Records of the Australian Museum (2009) Vol. 61 
most of the Ordovician (Nowlan et al., 1997; Zhen et al. in 
Webby et al., 2000). Restriction of S. diversus primarily to 
the South China and the Tarim plates also indicates a closer 
biogeographic relationship between these two Chinese plates 
in the early Floian, and agrees with the recent biogeographic 
analysis based on the trilobite geographical and stratigraphi- 
cal distribution data from the Chinese Cambrian (Zhou et 
al., 2008) and Ordovician (Zhou & Zhen, 2008). Trilobite 
data (Zhou & Zhen, 2008; Zhou et al., 2008) suggested that 
a broad undifferentiated biogeographic entity persisted in the 
Chinese plates and terranes positioned in eastern Gondwana 
from Cambrian to Tremadocian time, and became more di¬ 
versified in the early Floian when two subprovinces (North 
China and South China) could be recognized (Zhou et al., 
2008, fig. 2). Trilobite distributional patterns and sharing of 
several endemic forms also support a closer biogeographic 
tie between Australia and North China (Choi et al., 2001; 
Zhou & Zhen, 2008; Zhou et al., 2008). 
Material and methods 
All photographic illustrations shown in Figs 4 to 16 are 
SEM photomicrographs captured digitally (numbers with 
the prefix IY are the file names of the digital images). 
Conodont specimens for this study are from three samples 
from the Emanuel Formation that bear S. bilobatus. Both 
WCB705/133 and WCB705/243 are from Section WCB 
705 measured by following the line of the type section 
(Guppy & Opik, 1950; Grid Ref. for base: 18°39'50"S 
125°54'00"E; and top: 18°39T2"S 125°55’05"E), and 
sample 161-166 m is from a drillhole (BHP/PDH, grid 
ref. 18°40'30"S 125°33'00"E, from the middle part of the 
Emanuel Formation, see Nicoll et al., 1993) in the northern 
margin of the Canning Basin. Figured specimens bearing the 
prefix CPC are housed in the Palaeontology Collection of 
Geoscience Australia in Canberra. Those bearing the prefix 
“AM F” (Figs 12-14) are deposited in the collections of the 
Palaeontology Section at the Australian Museum, Sydney, 
and were collected from the Honghuayuan Formation in 
Guizhou (see Zhen et al. in press a for sampling locations 
and stratigraphic details). Several species documented herein 
only by illustration are rare in the collection [ Cornuodus 
sp. (Fig. 4G-F), gen. et sp. indet. A (Figs 8F-0, 10Y), 
Nasusgnathus dolonus (An, 1981) (Fig. 10S-U), Paltodus sp. 
(Fig. 10X), Protoprioniodus simplicissimus McTavish, 1973 
(Fig. 6M-R), gen.sp. indet. B (Fig. 10W), and Triangulodus 
bifidus Z h en in Zhen et al., 2006 (Fig. 15I-K)] or those which 
are discussed adequately elsewhere [Bergstroemognathus 
extensus (Graves & Ellison, 1941) (see Zhen etal., 2001; Fig. 
4A-F), Lissoepikodus nudus Nicoll & Ethington, 2004 (see 
Nicoll & Ethington, 2004; Fig. 10F-R), and Stiptognathus 
borealis (Repetski, 1982) (see Ethington et al., 2000; Fig. 
15A-H)]. Conodont terminology and notation employed in 
this contribution are conventional as defined in Treatise Part 
W (Clark et al., 1981), except for the M elements (makellate), 
whose orientation, morphology and the terminology was 
introduced by Nicoll (1990, 1992). 
Systematic palaeontology 
Phylum Chordata Balfour, 1880 
Class Conodonta Pander, 1856 
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., Findstrom in 
Ziegler, 1977; Ji & Barnes, 1994; Zhen etal., 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 phyloge¬ 
netic 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 con¬ 
sisting 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 aden¬ 
ticulate 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 Findstrom, 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); (7), 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, Acodusl 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 (IY 126-022); (R), CPC39805, outer lateral view (IY126-023). S-X, Acodus deltatus! Lindstrom, 1955. ( S ), M element, CPC39806, 
161-166 m, posterior view (IY130-034); (7), Sb element, CPC39807, WCB705/243, outer lateral view (IY130-045); ( U ), Sd element, 
CPC39808, 161-166 m, inner lateral view (IY132-010); V-Y, 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 pm. 
