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'Mammal-like reptiles' is a term used to describe the prehistoric animals that appear to be the reptilian ancestors of mammals. The term "mammal-like reptiles" is most commonly used to describe the group Therapsida, although it can be also used more broadly to describe non-mammalian Synapsids.
Mammal-like reptiles were the dominant terrestrial animals in the middle to late Permian period. As with almost all life forms then extant, their numbers and variety were severely reduced by the Permian extinction. Some species survived into the Triassic period, but saurischian dinosaurs quickly became the dominant animals. The last mammal-like reptiles probably survived no later than the late Triassic.
The term "mammal-like reptiles" is not considered a formal one by most experts; technically speaking, mammal-like reptiles were closer on the evolutionary branch to mammals than to reptiles as traditionally defined. They possessed glandular skin that lacked scales (thus they can be better visualized as being "naked lizards", both furless and scaleless.) However, their overall character is more like a modern lizard than a modern mammal, and the distinguishing features are relatively fine ones of internal structure. It is currently unknown whether mammal-like reptiles possessed mammalian characteristics like body hair and mammary glands, as the only real evidence is provided by fossils that to date only suggest differences in skeletal structure.

Contents

Classification

See also

References

Classification

Most paleontologists hold fossilized jaw remains to be the distinguishing feature used to classify mammal-like reptiles and reptiles. The jaw transition is a good classification tool as most other fossilized features that make a chronological progression from reptilian to mammalian follow the progression of the jaw transition. The dentary, or lower jaw, consists of a single bone in mammals, where the lower jaw of modern and pre-historic reptiles consists of a conglomeration of smaller bones.
Mammalian jaw structures are also set apart by the dentary-squamosal jaw joint. In this form of jaw joint, the dentary forms a connection with a depression in the squamosal known as the glenoid cavity. In contrast, all other nonmammalian jawed vertebrates, including reptiles, possess a jaw joint in which one of the smaller bones of the lower jaw, the articular, makes a connection with a bone of the skull called the quadrate to form the articular-quadrate jaw joint. In transitional forms between mammals and reptiles, the jaw joint is composed of a large, lower jaw bone (similar to the dentary found in mammals) that does not connect to the squamosal but connects to the quadrate with a receding articular bone.
Over time, mammal-like reptiles, as they became more mammalian and less reptilian, began to develop a secondary palate, separating the mouth and nasal cavity. In early mammal-like reptiles, a secondary palate began to form on the sides of the maxilla, still leaving the mouth and nostril connected.
Eventually, the two sides of the palate began to curve together, forming a U-shape instead of a C-shape. The palate also began to extend back toward the throat, securing the entire mouth and creating a full palatine bone. The maxilla is also closed completely. In fossils of one of the first mammal-like reptiles, Eutheriodont, the beginnings of a palate are clearly visible. The later Thrinaxodon has a full and completely closed palate, forming a clear progression.^[1]

See also

★ Mammal classification

★ Prehistoric mammal

★ Cynodonta

★ Pelycosauria

★ Synapsida

★ Therocephalia

★ Therapsida

★ Dimetrodon

★ Thrinaxodon

References

1. Hopson, James A. "The Mammal-Like Reptiles: A Study of Transitional Fossils." ''The American Biology Teacher'' 49.1 (1987): 16-26