Some new Namurian bivalve faunas and their significance in the origin of carbonicola and in the colonization of carboniferous deltaic environments

Abstract

Shells which have been attributed to the genus Carbonicola are known from certain Viséan and early Namurian (E 1 ) beds of Scotland, and were recorded from a single local Viséan horizon in the north of England. Re-examination of well preserved material from both these stratigraphic groups indicates that Viséan-early Namurian shells differ in internal features, as well as in external shape, from late Namurian (R 2 -G 1b ) and Westphalian Carbonicola , to which they do not appear to have been directly related. The former have been assigned to the new genus Paracarbonicola , with Pleurophorus elegans Kirkby 1880 as type species. Paracarbonicola is present in the Flora Beds of Poland, considered to be of E 1 age, and in Czechoslovakia, where it ranges upward into beds of E 2 age. In Britain and northern Europe no Carbonicola -like shells have been reported in measures lying unquestionably between the top of Stage E 2 and the bottom of R 2 . In beds of late R 1c age two newly found correlated assemblages of shells, the first in County Limerick, southwest Ireland, and the second near Glitheroe, northern England, are herein referred to cf. Sanguinolites Hind non M’Goy. These faunas existed in shallow-water marginally marine conditions, as evidenced by the associated Mollusca. Their shells were highly variable in both lateral outline and surface features. In the English Pennines, upward gradation of these faunas with unquestionable Carbonicola of R 2 and higher horizons may be demonstrated in terms of internal and external features of the shell. It therefore appears that British late Namurian Carbonicola faunas were probably derived from marine ancestors in late R 1 time. Limited evidence suggests a similar sequence of events in Belgium and the Netherlands. Both in lithological facies and in ranges of variation of shape of shell, the Irish and north Pennine faunas of cf. Sanguinolites Hind non M’Coy compare respectively with established and invading faunas of later Carbonicola . Moreover, the comparison is made more close by the fact that bivalve burrows, referred to Pelecypodichnus Seilacher, have not been seen on horizons lower than those of late R 1c , but recur above this horizon, tending to become more nearly vertical, more numerous and longer, as Pelecypodichnus escape shafts, on higher horizons. These trends culminate in the well known, extremely abundant burrows of the Haslingden Flags, mid-Pennines, of G 1 age. It is concluded that during the time when Carbonicola was probably evolving from cf. Sanguinolites Hind non M’Goy, which has been found without burrows in the Shale Grit (middle R 1c ) of Derbyshire, steep burrowing of bivalves was common, the long axes of the shell being near the vertical; that from this position rising movements of the shells took place, enabling the bivalves to escape burial from rapidly advancing delta lobes; furthermore, that in the process of rising the more elongate shells, having shorter anterior ends and relatively low measures of obesity, such as Carbonicola bellula (Bolton), held selective advantage over other shapes of shell. In these particular circumstances therefore the delta invaded the bivalves, rather than vice versa. During the deposition of bands of established faunas of Carbonicola , when rates of deposition were evidently lower than in the case of invading ones, shallow burrowing appears to have taken place obliquely, the long axis of the shell making characteristically a small angle with the plane of the substratum. From work on Recent analogous bivalve species, functional advantage in shallow downward burrowing may be assumed to have lain with the more oval and more obese varieties. Established faunas with the latter trends characterized lower-energy muddy environments of the delta, including those which immediately followed coal-forming conditions. These environments tended mainly to succeed the phases of rapid deltaic growth, of sands and silts in Namurian time, and are reflected in the prevalence of established faunas of Carbonicola in Westphalian time.