Integrated stratigraphic study of the Rhuddanian-Aeronian (Llandovery, Silurian) boundary succession at Rheidol Gorge, Wales : a preliminary report

Rheidol Gorge, approximately 17 km west of Aberystwyth, mid Wales, exposes a continuous succession of strata from the middle part of the upper Rhuddanian Coronograptus Cyphus Biozone through the lower Aeronian Demirastrites triangulatus (= Monograptus triangulatus) Biozone. Parts of the Aeronian succession are well known for their beautiful lower Aeronian graptolites preserved as pyrite internal moulds. We measured this section and sampled for graptolites, palynomorphs, and for lithological and geochemical analyses. One of our objectives was to assess the section for its suitability as a candidate for a new Global Stratotype Section and Point for the base of the mid-Llandovery Aeronian Stage. The succession alternates between bioturbated grey mudstones lacking in graptolites and laminated, graptolitic black shales. The black shales commonly show thin, interbedded siltstones. The grey mudstones are interpreted to represent deposition under oxic to dysoxic conditions, the black shales an anoxic seafloor environment. The strata have undergone low-grade metamorphism, commonly with a weakly to moderately developed cleavage, and the graptolites often show ductile and/or brittle deformation. Strate of the middle to upper cyphus Biozone are c 10.2 m thick and yield graptolite faunas of varying diversity and preservation quality. 0.8 m below the base of the D. triangulatus Biozone there is a change from predominantly organic-poor mudrocks with interbeds of darker shales with sparse graptolites to an interval of predominantly black shales with a relatively rich graptolite fauna. The graptolites in the black shale interval, which spans the zonal boundary, are flattened or in partial relief, commonly deformed, and the strata tend to break along cleavage rather than bedding planes. Nevertheless, a distinctive graptolite fauna occurs through the boundary interval that allows good correlation with successions in other parts of the world. The base of the D. triangulatus Biozone is marked by the first appearance of D. triangulatus. Other species first appearing just below the base of the D. triangulatus Biozone that are useful for international correlation include Pristiograptus concinnus and Pseudorthograptus finneyi. Strata rich in well-preserved, pyritic graptolites become common about 2.3 m above the base of the triangulatus Biozone. Chitinozoans are poorly to moderately well preserved in the section and indicate the Spinachitina maennili Biozone through the boundary interval, without any significant faunal changes, as is the case in many other parts of the world

Convergent evolution of two Silurian graptolites

The two graptolite speciesCochlograptus veles(Telychian, Upper Llandovery) and Testograptus testis(lower Homerian, upper Wenlock) are remarkably and uniquely similar in being strongly ventrally and planispirally coiled and in demonstrating an abrupt deflection in the immediate post−sicular regions of their rhabdosomes. The two species, however, are separated by a relatively large biostratigraphic gap and a global mass extinction, and they differ morphologically in the proportion of thecal overlap, different angles of inclination of the interthecal septa, relative proportions of the widths occupied by the free metathecae, the position of the sharp dorsal flexure relative to the tip of the sicula, and the presence of a distinctive, keel−like structure on T. testis. It is suggested therefore, that in spite of the strong proximal morphological parallelism between the two species, their origin is best explained as a remarkable example of convergent evolution. It is suggested that C. veles perhaps evolved from some modified monograptid such as Stimulograptus, whereas the small Testograptus group may have derived from some monograptid such as Monograptus flemingii. Cladistic analysis fully supports the independent derivation the two species. If correct, this hypothesis supports the validity of separate generic names for the two species, despite the close and unique rhabdosomal similarities, including proximal metathecal form