AGGLUTINATED PROTISTS FROM THE LOWER CAMBRIAN OF NEVADA

Abstract

Skeletonized protists (or protoctists) appear as body fossils in the late Neoproterozoic and the lower Cambrian (e.g., Allison and Hilgert, 1986; Lipps, 1992; Culver, 1994; Porter et al., 2003) and are well known from the post-Cambrian fossil record. However, molecular data and biomarkers of groups such as ciliates and foraminiferans indicate a long Precambrian history unrecorded by fossils, and also suggest that foraminiferans were important components of Neoproterozoic protistan communities (e.g., Summons et al., 1988; Wright and Lynn, 1997; Lipps, 2003; Pawlowski et al., 2003). The recent discovery of agglutinated microfossils interpreted as foraminiferans from the Neoproterozoic of Uruguay (Gaucher and Sprechmann, 1999) supports the inferences made from molecular data. Although foraminiferans are known for certain since the earliest Cambrian (e.g., McIlroy et al., 2001), their diversity and variability during the Cambrian is limited to a few agglutinated unilocular genera (e.g., Lipps, 1985; Culver, 1991, 1994; Cope and McIlroy, 1998; Zhigulina, 1999; McIlroy et al., 2001). Among these, Platysolenites Pander, 1851 is one of the best known genera because it has a wide distribution and high abundance in lower Cambrian sediments, especially those of Baltica and Avalonia (e.g., Rozanov, 1983; Lipps and Rozanov, 1996; McIlroy et al., 2001). Whether Platysolenites has an affinity with the foraminifera has been questioned by some authors (e.g., Eichwald, 1860; Rozanov, 1983; Brasier, 1989b), but the similarity of its morphology and wall structure to the foraminiferan genus Bathysiphon Sars, 1872 (e.g., Glaessner, 1978), and the presence of proloculi in Platysolenites (e.g., McIlroy et al., 2001) is strong evidence in favor of an affinity with the foraminifera. It should be emphasized that the preservation of proloculi in Platysolenites is rare, reported only in a few cases (McIlroy et al., 1994, …