Biostratigraphy and palaeoecology of some stropheodontid brachiopods from the Ludlow Rocks (Silurian) of the Welsh Borderland of Britain, and Sweden
This thesis provides detailed studies of the five stropheodontid species which occur commonly in the Ludlow rocks of the Welsh Borderland. These are Shaleria ornatella (Davidson), Leptostrophia filosa (J. de C. Sowerby), Pholidostrophia (Mesopholidostrophia) lepisma (J. de C. Sowerby), Amphistrophia funiculata (McCoy), and Strophonella euglypha (Dalman). About 2250 specimens have been collected from well-spaced geographical localities in the shelf facies of the Welsh Borderland (i.e. Ludlow, Abberley Hills, Woolhope inlier and Usk inlier) and, where possible, from different levels in the Ludlow succession. In addition, a large collection of Shaleria aff. ornatella Bassett & Cocks from Gotland, Sweden plus museum and other specimens of the Wenlock forms of S. euglypha and A. funiculata were studied statistically for comparison with the Welsh Borderland Ludlovian forms. After preparation, accurate measurements were made of: shell length and width, pedicle muscle field length and width, angle of divergence of the pedicle valve muscle field, width of denticulation, the number of ribs per mm at 5 or 10mm from the dorsal umbo, and some other special morphological features. All five species are fully described and illustrated: some previously undescribed morphological features have been recognised, discussed and figured. For each species, also, statistical analyses of the range of variation of the most significant morphological features have been carried out. In the case of P. (M.) lepisma, the emphasis is on the full description of the species and its variation as it has not previously been adequately described and figured. The study of L. filosa also concentrates on the description and the range of variation, together with a comparison with Wenlock forms of the same species. Shaleria ornatella is fully described and its range of variation is such that there seems little doubt that the Gotland form Shaleria aff. ornatella should be included in this species. The small but consistent difference in the Gotland form can be related to environmental rather than genetic controls. The intraspecific variation in a present-day species of Brachiopoda; Kraussina rubra (Pallas) from one locality was also studied; the range of variation is comparable to that in S. ornatella, thereby supporting the palaeontological interpretation of this (& other) species. In both S. euglypha and A. funiculata, particular interest lies in the morphological differences between the Wenlock and Ludlow forms of these species. These changes have been analysed in detail and attention directed to the containing sediments in order to determine whether changes of substrate or other environmental factors might explain the vertical changes. The accumulated evidence, however, seems to favour gradual evolutionary changes from the Wenlock to the Ludlow although there are too many gaps in the record to be certain. Finally, the functional morphology and palaeoecology of stropheodontid brachiopods in general, and of these five Ludlow specimens in particular, are discussed.