Mammal-like features:
* Dentition: Therapsids developed more specialized teeth with different shapes and functions (incisors, canines, premolars, molars) – a key adaptation for a more varied diet. This contrasts with the simple, conical teeth of most reptiles.
* Jaw structure: The jaw joint in therapsids started to shift from the quadrate bone (reptile-like) to the dentary (mammal-like) bone, which allowed for a stronger bite and greater chewing efficiency.
* Skulls: Therapsids had a more complex skull structure with a larger braincase and a more forward-facing position of the eyes, suggesting better sensory capabilities and possibly a more active lifestyle.
* Posture: Some advanced therapsids showed a more upright posture with limbs positioned beneath the body, similar to mammals, as opposed to the sprawling posture of most reptiles.
* Respiration: There is evidence that some therapsids had a diaphragm, a muscle essential for efficient breathing in mammals.
* Fur: Though fossil evidence is limited, some advanced therapsids likely had fur, a key adaptation for thermoregulation and insulation, especially in colder environments.
* Metabolic rate: Therapsids likely had a higher metabolic rate than reptiles, suggesting a warmer, more active lifestyle.
Reptile-like features:
* Laying eggs: Despite their mammal-like features, therapsids likely laid eggs, unlike most modern mammals.
* Absence of mammary glands: Therapsids did not have mammary glands, which are essential for nourishing young in mammals.
Transitional features:
* Evolution of the middle ear: In therapsids, the jaw joint started to shift from the quadrate-articular joint to the dentary-squamosal joint. This allowed for a stronger bite and freed up the quadrate and articular bones to evolve into the malleus and stapes of the mammalian middle ear, which is responsible for hearing.
* Evolution of the palate: Therapsids developed a secondary palate, a bony roof in the mouth separating the nasal passage from the mouth cavity. This allowed for breathing while chewing, an important adaptation for a more active lifestyle.
Overall:
While therapsids possessed features found in both reptiles and mammals, they showed a clear evolutionary trend towards a more mammalian form. These changes in their skull, teeth, jaw, posture, and other features suggest a gradual shift towards a more efficient and active lifestyle, laying the groundwork for the evolution of mammals.