Taxonomy And Evolution Of Cambrian Priapulids From South China

With only 19 known extant species from 7 genera, Priapulida is a small marine phylum lie within ecdysozoans of Animalia kingdom. However, endobenthic priapulids are especially abundance and diverse in Cambrian sea. Numerous soft-bodied priapulid fossils from both clastic platform facies and foreslope facies in the Yangzte Platform, South China, which are exemplified by a series of Burgess Shale-type Lagerstatten, are a key component in understanding the origin and evolution of the priapulids, ecdysozoans as well as the process of the "Cambrian explosion". However, establishing such an evolutionary lineage has proved more elusive. Here I describe new priapulid fossils from four celebrated Cambrian Burgess Shale-type biotas in the Yangzte platform of South China, i.e., the Shipai Fauna (Stage 4) from the Three Gorges area of Hubei Province and the Xixiang Fauna (Stage 4) from Xixiang County of southern Shaanxi Province, both are confined to the clastic platform facies, and the Jianhe Biota (upper Stage 4) and the Kaili Biota (lower Stage 5) from the Gedong area of Jianhe County, eastern Guizhou Province which are generally referred to as fossil assemblages of the foreslope facies. As with many Burgess Shale-type taxa these fossils are potentially central to the documentation of various aspects of early metazoan evolution.In this study, a total of 8 species in 5 genera of palaeoscolecids are systematically described, they are:Sanxiascolex papillogyrus gen. et sp. nov., Wronascolex yichangensis sp. nov. and W. spinosus (Ivanstsov and Wrona,2004) from the Shipai Fauna; Shaanxiscolex xixiangensis gen. et sp. nov. from the Xixiang Fauna; Guizhouscolex balangensis gen. et sp. nov. and Yunnanscolex sp. from the Jianhe Biota; and W. jianhensis sp. nov., and W. sp. from the Kaili Biota. Additionally, two taxa under open nomenclature, Gen. et sp. uncertain A and Gen. et sp. uncertain B from the Jianhe and Kaili biotas respectively, are also elucidated herein. What evident is that all of the taxa bearing a widespread cuticular sclerite, the Hadimopanella. However, detailed comparisons indicate that each scleritome from those taxa revealing some distinctions despite the overall similarity. It is the case coincidence with the fact that early paleozoic palaeoscolecids from different localities and horizons always exhibit different scleritomes. It is, therefore, likely that cuticular ornamentation of palaeoscolecids reflects habitation in a different type of substrate. The palaeogeographic distribution and stratigraphic range of Wronascolex are also elucidated. In spite of the fact that a re-evaluation of the taxonomy is required, current expressions have made the genus becoming a fairly diverse and cosmopolitan genus (including 8 species and 3 undeterminate species from South China, Siberia, Australia, Spain and North America) extending a geological period of Cambrian Stage 4 to Drumian. Nevertheless, an equivocal diagnosis of Wronascolex and the controversial taxonomy of the isolated Hadimopanella greatly diminish the biostratigraphic utility of the group. It is worth mentioning that scleritome scheme exemplified by W. spinosus from the Shipai fauna indicating that variations in growth pattern among different taxa of the molting palaeoscolecids might be expected.A case study on the palaeoscolecidan Mafangscolex yunnanensis (Luo et Zhang),1986 from the Chengjiang Lagerstatte of eastern Yunnan reveals notable information. A longitudinal row of conical sensilla and a possible lobopod appendage on the ventral side of M. yunnanensis are firstly observed, providing important morphological data on taxonomy, palaeoecology and phylogeny of the palaeoscolecids. New fossil evidence along with previous studies suggest that palaeoscolecidian worms have burrow dwelling behavior and some at least an occasional deposit-feeder.Irrespective of the debate over taxonomy of the palaeoscolecids, the form taxa Hadimopanella from the end of the Cambrian Stage 2 appears to be the oldest known examples of priapulids, being somewhat older than the Chengjiang Fauna. However, the diverse communities of the Chengjiang deposit represent the first evolutionary radiation of Priapulida. Strikingly, most Cambrian priapulid taxa are known only from some biotas or particular horizons, and only a few with pronounced longevity. A relatively high evolutionary rate during adaptation for a burrowing lifestyle seems adequate to account for this radiation. As an abundance of priapulid fossils have been collected from both the clastic platform and the foreslope facies of the Yangzte platform, an expectation that eastern Yunnan indicates a radiation centre of this animal group during the early Cambrian period should perhaps be reconsidered. Ottoia Walcott,1911 is a very important priapulid form of the Kaili Biota, which is the first reported occurrence of the genus outside the Cambrian of Laurentia. A total of 22 specimens are described as Ottoia guizhouensis sp. nov. The new species is distinguished from other Cambrian priapulids, including the type O. prolifica Walcott,1911 from the Burgess Shale, by the introvert that lacks a swollen base and bears a smaller number of longitudinal rows of hooks, and a more elongated trunk with more closely spaced annulations. Anatomically, a probable gizzard, a pair of posterior retractor muscles and muscle fibres of the gut wall are also observed in O. guizhouensis. Gut contents in O. guizhouensis suggest that this animal had an occasional deposit-feeding habit. Specimens buried in life position suggest that Ottoia may have burrowed and wriggled in all directions within the sediment and did not merely make U-shaped burrows.Several specimens with a dumb-bell shaped body from the Kaili Biota are assigned to Sicyophorus Luo et Hu,1999. However, a loricate trunk comprising about 50 longitudinal plates, an annulated neck and the presence of a caudal outgrowth preclude the Kaili material from its countertypes, the Sicyophorus rara Luo et al.1999 and Palaeopriapulites parvus Hou et al.,1999 from the Chengjiang Lagerstatte. It is, therefore, described as a new species, Sicyophorus guizhouensis sp. nov. The new species exhibits a mixture of characters typifying both the Priapulida (a spiny introvert and a caudal outgrowth) and the Loricifera (a loricate trunk), whilst a peculiar coiled gut is unknown from any other fossils and extant scalidophoran. Accordingly, it is a possibility of a neotenic origin of loriciferans from priapulids.After adding O. guizhouensis and S. guizhouensis into the original data matrix of cladistics of priapulids, analysis of their phylogenetic positions are available. The strict consensus of the most parsimonious trees shows that O. guizhouensis occupys the stem goup Scalidophora, whilst S. guizhouensis was placed at the most basal position of the crow Scalidophora where proximal to the Loricifera, which strongly supports the neotenic hypothesis. It is a remarkable fact that palaeoscolecidan taxa fall into different position within the phylogenetic tree. Cladistic analysis in conjunction with new fossil taxa seems likely to support a hypothesis that scalidophoran are descendents of the lobopod-bearing Cambrian ecdysozoans, the Xenusia. Based on morphological and anatomical data on intermediate fossil forms in the evolutionary lineage of Urbilateria - Xenusia - scalidophoran Fieldiai and Ancalagonia -Palaeoscolecida - Louisellia - Priapulida - Sicyophorus - Loricifera, it is likely that some palaeoscolecidan taxa are amongst the most primitive known ecdysozoans on account of their gross morphology and the new discovery of the possible lobopod appendage of M. yunnensis.