Urochordates include tunicates or sea squirts, small boxlike creatures that live as adults in colonies fixed to the seafloor. But their larvae swim actively, using the notochord and muscle fibers in a tail-like structure that is lost when they settle as adults. The genome of the sea squirt is entirely compatible with their zoological placement near the base of the chordates.
Another group of urochordates are the salps, which live in colonies that manage to swim. They have their own fascination, but in terms of the evolution of chordates we are discussing here, they have their place too. Recently it was found that their nervous system, including a primitive "brain", makes a good link between echinoderms and chordates.
Chordates were already known from the Chengjiang fauna, specifically in the form of Yunnanozoon, which is very much like Pikaia from the Middle Cambrian Burgess Shale and the living amphioxus, Branchiostoma, with recognizable gills and the characteristic zig-zag pattern of muscle bands that makes these animals chordates. A new and well preserved fossil called Haikouella was described late in 1999, and it seems to be close to the boundary between chordates and the first jawless fishes.
However, Haikouichthys is not merely a chordate but a lamprey-like jawless fish, with gill bars supporting its gills. And Myllokunmingia has pouch-like structures associated with its gills, which also makes it a jawless fish rather than a chordate.
These discoveries do not change any of our ideas about the evolution of fishes from chordates, but they change the timing. It's now clear that the origin of fishes is very early in the Cambrian, because there must have been time for them to evolve into two well-defined lineages before Chengjiang times. The apparent force of the "Cambrian explosion" is increased.
By the same token, the subsequent evolution of the fishes is now slower. It took about 30 m.y. before the ?cartilaginous structure of the earliest jawless fishes began to change into bone.
Obviously, we may never be able to trace the details of the evolution of immunity, because so many of the Paleozoic agnathan clades, and the early gnathostome clades, are extinct. It doesn't mean we can't wonder at the evolution itself!
Reference: Laird, D. J., et al. 2000. 50 million years of chordate evolution: seeking the origins of adaptive immunity. Proceedings of the National Academy of Sciences 97: 6924-6926. This is a commentary and quick summary of the state of our knowledge.
Many years ago, a construction crew grading a dirt road pushed over a slab of rock that had more than 100 beautifully preserved fossil fishes on it. So what did these fair dinkum Aussies do with it? They propped it against a fence-post and carried on with the job.
Much later, a local beekeeper moving his hives spotted the slab, recognized its importance, and called the Australian Museum in Sydney. The paleontologists were very excited, but by then the road had been re-routed and no-one could find the rock outcrop it had come from. It took several more years before the local Rotary Club organized a search and re-discovered a treasure trove of Late Devonian fishes, including magnificently preserved lobe-fin fishes. In 1993 they raised enough money and donated equipment and labor to dig up 70 tons of slabs, bearing thousands of fossil fishes. The fishes include placoderms and several new species of early sarcopterygians.
Some of the Canowindra fossils were studied by Zerina Johansen as her Ph.D. project, and are now being published. The fossils include the best-preserved rhipidistian (except for Eusthenopteron). Already they have filled in some of the details of late rhipidistian evolution. They hold out the tantalising possibility that important evolution toward tetrapods/amphibians may have been taking place in Australia as well as/instead of the northern continents, which is the story at present.
The citizens of Canowindra raised funding to build an "Age of Fishes" museum (the only museum in the world devoted to fossil fishes). Visit their Web site.
Unfortunately, further research at the site is being neglected. 70 tons of fossils collected at the site are piled up under the town's sports stadium. The Australian Museum announced that it had "reviewed its priorities and moved away from paleontology and geosciences to focus on ''more topical issues'' such as biodiversity conservation" [!!!]. See this article in the Sydney Morning Herald
Discovery of coelacanths on the coast of East Africa.
Discovery of shallow-water coelacanths on the South African coast.
References for Chapter 7
Page last updated April 7, 2013
Links checked March 16, 2013
[For Chapter 8, click here ]
[For Chapter 6, click here ]
[Return to UC Davis Geology Department] Home Page]