Human and animal bones are distinguished by gross skeletal anatomy, bone microstructure and bone macrostructure. Some human and animal bones are quite similar, making it difficult to identify isolated and fragmentary bones in the lab and the field. The three levels of classification help make distinctions among the most similar animal and human bones.
Humans and animals have anatomical differences in skeletal composition, which distinguishes bones throughout all parts of their bodies. Cranial morphology, for instance, differs between humans and animals because of the large brain size that humans have compared to their bodies. Humans generally have smaller faces compared to their cranial vaults; the opposite is true for most animals, with the exception of some primates. Human crania are also oriented on a vertical axis; the layout is horizontal for most animals.
Humans and animals also have different teeth layouts and mouth structures. This difference, called dentition, accounts for differences in human and animal dental formulas and tooth size and shape. For example, humans have small canine teeth and low molars, while animals have small or nonexistent canine teeth; these teeth are sharp and jagged in carnivores, and rounded or flat in herbivores.
Postcranial human bones of the upper limbs are less robust than those of animals. Humans have separate radial and ulna bones, while those two bones are fused in most animals.