Dineley, D. & Metcalf, S. GCR Editor: D. Palmer. 1999. Fossil Fishes of Great Britain. Geological Conservation Review Series No. 16. JNCC, Peterborough, ISBN 1 86107 470 0.
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Complete fishes are very common in Westerdale Quarry; Highland. This is a rare inland exposure of one of the earliest occurrences of a fish bed within the Orcadian sediments of the north of Scotland. The quarry is important because of its age, and because of the presence of the early bony (osteichthyan) fishes Thursius and Dipterus (
This small, flooded quarry exposes dark grey lacustrine siltstones of Eifelian age. Miles and Westoll (1963) place the Westerdale quarries 'no more than 850 ft below the Achanarras band' on the basis of regional dip. However, inland exposure is very poor in Caithness because of a thick cover of drift, and information from the faunal assemblage at the site is enigmatic, so that the stratigraphical position of the quarry remains uncertain.
Fish specimens occur in a 1 m thick bed of poorly fissile, laminated, calcareous grey siltstone within a sequence of dark grey laminated siltstones with subaqueous shrinkage cracks (Wick Flag type of Crampton and Carruthers, 1914, p. 64). This is detailed in the following section:
| Thickness (m) | |
| Dark grey laminated flagstones with shrinkage cracks | 0.9 |
| Light grey laminated siltstone Irregular surface | 0.12 |
| Light grey, calcareous siltstone, rare fishes | 0.25 |
| Mudstone parting | 0.01 |
| Dark grey, poorly fissile, varved calcareous siltstone; very fossiliferous, containing much disseminated scale material, plus ?coprolitic concentrations of scales; complete small Thursius sp. are common | 0.75 |
| Dark grey laminated flagstones, with shrinkage cracks | 3+ |
The beds dip to the north at a low angle, and the fish bed is thus exposed in the north-northeastern part of the quarry, where a bench has been formed by its removal.
Osteichthyes: Sarcopterygii: Dipnoi: Dipterida: Dipteridae
Dipterus valenciennesii Sedgwick and Murchison, 1828
Osteichthyes: Sarcopterygii: Osteolepiformes: Osteolepidae
Thursius macrolepidotus (Traquair, 1888a; Saxon, 1978)
Complete fishes are very common, but only two osteichthyan species of fish are recorded from here. Jarvik (1948a, p. 212) referred the specimens of Thursius to a new species, T. moythomasi, which occurs at only one other site, namely Sandside, the type locality for the species. This species is common at Westerdale; Dipterus is rare (
Thursius macrolepidotus and Dipterus valenciennesi are the characteristic fossils for Faunal Zone 1, the Lybster Subgroup of the Lower Caithness Flagstone Group. (Donovan et al., 1974). However, the faunal assemblage from Sandside is typical of Faunal Zone 5, the Ham-Scarfskerry Subgroup, which is at least 1000 m above the Lybster Subgroup
Westerdale Quarry probably reveals one of the oldest fish-bearing horizons in the Orcadian Basin. Consequently its conservation value lies in its fish fauna, which heralds the beginning of a major phase of fish evolution through the Mid-and Late Devonian in the north of Scotland. The site is still open, and has the potential for further research.