Mandibulas Evolucion

The problem of how, why and when the mammalian ear and jaw configuration arose from taht of reptiles has intrigued paleontologists and anatomists for more that a century. One hundred years ago comparative anatomists and embryologists determined that most of the bones of the reptilian and mammalian skulls were homologous or comparable. They could show for example that the bones forming the jawjoint in reptiles the articular and quadrante were the homologues of two of the bones in the mammalian middle ear the malleus and the incus. They did not; however explain why in the evolution of mammals from reptiles this dramatic change had taken place.
Documentation of the gradual evolution of mammals from reptiles required the discovery of many well preserved fossils. It has taken about onehundred years to accumulate the evidence, and only in recent years were the first examples of the oldest mammals found, adding the final and most important link in the transition from primitive reptiles to early mammals. The transition covers about 120 million years.
These discoveries together with the advent of new techniques such as cinefluoroscopy and electromyography to study the functioningof the chewing mechanisms of mammals and reptiles, have enabled us to begin to understand the evolution of the mammalian middle ear and masticatory apparatus from those of reptiles.
JAW STRUCTURE
Mammals have a three-boned middle ear and a lower jaw, or mandible, consisting of a single bone, the dentary, whereas reptiles have only one bone in the middle ear, the stapes, and a lower jaw made upof at least seven bones . the reptilian dentary supports the teeth and is followed by a series of postdentary bones. In mammals the jaw joint is formed between the temporal bone and the single bone of the lower jaw, the dentary . in reptiles the jaw joint lies between the last postdentary bone, the articular, and a skull bone, the quadrate. The quadrate is usually joined to the temporal bone andthe braincase. This comparison implies that during the evolution of mammals from reptiles the postdentary bones and the quadrate decreased in size and the dentary progressively increased until it reached the temporal bone, when a new jaw articulation developed. The small bones of the reptilian jaw joint were then incorporated into middle ear as the malleus and incus.
The evolutionary history ofthe mammalian jaw is in part recapitulated during the embryological development mammals. The malleus originates as part of the lower jaw, and only in later stage of foetal development is the separation between these elements achieved. In young marsupials, which start to suckle at a very inmature stage the initial functioning jaw joint lies between the articular and the incus the jaw joint at thesestages is like that of reptiles. Only in more advanced stages of developmet when the dentary establishes contact with the temporal bone, do the articular and incus separate from the lower jaw and move into the middle ear.
Before discussing whether the fossil evidence in fact show that these changes took place as suggested or attempting to explain why, it is necessary to review briefly seceralother differences between the masticatory apparatuses of reptiles and mammals.
Tooth replacement.
Figure 3 compares the tooth replacement patternd in mammala and reptiles. In most mammals the dentition is divided into four distinct regions: incisor, canine, premolar, and mola. The teeth in the first three have two distinct generations. The molars are never replaced and are added to the jaw fromthe front backward during growth. In marked contrast, the teeth of reptiles are replced continuously through life by alternate tooth replacement or simply adding teeth at the end of the row. Thus newly hatched or recently born reptiles start life with a full set of functional teeth that enables them to process their own food. Growing reptiles may increase their skull length by a factor as great as...