BRITISH AVONIAN CONODONT FAUNAS 55 
Western Europe (Voges 1959). Collinson, Scott & Rexroad (1962) noted that the 
first appearance of the genus Pseudopolygnathus in the Mississippi Valley was at 
the base of their Gnathodus n. sp. B.-Gnathodus kockeli Assemblage Zone, which is 
confined to the Glen Park and the basal Hannibal Formations and is correlated 
with the lowermost part of the Lower Carboniferous Cu I Zone of Western Europe. 
The upper part of the Pseudopolygnathus vogesi-Clydagnathus Assemblage 
Subzone has specimens of P. inornatus which Ziegler (1962), who referred to 
them as Polygnathus nodomarginatus, found in the Middle and Upper Spathognathus 
costatus Zones, as well as in the G. kockeli-P. dentilineatus Zone. Dr. C. W. 
Collinson (personal communication) has found similar specimens in the Mississippi 
Valley, where they appear to be forerunners of Siphonodella sulcata. These are 
found in abundance, and are associated with Gnathodus kockeli in the base of the 
Hannibal Formation at several localities (just below the lowermost Szphonodella 
Zone). 
The conodont fauna of the K and basal Z Zones of the Avonian is also difficult to 
correlate precisely with the American and West German successions, because of the 
rarity of the genus Szphonodella in the K Zone and its absence in the Z Zone. This 
genus is one of the most useful of all guide fossils in other areas, where individual 
species are distinctive, short ranging, and have a wide geographical distribution 
(Collinson, Scott & Rexroad 1962, Chart 2). In North America and West Germany 
ranges of individual species have been used to define the boundaries of conodont 
assemblage zones. 
In the Avonian, the genus Siphonodella is confined to the Siphonodella—Polygnathus 
inornatus Assemblage Zone. Specimens from the Avon Gorge, although fractured, 
have rostral ridges and are more advanced in development than Siphonodella sulcata, 
the earliest known species in North America, where it occurs near, but not at, the 
base of the Hannibal Formation. Specimens from the North Crop are identified as 
Siphonodella isosticha. This species in North America ranges from the base of the 
Upper Hannibal Formation to the top of the Upper Chouteau Formation. Thus the 
base of our Siphonodella—Polygnathus inornatus Assemblage Zone cannot be older 
than the base of the upper part of the Hannibal Formation, which is included in the 
lower part of the Szphonodella quadruplicata—Siphonodella crenulata Assemblage Zone 
of the Mississippi Valley. 
Correlation of the base of this zone with our Siphonodella—Polygnathus inornatus 
Assemblage Zone, would make it broadly equivalent to the lower part of the Cu II « 
Zone of the European Lower Carboniferous. This is supported, to some extent, by a 
consideration of the ranges of gnathodid species, which provide a possible correlation 
for the higher part of the Avonian section. 
Klapper (1966) has described a fauna from the Lodgepole Limestone of Montana 
and Wyoming, which is referable to the Lower Siphonodella crenulata Zone (Cu II «) 
of Germany and from the Mississippian part of the Dark Shale Unit, which is referable 
to the Siphonodella—P. triangulus triangulus Zone (Cu I). These faunas are in part 
similar to the Upper K Zone fauna of the Avonian. 
The conodont fauna of the Spathognathodus robustus—Spathognathodus tridentatus 
