Maotianshan Shales - Chengjiang Fauna

Chengjiang Fauna

Further information: List of Chengjiang Biota species by phylum

The Chengjiang biota comprises an extremely diverse faunal assembly, with some 185 species described in the literature as of June 2006. Of these, nearly half are arthropods, few of which had the hard, mineral-reinforced exoskeletons characteristic of all later arthropoda; only about 3% of the organisms known from Chengjiang have hard shells, and most of those are the trilobites, of which there are five species, all of which have been found with traces of legs, antennae, and other soft body parts, an exceedingly rare occurrence in the fossil record. Phylum Porifera (sponges; 15 species) and Priapulida (16 species) are also well represented. Other phyla represented are Brachiopoda, Chaetognatha, Cnidaria, Ctenophora, Echinodermata, Hyolitha, Nematomorpha, Phoronida, Protista, and Chordata. About one in eight animals are problematic forms of uncertain affinity, some of which may have been evolutionary experiments that survived for only a brief period as benthic environments rapidly changed in the Cambrian. Chengjiang is the richest source of the lobopodia, often considered a distinct phylum, with six genera represented: Luolishania, Paucipodia, Cardiodictyon, Hallucigenia (also known from the Burgess Shale), Microdictyon, and Onychodictyon.

Perhaps the most important fossils from Chengjiang are eight possible members of phylum Chordata, the phylum to which all vertebrates belong. The most famous is Myllokunmingia, possibly a very primitive agnathid (i.e., jawless fish). Similar to Myllokunmingia is Haikouichthys ercaicunensis, another primitive fish-like animal.

The enigmatic Yunnanozoon lividum is considered to be the earliest hemichordate, possessing many of the characteristic chordate features and providing an anatomical link between invertebrates and chordates. Haikouella lanceolata is described to be the earliest craniate-like chordate. This fish-like animal has many similarities to Y. lividum, but also differs in several aspects: it has a discernible heart, dorsal and ventral aorta, gill filaments, and a notochord (neural chord).

At present, there is no agreement as to the systematic placement of the Vetulicola, represented by seven species from Chengjiang: originally described as crustacean arthropods, the Vetulicola were later erected as a new phylum of primitive deuterostomes by D.G. Shu et al. (Shu 2001). Another researcher places them with the urochordates, based on putative affinity with the Phylum Chordata. They are thought to have been swimmers that either were filter feeders or detritivores.

Some two dozen animals from the Chengjiang biota are problematic regarding phylogenetic assignment. Among these, Anomalocaris saron, the alleged predatory terror of the early Cambrian, is the most famous. Shu (2006) recently described Stromatoveris psygmoglena as a possible bilateran missing link between Ediacaran fronds and Cambrian ctenophores.