New well-preserved scleritomes of Chancelloridae from the early Cambrian Yuanshan formation (Chengjiang, China) and the middle Cambrian wheeler shale (Utah, USA) and...

ABSTRACT-From the early Cambrian of Yunnan, China, new chancellorid scleritomes assigned to Allonnia junyuani new species are described and compared with scleritomes of Chancelloria eros Walcott, 1920 from the Middle Cambrian of Utah, USA and isolated phosphatized chancellorid sclerites from the Georgina Basin of North Australia. Characters of the entirely preserved chancellorids offer new insight into the paleobiology of Chancelloridae. An irregular thickening at the inferred base of the narrow body end of Chancelloria eros from Utah is interpreted as a root bulb that anchored the sessile animal in the soft bottom sediment. Sclerites show gradual increase in size from the narrower towards the broader upper end of the chancellorid body. A central "gastral" lumen was not directly observed in any of the investigated specimens, but the orientation of sclerites towards the outer body surface indicates that the fossils are secondarily flattened due to compaction and were originally hollow. Taphonomy of isolated chancellorid sclerites from the Georgina Basin, Australia, indicates a biomineralization very different from that of sponge spicules, including the spongin spiculoids of demospongid Keratosa. Allonnia junyaani from Chengjiang (Yunnan) exhibits a prominent epidermis, probably with stable epithelian cell-tocell connections. This feature excludes a sponge affiliation of the Chancelloridae and points towards a systematic position of this group within the epithelian-bearing animals and thus well above the Porifera.

INTRODUCTION

THE CHANCELLORIDAE are a problematic group stratigraphically restricted to the Cambrian. Their earliest records were reported from the first (Qian and Bengtson, 1989) and second (Qian et al., 1999) small shelly fossil assemblage of the Meishucunian Stage (equivalent to the Nemakit-Daldynian to Tommotian), where they occur disarticulated, and associated with rich small shelly faunas (Fuchs and Mostler, 1972; Mostler, 1980). During the Atdabanian, the Chancelloridae flourished in shallow marine environments (commonly as components of archaeocyathid mounds). Rich associations are known, especially from South Australia (Bengtson et al., 1990) and the Lower Cambrian of China (Qian, 1989; Qian and Bengtson, 1989; for well-preserved scleritomes figured as "chancellorid gen. et sp. nov." see Chen et al., 1996, p. 91; Chen and Zhou, 1997, p. 29). From the Middle Cambrian, isolated sclerites of the Chancelloridae have been reported worldwide, with high diversity for example from the Georgina Basin, Australia (Mehl, 1998). Entirely preserved chancellorid scleritomes of Middle Cambrian age occur in the Burgess Shale (Walcott, 1920), the Wheeler Formation, USA (Rigby, 1978), and the Kaili Formation, Guizhou, China (Zhao et al., 1999). The Late Cambrian record represents a rapid decline, with only a few occurrences of chancellorid taxa (e.g., Mostler and Mosleh-Yazdi, 1976). Most probably the group had become extinct before the end of the Cambrian.

Although the Chancelloridae are among the most common early and middle Cambrian "shelly" fossil groups, entirely preserved chancellorid skeletons seem to be rare. Natural sclerite assemblages constituting the chancellorid scleritomes, associated with other "soft tissue-fossils," are restricted to lagerstaetten resulting from extraordinary taphonomic events, such as a rapid burial or deposition under anoxic conditions. Such Cambrian localities are comparably rare, but are typically rich in well-preserved body fossils, including Chancelloridae. In addition to Chancelloria eros, three other chancellorid species from the Burgess Shale were described by Walcott (1920): C. drusilla, C. libo, and C. yorkensis. Probably at least three more taxa can be recognized in the original material because the type series of Chancelloria eros is not monospecific (Bengtson et al., 1990; Janussen, personal observation). Other localities of Burgess Shale type, which have delivered natural assemblages of chancellorid sclerites, are the Middle Cambrian Marjum and Wheeler formations, Utah, USA, and the Early Cambrian sections at Mt. Maotian (Chengjiang County) and Haikou (Dinning County) in Yunnan, China.

Taxonomic history.-The Family Chancelloridae currently comprises 13 partly controversial genera, the most common of which, besides Chancelloria Walcott, 1920, are the genera Allonnia Dore and Reid, 1965, and Archiasterella Sdzuy, 1969. Allonnia comprises 3+0, rarely 2+0 or 4+0, comparably large sclerites normally with all their rays bending away from the basal plane at sharp angles. Archiasterella Sdzuy, 1969, shows 4+0 and 5+0, rarely 2+0, sclerites with one marginal ray curving away sharply from the basal plane. Due to the fact that chancellorid sclerites, along with other small shelly fossils, are very common components of Cambrian sediments, most taxa are based on isolated sclerites only and may be artificial.

Walcott (1920) originally described the Chancelloridae as a family of the Heteractinellida (Phylum Porifera). This systematic classification within the sponges was widely accepted in subsequent literature. Due to their star-shaped, supposedly calcareous sclerites, the Chancelloridae were attributed to the extinct order Heteractinellida within the Calcarea (e.g., Mostler, 1995; Mostler and Mosleh-Yazdi, 1976; Reid, 1959; Rigby, 1978; Rigby and Nitecki, 1975). According to Sdzuy (1969), chancellorid sclerites originally had calcareous double-layered walls. Because of their segmented composition with large internal foramens and basal pores, they would most likely have been secreted by tissues occupying their inner cavities. He considered the Chancelloridae to be primitive sponges, and perhaps the stem group of all modem Porifera, in which case the mineralogy would have changed during phylogeny from calcareous chancellorid sclerites into siliceous spicules of the modern Hexactinellida and Demospongiae. However, as stated by Goryansky (1973) and Bengtson and Missarzhevsky (1981), the poriferan mode of spicule-secretion by enveloping sclerocytes is entirely different from the assumed mineralization of chancellorid sclerites, which would rather be indicative of an external protective skeleton.