Anagaudryceras calabozoi 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 : 532-535

Anagaudryceras calabozoi Raffi and Olivero sp. nov. [M and m]

Figs. 6A, B , 7 , 8 , 9A.

1992 Anagaudryceras subsacya ( Marshall, 1926) ; Marenssi et al. 1992: 92.

2018 Anagaudryceras sp. ; Olivero and Raffi 2018: 86, fig. 7H.

ZooBank LSID: urn:lsid:zoobank.org:act:366EF942-7214-40C1-9FB0-0E0E52085EC5

Etymology: In memory of Fernando Calabozo (1986–2016), an outstanding Argentinian geologist of Antarctica and friend, whose early death is much regretted.

Type material: Holotype ( CADIC PI 411 ), large macroconch (D 102 mm) with complete phragmocone and incomplete body chamber ( Fig. 7A ) . Paratype ( CADIC PI 472 ), moderate-large microconch (D max 93.7 mm) with complete phragmocone and incomplete body chamber ( Fig. 8A ) .

Type locality: Redonda Point locality, southeastern James Ross Island, Antarctica .

Type horizon: Mid-Campanian (Cretacaous), from the base of the informal Member III of the Rabot Formation, Ammonite Assemblage 7.

Material. — A total of 53 specimens that include 10 internal molds of macroconchs [M] with phragmocone and body chamber, preserving patches of the shell ( CADIC PI 296 , 411 , 456–463 ) and 8 fragments of internal molds [M] ( CADIC PI 464–471 ) , 11 internal molds of microconchs [m] with phragmocone and body chamber preserving patches of the shell, ( CADIC PI 472−481 , 493 ) , 12 fragments of body chamber ( CADIC PI 494–505 ) ; 12 internal molds of juvenile specimens with phragmocone and part of the body chamber preserving patches of the shell ( CADIC PI 302 , 482−492 ) . All from type locality and horizon. Fragment of body chamber collected 1956–1975 by Antonio Cañon Martinez in Magallanes Basin Chile (ACM-118) ,

Diagnosis. —Moderate to large shell, compressed whorl section with dense and prorsiradiate lirae, slightly bifurcated, accompanied by four or five collars per whorl. Neanoconch with five or less ribs accompanied by fine lirae. Macroconch body chamber with band-like ribs that become wider towards the venter, where they project aperturally. Microconch body chamber with two types of ornamentation, strong asymmetrical ribs, slightly flexuous, preceded by constrictions and almost imperceptible spiral ornamentation.

Description—Early and mid growth stage (D up to 65 mm): The coiling is evolute, with fairly depressed whorl section (Wb/Wh ~1.07–1.27; Table 4). The umbilicus is moderately wide (U ~45%; Table 4) and shallow, with rounded umbilical wall and umbilical shoulder. The flanks are subparallel and mildly rounded but slightly convergent to a moderate rounded venter. The ornamentation in the juvenile shell can be clearly differentiated in two stages. The neanoconch presents five or less ribs accompanied by fine lirae. Beyond the neanoconch, the ornamentation consists of fine, flexuous, prorsiradiate lirae with some bifurcation and intercalate lirae at the first third of the flank and into the ventrolateral shoul- der. Accompanying the lirae and parallel to them, there are up to five collars per whorl. The ribs are markedly asymmetrical and projected slightly aperturally on the venter.

Later growth stage (D more than 65 mm): Macroconch M], as the diameter increases, the whorl section becomes more compressed (Wb/Wh ~0.80; SOM: fig. 5; Table 4), with flanks that converge to a less rounded ventrolateral shoulder and slightly arched venter. The ornamentation in the body chamber consist of flexuous broad ribs (band-like ribs of Kennedy and Klinger 1979), that becomes wider towards the venter, where they project aperturally. Each rib is preceded by a marked constriction that runs parallel to the rib following its shape. As the shell diameter increases, the ribs become less distant. Microconch [m], the coiling becomes slightly more involute at 75–90 mm of diameter U ~36%; Table 4; SOM: fig. 6), the whorl section is fairly depressed (SOM: fig. 5) or as high as broad (Wb/Wh ~1.18– 1.03; Table 4), with umbilical and ventrolateral shoulders very rounded. At the body chamber there are strong asymmetrical ribs, slightly flexuous, preceded by constrictions and almost imperceptible longitudinal striation, consisting of an alternation of fine ridges and broad and shallow sinus.

Remarks. —The most notable aspect of our collection is that the two markedly different morphotypes of Anagaudryceras have the same stratigraphic range, and have more or less identical early developmental stage but differs in adult morphology. These facts, plus a similar ratio of specimens of each morphotype, are strong evidences of sexual dimorphism. The micro- and macroconchs of Anagaudryceras calabozoi Raffi and Olivero sp. nov. are morphologically almost identical and cannot be discriminated from each other at 60–65 mm shell diameter or smaller. Unfortunately, the preservation of the peristome in Antarctic lytoceratids is extremely rare, and consequently we do not have lappets that would confirm the microconch condition. Nonetheless our specimens present a remarkable differentiation in adult sizes.

