Sonninia (Euhoploceras), 1913

Sandoval, José, 2022, Sonniniidae Ammonitina, Middle Jurassic from Southern Spain: taxonomic, biostratigraphical and palaeobiogeographical analysis, Geodiversitas 44 (27), pp. 801-851 : 815-816

Subgenus Sonninia (Euhoploceras) Buckman, 1913 [M] & [m]

Sonninia (Euhoploceras) Buckman, 1913: 4 .

? Stiphromorphites Buckman, 1923 [M]: pl. 398. — Type species: S. nodatipinguis Buckman, 1923 (OD).

Sherbonites Buckman, 1923 [M]: pl. 411. — Type species: S. projectifer Buckman, 1923 (OD).

Nannoceras Buckman, 1923 [m]: pl. 445, n. syn. — Type species: N. nannomorphum Buckman, 1923 (OD).

TYPE SPECIES. — Sonninia acanthodes Buckman, 1889 ( Buckman 1889: 658 [OD]); HT figured in Buckman (1892: pl. 60, pl. 63, fig. 1).

DESCRIPTION

Macroconchs evolute to moderately involute with rectangular (sometimes almost square) to ovate, slightly compressed whorl sections. The venter is subtabulate, often slightly bisulcate and possesses a low hollow keel persisting to end of the BC. The inner whorls have weak primary ribs and many shells have well-developed tubercles or spines. The length of the spinose/ tuberculate stage varies widely and it is not uncommon to find specimens that retain strong ribbing beyond the end of the PH. The BC commonly has single or, more rarely, some bifurcate ribs, which extend as far as the peristome. In others, tubercles give way to ribs, which progressively fade, the BC becoming striated or completely smooth. Conversely, many lack strong ornamentation throughout ontogeny occasionally with tiny spines present only on the earliest inner whorls. Secondary ribs range from strong to weak. The septal suture is relatively complex, with a ramified deep L, and slightly retracted U2-U5 lobes. Complete macroconchs have simple, only very slightly constricted peristomes, which appear to be rather weak and are consequently rarely preserved. The microconchs ( Nannoceras ) are much smaller, with more persistent ribbing and spatulate lateral lappets.

REMARKS

According to some authors (Buckman 1928, T.A. 7: 13; Fernández-López 1985: 94), Nannoceras probably represents the dimorphous microconch of S. (Euhoploceras). Actually, other authors ( Dietze et al. 2005: 59; Dietze et al. 2007: 16, 2011a: 217; Chandler et al. 2006: 370; Howarth 2013 in the Treatise) have suggested that Fontannesia Buckman [M] and the lappeted Nannoceras Buckman [m] constitute a dimorphous pair, which in turn are the most plausible ancestors of Witchellia [M] – Pelekodites [m]. Chandler (2019: 181, fig. 8e-g) figured three specimens of S. (Euhoploceras) [m], which are lappeted and have strong ribbing, and two have tuberculate inner whorls, which undoubtedly correspond to “ Nannoceras ”. Dr. R.B. Chandler confirmed to me (personal communication, March 2021) that, as Fernández-López (1985) had indicated, Nannoceras [m] is the dimorphic pair of S. (Euploceras) [M] and he generously sent me some plaster casts and photographs of typical Nannoceras from the upper Aalenian (Concavum Zone, Limitatum Subzone) and Lower Bajocian (Discites Zone) of Dorset (S England), which stratigraphically coexist with S. (Euhoploceras).

DISTRIBUTION

In agreement with the Treatise by Howarth (2013: 116), the present study concludes that the stratigraphic range of S. (Euhoploceras) extends throughout the upper Aalenian (Concavum Zone) to the lower Bajocian (Laeviuscula Zone) of Europe, North Africa ( Atlas Ranges ), Arabian Peninsula, China (Tibet), western Australia, Canada (British Columbia, Alberta), United States (Alaska, Oregon, California), Chile, and Argentina.