbilicus of c. 25% of the total diameter of the shell. In addition, the suture line of Anagaudryceras subcompressum Raffi and Olivero sp. nov. has two bipartite saddles whereas Anagaudryceras yamashitai has three bipartite saddles at same diameters.

Stratigraphic and geographic range. —Early Maastrichtian of the Sanctuary Cliffs Member, Ammonite Assemblage 8.2, Snow Hill Island Formation, Antarctica. The first record of the species is just above the C32/C31 magnetic chron boundary ( Milanese et al. 2017b).