Academic literature on the topic 'Fossil Amniotes'

There are large biological differences between the mammals and the primitive living amniotes as represented by turtles, lizards, and crocodiles ● Differentiated dentition with occluding post-canine teeth, and radical reorganisation of jaw musculature to operate them ● Differentiation of vertebral column and limb musculature, and repositioning of limbs to bring feet under the body, increasing agility of locomotion ● Relatively huge brain and highly sensitive sense organs ● Endothermic temperature physiology, with very high metabolic rates, insulation, and high respiratory rates ● Precise osmoregulatory and chemoregulatory abilities using loops of Henle in the kidney and an array of endocrine mechanisms Incomplete as it is, the fossil record of the mammal-like reptiles, or ‘non-mammalian synapsids’ permits the reconstruction of a series of hypothetical intermediate stages that offers considerable insight into how, when, and where this remarkable transition occurred. Deriving these stages starts with a cladogram of the relevant fossils that is then read as an evolutionary tree, with hypothetical ancestors represented by the nodes. The characters that define a node, plus the characters of the previous nodes, constitute the reconstruction. The differences in characters between adjacent nodes represent the evolutionary transitions that by inference occurred, and the whole set of successive nodes generates all that can be inferred about the sequence of acquisition of characters. If a hypothetical ancestral synapsid is placed at the base of the cladogram, and a hypothetical ancestral mammal as the final node, then the set of nodes in between represents everything the fossil record is capable of revealing about the pattern by which mammalian characters evolved: the sequence of their acquisition, the correlations between characters, and possibly the rates of their evolution. Of course, the inferred pattern of evolution of characters is only as reliable as the cladogram which generated it, and that in turn is only as realistic as the model of evolution used in its construction from the character data. And of course, there must have been many intermediate stages in the transition than cannot be reconstructed for want of appropriate fossil representation of those particular grades. Nevertheless, limited as it may be, this is what can be known from the fossil record.

‘The evolution of amphibians’ traces the evolutionary origin of living amphibians: anurans, urodeles, and caecilians. The comparison of the DNA sequence of their genes shows that the living amphibians taken together are a monophyletic group. This means that they all go back to a single common ancestor that had already separated from the common ancestor of the amniotes in the evolutionary tree. However, molecular evidence is little help in discovering which of the ancient tetrapod groups are related to the modern groups. Moreover, the earliest fossils of the three modern groups do not occur until far later, tens of millions years later, than any plausible relatives amongst the Carboniferous and Permian tetrapods.