202 BRITISH AVONIAN CONODONT FAUNAS 
THE EVOLUTIONARY DEVELOPMENT OF PSEUDOPOLYGNATHUS 
There have been three distinct developments of the genus Pseudopolygnathus in 
the Avon Gorge : one in the lower and middle parts of the K Zone, referred to as the 
Pseudopolygnathus vogesi sp. nov. development, a second near the base of Z, the 
Pseudopolygnathus primus development and a third in the upper part of the Z Zone 
the Pseudopolygnathus multistriatus development. 
The three developments have been along similar lines, the starting point of each 
being a straight, bladed spathognathodid with lateral denticles. Specimens of 
growth series from the three developments are, with the exception of adults, 
extremely difficult to distinguish (Fig. 43). 
THE EVOLUTION OF PSEUDOPOLYGNATHUS PRIMUS 
Voges (1959) noted that in the Upper Devonian the Spathognathodus costatus 
(sensu Bischoff & Ziegler) group marks the starting point in the evolutionary develop- 
ment of Pseudopolygnathus primus, and other pseudopolygnathids, which deviate 
from the typical bilateral symmetry of the spathognathodids. Voges recognized 
two groups within the Spathognathodus costatus group, right forms and left forms, the 
basal cavities of which are bilaterally symmetrical elements. The outer margin of 
the platform of right forms and the inner margin of left forms are both convex, and 
are thus bilaterally asymmetrical elements. In the left forms, there is a row of 
nodes or ridges on the inner side of the platform, which extends for three quarters the 
length of the unit. In the right forms, this row of nodes or ridges is on the outer side 
of the platform. In left forms, nodes are usually absent on the outer side of the 
platform, but in large specimens one or two nodes may be present on the outer side of 
the platform, situated near the basal cavity. Voges recognized two varieties of right 
forms, which correspond to the subspecies Spathognathodus spinulicostatus spinuli- 
costatus (Bischoff 1957) and S. spinulicostatus ultimus (Bischoff 1957). In both these 
subspecies a secondary row of nodes or small ribs extends from the middle of the 
basal cavity to the posterior extremity on the inner side of the platform. In 
Spathognathodus spinulicostatus ultimus they are more strongly developed, and a 
furrow replaces the nodes along the posterior third of the blade. 
Voges believed that S. spinulicostatus spinulicostatus and S. spinulicostatus ultimus 
were stages in the ontogenetic development of a right form, to which Spathognathodus 
costatus may be added as a left form. The opinion of Voges that S. spinulicostatus 
ultimus May represent an ontogenetic senile form of S. spinulicostatus spinulicostatus 
developed in the Wocklumeria Stage is not held by Ziegler (1962) who thinks it is 
unlikely that a fauna consisting entirely of senile members of S. costatus (spinuli- 
costatus) spinulicostatus occurs at one horizon, while at a stratigraphically lower 
horizon, it consists entirely of non-senile members of S. costatus (spinulicostatus) 
spinulicostatus. In addition, Ziegler has found a few specimens of right forms of S. 
costatus costatus and a few left forms of S. costatus (spinulicostatus) spinulicostatus and 
S. costatus (spinulicostatus) ultimus. 
Voges believed that from the first stage, represented by the S. costatus group, there 
