Cambrian chordates

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The Cambrian chordates are an extinct group of animals belonging to the phylum

Chengjiang biota of China in 1991,[3] which were later found to be of chordates,[4][5] several Cambrian chordates are known, with some fossils considered as putative chordates.[6]

The Cambrian chordates are characterised by the presence of segmented muscle blocks called myomeres 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."[7]

Discovery

Charles Doolittle Walcott (left) at a fossil quarry excavating the Burgess Shale in British Columbia, Canada.

Pikaia gracilens was the first Cambrian chordate known. It was discovered by

parapodia [paired protrusions on the sides of polychaete worms] on the body segments back of the fifth."[8]

Wonderful Life: The Burgess Shale and the Nature of History,[13] from which Pikaia became known as the "most famous early chordate fossil,"[14] or the earliest chordate,[15] or the oldest ancestor of humans.[16][17]

The second Burgess shale chordate was also discovered by Walcott but left 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.[18] He and Emilio Insom at the University of Camerino published the classification in 1993, giving the name Metaspriggina walcotti, an animal with unknown identity.[2] Conway Morris recovered additional specimen with which he described the species as that of chordate in 2008.[18]

Maotianshan Shale, site of the discovery of the Chengjiang Biota, (UNESCO world heritage site)

In 1991,

vetulicolians of uncertain classification.[6]

Burgess Shale chordates

Pikaia gracilens

Pikaia fossil from the Royal Ontario Museum's collection.

Pikaia gracilens was a primitive chordate having a lancelet-like body that lacked a well-defined head components and averaged about 3.8 cm in length. On each side of its head is a pair of large, antenna-like tentacles similar to those of snails.[21] 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.[22]

A hollow tubular structure running from its anterior part of the body to the tail was earlier believed to be an indication of the presence of a

vertebrates only. No eyes can be observed.[24]

The muscle orientation and flat shaped body indicate that Pikaia was an active and free swimmer.[8] It would have swum by throwing its body into a series of S-shaped, zigzag curves, as do living eels. However, its myomere arrangement suggests that Pikaia could not be a fast swimmer.[25]

Metaspriggina walcotti

Metaspriggina fossil from the Royal Ontario Museum's collection.

Metaspriggina Walcotti is fish-like and measures up to 6 centimetres (2.4 in) in length and 1 centimetre (0.39 in) in breadth.[26] 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 than the others and do not support any gills, a characteristic that suggests a distant relationship to gnathostomatans (jawed vertebrates from fish to humans).[18] It lacked fins and it had a weakly developed cranium, but possessed 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.[26]

Maotianshan Shale chordates

Yunnanozoon lividum

Yunnanozoon lividum fossil.

Yunnanozoon lividum is small, measuring 2.5 to 4 cm long. It is described as a

Haikouella.[19]

An analysis in 2015 concluded that

Haikouella is a junior synonym of Yunnanozoon, and that its two species (H. lanceolata and H. jianshanensis) are members of the genus Yunnanozoon.[20][28]

Myllokunmingia fengjiaoa

Myllokunmingia fossil.

Myllokunmingia fengjiaoa is 2.8 cm long and 0.6 cm high. It is among the oldest possible

digestive tract that may run all the way to the rear tip of the animal. Mouth is not identified.[30]

Zhongjianichthys rostratus

Zhongjianichthys rostratus is generally regarded as an early fish, and therefore as one of the first vertebrates. It has an eel-like body, 1.1 cm in length, and covered with a thick integument. It has no visible myomeres. Its ventral fin is low and runs much of the body's length. It has reduced fins that indicates that it was mainly bottom-dwelling and did not swim much.[6][31]

Zhongxiniscus intermedius

Zhongxiniscus intermedius had a small, broad and short, fish-like body that was roughly 10 mm in length. It has S-shaped myomeres, dorsal and ventral fins. It is tentatively interpreted as an intermediate form between Cathaymyrus and two vertebrates Haikouichthys and Myllokunmingia.[32]

Haikouichthys ercaicunensis

Haikouichthys ercaicunensis looks like Myllokunmingia with which it was discovered from the same beds. The body about 2.5 cm (1 in) long and is narrower than Myllokunmingia. The head and tail are distinct at the opposite ends. There are at least six to nine probable gills. The myomeres are with rear directed chevrons in the tail. It has a long notochord along its body length.[33] It has a prominent dorsal fin with fin radials but not as well developed in those of hagfish and lampreys.[34]

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