Dan-lan-y-castell

[SN 3640 1639]

Introduction

The quarry at Dan-lan-y-castell is of special importance on account of the diverse fauna obtained there. The locality lies in the northern part of the Llangynog Inlier of Precambrian to Arenig rocks (Cope, 1982) and exposes early Arenig (Moridunian Stage) siltstones and mudstones belonging to the Bolahaul Member of the Ogof Hên Formation. It was noted by Cantrill and Thomas (1906, p. 227) and Thomas (in Strahan et al 1909, p. 16), who listed a meagre fauna and assigned the rocks to the 'Tetragraptus Beds'. Exhaustive collecting by Dr J.C.W. Cope from over 20 tonnes of rock has afforded the largest and most diverse early Arenig fauna known in Britain. Especially important and dominant is the large bivalve fauna, among which are the earliest known representatives of several major groups.

Description

A small disused quarry 500 m SSE of Dan-lan-y-castell Farm shows interbedded siltstones and mudstones of the Bolahaul Member (Arenig, Moridunian Stage), overlying conglomerates of the Allt Cystanog Member of the Ogof Hên Formation. Thin stringers of conglomerate and occasional pebbles occur within the Bolahaul Member, and bioturbation is widespread.

The fauna includes the stratigraphically important trilobites Merlinia murchisoniae (Murchison), Ampyx cetsarum Fortey and Owens and Neseuretus ramseyensis Hicks (see Fortey and Owens, 1978, 1987), along with the brachiopod Paralenorthis alata (J. de C. Sowerby). Especially interesting is the rich bivalve fauna (Figure 8.4)a, described by Cope (1996), and comprises abundant actinodontoids together with palaeotaxodonts, rarer modiomorphoids, and the earliest representatives of the anomalodesmatans, cyrtodontoids, solemyoids and pterioids. Many of the taxa were recognized for the first time from this locality, including 12 species and nine genera, two families, two superfamilies and one order, and the fauna forms the basis of Cope's major reassessment of the early history, diversification and high-level classification of the Class Bivalvia. In addition, the fauna includes the earliest known bryozoan from Britain (Taylor and Cope, 1987) and a parablastoid (Paul and Cope, 1982), orthoconic nautiloids, gastropods, monoplacophorans, a rostroconch, dendroid graptolites, conulariids, hyolithids and several other groups (Cope, 1996). Riding et al. (1998) described Arenigiphyllum, the oldest definite example of a calcified red alga, from this quarry.

Interpretation

The Ogof Hên Formation is interpreted as being deposited in a transgressive inshore, shallow-water environment, the fauna of which was described as the Neseuretus community by Fortey and Owens (1978). The early Arenig age of the sediments is determined by the species of the trilobites Merlinia, Ampyx and Neseuretus, most of which are common to several localities in the Ogof Hên Formation of the Carmarthen district and in Pembrokeshire. The diverse bivalve fauna is known only from Dan-lan-y-castell. Cope (1996) considered that exceptional diversity occurs here because the sediments were deposited in an environment close inshore, such as is not often preserved in the geological record, and that it may represent a habitat where major evolutionary change was occurring.

Conclusions

Dan-lan-y-castell is a site of international palaeontological significance because it provides a unique 'window' on a close-inshore facies, such as is not commonly preserved. The Arenig shelly fauna is uniquely rich and includes the earliest species of certain animal groups and, especially, a rich bivalve fauna that has an important bearing on the early evolution of the Class Bivalvia.

References