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Fig. 21 Distribution of ammonites in the Lower Sinemurian of Robin 

 Hood's Bay. 



characteristic of the Sauzeanum Subzone. Amioceras semicostatum 

 is common through most of the subzone. 



TURNERI ZONE, Brooki Subzone, beds 429.7-433.2. The only 

 ammonites found in the beds that are allocated to this subzone are 

 seven Caenisites brooki in the middle part and two Amioceras sp. 

 indet. in the middle and lower beds. Caenisites brooki probably only 

 occurs in the upper or top part of the subzone (Dean et al, 1961: 

 453), and the ammonite Caenisites preplotti Spath, which is charac- 

 teristic of the base of the subzone, does not occur in Robin Hood's 

 Bay. So the Brooki Subzone has to be defined according to the 

 boundaries of the adjoining subzones: the highest occurring 

 Coroniceras (Arietites) alcinoe in bed 429.64 at the top of the 

 Sauzeanum Subzone defines the base of the Brooki Subzone at the 

 bottom of bed 429.7, and the appearance of Microderoceras birchi in 

 bed 433.3 at the base of the Birchi Subzone defines the top of the 

 Brooki Subzone at the top of bed 433.2. 



Birchi Subzone, beds 433.3^146.2. This subzone is generally con- 

 sidered to correspond to the range of Microderoceras: five M. birchi 

 occur in bed 433.3, so defining the base of the subzone, and a single 



M. scoresbyi occurs in bed 441.2 at the middle of the subzone. The 

 top of the subzone is delimited by the appearance of the first 

 Asteroceras at the base of the Obtusum Zone. Caenisites brooki 

 persists into the basal bed (433.3) of the Birchi Subzone, and the 

 same bed also contains 24 examples of Caenisites turneri. 

 Promicroceras capricornoides appears just above the lowest part 

 and extends to the top of the subzone. There are no other ammonites 

 in the subzone. 



UPPER SINEMURIAN 



OBTUSUM ZONE, Obtusum Subzone, beds 446.31-446.5. The 

 base of both zone and subzone is drawn at the first appearance of a 

 single Aste roceras in bed 446.3 1 ; that specimen is a definite example 

 of the genus, but is not specifically determinable. The only specimen 

 of A. obtusum that was found occurs in the overlying bed 446.32, and 

 A. confusum is more common in beds 446.32 and 446.33. 

 Promicroceras capricornoides persists into the lowest two beds of 

 the Obtusum Subzone, then is immediately replaced by P. planicosta 

 for the remainder of the subzone: the two species do not overlap. 

 Other ammonites are Xipheroceras dudressieri (confined to the 

 subzone) and X. zipluts, Epophioceras landrioti in the upper half and 

 Cymbites laevigatas at the top of the subzone. 



Stellare Subzone, beds 447-455.1. The base of the subzone is 

 placed at the first appearance of the distinctive index species 

 Asteroceras stellare. which ranges up to the middle of the subzone, 

 and the top is limited by the first Eparietites at the base of the 

 Denotatus subzone. In the upper half of the subzone the index 

 species is replaced by Asteroceras blakei, which persists into the 

 overlying subzone. Aegasteroceras crassum appears at about the 

 middle of the subzone, then A. sagittarium occurs in the top part. 

 Promicroceras planicosta is very common in all but the highest beds 

 of the subzone, and 262 specimens were collected by Bairstow. 

 Other ammonites in the subzone are Cymbites laevigatus, Xiphero- 

 ceras ziphus, and Epophioceras landrioti near the base. 



Denotatus Subzone, beds 455.2—162. The base of this subzone is 

 placed at the first appearance of the genus Eparietites, ie. the new 

 species E. bairstowi, which is more evolute and has thicker and more 

 massive whorls than any other Eparietites. The main species ranging 

 through the middle and upper parts and up into the Simpsoni Subzone 

 is E. impendens. From the subzone below Asteroceras blakei, 

 Aegasteroceras crassum and A. sagittarium persist into the bottom 

 and middle parts of the Denotatus Subzone. Cymbites laevigatus 

 occurs throughout the subzone, and the Schlotheimid Angulaticeras 

 sp. indet. occurs in the top two beds. 



OXYNOTUM ZONE, Simpsoni Subzone, beds 463-171. The 

 base of the subzone is placed at the first appearance of the index 

 species Oxynoticeras simpsoni in bed 363. where there are two large 

 specimens that show typical characters of the species; there are four 

 more specimens in bed 464.3, poorly preserved examples in beds 

 465 and 466, then the species becomes common in bed 467 and 468 

 in the mid to upper part of the subzone. 



From the subzone below, Eparietites impendens persists into beds 

 463—164.32, where it overlaps with O. simpsoni in the bottom 1 .68 m 

 of the Simpsoni Subzone. In fact at its highest level in bed 464.32 

 there are many typical E. impendens. A similar overlap between E. 

 impendens and O. simpsoni is also found in the top part of the 

 Frodingham Ironstone near Scunthorpe, Lincolnshire. 



Gagaticeras is characteristic of the upper half of the Simpsoni 

 Subzone. from bed 467 upwards, where there are many specimens 

 belonging to four species. Palaeoechioceras occurs in bed 467, and 



