BRITISH AVONIAN CONODONT FAUNAS 57 
abundant in the overlying Bactrognathus—Polygnathus communis Assemblage Zone. 
In the Avonian of the Avon Gorge a similar change in the gnathodid fauna is seen. 
Gnathodus delicatus occurs near the base of the Zz Subzone, Gnathodus semiglaber is 
present in the upper part of Zz and Guathodus antetexanus first appears in the upper 
part of Zz and ranges into the lower part of C, (Fig. 11). 
The occurrence of gnathodids in Germany provides conflicting data for comparison. 
Ziegler (1960, 1963) has described gnathodids identical to those of the middle Poly- 
gnathus lacinatus Zone of the Avonian (Samples Z 28-Z 30). One of his faunules of 
anchoralis age (1960) also contained Siphonodella and Mestognathus, but the other 
yielded no representatives of Siphonodella. The anchoralis fauna of Germany, as at 
present understood, contains the simultaneous first appearance of several species of 
Gnathodus, which appear at different horizons in the Lower Carboniferous of Britain 
and North America. We interpret this as partly the result of Bischoff & Voges’ 
limited stratigraphic sections and partly the result of the more recent taxonomic 
refinements, which would now require the revision of the earlier specific nomenclature 
of these authors. 
Specimens referable to G. semiglaber, G. typicus, G. antetexanus and G. girtyi all 
first appear at the base of the anchoralis Zone. It appears that the anchoralis Zone 
as presently defined in Germany occupies a greater period of time than hitherto 
thought and that the Cu II 6—-y with which it is generally equated could include the 
time interval Upper Z2-Upper Cz in the Avonian and at least from the top of the 
Chouteau Formation to the top of the Burlington Formation in North America. 
If the first appearance of species is used as the basis of correlation, the oldest 
occurrence of Guathodus delicatus in the Avonian is in the Spathognathodus costatus 
costatus—Gnathodus delicatus Zone. In North America it first occurs in the Siphono- 
della isosticha—S. cooperi Zone, which might thus be interpreted as being equivalent 
to the Avonian Zone. This would make it equivalent to the Upper Chouteau, and 
to the upper part of Cu II « of the German succession (Figs. 12, 16). Collinson, 
Rexroad & Scott (1962) noted that the top of the Siphonodella isosticha—Siphonodella 
coopert Assemblage Zone was marked by a major unconformity in the Mississippi 
Valley and was a cut-off horizon for Gnathodus delicatus. This is one possible 
correlation, but the rarity of siphonodellids in the Avonian makes any such correla- 
tion based on “ first appearances ”’ of species tenuous, and other aspects of the fauna 
suggest a somewhat younger (Chouteau) age for the Avonian Zone. 
The Siphonodella—Polygnathus inornatus Zone is also characterized by the presence 
of Polygnathus inornatus s.l. and Siphonodella isosticha, both of which are character- 
istic species of the S:phonodella isosticha—S. cooperi Zone of the Mississippi Valley 
(Collinson, Scott & Rexroad 1962 : 21). Correlation of these two zones would thus 
appear an alternative valid solution to the problem, the general absence of earlier 
transitional siphonodellids in the Avonian being interpreted as a result of geographic 
faunal variation, either in the absence of whatever conodont-bearing group they 
represented, or, more probably, their “local ’’ functional replacement by broadly 
homologous polygnathids (Fig. 16). 
Such a possible correlation is supported by the fact that in the Bonaparte Gulf of 
