16 SHELVE DISTRICT 



Bohemian stock as Drabovia is represented in the Costonian Spy Wood Grit by D. 

 fascicostata, which is known in Czechoslovakia only from the allegedly younger 

 Letna Formation. 



In summary then, it appears that although the W. Shropshire assemblages of 

 Ordovician brachiopod species are strongly endemic and not reliable indices for 

 correlation outside the Anglo-Welsh Basin, they do confirm the time-stratigraphic 

 classification of the Shelve succession proposed by Dean (in Whittard 1967 : 317) 

 except for the equation of the Rorrington Beds with the Middle and Upper Llan- 

 deilian rather than the basal Caradocian (Text-fig. 2). 



III. FAUNAL ASSOCIATIONS 



The calculation of density distributions of species solely according to the frequency 

 of their presence in localities for any given formation, as in the preceding section, is 

 effective enough for determining the degree to which a taxon is diagnostic of a 

 particular stratigraphic horizon. Such estimates, however, do not give any indica- 

 tion of the relative commonness of occurrence of species recovered from any single 

 locality : of how, for example, Palaeoglossa myttonensis and Protoskenidioides 

 revelata are recorded from 13 and 3 localities of Mytton Flags respectively, although 

 the former species is represented in the collection by only 16 pedicle or brachial 

 valves compared with 25 valves of the latter. Such data may be very important. 

 They may reflect differences in the transporting capacity of sea currents. Alter- 

 natively, the Palaeoglossa populations may have consisted of thinly spread individuals 

 tolerant of widely differing environments whereas Protoskenidioides may have existed 

 as high density clusters of shells in rigorously circumscribed ecological niches. 



Different kinds of data are required to promote investigations into the palaeo- 

 environment on the one hand and palaeoecosystems on the other. Palaeoenviron- 

 mental studies require information on the number of each kind of valve as well as 

 the proportion of complete to fragmented shells composing collections. This 

 demand is well served by data prepared in the manner adopted by Temple (1968 : 9 ; 

 1970 : 8) for Lower Llandovery brachiopods. Yet for this approach, information 

 on the size range frequencies of species is equally important ; and since such details 

 cannot be derived from normal statistical tables, they are rarely available and the 

 exercise correspondingly diminished in its efficacy. In contrast, studies of relation- 

 ships within and between communities mainly require estimates of species distribu- 

 tion among communities ; frequencies of individuals representing a species within 

 an association only indicate its importance according to numerical ranking. Never- 

 theless, such data are valuable from both palaeoecologic and stratigraphic view- 

 points. Variation in the numbers of individuals present in fossil communities or 

 their residues may be aids in determining the optimum conditions for the main- 

 tenance of a species in space, or in identifying changes in the interrelationship between 

 species of an evolving association in time. In his study of the Shelve trilobites, 

 Whittard (1966 : 298) illustrated variation in the number of specimens collected from 

 each formation by time-histograms. Brachiopods pose a different problem because 



