Chengjiang Maotianshan Shales

An Early Cambrian Lagerstatte and Window to the "Cambrian Explosion"

The fossils of the Maotianshan Shales and Burgess Shale Fauna of Field, British Columbia provide a lens to view the appearance on earth of all the major phyla in existence today, organisms that remain of enigmatic origin, as well as forms that did not persist beyond the early Cambrian Period. The Chengjiang Biota's diversity suggests a stable ecosystem occuring after the Cambrian Explosion when life's major phyla appeared in what seems like the blink of the eye compared to preceding four billion years of geological time on earth. Importantly, since Chengjiang includes all major animal groups found in the Burgess Shale, discoveries in this laggerstatte imply earlier diversication and/or diversification over a shorter time interval than can be inferred from the Burgess Shale. One theory for this change is the fact that the rising concentration of oxygen, a result of bacterial and plant photosynthesis over the eons (also see stromatolites), allowed for organisms to be larger in size and lead a more active lifestyle. According to this theory, oxygen finally reached a concentration where the aerobic machinary, acquired by eukaryotic cells in the precambrian and contained in the mitochondria through endosymbiosis, could provide a much higher energy level. Larger organisms more easily leave traces in the fossil record. The Cambrian Explosion also encompasses the evolutionary emergence of hard body parts, thought by some to be a consequence of the selection pressures brought about by predation; solid structure support could also lead to increased size. Other mechanisms are theorized to have contributed to the Cambrian explosion, such as a cooling of the Earth and an increase in calcium in marine environments. While the Cambrian Explosion may always be shrouded in some mystery, it is clear that it represents a profound period for the evolution of life on Earth, where the basic body plans of the major animal phyla appear by the fossil record to have been established over a brief 10 million year span. All the major animal phyla that exist today -- about three dozen -- evolved from these early Cambrian fauna.

Maotianshan Shales fossils exhibit impressive animal diversity, many with their soft-tissue parts preserved. The faunal distribution spans across annelid worms, arthropods (including trilobites), primitive chordates, echinoderms, hemichordates, medusiforms, priapulids worms and sponges, and many more -- see an up to date list of Chengjiang Biota here. Of particular significance are the putative first agnathan fish (jawless fish). There are also numerous problematic animals, some of which may have left no descendents, and therefore constitute failed evolutionary attempts at diversity. As a whole, scientists have enormous work ahead interpreting the Chengjiang fossil record, and unraveling the many mysteries in this exquisite rock record of early Cambrian life.

The fossils are found in a 50 meter thick section of mudstone known as the Maotianshan Shale, named for a distinctive hill in Chengjiang County, Yunnan Province, China in the Qiongzhusi Section of the Yu'anshan Member of the Heilinpu Formation. Fossils occur in widely scattered outcrops within thin beds only 1 to 2 cm thick, with the soft parts preserved as aluminosilicate films where the structure has been infilled by iron-rich clay minerals or hematite. The fossils are commonly weathered to a reddish color, possibly as a result of oxidized pyrite, on a distinctive yellow matrix. Analysis of some specimens has shown an iron content as high as 43%, a five-to-eightfold increase over the background level of the matrix.

Although fossils from the region have been known since the early 1900s, it was the discovery of the trilobita form Misszhouia (Naraoia) longicaudata on July 1, 1984 by Hou Xian-guang that led to the recognition of this exceptional soft-body lagerstatte. Unlike many fossil deposits, those of Chengjiang and the younger Burgess Shale show exceptional preservation of nonmineralized body parts and internal soft tissues, affording us far more information than could be gleaned from preservation of hard skeletal elements alone.

The region that became the Heilinpu Formation was bounded on three sides by relatively steep mountains, with an opening to the ocean on the east. Various events of shallowing are evident in strata above and below that are indicative of both sea-level changes and tectonic activity. The climate of the region is thought to have been tropical due to proximity to the equator. The Chengjiang fauna is believed to have been mainly benthic (bottom-living), with most taxa living directly on the soft, level seafloor. Few animals that lived above the seafloor were preserved, perhaps because they were able to flee from the periodic storm-induced turbidity flows that are presumed to have resulted in the preservation seen. Most fossils show evidence of only short-distance post-mortem transportation. Some sponges have been preserved flat and current-aligned, indicative of some transport prior to burial, as is also the case for various burrowing worms. Some specimens, however, have been found exactly where they had lived, especially in the case of some lingulid brachiopods that were found with their shells lying flat and their pedicles extending into the sediment. Most specimens of all types are preserved with their flattest surface parallel to the bedding plane of the matrix.

The Yu'anshan Member extends over tens of thousands of square kilometers of eastern Yunnan Province, and thus the Chengjiang biota is now known from various locations within the region. Nonetheless, the Chengjiang biota retains the name of the location at which it was first discovered. Some important locations are at Eriacun, Mafang, Haikou, and Kafuquing. It is at Eriacun that the world's oldest known vertebrates have been found.

As of June 2006, some 185 species of organisms were described in the scientific literature. Of these, 45% are arthropods and 13 % are enigmatic. Approximately 65% of all fossils preserved are arthropods of exceptional diversity among which the most abundant forms are small bivalved animals known as bradoriids that had pliable exoskeletons. Few arthropods from Chengjiang had the hard, mineral-reinforced exoskeletons as found in trilobites. Rather, Early Cambrian arthropods mostly had relatively pliable, nonmineralized exoskeletons like those of insects. Only about 3% of the organisms known from Chengjiang have hard shells, and most of those are the trilobites. The mud-filled guts of some arthropods (such as Fuxianhuia protensa) suggest they fed by swallowing mud to extract the in situ nutrients. Others, such as the relatively huge Anomalocaris saron were active, free-swimming predators; note that some researchers place the still enigmatic anomalocarids as distinct from the Arthropoda. As one of the few hard-shelled members of the fauna, the trilobites are represented by Eoredlichia, Kauyangia, Wutingaspis, Palaeolenus, and Yunnanaocephalus. Examples of all have been found with traces of legs, antennae, and other soft body parts that are only extremely, rarely observed in the fossil record.

The sponges (Phylum Porifera) are the second most diverse group after the arthropods with the Desmospongea (tubular sponges) the dominant type. Leptomitus. Leptomitella, and Choialla are examples. The Hexactinellids (glass sponges) are found as well, although they are far more rare. Haliochondrites and Quadrolaminella are examples of this type. Many sponges have been found buried where they lived, and so are often found articulated.

The worms are represented by numerous specimens from the Chengjiang biota. The phylum Nematomorpha has been attributed to the genus Cricocosmia which has a long proboscis armed with spines. Some scientists ally it with the Priapulida (priapulid worms), and others with the paleoscolcid worms, indicative of the fact that the taxonomy of members of this fauna is still in a state of flux. Other examples are Maotianshania and Paleoscolex. The Priapulida are represented by Paleopriapulites, Acosmia, Sicyophorus, and others. The priapulids were a common component of the Cambrian marine faunas throughout the world.

Some of the more unusual members of the fauna are the Lobopodians, creatures that have been commonly termed "velvet worms", after extant members of worms with legs (in Phylum Onychophora) that they closely resemble. Their vermiform bodies bear appendages, hence the other common name of "worms with legs". Six genera come from the Cambrian of Yunnan Province, making it the richest source of these ancient Lobopodian fossils. The Chengjiang lobopods are Luolishania, Paucipodia, Cardiodictyon, Hallucigenia (also known from the Burgess Shale), Microdictyon, and Onychodictyon.

The Phylum Chordata is that to which all vertebrates belong. Several examples are found within the Chengjiang biota. As the earliest known vertebrate, the most renowned is Myllokunmingia, which was only recently discovered in 1999. Some 500 examples are known, most of which are flattened imprints of lateral aspect. A series of V-or W-shaped muscle blocks are evident in most specimens, with a distinct dorsal fin which has traces of ray-like supports. At the anteroventral end of the trunk there is a set of six or seven gill pouches containing feathery gill filaments. All of these features distinguish this taxon as a vertebrate, probably a primitive agnathid (jawless) fish, one thought to have affinities with the modern-day hagfish.

Several enigmatic taxa are known as well. Allonia is a chancelloriid that was initially interpreted as a sponge. Other workers believe that they have their affinity with the tunicates, and still others with the halkeriids. Batofasciculus looks like nothing more than a spiky balloon; the nature of this organism is truly an enigma. Dinomiscus looks much like a typical crinoid, but most researchers are unsure of its taxonomic placement. One worker allies it with Eldonia in the newly-erected class Eldonoidea. Eldonia itself is frequently seen in the biota, sometimes occurring in associations of several individuals. It is typically preserved as a flat, discoidal impression. It was originally described as a medusiod named Stellostomites, but Simon Conway Morris considered it to be a species of Eldonia, a well-known component of the Burgess Shale. The medusoid shape suggests that Eldonia was pelagic; other researchers have argued for an entirely different existence, considering the eldonioids to be benthic, lying passively on the surface of the seafloor.

The Vetulicolians are an enigmatic group that some scientists place in their own phylum (Phylum Vetulicolia). They are thought to have been swimmers that either were filter feeders or detritivores. Originally described as crustacean arthropods, Shu (2001) later erected the Vetulicola as a new phylum of primitive deuterostomes. Another researcher places them with the Urochordates based on putative affinity with the Phylum Chordata. At present, there is no agreement as to their systematic placement.

Shu (2006) recently described an Ediacaran-vendiobont-like animal, Stromatoveris psygmoglena, gen. sp.nov., with features reminiscent of ctenophores which resemble jellyfish (radiate), but other characteristics suggestive of Bilaterans (Triploblasts with bilateral symmetry). Stromatoveris is a putative missing link between Ediacaran fronds and Cambrian ctenophores.

The Chengjiang Biota is an incredible lagerstatte by all measures, including the enormous diversity and early appearance in the fossil record, and also the exquisite degree of detail of the preservation of its specimens. It affords us a unique opportunity to learn about these wonderful creatures so near the initial radiation of life to the forms of today known as the Cambrian Explosion. This amazing diversity was hailed throughout the world as one of the most important fossil finds of the 20th Century. Because of its unique value to science, the Chengjiang lagerstatte was placed on China's first list of A-class national geological parks in 2001. Chengjiang County has plans to apply for a World Natural Heritage designation for its deposits. Chengjiang is an underdeveloped county having rich phosphate deposits that are found both above and below the formation holding the lagerstatte. They have been exploited in part through efforts that began at about the same time that Hou Xian-guang discovered the deposits that bear these exceptional fossils, with phosphate mining bringing in some 2/3 of the county's revenue in 2003. Efforts are underway to close the region to mining to support the county's efforts to attain Heritage Status. A consequence of this has been renewed mining efforts in the region which threaten the fossil-bearing strata due to erosion, slumping of overburden, and simple destruction by the mining efforts. Chengjiang County faces the dilemma between calls for preservation of the treasure trove of early Cambrian fossils to which it is steward and the economic reliance it has on the phosphate industry. It is hoped that a balance between exploitation and restoration of the land can be achieved while there is still time.

References: