. Bulletin of the British Museum (Natural History), Geology. . Fig. 13 Atheloptic assemblage of blind, or nearly blind, benthic trilobites (a-f)* together with free-swimming, large-eyed mesopelagic forms (g-i). a, Illaenopsis; b, Shumardia; c, Bergamia; d, Colpocoryphe taylorum; e, Ormathops nicholsoni; f, Ampyx; g, Pricyclopyge; h, Bohemilla (Fenniops); i, Degamella. The regression near the Arenig-Llanvirn boundary presumably reduced the water depth sufficiently for normal-eyed trilobites to thrive during the deposition of the Llanfallteg Forma- tion. Since the cyclopygids are also numerous t
. Bulletin of the British Museum (Natural History), Geology. . Fig. 13 Atheloptic assemblage of blind, or nearly blind, benthic trilobites (a-f)* together with free-swimming, large-eyed mesopelagic forms (g-i). a, Illaenopsis; b, Shumardia; c, Bergamia; d, Colpocoryphe taylorum; e, Ormathops nicholsoni; f, Ampyx; g, Pricyclopyge; h, Bohemilla (Fenniops); i, Degamella. The regression near the Arenig-Llanvirn boundary presumably reduced the water depth sufficiently for normal-eyed trilobites to thrive during the deposition of the Llanfallteg Forma- tion. Since the cyclopygids are also numerous there the depth could not have been less than about 200 m. The change during the regression need not have been very great if the atheloptic assemblage lived just below, and the normal-eyed assemblage just above, the critical depth of light penetration. Because they were inhabitants of the water column well below the surface and somewhat insulated from the constraints of surface temperature conditions, the cyclopygids are not confined to the Gondwanan plate in the early Ordovician, as are the shallow shelf genera such as Neseuretus. They were capable of living in the appropriate sites at former temperate latitudes around Baltica and Kazakhstania, as indicated above. They did not penetrate into tropical latitudes at that time, however. One of us (RAF) has made an intensive search in the appropriate lithology in the Cow Head Group of western Newfoundland, which accumulated off the shelf of the North American continent during the early Ordovician. No fossils of cyclopygids were discovered. The oldest occurrences in North America are Middle Ordovician: the earliest of these is probably from the Normanskill Shale {Niobel huberi Roy, 1929 is likely to be a Degamella sp.), while typical cyclopygid biofacies are known from the Ashgill of Quebec (Cooper & Kindle 1936) and the Whitehouse Formation in the Girvan district of Scotland. The early history of the group is associated with hig
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