56 BRITISH AVONIAN CONODONT FAUNAS 
Assemblage Zone is similar to that described by Branson & Mehl (1934A) from the 
Bushberg Sandstone of Missouri, while the fauna of the basal Spathognathodus 
costatus sulciferus-Gnathodus delicatus Assemblage Zone is similar to that of the 
Hannibal Formation of Missouri described by E. R. Branson (1934). 
Our faunas from the K and Z Zones seem to agree closely with those described by 
Branson & Mehl (1934A) from the Bushberg, E. R. Branson (1934) from the Hannibal, 
and Cooper (1939) from the pre-Welden Shale Formations. This suggests that the 
biostratigraphic zones set up by Collinson, Scott & Rexroad (1962) may not be fully 
representative of the conodont faunas of the lowermost Lower Carboniferous of the 
whole of North America, as opposed to the Illinois Basin. Further work on Missouri 
and Oklahoma sections would probably prove useful in correlation between North 
America and the South West Province of Great Britain. 
Gnathodus delicatus makes its first appearance in the Avonian 34 ft. (Sample ZLA 6 
North Crop) from the top of the Spathognathodus costatus costatus—Gnathodus delicatus 
Assemblage Zone. In North America this species first appears at the base of the 
Chouteau Formation, in the top of the Siphonodella quadruplicata—Siphonodella 
crenulata Assemblage Zone, but little is known about the distribution of Upper 
Hannibal gnathodids, and it is therefore possible that Gnathodus delicatus may 
appear lower in the section. The top of the Hannibal Formation is the zone of few 
gnathodids of Collinson, Scott & Rexroad (1962). A similar zone of few gnathodids 
is also present in Germany. Branson & Mehl (1938A : 136) remarked that the 
Chouteau conodont assemblage is characterized by the introduction of Gnathodus. 
The base of the Chouteau Formation also corresponds to the oldest stratigraphical 
occurrence of the upper zone of Guathodus shown by Collinson, Scott & Rexroad 
(1962, Chart 3). 
The earliest occurrence of Guathodus delicatus in the Avon Gorge, which is some- 
what higher (base Sample Z 29) in the section than the earliest occurrence on the 
North Crop, is represented byrelativelylarge numbers of specimens, in contrast to the 
few specimens found in its first occurrence on the North Crop. The earliest occur- 
rence of Gnathodus delicatus on the North Crop may be correlated to the Siphonodella 
quadruplicata—Siphonodella crenulata Assemblage Zone occurrence of North America, 
which lies within the Zone of few gnathodids, whereas the Avon Gorge first occurrence 
may be correlated with the abundant occurrence of Gnathodus delicatus in North 
America, near the upper limit of the Siphonodella isosticha—Siphonodella cooperi 
Assemblage Zone, which occupies the upper part of the Chouteau Formation and is 
correlated with the upper part of the European Lower Carboniferous Cu II « Zone. 
Rexroad & Scott (1964) described the conodont faunas from the Siphonodella 
isosticha—Siphonodella coopert, Gnathodus semiglaber—Pseudopolygnathus multistriatus, 
Bactrognathus—Polygnathus communis and Bactrognathus-Taphrognathus Assemblage 
Zones, when they described the conodont fauna of the Rockford Limestone and New 
Providence Shale. They noted (Table 2: 15) that Gnathodus delicatus was most 
abundant in the Siphonodella isosticha—Siphonodella cooperi Assemblage Zone, 
Gnathodus semiglaber most abundant in the overlying Gnathodus semiglaber—Pseudo- 
polygnathus multistriatus Assemblage Zone and Gnathodus antetexanus most 
