A new method of calculating the MNI and a full lifespan mortality profile in assemblages of semi-hypsodont rodents is proposed. Fossil jaws of the Paleogene theridomyid genera Isoptychus, Theridomys? and Pseudoltinomys show similar patterns of dental replacement, eruption and wear for all three genera. Deciduous premolars on the point of being replaced by their permanent successors coexist in jaws with erupting, unworn, unrooted third molars. The minimum number of individuals (MNI) in a theridomyid assemblage, of which the local origin is demonstrated, can therefore be calculated using the sum of deciduous premolars plus the most abundant of the permanent premolars or third molars. The teeth used to estimate the MNI of a species can also be used for the construction of its mortality profile. The ratio of an age-dependent crown height measurement to an age-independent crown width measurement is used as an age proxy for the establishment of 'age groups'. Wear patterns correspond well to age groups and, thus, broken unmeasurable specimens need not be excluded, as their wear stage can be used to assign them to 'age groups'.
Using these methods, the MNI and mortality profiles of one Isoptychus sp. and two Thalerimys fordi assemblages from the Late Eocene Solent Group (Hampshire Basin, Isle of Wight, southern England) were reconstructed. The mortality is attritional, showing a characteristic 'U-shape' in the distribution of the individuals in 'age groups'. The members of the three species, therefore, died of biological natural causes and not by a catastrophic event. This method can be applied to fossil semi-hypsodont micromammalian species, provided dental ontogeny is known. The method enables the construction of mortality profiles for the complete age range and, consequently, allows the analysis of the accumulation mechanisms of assemblages of semi-hypsodont rodents, with deciduous and permanent premolars. It can readily be applied to assemblages consisting only of isolated teeth.