Foordiceras decacanthum Korn & Hairapetian, 2025, sp. nov.

Foordiceras decacanthum sp. nov. urn:lsid:zoobank.org:act: FEB57A12-32B4-461B-968A-0037BCC865E6 Fig. 17; Table 11 Metacoceras sp. – Korn et al. 2021: text-fig. 17b. Diagnosis Species of Foordiceras with thinly discoidal, subevolute conch (ww/dm ~0.40; uw/dm ~ 0.44), weakly depressed whorl profile (ww/wh ~ 1.25) and high coiling rate (WER ~ 2.10) at a conch diameter of 60 mm. Whorl profile trapezoidal with divergent, convex flanks; venter flattened, umbilical margin incipient. Sculpture with about 10 very coarse, blunt ventrolateral ribs per volution. Suture line with a very shallow external lobe and a very shallow, broadly rounded lateral lobe. Etymology From the Greek ‘δέκα’=‘ten’ and ‘ἄκανθα’=‘thorn, spine’; because of the ten ventrolateral nodes per volution. Type material Holotype IRAN – Esfahan Province • Baghuk Mountain; Hambast Formation, Vedioceras beds (late Wuchiapingian); 2010; Korn et al. leg.; illustrated in Fig. 17; MB.C.30224. Description Holotype MB.C.30224 is an incomplete, but rather well-preserved internal mould with a diameter of 62 mm (Fig. 17B). The conch is thinly discoidal and subevolute (ww/dm= 0.41; uw/dm =0.44) with a high coiling rate (WER= 2.08); the whorl profile is weakly depressed (ww/wh =1.27) and rounded trapezoidal with a flatly rounded venter and a broadly rounded ventrolateral shoulder. The divergent flanks are convex; a rudimentary umbilical margin is developed (Fig. 17A). The sculpture consists of about 10 very large, but low ventrolateral nodes per volution; these nodes have a rounded conical shape and continue as very low ribs on the flank. They are much wider than the spaces between them. These ribs are considerably more prominent on the penultimate whorl, at a conch diameter of about 25 mm, than on the last whorl. The suture line shows a very shallow external lobe and a very shallow lateral lobe (Fig. 17C). The last half volution of the phragmocone has about 12 chambers (CLI = 15). Remarks Foordiceras decacanthum sp. nov. differs from F. eicosacanthum sp. nov., which possesses a similar conch geometry, in the number and formation of the ventrolateral nodes. Foordiceras decacanthum has only five very coarse nodes on half a whorl, whereas F. eicosacanthum has ten much smaller nodes. Foordiceras decacanthum has a very similar sculpture to F. ascetum sp. nov., but shows a depressed whorl profile (ww/wh ~1.25), whereas in F. ascetum this is almost equidimensional (ww/wh ~1.00). Another criterion to separate F. decacanthum from the rather similar F. ascetum is in the width of the nodes; in F. decacanthum they are wider than the interspaces, while they are only half as wide as their interspaces in F. ascetum. Foordiceras decacanthum sp. nov. differs from the superficially similar F. goliathus, which also possesses very coarse ventrolateral nodes, in the much wider umbilicus. The uw/dm ratio is about 0.45 in F. decacanthum, but only around 0.30 in F. goliathus.