Anagaudryceras calabozoi Raffi and Olivero sp. nov. has intermediate morphological characters between Anagaudryceras sacya and Anagaudryceras subsacya ( Marshall, 1926) . A. sacya has six ribs per whorl, covered by fine lirae in the inner whorls and, flexuous, flattened band-like ribs on the mature body chamber. This completely different style of ornamentation at the adult body chamber, concurs with macroconch ornamentation of A. calabozoi Raffi and Olivero sp. nov. but ribs are wider. A. subsacya , from the Santonian– Campanian of Natal and early to mid-Campanian of New Zealand, also presents two types of ornamentation. However, its internal whorls have five ribs that increase in number in the body chamber, with a slight aperturally projection on the venter. Anagaudryceras compressum Shigeta and Nishimura, 2014 , from the lower Maastrichtian of Hokkaido, also has band-like ribs in the adult body chamber. However, they are narrower and less flexuous, and have a more compressed whorl section (Wb/Wh ~0.65). Similar flattened but wider ribs are observed on the last whorl of A. matsumotoi Morozumi, 1985 , from the upper Maastrichtian ( Maeda et al. 2005; Shigeta and Nishimura 2014). The Maastrichtian Anagaudryceras lueneburgense ( Schlüter, 1872) has dense flexuous ribs accompanied by constriction at the adult body chamber, but these are less broad than in A. calabozoi Raffi and Olivero sp. nov. (see Birkelund 1993: pl. 1: 3–5; Kennedy and Summesberger 1986: pl. 3: 6, pl. 15: 4). Anagaudryceras subtililineatum differs from the juvenile stages of A. calabozoi Raffi and Olivero sp. nov. by the more depressed whorl section (Wb/Wh ~1.40) at same diameters. Also, A. calabozoi Raffi and Olivero sp. nov. presents bifurcate and intercalate lirae. Anagaudryceras mikobokense Collignon, 1956 from the Maastrichtian of Madagascar also resembles A. calabozoi Raffi and Olivero sp. nov. in present radial striae and flexuous ribs, but this are not asymmetrical and without associated constrictions. The same character is present in Anagaudryceras aurarium ( Anderson, 1938) but the latter species is ornamented by distant constrictions separating very broad flattened ribs and it is more involute (U ~29%) and compressed (Wb/Wh ~0.75) than A. calabozoi Raffi and Olivero sp. nov. at same diameters.

Anagaudryceras pulchrum ( Crick, 1907) closely resembles A. calabozoi Raffi and Olivero sp. nov. in its ornamentation style with a shell covered by thin sinuous lirae associated with 4 or 5 lirated and asymmetrical ribs followed by constrictions. Still, A. pulchrum is devoid of longitudinal striation. The topotype material mentioned by Marenssi et al. (1992) for the Rabot Formation as A. subsacya , probably corresponds to the microconch of A. calabozoi Raffi and Olivero sp. nov.

According to Matsumoto (1995), ancestral characters in ammonites apparently reappear at the last growth stage of descendants. In Antarctica, the record of the A. sacya (= A. buddha of Medina et al. 1982 and Ineson et al. 1986) from the deep-marine Albian Lower Kotick Point Formation, Gustav Group supports the evolutionary concept of Matsumoto (1995), reinforcing the idea that the band-like ornamentation is a basal feature, probably expressed in different species controlled by environmental factors. In this case environmental factors could be the similar habitat-depth for both the Albian A. sacya and the Campanian A. calabozoi Raffi and Olivero sp. nov (see Olivero and Raffi 2018). The presence of different states of this feature (band-like rib) may suggest a close phylogenetic relationship between these taxa (see also Shigeta and Nishimura 2014), and probably our new species is a descendant of A. sacya .

Stratigraphic and geographic range. —Late early to mid-Campanian, around the C33R/C33N magnetic chron boundary ( Milanese et al. 2017a, b), Rabot and Santa Marta Formation, Ammonite Assemblage 6 Natalites spp. Group 2 and Ammonite Assemblage 7 Neokossmaticeras redondensis, James Ross Island, Antarctica. Campanian of the Cerro Toro Formation, Lake Pehoe locality, Magallanes Basin, Chile.

Figures

Fig. 6. The gaudryceratid ammonoid Anagaudryceras spp., from the Campanian (Cretaceous), Antarctica, Rabot Formation, Redonda Point locality (A–C), and from the late Campanian–early Maastrichtian (Cretaceous), Antarctica, Snow Hill Island Formation, Sanctuary Cliffs locality (D, E). A, B. Anagaudryceras calabozoi Raffi and Olivero sp. nov. A. CADIC PI 411, holotype, adult macroconch; suture line (A1), transversal section (A2). B. CADIC PI 472, adult microconch; suture line (B1), transversal section (B2). C. Anagaudryceras cf. A. politissimum (Kossmat, 1895), CADIC PI 455, transversal section. D. Anagaudryceras sanctuarium Raffi and Olivero sp. nov., CADIC PI 604, transversal section. E. Anagaudryceras subcompresum Raffi and Olivero sp. nov., CADIC PI 506, holotype, adult specimen; suture line (E1), transversal section (E2). Scale bars 10 mm. Abbreviations: E, external lobe; I, internal lobe; L, lateral lobe; U, umbilical lobe; Us, septal lobe.

Fig. 7. Macroconchs of the gaudryceratid ammonoid Anagaudryceras calabozoi Raffi and Olivero sp. nov., from early Campanian (Cretaceous), Antarctica, Rabot Formation, Redonda Point locality (A) and Hamilton Norte locality (B, C). A. CADIC PI 411, holotype, phragmocone and part of the body chamber in lateral (A1) and ventral (A2) views. B. CADIC PI 456, phragmocone and part of the body chamber in lateral view. C. CADIC PI 464, phragmocone and part of the body chamber in lateral view. Arrows mark the beginning of the body chamber.

Fig. 8. Microconchs of the gaudryceratid ammonoid Anagaudryceras calabozoi Raffi and Olivero sp. nov. from early Campanian (Cretaceous),Antarctica, Rabot Formation, Redonda Point locality. A. CADIC PI 472, holotype, phragmocone and part of the body chamber in lateral (A1) and ventral (A2) views. B. CADIC PI 473, phragmocone and part of the body chamber in lateral (B1) and ventral (B2) views. Arrows mark the beginning of the body chamber.