| One feature of mammals that distinguishes them from most other toothed vertebrates is reduced tooth replacement. It is generally thought that a single replacement of the antemolars and no replacement of the molars (i.e. diphyodonty) is characteristic of mammals. The present work seeks to delineate the evolutionary history of mammalian tooth replacement. A review of the literature on Mesozoic mammals found that the most primitive well-known member of the mammalian clade (Sinoconodon) is polyphyodont and that diphyodonty may have first appeared in the somewhat more derived form, Morganucodon. Definitive evidence of diphyodonty does appear in a variety of Mesozoic groups and the primitive replacement patterns of marsupial and placental mammals seem to have evolved in the Cretaceous. Many have argued that the reduction of replacement in the mammalian lineage is related to changes in a package of other aspects of the biology of early mammals, including lactation and rapid growth to a fixed adult size. Marsupials have a distinctive pattern of further reduced replacement in which (at most) only the hindmost premolars are replaced. This pattern has been related to marsupial mode of reproduction, specifically to suppression of odontogenesis during the period when young are attached to the nipple. The relationship of suckling and tooth replacement was investigated through behavioral observations on the small South American opossum, Monodelphis domestica. Observations of mothers and young, examination of the histological development of the dentition and determination of ages of tooth eruption allowed the timing of events in suckling and dental development to be determined. Dental development, in general, is not suppressed during the period of suckling. A survey of the literature showed that reduction of functional replacement, similar to that in marsupials, is common in a wide variety of placental mammals. These findings cast doubt on the use of tooth replacement pattern to reconstruct reproductive pattern in fossil mammals. To date there appears to be no general explanation for the loss of diphyodonty in mammals. Lastly, a method for relating somatic growth with growth of the functional dentition is presented. |
