Acodus Pander, 1856

Y Y Zhen, 2005, Revision of Two Prioniodontid Species (Conodonta) from the Early Ordovician Honghuayuan Formation of Guizhou, South China, Records of the Australian Museum 57, pp. 303-320 : 306-307

Acodus Pander, 1856

Type species. Acodus erectus Pander, 1856 .

Remarks. The generic definition of Acodus and other related genera like Tripodus Bradshaw, 1969 and Tropodus Kennedy, 1980 has been a subject of disagreement among conodont workers (e.g., Lindström in Ziegler, 1977; Sweet, 1988, Kennedy, 1980; Albanesi in Albanesi et al., 1998; Johnston & Barnes, 2000; X.H. Chen et al., 2003). It seems likely that there are more than one genus involved in these species embraced under a rather broad definition of Acodus given by Zhen et al. (2004). However, a serious revision of this species group, which will certainly need a monographic treatment, is far beyond the scope of the present project, and the level of our current understanding of these species does not yet support such a major revision. Acodus , defined as a multielement genus by Lindström (in Ziegler, 1977), is deeply entrenched in the Ordovician conodont literature. It was widely distributed in the Early and Middle Ordovician, and played a crucial part in the origin and early evolution of several major pectiniform clades like prioniodontids and balognothids (Stouge & Bagnoli, 1999). We retain Acodus as a valid genus, although Pander’s original specimens of the genotype, A. erectus , are irretrievably lost.

The type species of Tripodus , T. laevis Bradshaw , was originally defined as a form species on the basis of three specimens which show sharp anterior and posterior margins, a sharp blade-like costa on the outer lateral face situated more towards anterior margin, and a broad carina on the inner lateral face ( Bradshaw, 1969). Based on material from the Ibex area of Utah, Ethington & Clark (1982) revised the type species of Tripodus as consisting of a quinquimembrate apparatus. This is interpreted herein as including geniculate M (oistodiform element, see Ethington & Clark, 1982, fig. 33A,B), alate triform Sa (trichonodelliform element, their fig. 33C), asymmetrical tetra-costate Sb (distacodiform element, their fig. 33F,G), asymmetrical laterally compressed Sc (drepanodiform element, their fig. 33H), and asymmetrical multi-costate Sd (paltodiform element, their fig. 33D,E) elements. Apparently the type material of T. laevis would fall into the definition of the Sb element of the multi-element concept of the species, and the types of Scolopodus alatus Bradshaw, 1969 , which Ethington & Clark (1982) regarded as part of the T. laevis apparatus, include both Sa ( Bradshaw, 1969, pl. 132, fig. 4, paratype) and Sd ( Bradshaw, 1969, pl. 132, figs 1–3, holotype) elements. Ethington & Clark (1982) also regarded Triangulodus van Wamel, 1974 as a junior synonym of their revised Tripodus . This raised the question as to whether Tripodus is likely a senior synonym of Triangulodus or a junior synonym of Acodus .

Based on material from western Newfoundland, Stouge (1984) regarded Tripodus as a junior synonym of Acodus , and suggested that Acodus combsi Bradshaw, 1969 , T. laevis Bradshaw , and S. alatus Bradshaw represented different elements of one species apparatus which he called A. combsi . By assigning the prioniodiform P element represented by the form species A. combsi to the species apparatus, Stouge (1984) expanded the definition of T. laevis given by Ethington & Clark (1982), although at the time he was unaware of their work. Further differentiation of the prioniodiform Pa and Pb elements has completed the species apparatus as septimembrate (Albanesi in Albanesi et al., 1998). Acodiform specimens assignable to the form species, A. combsi Bradshaw, 1969 were also reported from the Ibex area of Utah under the name “ Scandodus ” robustus Serpagli, 1974 (Ethington & Clark, 1982, pl. 10, fig. 25). This species definition for T. laevis has been more or less accepted by many Ordovician conodont workers ( Sweet, 1988, Stouge & Bagnoli, 1988; Albanesi in Albanesi et al., 1998; Bergström & Albanesi, 2001; Pyle et al., 2003), although others (Johnston & Barnes, 2000; X.H. Chen et al., 2003) retained the original concept of oistodiform M and costate S elements only as defined by Ethington & Clark (1982).

By taking A. combsi as the name bearer of the species, Stouge’s work (1984) raised two questions: firstly, should T. laevis or A. combsi be the name bearer of the revised multi-element species, and secondly, to which genus should this species be assigned? In a recent documentation, Bergström & Albanesi (2001) reviewed the validity of the species name Tripodus laevis , and concluded that based on the most recent edition of ICZN rules, T. laevis is the valid name and A. combsi is a junior synonym of T. laevis . As for the second question, some conodont workers follow Sweet (1988) in maintaining Tripodus as a valid genus and assigning Acodus as a nomina dubia, whereas others (e.g., Stouge & Bagnoli, 1988; Johnston & Barnes, 2000) utilize both Acodus and Tripodus by consigning species to the respective genus on historical reasons or on the details of the P and S elements. Based on the definitions of Acodus given by Lindström (in Ziegler, 1977) and of Triangulodus given by van Wamel (1974), both bear costate S and geniculate M elements, but Acodus has prioniodiform (acodiform) P elements instead of scandodiform P elements without costa on lateral faces as in Triangulodus . However, as discussed above, the form species of T. laevis may represent the Sb element of the multi-element species apparatus adopted herein. If Stouge’s (1984) revision of T. laevis is accepted, Tripodus is likely a junior synonym of Acodus . Ethington (pers. comm., 2004) suggested that the S elements of Tripodus were much more ornate in their surface morphologies than typical Acodus as demonstrated by A. deltatus Lindström, 1955 and A. triangularis Ding described herein, and the P elements of T. laevis ( A. combsi s.f. of Bradshaw) were deeply albid with very shallow basal cavities and without prominent carinae on the cusps. Until the type species of Tripodus can be revised in detail the taxonomic relationship of these two genera remains uncertain.

In the type sections of the Marathon Basin of Texas, all three form species ( T. laevis , A. combsi and S. alatus ) were established based only on a few specimens, and T. laevis and A. combsi were not found in association ( Bradshaw, 1969). Furthermore, co-occurrence of Histiodella sinuosa (Graves & Ellison) and Periodon aculeatus Hadding, 1913 in the Fort Peña fauna also suggests a younger age (Middle Ordovician, Yapeenian equivalent). Considering that the FAD (first-appearance datum) of T. laevis is a potential candidate for defining the base of the Middle Ordovician, detailed revision of this species at the type locality is urgently needed.