[edit] Living fossils
Main article: Living fossil
Ginkgo adiantoides Eocene fossil leaf from the Tranquille Shale of British Columbia, Canada.
Living fossils, like the coelacanth, ginkgo, and horseshoe crab, are examples of organisms that are relatively unchanged from their distant ancestors.
We now view these XY gene pairs as a kind of living fossil. They give us a glimpse back into the past when the X and the Y had an absolutely identical gene content. Today we can see a small remnant of that shared gene content.
The coelacanth, a primitive fish, a "living fossil" believed to have been extinct for 65 million years, was caught in a fishing net in 1938 off the coast of Africa.
Most familiar freshwater and sea water fishes belong to this group. The living fossil Coelacanth is a bony fish whose relatives (lobe-finned fishes) can be traced back to the Devonian geological period (363-409 Mya).
The long, unsegmented telson projects to the rear. They possess book gills that resemble the pages in a book. Limulus is considered a living fossil due to its great similarity to fossil forms from the Paleozoic Era.
Features shared with bacteria include the lack of a nucleus and strongly similar metabolic genes. "We can look at the Archaea as the living fossils of our prokaryotic [bacterial] ancestors," observed Carl Woese (UIU).
 See also: Animals, Fossil, Animal, Species, Environment
  
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