Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Vertebrates

According to Benton, dermal armour that was found as isolated fragments in North American Upper Cambrian deposits, as well as organisms in the Burgess Shale, Middle Cambrian deposits from Canada, such as Pikaia, that was superficially amphioxus-like, comprised the only evidence of vertebrates from this period. More recently the range of vertebrates was extended back to the Early Cambrian by discoveries in the Chengjiang deposits of China, which also produced basal deuterostomes that were very well preserved (Shu, 2003).

Also found in the Chengjiang deposits is a streamlined vertebrate that was 28 mm long, Myllokunmingia, described by Shu et al. (1999) from a single specimen, with a head that was poorly defined, though there is what appears to be a mouth at the anterior end, and 5 or 6 gill pouches. Its body has double-V-shaped myomeres, 25 extending along most of the body. Among the internal organs there are a heart cavity, a broad gut and a possible notochord, and along the anterior 2/3 of the body there is a low dorsal fin, and along the posterior 2/3 a possible ventro-lateral fin.

There are now more then 500 specimens of Haikouichthys, also found in the Chengjiang deposits (Shu et al., 1999, 2003a). This animal has a head with eyes, a nasal capsule, that was small and paired, cartilaginous protective tissue surrounding a brain, and extending for about 1/3 of its body length there were 6 pharyngeal gill arches. It also had a notochord, and up to 10 separate squarish elements across the notochord, suggested to be cartilaginous vertebrae. The posterior half of the body, that was slightly rounded at the end, had a series of a W-shaped myomeres. Traces of a number of structures in the body have been suggested to be a heart, gut and a series of gonads.

A second specimen led to the suggestion that Myllokunmingia and Haikouichthys are in fact the same species (Hou et al., 2002). These fishes from Chengjiang, as well as a new taxon, Zhongjianichthys,  a small animal that looks somewhat like an eel, with 2 eyes, possible nasal sacs and possible vertebral arches, have been grouped into the Myllokunmingiida.

In all vertebrates the most significant characteristic is the presence in the embryonic phase of neural crest tissue, with the ability to develop into mineralised bone, dentin in teeth, epidermal cartilages that are calcified, and epidermal placodes, that develop into the major sensory organs of vertebrates - paired eyes, nasal capsules, as well as the lateral line system (Long, 2011). Long suggests that in most chordates probably the most characteristic is their ability to secrete phosphatic hard tissue during the embryonic stage, especially bone, the most advanced tissue. These tissues, derived from mesoderm or neural crest cells, are not present in any known living invertebrate.

Sources & Further reading

  1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3rd Ed. , Blackwell Science.
  2. Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, 2nd ed, University of New South Wales Press.

 

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Last Updated 11/09/2011

 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading