Cambrian chordates

The Cambrian chordates are an extinct group of animals belonging to the phylum Chordata that lived during the Cambrian, between 485 and 538 million years ago. The first Cambrian chordate known is Pikaia gracilens, a lancelet-like animal from the Burgess Shale in British Columbia, Canada. The discoverer, Charles Doolittle Walcott, described it as a kind of worm (annelid) in 1911, but was later realised to be a chordate. Since the discovery of other Cambrian fossils from the Burgess Shale in 1991, and from the Chengjiang biota of China in 1996, which were later found to be of chordates, several Cambrian chordates are known, with some fossils considered as putative chordates.

The Cambrian chordates are characterised by the presence of segmented muscle blocks and notochord, the two defining features of chordates. Before the full understanding of Cambrian fossils, chordates as members the most advanced phylum were believed to appear on Earth much later than the Cambrian. However, the better picture of Cambrian explosion in the light of Cambrian chordates, according to Stephen Jay Gould, prompted "revised views of evolution, ecology and development," and remarked: "So much for chordate uniqueness marked by slightly later evolution."

Discovery

Pikaia gracilens was the first Cambrian chordate known. It was discovered It was discovered by Charles Doolittle Walcott from the Burgess Shale in the mountains of British Columbia, Canada. Walcott reported the specimen with taxonomic description in 1911. He used the series of transverse body segments as a feature of annelid worms and classified it as a polychaete worm. He gave the name after Pika Peak, a mountain in Alberta, Canada. However, he was also aware of the unique differences from living worms that he prudently remarked: "I am unable to place it within any of the families of the Polychaeta, owing to the absence of parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."

University of Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris re-analysed the specimens and came to an opinion in 1977 that Pikaia was obviously a chordate: the body segments were similar to muscle blocks in chordates, and a rod-like structure spans the body length, an indication of notochord, a defining structure of chordates. Their article in the Scientific American in 1979 firmly concluded that it was a chordate, saying: "The chordates are represented in the Burgess Shale by the genus Pikaia and the single species P. gracilens." Conway Morris published the formal classification as a chordate in 1979. Harvard University palaeontologist Stephen Jay Gould popularised Pikaia as an ancestral species of chordates in his book 1989 Wonderful Life: The Burgess Shale and the Nature of History, from which Pikaia became known as the "most famous early chordate fossil," or the earliest chordate, or or the oldest ancestor of humans.

The second Burgess shale chordate was also discovered by Walcott, who left it unexamined. The date of discovery was not recorded. Italian palaeontologist Alberto M. Simonetta while working at the US National Museum of Natural History, where Walcott's collections are maintained, became the first to analyse the specimen in 1960. He and Emilio Insom at the University of Camerino published the classification in 1993, giving the name Metaspriggina walcotti, an animal with unknown identity. Conway Morris recovered additional specimen with which he described the species as that of chordate in 2008.

In 1991, Hou Xian-Guang (of the Chinese Academy of Sciences), Lars Ramsköld and Jan Bergström (of the Swedish Museum of Natural History) reported a series of discoveries of Cambrian fossils from the Maotianshan Shales in Chengjiang County, Yunnan Province, China. One specimen which they named Yunnanozoon lividum was a worm-like animal that could not be easily fit into any known type of animals. They remarked:

We are unaware of any described animal similar to Yunnanozoon gen. n., and are unable to refer it to a particular phylum. This is, however, no valid reason to regard it as presenting an unknown phylum. It may be noted that a thick cuticle with repetitive pattern is characteristic of some aschelminth groups, but of virtually no other extant animals apart from arthropods.

They speculated that the animal was most related to roundworms (aschelminthes). Four years later, Ramsköld and his team reanalysed the anatomical details and concluded that it was a chordate. The Maotianshan Shales have yielded other Cambrian chordates and chordate-related animals including Cathaymyrus species (C. diadexus and C. haikoensis), Haikouella species (H. lanceolata and H. jianshanensis, but possibly a type, synonym, of Yunnanozoon), Myllokunmingia fengjiaoa, Shankouclava anningense, Zhongjianichthys rostratus, Zhongxiniscus intermedius, and a group called vetulicolians of uncertain classification.

Burgess Shale ch0rdates

Pikaia gracilens

Main article: Pikaia

Pikaia gracilens was a primitive chordate having a lancelet-like body that lacked a well-defined head components and averaged about 38 mm in length. On each side of its head it bears a pair of large, antenna-like tentacles that look like those of snails. There are a series of short appendages on either side of the underside of the head just after the mouth, and their exact nature or function is unknown. The pharynx is associated with six pairs of slits with tiny filaments that could be used for respiratory apparatus.

The chordate nature was discovered by Cambridge palaeontologist Harry B. Whittington and his student Simon Conway Morris in 1979. According to their interpretation, a hollow tubular structure running from its anterior part of the body to the tail is an indication of the presence of a notochord, a defining feature of all chordates from protochordates to mammals. In 2012, Conway Morris and Jean-Bernard Caron found additional chordate nature that the body segments are anatomically blocks of skeletal muscles, the myomeres, which are found in vertebrates only. However, the hollow tubular rod was redescribed as a dorsal organ of unknown function, and that the notochord lies beneath it.

The muscle orientation and flat shaped body indicate that Pikaia was an active and free swimmer. It would have swam by throwing its body into a series of S-shaped, zigzag curves, as do living eels. However, its myomere arrangement suggest that Pikaia could not be a fast swimmer.

Metaspriggina walcotti

Metaspriggina Walcotti is fish-like and measures up to 6 centimetres (2.4 in) in length and 1 centimetre (0.39 in) in height. It possesses a notochord, along with seven pairs of pharyngeal bars, possibly made of cartilage. The pharyngeal bars were formed of multiple separate pairs of bones. The first two pairs are larger that the others and do not support any gills, a characteristic that suggests a distant relationship to gnathostomatans (jawed vertebrates from fish to humans). It lacked fins and it had a weakly developed cranium, but it did possess two well-developed upward-facing eyes with nostrils behind them. Unlike in Pikaia in which myomeres are numerous and V-shaped, the myomeres have a W-shaped configuration and are 40 in number.