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Evolutionary Descent of Mammals
Mammals: Amniotes is a group of tetra pod vertebrates which includes reptiles, birds, and mammals. The eggs of amniotes have extra embryonic membranes, which guard the embryo and provide nourishment to it and excretes out the waste material (Perry et al., 2010).
Amniotes lay eggs having four extra embryonic membranes. The initial amniotes were small in size and lizard-like, however they adopted during the early Permian period and divided themselves into several distinguished morphologies (Butler & Hodos, 2005).
Amniotes were further classified into Anapsids, Diapsids and Synapsids according to increasing skull fenestrations in the temporal region with none, two and three fenestrations respectively (Rougier et al., 2007).
The third skull fenestration condition is synapsid and the synapsids include mammals and their extinct relatives, the Therapsids and Pelycosaurus. Synapsids were the first Amniote group to endure extensive adaptation and were the leading large amniotes of the late Paleozoic era. Pelycosaurus, amniotes later developed into Therapsids in the Mesozoic era, which were reptile like mammals (Jenkins Jr, 2013).
Fossil Records of Mammal Descent
Fossil evidence alone is not adequate to prove that evolution ever took place, but it supports the theory of progressive increase in the intricacy of organisms. The oldest fossil-bearing rock contains a very few types of fossilized organisms and these all have an uncomplicated structure.
Younger rocks contain a greater diversity of fossils with increasingly complex structures (Wilson, 2006). One of the major objections upon the evidence of fossils is the absence of a continuous record. Gaps in the fossil record are taken as strong evidence against a theory of descent by modification.
Some of the factors of missing fossil record are facts such as dead organisms decay quickly; these are eaten by vultures; the soft bodied organisms do not solidify into fossils and only a small percentage of organisms actually die in such conditions which favour the development of fossils (Sahney & Benton, 2008).
The most appropriate example of evolutionary history (phylogeny) is of the horse. It is supported by a complete record of fossils which were found in North America. The oldest recognizable horse-like fossils belong to a genus called Hyracotherium, widely distributed throughout North America and Europe (Berkeley, 2015).
Pulses 1 to 7
Pulse 1 (Permian Era)
The last significant period of time in the Paleozoic Era was the Permian period and it encompassed a period from 299 to 251 million years ago. The dissimilarity between the Paleozoic and the Mesozoic era can be judged by the largest process of extinction which has been recorded in history (Jin et al, 1994). The new environment was extremely beneficial for the early reptiles.
Therapsids discovered an internal method to keep them heated and warm by conserving heat which is produced by the metabolism of food, especially fat. These more metabolically active reptiles became the prevailing land animals of the late Permian era (Kump, Pavlov & Arthur, 2005).
Pulse 2 (Early to Middle Triassic Era)
The replies became the source of ancestry for the rest of the organisms which came later in the Mesozoic period. Lystrosaurus was the most common and pervasive animal. It used to be a medium sized, grass eating herbivore and was also distantly related to mammal species.
By the end of the epoch, the archosaurs had replaced synapsids as the dominant tetrapod group. Therocephalians became extinct and Dicynodonts began to decline after most probably a large scale radiation exposure (Romano et al., 2013)
Pulse 3 (Late Triassic)
The Late Triassic was the beginning of the” ‘Age of Dinosaurs”, and constitute one of the most important substitutes amid non-marine tetrapods. Many significant groups appeared abruptly in the Carnian, from which the pseudosuchian thecodonts dominated the land and phytosaurs took over the rivers and lakes. Dinosaurs remained..
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