Acodus Pander, 1856

Zhen, Yong Yi & Nicoll, Robert S., 2009, Biogeographic and Biostratigraphic Implications of the Serratognathus bilobatus Fauna (Conodonta) from the Emanuel Formation (Early Ordovician) of the Canning Basin, Western Australia, Records of the Australian Museum 61 (1), pp. 1-30 : 8-11

Acodus Pander, 1856

Diaphorodus Kennedy, 1980: 51 .

Type species. Acodus erectus Pander, 1856 .

Remarks. The name Acodus , although widely cited in Ordovician conodont literature, has been a subject of debate and has remained one of the most controversial conodont genera for many years. As its type species A. erectus was poorly known, Kennedy (1980) regarded Acodus as a nomen dubium, and this view was accepted by several subsequent workers (e.g., Sweet, 1988), while others (e.g., Lindström in Ziegler, 1977; Ji & Barnes, 1994; Zhen et al., 2004) retained Acodus as a valid genus, but with varying definitions.

During the late Tremadocian to early Floian, conodonts evolved rapidly and experienced the greatest diversification event in their evolutionary history of some 300 million years. Several important clades, particularly the Prioniodontida , evolved from forms previously gouped with Acodus . Understanding of the apparatus composition and structure of Acodus and related taxa is crucial in depicting the phylogenetic relationships and evolutionary history of these related clades (Stouge & Bagnoli, 1999). Therefore, a narrower rather than broader generic concept is needed for Acodus , which is restricted herein to forms with the same apparatus composition and structure as Prioniodus but typically consisting of adenticulate elements.

Recent study by Nicoll & Ethington (2004) shows that Oepikodontidae diverged from the main clade of Prioniodontoidea in the late Tremadocian through an adenticulate stage represented by Lissoepikodus (recognized in the Emanuel Formation), and supports the hypothesis that both Prioniodontoidea and Balognathoidea might have evolved from a common ancestor (Stouge & Bagnoli, 1999), most likely Acodus (= Diaphorodus + “ Acodus ” deltatus of Stouge & Bagnoli, 1999).

As Pander’s type specimens of the genotype, A. erectus , are lost, our current understanding of Acodus is largely based on several subsequent works on A. deltatus Lindström, 1955 and other related species documented by McTavish (1973),

[ Fig. 4 , caption continued]. (G), P element (short based), CPC39796, inner lateral view (IY129-024). H–J, Sa element; (H), CPC39797, lateral view (IY129-009); (I), CPC39798, lateral view (IY129-11); (J), CPC39799, basal view (IY129-014). (K), P element (short based), CPC39800, inner lateral view (IY129-020). (L), Sb element, CPC39801, inner lateral view (IY129-018). M–R, Acodus ? transitans McTavish, 1973 . M–Q, Pa element; M,N, CPC39802; (M), basal-outer lateral view (IY126-018); (N), outer lateral view (IY126-020). (O), CPC39803, inner lateral view (IY126-029). P–R, Sc element; P,Q, CPC39804; (P), outer lateral view (IY126-021); (Q), basal-outer lateral view (IY126-022); (R), CPC39805, outer lateral view (IY126-023). S–X, Acodus deltatus? Lindström, 1955 . (S), M element, CPC39806, 161– 166 m, posterior view (IY130-034); (T), Sb element, CPC39807, WCB705/243, outer lateral view (IY130-045); (U), Sd element, CPC39808, 161– 166 m, inner lateral view (IY132-010); V–X, P element; V,W, CPC39809, 161– 166 m; (V), anterior view (IY130-037); (W), inner lateral view (IY130-038); (X), CPC39810, 161– 166 m, outer lateral view (IY130-028). Scale bars 100 µm.

