New fossil records of Cambrian Lophotrochozoans (Spiralian: Entoprocta) and their implication for pseudocoelomate origin
ZHANG Zhi-Fei;ZHANG Zhi-Liang
2014, 36(1):
34-41.
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Lophotrochozoa (Spiralian), a clade of non-ecdysozoan protostomes, comprises a large number of animal phyla, recently sub-grouped as tree lineages including Trochozoa (Mollusca, Annelida, Brachiopoda, Phoronida), Polyzoa (Cycliophora, Entoprocta and Bryozoa) and many small Platyzoan phyla. Although considerable advance have recently been made in unveiling the Cambrian morphology and phylogeny of several pivotal spiralian animals, the most conspicuous phylogenetic gap in the fossil records is for Platyzoa and Polyzoa. Recently, the problematic animal Cotyledion tylodes from the Chengjiang fanua (ca. 520 Ma old) has been reinterpreted as sclerite-bearing stem group entoproct based on observations of ca. 400 specimens collected by the working group of Early Life Institute, Northwest University, Xi’an of China. The animal’s body is goblet-shaped, with an upper cup-like calyx (cavity) and lower cylindrical stalked ended by a knob-shaped holdfast providing for exoskeletal attachment. The Calyx contains a U-shaped gut with a mouth and anus surrounded by a crown of flexible, retractable tentacles. When compared to living entoproct cousins, Cotyledion is larger in size, measuring between 8 and 56 mm and with a body covered by loosely or tightly-spaced, rounded or elliptical sclerites. In addition, Cotyledion appears to be coelomate, with a coelomic extension of calyx to the centre of stalk.
It is therefore assumed that the loss of a coelom may represent an apomorphic character in living entoprocts, and their extreme body decrease and severe structural simplification of anatomy and body wall could be taken as a result of miniaturization that is a widespread phenomenon in animals, and an adaptation that may be associated with a change to colonial life in living colonial entoprocts. Thus, the description of entoprocts from the Chengjiang biota has important implications for the earliest evolution of Lophotrochozoans.
