The enamel structure, Hunter-Schreger bands (HSB) in particular, of early Tertiary mammals is surveyed and the correlations between HSB occurrence and body size, chewing mechanisms and other factors are discussed. The previously proposed positive correlation between body size and HSB occurrence is generally supported, as well as the stated exceptions; a causal relationship is not assumed. A positive correlation between the occurrence of HSB and divergence from the structure of the primitive tribosphenic molar and insectivorous feeding can also be observed, but exceptions occur in this respect as well. Poorly developed HSB, well developed HSB, and a specialized bending of prisms in advanced periptychids probably evolved independently and directly from radial enamel. Observations indicate convergent evolution of HSB within different families of the archaic ungulates as well as in other mammalian groups, for example, the Pantolesta, Pantodonta, Primates, Rodentia, and Tillodonta. Within Ungulata underlying synapomorphies or developmental constraints may faciliate parallel acquisition of HSB in several families.

It is hypothesized that with several co-occurring changes in dental morphology, body size, overall ecology, and chewing pattern in eutherian mammals of the early Tertiary different combinations of these factors in different lineages resulted in the evolution of HSB. Trough biomechanic constraints, either one factor or a combination of factors could have caused the alteration of the enamel structure to HSB or specialized bending.