|MadSci Network: Evolution|
Hi Phil, Thanks for your question. Coelacanths (order Coelacanthiformes) are interesting fishes recognizable for their distinctive lobed fins, among other unusual (for fishes) characteristics. Coelacanths first appear in the fossil record about 360 million years before the present, and were once believed to have gone extinct as a lineage about 80 million years ago, when they disappear from the known fossil record. However, in 1938, a modern coelacanth was discovered among the catch of a South African fishing vessel. Subsequently, many more specimens of that species, now called Latimeria chalumnae, were recovered from the Comoros (a group of islands near Madagascar) and in 1998 a second species (Latimeria menadoensis) was found on the other side of the Indian Ocean, near the island of Sulawesi in Indonesia. The short answer to your question is yes, there are differences between fossil coelacanths and the two extant (living) species. Coelacanths have been around for a long time, and over 120 coelacanth species have been identified in the fossil record. Many of them look strikingly similar to the extant species, but some, such as those in the genus Allenypterus, have a much different body shape. Latimeria is also significantly larger than most fossil species, although a genus of ‘giant’ coelacanth has been reported from the Cretaceous. Although so many coelacanths look similar at first, it is straightforward to diagnose species, both extinct and extant, based on features of their anatomy. The lineage including Latimeria is estimated to have diverged from the most closely related known fossil about 80-85 million years ago. One of the most striking differences between Latimeria and known coelacanth fossils is their habitat. Most known coelacanth fossils are from shallow-water marine environments, such as bays, and sometimes from fresh water. Modern coelacanths live in deeper water, often in caves. This difference also helps in understanding the long gap in the fossil record. That kind of environment is not ideal for the formation of fossils, and when they do form, they would be more likely to be found on the ocean floor today, or to have been subducted under the earth’s crust through the processes of plate tectonics. It is not unreasonable to suppose that many extinct species lived in areas that will make it unlikely for us to ever find their fossils. Coelacanths are really interesting and cool fishes, and I hope you will continue to read about them and ask further questions if you like. Good luck in your studies. Best wishes, Shannon DeVaney, Ph.D. candidate Division of Ichthyology Natural History Museum and Biodiversity Research Center Suggested starting points for further information: Weinberg, Samantha. 2000. Fish Caught in Time: The Search for the Coelacanth. New York: HarperCollins. 220 pp. Forey, Peter L. 1998. History of the Coelacanth Fishes. London: Chapman & Hall. 419 pp. Smith, J.L.B. 1956. Search Beneath the Sea: The Story of the Coelacanth. New York: Holt. 260 pp. http://www.dinofish.com “Dinofish: The Fish Out of Time” coelacanth information site. http://en.wikipedia.org/wiki/Coelacanth a> Wikipedia: Coelacanth http://www.pbs.org/wgbh/nova/fish/ “Ancient Creature of the Deep” – coelacanth page associated with PBS’s NOVA program.
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