Parapanderodus Stouge, 1984

Zhen, Yong Yi, 2011, Middle to Late Ordovician (Darriwilian-Sandbian) Conodonts from the Dawangou Section, Kalpin Area of the Tarim Basin, Northwestern China, Records of the Australian Museum 63 (3), pp. 203-266 : 229-231

Parapanderodus Stouge, 1984

Type species. Parapanderodus arcuatus Stouge, 1984 = Parapanderodus striatus (Graves & Ellison, 1941) emended by Smith (1991).

Remarks. Parapanderodus was erected by Stouge (1984) to accommodate several coniform species characterized by a posterior groove, an unexpanded base, and fine striation on the surface. The type species P. arcuatus was interpreted as having two element types, but the other four species documented from the Table Head Formation of western Newfoundland— P. cf. consimilis ( Moskalenko, 1973), P. elegans Stouge, 1984 , P. striatus (Graves & Ellison, 1941) , and P.? aff. triangularis (Ethington & Clark, 1964) —were described as form species. Stouge & Bagnoli (1988) revised the genus and considered the two species referred to Semiacontiodus by Stouge (1984), S. asymmetricus (Barnes & Poplawski, 1973) and S. preasymmetricus Stouge, 1984 , to form part of the P. arcuatus species apparatus, thereby regarding P. arcuatus as a junior synonym of P. asymmetricus (Barnes & Poplawski, 1973) . Subsequently however, Stouge & Bagnoli (1990, p. 21) and Bagnoli & Stouge (1997, p. 151) treated P. arcuatus Stouge, 1984 as a valid species.

Based on study of a large collection from Greenland including fused clusters with the occurrence of all the Parapanderodus species described by Stouge (1984), Smith (1991) considered all five Parapanderodus species , as well as Semiacontiodus asymmetricus (Barnes & Poplawski, 1973) and S. preasymmetricus Stouge, 1984 , described by Stouge (1984) from the Table Head Formation, to represent a single species. Consequently Smith (1991) regarded the originally-designated type species ( P. arcuatus ) and subsequently-redesignated type species ( P. asymmetricus ) as junior synonyms of P. striatus , although he acknowledged that the Middle to early Late Ordovician (Whiterockian) representatives described by Barnes & Poplawski (1973) and Stouge (1984) tended to be albid. However, Ji & Barnes (1994) restricted P. striatus to the Early Ordovician hyaline elements. As Parapanderodus is only represented by a few specimens in the Dawangou material, it is impossible to re-assess the uncertainties and debates arising from the earlier work regarding to the species concept of P. striatus , P. arcuatus and P. asymmetricus and their taxonomic relationships. Therefore Smith’s (1991) definition of P. striatus is followed herein until a more comprehensive revision of these related species is undertaken.

Smith (1991) erected Toxotodus as a coniform genus characterized by having antero-posteriorly compressed symmetrical and asymmetrical elements with a striate cusp and a very shallow or reduced basal cavity. However, the type species of Toxotodus , Scolopodus carlae Repetski, 1982 was regarded as a species of Parapanderodus by Stouge & Bagnoli (1988) and Ji & Barnes (1994). Considering that the species of Toxotodus show some rather distinctive characters that readily distinguish them from the type species of Parapanderodus , Toxotodus is considered herein as a valid genus. Consequently, several species that were assigned to Parapanderodus previously by various authors are here regarded as species of Toxotodus , such as Parapanderodus carlae (Repetski, 1982) , and P. retractus Ji & Barnes, 1994 , from the upper part of the St. George Group of western Newfoundland.

Parapanderodus and Toxotodus were sister taxa as evidenced by sharing of several distinctive characters, such as a striate cusp, a conspicuous posterior groove and a shallow basal cavity. It is likely that they are also closely related to Early Ordovician species of Decoriconus that, as revised by Löfgren (1998), consist of a seximembrate apparatus.