CONULARIIDS OF THE FARDEN MEMBER OF THE SOUTH THREAVE FORMATION (UPPER ORDOVICIAN, RAWTHEYAN), SW SCOTLAND

The Upper Ordovician (Rawtheyan) Lady Burn starfish beds (South Threave Formation, Farden Member), exposed in the Craighead Inlier near Girvan, SW Scotland, host one of the most diverse conulariid assemblages ever documented. Taxa present in very fine-grained feldspathic greywackes include one species each of Archaeoconularia, Ctenoconularia and Pseudoconularia and two species of Conularia. Conularina or Eoconularia and Glyptoconularia may also be present. A. mirifica (Reed) and P. currieae Begg, both erected on the basis of incomplete single specimens, probably are conspecific with specimens assigned to A. slateri (Reed) and P. megista (Lamont), respectively. Many Archaeoconularia show plicated closure of their apertural end, with moreover the low internal carina at the facial midline being smoothly bent but unbroken. Some Conularia also show plicated closure. The Ctenoconularia include some of the largest specimens of this genus ever found, having originally measured about 250 mm long. Pseudoconularia, some again showing plicated closure, attained similar lengths. Apparently, conulariids forming monospecific V-like pairs or radial clusters have not been found, possibly owing to pre-burial transport.

The Farden Member conulariid assemblage represents a unique combination of genera characteristic of or present in Upper Ordovician conulariid assemblages of cratonic North America (Conularia and Ctenoconularia), Baltica (Conularia and Pseudoconularia), and Perunica and Perigondwana (Archaeoconularia and Pseudoconularia). Another conspicuous difference between the Farden Member assemblage and those of the rest of Laurentia is the (apparent) absence in the Farden Member of Climacoconus and Metaconularia, which occur throughout cratonic North America, mainly in carbonate strata. Interestingly, Climacoconus does occur, together with Eoconularia, in older (Burrellian) mudstones of the Balclatchie Formation in the Stinchar Valley, near Girvan.

The distinctive Farden Member conulariid assemblage, the only Laurentian conulariid assemblage originally located near a convergent plate boundary, may provide additional evidence bearing on the paleogeographical distribution of conulariid-bearing latest Ordovician terranes and on latest Ordovician paleooceanography.