Mammal-like reptiles is a term used to describe the prehistoric animals that appear to be the reptilian ancestors of mammals. They were the dominant terrestrial animals by the Middle Permian period. 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. The Cynodonts were the most mammal-like of the Therapsids. Probably the most famous mammal-like reptile is Dimetrodon, which is often wrongly described as a dinosaur. While Dimetrodon is not thought to be a direct ancestor of mammals, it illustrates the differences between the mammal-like reptiles and mammals.
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, as 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.
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, had a secondary palate, separating the mouth and nasal cavity, begin to form. 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.
- Mammal classification
- Prehistoric mammal
- Hopson, James A. "The Mammal-Like Reptiles: A Study of Transitional Fossils." The American Biology Teacher 49.1 (1987): 16-26