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Evolutionary theory predicted that since amphibians evolved from fish, an intermediate form should be found in rock dated between 365 and 385 million years ago. Such an intermediate form should have many fish-like characteristics, conserved from 385 million years ago or more, but also have many amphibian characteristics as well.
Acanthostega (meaning "spiny roof") is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs.It appeared in the late Devonian period (Famennian age) about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land.
These fish evolved in shallow and swampy freshwater habitats, where they evolved large eyes and spiracles. Primitive tetrapods ("fishapods") developed from tetrapodomorphs with a two-lobed brain in a flattened skull, a wide mouth and a medium snout, whose upward-facing eyes show that it was a bottom-dweller, and which had already developed ...
[4] [5] Charles Darwin more famously proposed the theory of universal common descent through an evolutionary process in his book On the Origin of Species in 1859: "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first ...
These animals are called "secondarily aquatic" because although their ancestors lived on land for hundreds of millions of years, they all originally descended from aquatic animals (see Evolution of tetrapods). These ancestral tetrapods had never left the water, and were thus primarily aquatic, like modern fishes.
Evolution is the change in the heritable characteristics of biological populations over successive generations. [1] [2] It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. [3]
The first ancestors of fish, or animals that were probably closely related to fish, were Haikouichthys and Myllokunmingia. [ 6 ] [ 3 ] These two genera all appeared around 530 Mya . Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan : a notochord , rudimentary vertebrae, and a well-defined head ...
After Man: A Zoology of the Future is a 1981 speculative evolution book written by Scottish geologist and paleontologist Dougal Dixon and illustrated by several illustrators including Diz Wallis, John Butler, Brian McIntyre, Philip Hood, Roy Woodard and Gary Marsh.