1. Jaw Structure:
* Lower jaw: Mammals have a single bone in their lower jaw called the dentary. This is a defining feature that sets them apart from other vertebrates like reptiles, which have multiple bones in their lower jaw.
* Attachment: The dentary in mammals connects directly to the skull via a joint called the squamosal-dentary articulation.
* Teeth: Mammals have specialized teeth, including incisors, canines, premolars, and molars, with varying shapes and functions. These teeth patterns are often distinctive within specific mammal groups.
2. Middle Ear Bones:
* Unique Structure: Mammals possess three middle ear bones (malleus, incus, and stapes) that are not present in other vertebrates. These bones evolved from bones in the reptile jaw and are crucial for hearing.
* Location: These bones are embedded within the skull and are easily identifiable in fossils.
3. Skull Shape:
* Braincase: Mammals have a relatively large braincase compared to other vertebrates, which houses their complex brains.
* Openings: The skull of mammals has distinct openings (fenestrae) that are associated with the jaw muscles and other features, such as the temporal fenestrae.
4. Other Skeletal Features:
* Limbs: Mammalian limbs typically have a distinctive five-fingered (pentadactyl) pattern.
* Posture: Most mammals have limbs positioned underneath their bodies, allowing for efficient walking and running.
* Fur: While hair (fur) is a soft tissue that doesn't fossilize easily, sometimes imprints or traces of fur can be preserved in sedimentary rocks.
5. Other Clues:
* Footprints: Mammalian footprints are distinct from other vertebrates and can provide information about their gait, size, and behavior.
* Coprolites (fossilized dung): The shape, size, and contents of fossilized dung can sometimes reveal the diet and morphology of the mammal that produced it.
Overall, a combination of these skeletal characteristics helps paleontologists identify and classify mammal fossils, even when they are incomplete or fragmentary.
It's important to note that some early mammals might have possessed features that were more reptilian-like, making their identification more challenging. However, the evolution of these distinctive traits firmly established the lineage of mammals in the fossil record.