Dunside

[SO 746 362], [SO 751 371]

(Potential GCR site)

Highlights

This site, near Lesmahagow in Strathclyde, is the original type locality of the rare primitive thelodont Loganellia scotica. Other early fishes have been found here, and the site is internationally important because of the rarity of such ancient fish assemblages.

Introduction

A stream section on Logan Water in the Lesmahagow inlier exposes the only known outcrop of the fish-bearing Ceratiocaris Beds. These are the lowest beds of the Kip Burn Formation of the Priesthill Group, of latest Llandovery or earliest Wenlock age (Figure 2.8). Murchison visited the area following the discovery of eurypterids by Robert Slimon, and subsequently published a description of the geology (Murchison, 1856b). Hunter (1885) gave the first written record of fishes collected from a nodule horizon at this site. Slimon had collected specimens of Thelodus and Birkenia during his original survey, but these remained unrecognized until his collection was re-examined following the description of fishes from other collections (MacNair, 1905). At this stage it was not realized that the Birk Knowes exposures were older than those on Logan Water and the fish species were all grouped together.

The geology of the site has been described by Murchison (1856b) and more recently by Jennings (1961), and the fish fauna by Hunter (1885), Traquair (1899b, 1905a) and MacNair (1905; see (Figure 2.4) and (Figure 2.8)).

Description

The Ceratiocaris Beds consist of dark grey, carbonaceous, laminated siltstones with a very few olive mudstone bands. They are very fossiliferous, with common eurypterids, but relatively rare fishes (Jennings, 1961). This is the type locality for Loganellia scotica Traquair, 1899 and Thelodus planus Traquair, 1899. Birkenia elegans Traquair, 1899 and a possible second species of Birkenia also occur.

The first descriptions of the Lesmahagow vertebrate fauna were by Traquair (1899b, 1905a). Since the true stratigraphical relationships between the different fossiliferous horizons in the area were not known then, the fishes were divided into those from the 'Ludlow' Beds and those from the 'Passage' or Downtonian beds.

Fauna

AGNATHA

Thelodonti: Thelodontida: Loganellidae

Loganellia scotica Traquair, 1898

Anaspida: Birkeniiformes: Birkeniidae

Birkenia elegans Traquair, 1898

Traquair described Thelodus scoticus and T. planus, both of which must have come from this site, and an indeterminate form from the 'Ludlow' Beds, and he listed Birkenia elegans as coming from the 'Passage' Beds.

Loganellia scotica is the earliest, complete British articulated thelodont, and it is known also from the Jamoytius Horizon at Birk Knowes (q.v.). Thelodus planus Traquair, 1899 was described from a single specimen found at the site in 1898 by A. Tait. The species is defined on the ornamentation of the scales, and may well prove to be a nomen nudum on further research (Turner, 1991).

Birkenia is rare in the Ceratiocaris Beds, but is the most common fish in the overlying fish beds of Lesmahagow and the Hagshaw Hills (see Shiel Burn and Slot Burn reports). The finds here are the stratigraphically lowest record of Birkenia in Scotland, and the material may represent a new species, rather than B. elegans as recorded ((Figure 2.11)A, B).

Interpretation

The locality at Dunside has produced a very similar vertebrate and arthropod fauna to that at Birk Knowes, but it dates from a slightly younger formation. The main differences between the two assemblages lie in the absence of Jamoytius and Ainiktozoon at Dunside, and the presence there of Birkenia. Birkenia cf. elegans was recently reported together with thelodont denti-cies from County Mayo, north-western Ireland (Palmer et al., 1989), in a postulated extension of the Midland Valley of Scotland. The implication is of an aquatic connection between these two areas, some 500 km or more in length (see (Figure 2.2)).

Conclusion

The conservation value of this key early Silurian fish site lies in its remarkable fauna of up to four fish species, associated with predatory eurypterid arthropods. It differs from the nearby older site, Birk Knowes, in the presence of the anaspid fish Birkenia.

References