Erraticodon Dzik, 1978

Zhen, Yong Yi, Percival, Ian G. & Webby, Barry D., 2003, Early Ordovician Conodonts from Far Western New South Wales, Australia, Records of the Australian Museum 55 (2), pp. 169-220 : 194-195

Erraticodon Dzik, 1978

Type species. Erraticodon balticus Dzik, 1978 .

Diagnosis. Septimembrate apparatus with a ramiformramiform structure including digyrate (or modified) P, makellate M, alate Sa, bipennate Sb and Sc, and tertiopedate Sd, hyaline elements with a prominent cusp, discrete peglike denticles on the processes, and a shallow basal cavity.

Remarks. The type material of E. balticus was recovered from an erratic boulder found near Kartuzy, Pomerania, Poland, but believed to be transported from the Baltic region. Dzik (1978) originally recognized a seximembrate apparatus for the species, consisting of a makellate M, which has a denticulate outer lateral process but apparently lacks an anticusp ( Dzik, 1978, pl. 15, fig. 5, text-fig. 6d), bipennate Sb and Sc with a denticulate posterior process and an anterolateral process, tertiopedate Sa and Sd with a denticulate posterior process and a lateral process on each side, and digyrate Pb elements ( Dzik, 1978, text-fig. 2, spathognathiform element). Subsequently, Dzik (1991) indicated that the species had a septimembrate composition, but unfortunately provided neither description nor further details of the revised apparatus. Based on his schematic illustration ( Dzik, 1991, p. 299, text-fig. 12A), both Pa and Pb elements are digyrate; the Pb ( Dzik, 1991, text-fig. 12 A-sp) has a straight basal margin, and an anterior costa which extends to the anticusp, while the Pa ( Dzik, 1991, text-fig. 12A-oz) has a rather strongly arched basal margin and lacks an anticusp. The other elements he illustrated are reinterpreted here as ne=M, tr=Sa, pl=Sd, ke-hi=Sb, and lo=Sc.

Watson (1988, pl. 8, fig. 13) recovered a very rare bipennate element with a long, curved posterior process from the Goldwyer Formation of the Canning Basin which he attributed to E. balticus . This may represent a variant of the Sb element .

Erraticodon balticus is characterized by having an accentuated denticle on the posterior process of the Sa, Sb and Sc elements . The species is widely distributed in the Baltic region and the Siberian Platform ( Dzik, 1978), the Canning Basin of Western Australia ( Watson, 1988), South China (Ding et al., in Wang, 1993), Argentine Precordillera ( Lehnert, 1995; Albanesi, in Albanesi et al., 1998), Utah (Ethington & Clark, 1982), and western Newfoundland ( Stouge, 1984; Pohler, 1994; Johnston & Barnes, 2000).

Three additional named species are ascribed to Erraticodon . Locally these include E. patu Cooper , of O. evae Zone age, from the Horn Valley Siltstone ( Cooper, 1981), the Tabita Formation at Mount Arrowsmith and an unnamed unit at Koonenberry Gap. Records of this species outside Australia are in need of reassessment. The stratigraphically younger E. balticus supposedly co-occurs with E. patu at the top of the San Juan Formation in the Argentine Precordillera, but of the four illustrated specimens, only an Sa element ( Lehnert, 1995, pl. 10, fig. 11) can be assigned doubtfully to E. patu . Illustrated P elements from the Suri Formation ( navis Zone ) in the Famatina Range of western Argentina (Albanesi & Vaccari, 1994) seem comparable with those of E. patu from western New South Wales, but associated S and M elements from western Argentina do not conclusively belong to E. patu . Bauer (1990) reported E. patu from the McLish Formation of the Arbuckle Mountains, Oklahoma. A mid-late Darriwilian age was suggested for this unit, based on the occurrence of Eoplacognathus and other typical Middle Ordovician taxa ( Bauer, 1987). Of the two figured specimens referred to E. patu , even the generic assignment is doubtful for one ( Bauer, 1990, pl. 2, fig. 17). The other figured specimen is a Pb element, resembling E. patu but probably more closely related to an unpublished new species of Erraticodon from the Oakdale Formation (mid-late Darriwilian) of central New South Wales (Zhen & Percival, in prep.). Cooper (1981) indicated that E. patu also occurred in the Whiterockian Bay Fiord Formation of Arctic Canada ( Nowlan, 1976). The description and illustration of the Canadian specimens ( Nowlan, 1976: 442–445, pl. 11, figs.

1–11) indicate that they too are closely related to the unpublished species from the Oakdale Formation. The Pa and Pb elements of this Canadian species are comparable with those of E. patu , but S and M elements of these two species are readily distinguishable.

Erraticodon tangshanensis Yang & Xu , in An et al., 1983, first described from the Beianzhuang Formation ( evae to originalis zones) and the Majiagou Formation (Middle Ordovician) of North China, consists of a septimembrate apparatus, which differs from that of E. balticus in having a modified digyrate Pa element , and in possessing stout denticles on the Sa element . It has also been reported from Korea ( Lee, 1975, 1976, 1979), and from the Coolibah Formation of the Georgina Basin (Stait & Druce, 1993), but this latter occurrence in centralAustralia is questionable, given that it is based on four poorly preserved specimens. The two figured specimens (Stait & Druce, 1993, fig. 21B,C) do not appear to be conspecific with E. tangshanensis .

The other species, E. hexianensis An & Ding, 1985 , consists of a quinquimembrate apparatus. The type material was from the Lower Ordovician Xiaotan Formation of Anhui Province (An & Ding, 1985), but the full stratigraphic range extends from the upper Lower Ordovician (Dawan Formation and equivalents) to the Middle Ordovician (Kuniutan Formation and equivalents) of South China (Ding et al., in Wang, 1993). S elements (Sa, Sb, Sc, Sd) of E. hexianensis resemble those of E. patu , except that the Sc element of E. hexianensis has a shorter anterior process. The M element of the Chinese species has an erect cusp with rounded inner lateral corner, and apparently has neither anticusp nor denticles on the inner lateral corner. P elements of E. hexianensis were not adequately illustrated or documented; the only figured Pb element (An & Ding, 1985, pl. 1, fig. 15) seems digyrate with a robust cusp, a sharp anterior costa, and a lateral process on each side with three denticles. However, in a later publication, An (1987, pl. 22, fig. 12) illustrated this same specimen but referred it to Phragmodus sp. Further confusion arises because although this and the immediately preceding figure are stated in the caption to plate 22 of An (1987) to be different views of the one specimen, this does not seem to be the case. Nicoll (pers. comm. 2002) suggested that this and another specimen (figured by An, 1987, pl. 22, fig. 15 as Oulodus sp. ) might represent the Pb and Pa elements of E. hexianensis respectively.

A potential fifth species of the genus, referred to herein as E. sp. A, is a rare component in the Tabita Formation fauna, but due to limited material is not formally named. Features distinguishing it from E. patu are discussed below.