Restudy of the worm-like carbonaceous compression fossils Protoarenicola, Pararenicola, and Sinosabellidites from early Neoproterozoic successions in North China

摘要

The carbonaceous compression fossils Protoarenicola baiguashanensis Wang, 1982, Pararenicola huaiyuanensis Wang, 1982, and Sinosabellidites huainanensis Zheng, 1980, from the early Neoproterozoic Liulaobei and Jiuliqiao formations in northern Anhui, North China, were previously interpreted as worm-like metazoans, largely on the basis of transverse annulations and purported proboscis structures. If correct, these would be some of the earliest known bilaterian animals and would provide a key paleontological calibration to molecular clock analyses. In this study, we examine a large population of these carbonaceous fossils, clarify their taxonomy, and provide new insights into their morphological, paleoecological, and phylogenetic interpretations. Although all three species are characterized by annulated tubes, P. baiguashanensis bears a bulbous terminal structure at one end of its tube. P. huaiyuanensis is characterized by a constricted opening at one end and a closed termination at the other. The two ends of S. huainanensis tubes are both closed and round. The bulbous terminal structure in P. baiguashanensis was previously interpreted as an animal proboscis, but new observations suggest that it was more likely a holdfast structure analogous to discoidal holdfast structures of the Mesoproterozoic Tawuia-like fossil Radhakrishnania Kumar, 2001, and the frondose Ediacara fossil Charniodiscus Ford, 1958. Furthermore, it is possible that at least P. baiguashanensis and P. huaiyuanensis may represent reproductive or taphonomic fragments of the same organism. This reinterpretation weakens the previous interpretation that P. baiguashanensis and P. huaiyuanensis were worm-like bilaterian animals. Instead, they can be alternatively interpreted as erect epibenthic organisms, possibly coenocytic algae reaching a tiering height of 30 mm. The predominance of discoidal holdfasts, as opposed to rhizoidal holdfasts, in pre-Ediacaran epibenthic organisms was probably related to more stable substrates in the presence of microbial mats and in the absence of bioturbating animals. (C) 2007 Elsevier B.V. All rights reserved.