Ataxioceratinae Buckman, 1921

Moliner, Luis & Olóriz, Federico, 2010, New Lower Kimmeridgian ataxioceratin ammonite from the eastern Iberian Chain, Spain: Systematic, biogeographic, and biostratigraphic relevance, Acta Palaeontologica Polonica 55 (1), pp. 99-110 : 102-103

Subfamily Ataxioceratinae Buckman, 1921

(sensu Spath 1930; emend. Zeiss 1968)

Genus Geyericeras gen. nov. [m, M]

Etymology: The new taxon Geyericeras is dedicated to the late Professor Otto Franz Geyer (1924–2002), ammonitologist and stratigrapher of Upper Jurassic deposits in Iberia and South America.

Type species: Geyericeras aragoniense sp. nov.

Diagnosis.—Micro− and macroconchiate ataxioceratids of small size showing moderate to loose coiling. Whorl section subrectangular, narrower in microconchs than in macroconchs. Ribbing dense and delicate on the inner whorls. On the phragmocone ribs are mainly bifurcate, some polygyrate and less frequently subpolyplocoid, which also occur on the inner whorls in macroconchs. Intercalatory ribs scarce. On the body−chamber ribs are stronger, rib inter−space slightly wider and subpolyplocoid ribs exist. Ribbing index commonly lower than 4. Rib−curve decreasing from shell size less than 50 mm. No parabolic structures are present. Constrictions common. Lappeted peristome in microconchs.

Remarks.— Geyericeras gen. nov. microconchs [m] are smaller than 60 mm while macroconchs [M] have a shell size up to 152 mm. The main phenotype traits defining Geyericeras [m, M] are the relatively tight coiling on the inner whorls, the fine and delicate ribbing in microconchs and immature macroconchs, the occurrence of subpolyplocoid ribs, and the stratigraphic range within the upper part of the Ataxioceras lothari Biozone identified in the eastern Iberian Chain. At present, Geyericeras gen. nov. includes a single species, Geyericeras aragoniense sp. nov. [m, M]. The occurrence of subplyplocoid ribs in microconchiates is interpreted as forced by high−dense ribbing, and is unconnected to dense ribbing in the outer whorls of macroconchiates. In macroconchs, subpolyplocoid structures result from the complete−to−defective connection of secondary ribs with the adjacent, aboral primary rib. Thus, the occurrence of subpolyplocoid ribs in microconchs resembles the dense−ribbing effect which is well known in Schneidia [m. M]. In contrast, in macroconchs the origin of subpolyplocoid ribs is similar to that known in some new endemic taxon from the Sutneria platynota Biozone , which clearly differs in ribbing style (under study by LM).

Geyericeras gen. nov. [m] is interpreted as a result of in−situ morphological evolution of Ataxioceratinae on epicontinental shelves of the present−day eastern Iberian Chain, and therefore represents phenotype dynamics related to endemism (see Olóriz et al. 1988; Moliner and Olóriz 1999; Olóriz 2000 for paleoenvironmental considerations). Microconchs show morphological convergence with evolute phenotypes belonging to the stratigraphically older Schneidia Atrops, 1982 [m], which developed during the Schneidia guilherandense Subchronozone of the Sutneria platynota Chronozone. By analogy with Schneidia Atrops, 1982 [m, M], Geyericeras gen. nov. [m, M] originated from the last−known cladogenetic event in the phylogenetic branch of Ardescia Atrops, 1982 [m, M], most probably through: (i) increased discocone structuring of shells and its effects on ribbing before maturity; and (ii) late ontogenetic innovation comprising the development of subpolyplocoid structures manifested with variable timing in the ontogeny of macroconchiate specimens (heterochrony?). Thus, the new genus Geyericeras [m, M] is interpreted as having originated within the branch of the older endemic Ardescia during the latest Ataxioceras hippolytense Chronozone or, alternatively, later within the Ataxioceras lothari Chronozone. Geyericeras gen. nov. [m, M] represents one of the cul−de−sac phenotype specializations which are common in Lower Kimmeridgian Ataxioceratinae —i.e., the acquisition of double furcations or real subpolyplocoid ribbing.

The resulting extreme phenotypes left no descendants, most probably because they reached a maximum for ribbing specialization in Ataxioceratinae through the combination of shell type and sculpture. Earlier in the Kimmeridgian, this ammonite group experienced analogous evolutionary changes in phenotype expression, resulting in the origination of other genera such as Schneidia [m, M] and new endemic forms during the Sutneria platynota Chronozone.

Geyericeras gen. nov. [M] differs from its microconchs through both the significantly larger adult size and wider, coarser but comparatively weakened primary ribs. However, difficulties arise in differentiating incomplete or immature specimens of similar shell size. Whatever the case, macroconchs of Geyericeras have more robust shells and show less frequent constrictions.

Geyericeras gen. nov. [m] is clearly separated from Ardescia Atrops, 1982 [m] on the basis of a finer and tighter ribbing, as well as by the more or less regular development of subpolyplocoid ribs.

Geyericeras gen. nov. [m] is typically more evolute than Schneidia Atrops, 1982 [m] at their respective largest shell sizes. These two taxa are recorded from the eastern Iberian Chain, the former within the uppermost Ataxioceras lothari Biozone and the latter in deposits from the upper part of the Sutneria platynota Biozone , as observed elsewhere (e.g., Atrops 1982; Marques 1983; Olóriz and Rodríguez−Tovar 1993; Bachnou and Atrops 1996; Gradl and Schairer 1997; Villaseñor et al. 2000). Schneidia [M] shows short, reinforced, periumbilical ribs that are unknown in Geyericeras [M].

Lithacosphinctes Olóriz, 1978 [m, M] is more evolute than Geyericeras gen. nov. [m, M] and developed larger and stouter shells with less dense and coarser ribbing. Moreover, Lithacosphinctes [m, M] developed parabolas and has no polyplocoid or subpolyplocoid ribbing. Finally, the majority of the stratigraphical range of Lithacosphinctes [m, M] predates the appearance of Geyericeras gen. nov. [m, M] in the eastern Iberian Chain.

Ataxioceras Fontannes, 1879 [m] shows less dense ribbing than Geyericeras gen. nov. [m], and true polyplocoid ribs are typical in large microconchs of Ataxioceras . Ataxioceras [M] exhibits rib curves similar to those of Geyericeras [M], although its ribbing is very different, with a typical trend toward reducing reinforced primary ribs to the periumbilical zone in late ontogeny. In addition, Ataxioceras [M] shows a narrow−oval whorl section with a narrow venter and slightly convex flanks, whereas the whorl section in Geyericeras [M] is subrectangular with flattened flanks and a wide, slightly arched venter.

Parataxioceras Schindewolf, 1925 [m, M] developed larger and more evolute shells than Geyericeras gen. nov. [m, M], and has ribs which are stronger, less dense, and show parabolic structures. In addition, subpolyplocoid ribbing in Parataxioceras [m] is not related to dense ribbing as it is in Geyericeras [m].

No ammonites similar to Geyericeras gen. nov. [m, M] have been described from equivalent horizons in epicontinental shelves from southern Europe. Therefore, Geyericeras is interpreted as endemic, restricted to the eastern Iberian Chain.

Stratigraphic and geographic range.—Uppermost part of the Ataxioceras lothari Biozone (youngest part of the Ataxioceras hypselocyclum Chronozone ) in the eastern Iberian Chain.