Early cambrian sponge spicule fossils from Zhenba county, southern Shaanxi Province

Abstract

The abrupt diversification of animals at the Ediacaran-Cambrian transition is one of the most important events in the history of life, and this event has been called the Cambrian Explosion (or Radiation) in literature. Besides bilaterian animals, nonbilaterians, such as cnidarians and sponges, are also part of the diversification event (Xiao et al., 2005). During past years, many progresses on the study of Early Cambrian sponges have been made along with discovery of diverse sponge fossils of exceptional preservations, e.g. the Chengjiang Biota, Niutitang Biota, Hetang Biota. All these researches show that the sponge radiation event occurred in the Qiongzhusian Age. Unlike body fossils, the work on disarticulated sponge spicules is relatively limited, although sponge spicules could be very abundant and diverse through acid maceration of Early Cambrian carbonate samples in laboratory. The purpose of this paper is to describe the sponge spicules collected from the Lower Cambrian of southern Shaanxi, to document the preservation mechanism of these sponge spicules, and to discuss the implication for the early Cambrian sponge radiation.Sponge spicules studied herein were collected from dolomites of the Lower Cambrian Xihaoping Member and from nodular limestones of the lower Shuijingtuo Formation at the Xiaoyang and Yudu sections, Zhenba County, Shaanxi Province (Text-figs.1, 2). Both sections are situated on the present-day northern margin of the Yangtze Platform. Biostratigraphic data of both trilobites and bradoriids indicate that these sponge spicules are of the Early Cambrian Qiongzhusian age (Li et al., 2004). The spicules were extracted from dolomites and limestones with 5%—7% acetic acid, and examined under SEM after being coated with gold. The preservation of spicules is variable. The original composition of the spicules is opal (SiO_2·nH_2O), which is unstable mineral. Subsequently, opal alteration during diagenesis usually results in the formation of quartz spicules, which were formed through the dehydration and recrystallization. The quartz spicules are white in colour. Some spicules are preserved as internal phosphate infillings of axial cavities due to the dissolution of opal spicules in local alkali microenvironments. Spi- cules preserved as internal phosphate infillings are black in colour.Although sponge spicules are commonly found in the strata ranging from the Early Cambrian to the Quaternary (Bengtson et al., 1990), it is unpractical to simply use their morphology to identify particular taxa since many types of disarticulated sponges could occur in different taxa and one taxon can have several types of spicules. Therefore, here spicules are described according to their morphology except the typical anchor-like spicules Nabaviella sp. Altogether, 10 basic type of spicules are represented in present collection, including 3 kinds of hexactinellid spicules, tetractines (stauractines) (Pl.I, figs.C,H; Pl.II, figs.U,V), hexactines (Pl.I, fig.N; Pl.II, figs.C,D,T) and pentactines (Pl.I, figs.E,J,M,O; Pl.II, figs.A,B,E,J); 6 kinds of demosponge spicules, triaenes (Pl.I, fig.R; Pl.II, fig.R), orthopolyaenes (Pl.I, fig.A; Pl.II, figs.M,N), dichotriaenes (Pl.II, fig.I), dichotriactines (Pl.I, figs.G,I), dichostauractines (Pl.II, fig.P), orthopolyactines (Pl.I, figs.K,P,Q; Pl.II, figs.K,L,O,Q); and one kind of spicules with uncertain affinity, Nabaviella sp.(Pl.I, figs.F,L; Pl.II, fig.W). Although morphologically the demosponge spicules are more diverse than the hexactinellid ones, the later ones are much abundant. Among them, tetractines (stauractines) are the most abundant, representing more than half of the total spicules in number. The spicules in the Shuijingtuo Formation are more diverse and abundant than in the Xihaoping Member. These diverse spicules show that the explosive radiation of sponges happened in the Qiongzhusian age, and it is consistent with the soft-body preservations, e.g. the Chengjiang Biota, Hetang Biota.