Australia: The Land Where Time Began

A biography of the Australian continent 

Cephalochordates - Lancelets 

When Amphioxus and the lancelets were first discovered in 1770 they were believed to be slugs. They were shown to be allies of vertebrates by Alexander Kowalevsky in 1866. Since that time they have been considered to be the closest extant ancestor of vertebrates, as they are small animals similar in appearance to eels with myomeres that are well-developed with V-shaped muscle blocks, many gill slits along a pharynx that is well developed, hollow tubes supporting a long median dorsal fin, and a notochord extending the length of the body from within the head to the tail tip. The name lancelet comes from the lance shape of their bodies, an appearance very similar to the shape seen in primitive fish. There are about 25 species in 2 known living genera Branchiostoma, previously called Amphioxus and Epigonichthys. All known species are marine and grow to a maximum length of about 7 cm. They are filter feeders that bury their bodies in the sandy sea floor in shallow areas, only their mouths protruding from the sand to allow them to feed while remaining buried.

They have separate sexes, reproducing by shedding their eggs and sperm into the water. The larval stage has a powerful tail and is very fish-like, and have fewer gill slits than the adults. To generate a current to increase the flow of water to the mouth it has a circular ring of cirri, small tentacles. Unlike fish they have no heart, otherwise their circulatory system is very similar to that of typical vertebrates, having a ventral aorta, a large central artery. In the fossil record lancelets are virtually unknown, the oldest forms that could be lancelets date from the Chengjiang Fauna of the Early Cambrian of southern China. These are Yunnanozoon, described as enigmatic by Long, and from the Burgess Shale in British Columbia, Canada, of Middle Cambrian age, Pikaia. Several features of Pikaia give indications of its affinity to chordates, though it has the superficial appearance of a worm. It appears to have cartilage rods supporting what is believed to be a tail fin and appears to have a notochord. In its body form and its overall anatomy it seems similar to lancelets of the present, though at the time of writing it has not been studied in detail, resulting its evolutionary position remaining enigmatic. In the Early Cambrian deposits of China Cathaymyrus was discovered in the middle 1990s. The discovery of this animal demonstrates that animals similar to Pikaia had evolved by at least 525 Ma.

A notochord is believed to have extended far forward in the head of Yunnanozoon and it apparently had sets of paired organs that have been suggested to have been either gonads or slime glands. Though it could be an early, possibly basal, deuterostome (an organism with 2 openings, that includes all chordates, as well as 3 other phyla) it was previously classified as a cephalochordate Pikaia is another fossil that has been referred to as "a squashed slug" kind that has been reported to have a notochord. Long reports that after examining all the main fossils of this animal Philippe Janvier of Paris has said he cannot confirm that any of them had any chordate features, suggesting Pikaia could have been closely related to Yunnanozoon. 

Another animal that displays some similarities to lancelets of the present is Palaeobranchiostoma of Permian age in South Africa. It has a larger well-developed ventral fin than lancelets, and a dorsal fin that is also larger and has many small barbs. Some palaeoichthyologists have doubts about the interpretation of these fossil animals as perseveration of the known specimens is not good.

Haikouella from the Early Cambrian Chengjiang fossil site in China has previously been classified as closely related to Yunnanozoon. A later study has resulted in a proposal that it is actually more closely related to craniates, suggesting it is more specialised than other cephalochordates.

As with the tunicates, these animals resemble vertebrates more closely in their larval stage, though Long suggests a closer relationship to the main line of the vertebrates is precluded by the asymmetry that is a feature of cephalochordate larvae. In an evolutionary sense tunicates are more advanced than lancelets, as they have developed some phosphatic tissue, albeit primitive, that approaches the level of vertebrate bone.

Sources & Further reading

1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011