van Wamel (1974), Lindström (in Ziegler, 1977), Stouge & Bagnoli (1999), Zhen et al. (2003), Nicoll & Ethington (2004) and Zhen et al. (2005). McTavish (1973) described A. deltatus and several other species of Acodus from the Emanuel Formation of the Canning Basin, and suggested a seximembrate apparatus for A. deltatus including prioniodiform (= P of our interpretation), ramiform (trichonodelliform = Sa, gothodiform = Sb, cordylodiform = Sc, and tetraprioniodiform = Sd of our interpretation), and oistodiform (= M of our interpretation) elements. Based on the stratigraphical distribution and morphological changes of various Acodus species recognized in the Emanuel Formation, McTavish (1973, fig. 7) indicated that both Prioniodus and Baltoniodus might have evolved from Acodus . Some authors went even further by attributing A. deltatus to either Prioniodus ( van Wamel, 1974) or Baltoniodus ( Bagnoli et al., 1988) . In their revision of A. deltatus, Bagnoli et al. (1988) considered it to have a seximembrate apparatus without an Sa element, and included in this species some specimens from the Emanuel Formation illustrated by McTavish (1973). Subsequently Stouge & Bagnoli (1999) indicated that forms referred to A. deltatus by McTavish (1973), Ethington & Clark (1982) and Repetski (1982) were actually species of Diaphorodus and not congeneric with their revised “ A.” deltatus .

Kennedy (1980) proposed Diaphorodus as the replacement of Acodus and selected A. delicatus Branson & Mehl, 1933 as the type species. In his revision of this species from the Early Ordovician Jefferson City Formation of Missouri, he recognized a seximembrate apparatus including the following types of elements: oistodiform (= M of our interpretation) represented by the form species Oistodus expansus Branson & Mehl, 1933 (see Kennedy, 1980, pl. 1, figs 20–22); acontiodiform (= Sa herein; see Kennedy, 1980, pl. 1, figs 14–17); drepanodiform (=?Sc herein) represented by the form species Cordylodus simplex Branson & Mehl, 1933 (see Kennedy, 1980, pl. 1, figs 9–11); distacodiform (= Sd herein) represented by the form species Paltodus distortus Branson & Mehl, 1933 (see Kennedy, 1980, pl. 1, figs 18, 19); acodiform (= Pa herein) represented by the form species A. delicatus Branson & Mehl, 1933 (see Kennedy, 1980, pl. 1, fig. 3); and oistodiform (=?Pb herein) represented by the form species Oistodus vulgaris Branson & Mehl, 1933 (see Kennedy, 1980, pl. 1, figs 23–25). This multi-element species reconstruction is more or less agreeable with our current understanding of Acodus , except that the asymmetrical tricostate Sb element was absent in the type species of Diaphorodus defined by Kennedy (1980). However, Stouge & Bagnoli (1999) considered both Diaphorodus and Acodus (represented by “ Acodus ” deltatus ) as valid genera. According to their illustrations of both Diaphorodus sp. A from the Cow Head Group of western Newfoundland (Stouge & Bagnoli, 1999, pl. 1, figs 1–7) and “ A.” deltatus from the Köpingsklint Formation of Öland ( Bagnoli et al., 1988; Stouge & Bagnoli, 1999, text-fig. 2), both Diaphorodus with a septimembrate apparatus and Acodus with a seximembrate apparatus have a nearly identical species apparatus and element morphology, except there is no Sa element for “ A.” deltatus . As argued by Zhen et al. (2005, p. 306), we prefer to retain Acodus as a valid genus and consider Diaphorodus tentatively as a junior synonym of Acodus pending further studies of these and other closely related forms.

Figures

Fig. 4.A–F, Bergstroemognathus extensus (Graves & Ellison, 1941).All from sample WCB705/243.(A), M element, CPC39790,posterior view (IY116-046). (B), Sa element, CPC39791, posterior view (IY116-047).(C), Sb element, CPC39792, inner lateral view (IY116-048). (D), Sc element, CPC39793, inner lateral view (IY116-049).(E), Pb element, CPC39794, inner lateral view (IY116-042).(F), Pa element, CPC39795, inner lateral view (IY116-043). G–L, Cornuodus sp. All from WCB705/243; …[continued on facing page]