MACROPYGE (PROMACROPYGE) SCANDINAVICA NEW SPECIES; THE FIRST MACROPYGINID TRILOBITE RECORDED FROM THE FURONGIAN OF BALTICA

Journal of Paleontology, Nov 2007 by Terfelt, Fredrik, Ahlgren, John

ABSTRACT-

Macropyge (Promacropyge) scandinavica new species is described from the Furongian Peltura minor Zone on northwestern Mount Kinnekulle, southcentral Sweden. It represents the first macropyginid trilobite recorded in Baltica. M. (P.) scandinavica closely resembles other species of the same subgenus from southeast China, suggesting a correlation between the Scandinavian P. minor Zone and the Lotagnostus americanus-Hedinaspis regalis Zone of China. The specimens are preserved in two limestone lithologies, interpreted as representing alternating normal and storm deposits, supporting the idea of an intrabasinal paleohigh in Västergötland, southcentral Sweden.

INTRODUCTION

IN FURONGIAN (late Cambrian) times, Scandinavia was dominated by highly provincial olenid and, to a lesser extent, agnostoid trilobite faunas (e.g., Westergård, 1922; Henningsmoen, 1957a; Bergstrom and Gee, 1985), whereas the non-trilobitic faunas were dominated by proto-, para-, and euconodonts (Müller and Hinz, 1991; Szaniawski and Bengtson, 1998), with minor occurrences of brachiopods (Westergård, 1922; Henningsmoen, 1958; Holmer and Popov, 1990) and small phosphatized crustaceans and phosphatocopids (Müller and Walossek, 1985). Except for Britain, Poland, and eastern maritime Canada, mixing of these faunas with trilobites from outside Baltica was limited and basically restricted to a few cosmopolitan agnostoids (Ahlberg, 2003). Nonetheless, very rare occurrences of 'exotic' polymerid trilobites such as Pedinocephalus (Ivshin, 1956), Irvingella (Ulrich and Resser in Walcott, 1924), and Maladioidella (Endo in Endo and Resser, 1937) have been reported (Henningsmoen, 1957b, 1958; Westergård, 1947a; Rushton, 1983; Rushton et al., 2002). These trilobites indicate sporadic faunal connections with other trilobite realms or provinces. Maladioidella has a widespread peri-Gondwanan distribution (Rushton and Hughes, 1996) and was recently recorded in Sweden (Rushton et al., 2002). Pedinocephalus is known from Antarctica (Shergold et al., 1976), Afghanistan (Ivshin, 1956), Iran (Wittke, 1984), and India (Shah et al., 1991). Irvingella has a pandemic distribution (Hong et al., 2003), though Scandinavian representatives are very scarce. It is possible that the rare occurrences of these species reflect their actual abundance in the Baltic trilobite realm rather than representing occasional 'exotic' invaders from outside Baltica.

During field work in Västergödand, southcentral Sweden (Fig. 1) the second author, JA, found two cranidia and five pygidia of an unusual trilobite in the Peltura beds at Hällekis, Kinnekulle (Fig. 1). This 'exotic' trilobite, belonging to the subgenus Macropyge (Promacropyge), is of particular interest since it shows close affinity with other species of the same subgenus from southeast China. Above the Olenus Zone in Scandinavia, agnostoids become rare and the species recorded from the middle and upper Furongian permit only broad correlations with other continents (Ahlberg and Ahlgren, 1996; Ahlberg, 2003). The occurrence of Macropyge (Promacropyge) in the Scandinavian Peltura beds (middle Furongian) is therefore of importance for the correlation with coeval strata outside Scandinavia.

In this paper we describe a new species of Macropyge (Promacropyge) from Sweden, representing the first Furongian macropyginid trilobite recorded from Baltica. The potential of M. (P.) scandinavica n. sp. as a tool for intercontinental correlation is discussed, as well as the depositional environment of the sampled, trilobite-yielding strata.

GEOLOGICAL SETTING AND LOCALITIES

Lithologically, the Furongian of Scandinavia consists almost exclusively of a condensed succession of alum shale with beds and concretions of limestone (anthraconite or stinkstone). The most substantial successions are in the Oslo Region of Norway and in Scania, the southernmost province of Sweden, where the Furongian has a thickness of approximately 50 m (Westergård, 1944; Henningsmoen, 1957: the upper Cambrian of these authors with the Agnostus pisiformis Zone subtracted). It thins out towards the east to become less than 1 m thick on northern Oland (Fig. 1; Westergård, 1947b). This variation in thickness reflects a transition from outer to inner shelf facies, where the outer shelf facies is characterized by a greater thickness, less than 10% of limestone, and a high degree of stratigraphie continuity. By contrast the inner shelf facies rarely exceeds 25 m in thickness, contains up to 50% limestone, and includes several stratigraphical gaps (Buchardt et al., 1997).

The lower Paleozoic of Västergötland occurs as erosional outliers and forms five geologically famous table mountains: Kinnekulle, Billingen, Halleberg, Hunneberg, and Lugnåsberget (see Martinsson, 1974; Weidner et al., 2004). These comprise lower Cambrian to Silurian deposits resting on the Precambrian crystalline basement. Except for Lugnåsberget, they are all capped by dolente intruded as sills in Permo-Carboniferous times (Andersson et al., 1985). Kinnekulle is situated southeast of Lake Vänern (Fig. 1) and is a classical area for paleontological studies (e.g., Linnarsson, 1868; Westergård, 1922; Miiller and Walossek, 1987; Ahlberg and Ahlgren, 1996; Clarkson et al., 2003; Terfelt, 2003; Ahlberg et al., 2005). The Furongian of Kinnekulle has a maximum thickness of approximately 12 m (Martinsson, 1974; the upper Cambrian of this author with the A. pisiformis Zone subtracted). The strata are more or less horizontal and tectonically undisturbed.