How would a biologist determine whether a fossil represents a reptile or a mammal?

The correct answer highlights the importance of dental morphology in distinguishing between reptiles and mammals. Reptilian teeth are typically conical and uniform in size, which is well suited to their diet and method of feeding, as these teeth are designed primarily for grasping and holding prey. In contrast, mammalian teeth exhibit a diverse range of shapes and sizes, including incisors for cutting, canines for tearing, and molars for grinding food. This specialization allows mammals to have a more varied diet and to process food more efficiently.

Fossils that display a variety of tooth shapes would indicate mammalian characteristics, while a uniform arrangement would suggest a reptilian origin. This is a critical aspect of paleontology and evolutionary biology, as these dental features are often well-preserved in fossils and can provide key insights into the organism's lifestyle and ecological niche.

The other options, while related to characteristics of mammals or reptiles, are less effective for fossil identification. For instance, the presence of a four-chambered heart or evidence of a placenta is not directly observable in most fossil specimens, while molecules like keratin, primarily related to the integumentary system, are less definitive indicators of overall classification.