Dorsetensia Buckman, 1892

Sandoval, José, 2022, Sonniniidae Ammonitina, Middle Jurassic from Southern Spain: taxonomic, biostratigraphical and palaeobiogeographical analysis, Geodiversitas 44 (27), pp. 801-851 : 840-841

Genus Dorsetensia Buckman, 1892 [M] & [m]

TYPE SPECIES. — Ammonites edouardianus d’Orbigny, 1846 in d’Orbigny (1842-1851: 393, pl. 130, figs 3-5) ( OD).

DESCRIPTION

Macroconch [M] of medium-sized to large shells. The inner whorls are evolute with simple or, rarely, bifurcate (only in the earliest forms), radial or slightly flexuous ribs and no tubercles. The middle and outer whorls are more involute and compressed, with a sharp, sometimes undercut, umbilical edge, more or less flattened flanks, and a narrow venter with a full keel at all the stages. The ribbing usually weakens throughout ontogeny becoming smooth on the outer whorls. The mouth has plain borders.In some species, ribs are replaced by striation in the BC. The dimorphic microconch [m], traditionally classified with the name Nannina (gr. of Witchellia regrediens Haug, 1893 ) are small and evolute, have ribs to the end of the BC and have lateral lappets on the mouth border. The septal suture, though relatively simple, is slightly more complex than in Witchellia with wide and a rather shallow L, and the umbilical lobe is barely retracted.

REMARKS

Dorsetensia , included in the subfamily Sonniniinae in the Treatise ( Howarth 2013), is here formally transferred to Witchelliinae , because it shares many characters with Witchellia , such as: coiling; whorl section; ribbing; keel; and type of dimorphism with medium-sized macroconchs that have a simple aperture, and much smaller microconchs that have a complex aperture and well-developed lateral lappets. The primary differences are that Dorsetensia never has tubercles and the venter is not tabulate, whereas Witchellia has a relatively simpler septal suture. Some forms of Sonninites can also be quite similar to Dorsetensia , but Sonninites , usually more involute, has different ribbing with several furcate ribs in the inner whorls, may have tubercles in inner whorls, and has a more complex suture line.

Some authors (e.g. Buckman 1927; Huf 1968: 78, Pavia 1983: 62; Fernández-López 1985: 21; Fernández-López & Mouterde 1994: 124; Sadki 1996: 170; Myczynski 2004: 111) have considered Nannina to be the dimorphous partner [m] of Dorsetensia [M]. However, Howarth (2013, in Treatise: 119) deemed Nannina Buckman, 1927 as a junior synonym of Fontannesia Buckman, 1902 . Clearly, Nannina is not the dimorphic microconch of Dorsetensia because the inner whorls of the type species of the two genera are different. Furthermore, Nannina has a small coronate stage, a character that makes it closer to Sonniniinae and farther from Dorsetensia . Nor is Nannina synonymous with Fontannesia , as Howarth (2013) postulated, because the type species of the two taxa are quite distinct: Fontannesia has inner whorls with well-defined simple ribs that are in no case tuberculate, differentiating this species from Nannina (type species N. evoluta Buckman, 1927 [Buckman 1927: T.A. 7, pl. 752]). Furthermore, the stratigraphic ranges also substantially differ, as Fontannesia is restricted to the interval between the Concavum (Upper Aalenian) and Ovale zones, whereas the first stratigraphic records for Nannina do not appear until the upper part of the Propinquans Zone (seePavia 1983: 62;Fernández-López 1985: 115). The above mentioned shows that the taxonomic status of the HT of Nannina evoluta is not clear, but given its coiling, the coronate stage on the inner whorls and its stratigraphic distribution (Propinquans Zone), it could represent a dimorphic microconch of S. ( Sonninia ).

DISTRIBUTION

Lower Bajocian (uppermost Propinquans Zone-Humphriesianum Zone): Europe, North Africa ( Atlas Ranges ), Kenya, Madagascar, Saudi Arabia, Russia (south-eastern Pamirs), the Caucasus, China (Tibet), United States (Alaska, California, Oregon), Chile, and Argentina. In the Subbetic domain, Dorsetensia shares this stratigraphic distribution, appearing in the three Subbetic subdomains (External, Median, and Internal), but the genus is scarce except in the Pinguis horizon of the central sector of the Median Subbetic (Sierra de Alta Coloma area).