How elephants grow: heterochrony and the calibration of developmental stages in some living and fossil species

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Abstract
I present 1) baseline information on ontogeny in Recent elephants, and 2) some preliminary comparisons with fossil specimens. Relationships between size, state of skeletal fusion, and dental stage are similar in the two Recent species, Loxodonta africana and Elephas maximus. Limb bones that complete epiphyseal fusion late in ontogeny also grow more in absolute terms. Growth and dental progression are linearly related and probably gradual with respect to time, whereas skeletal fusion appears to be concentrated within a short period. Females are small and advanced in skeletal fusion for their dental stages. Limb proportions in elephants change with increasing size: the scapula becomes longer relative to other bones, and the femur becomes longer relative to the tibia. Among L. africana, E. maximus, and Mammuthus columbi, animals similar in size have similar limb proportions; the largest individuals, of L. africana and M. columbi, have exaggerated proportions and are therefore peramorphic. One might therefore expect insular dwarf species to be paedomorphic, but they are not. Although the scapula/humerus ratio for one mature specimen of Mammuthus exilis does not differ significantly from those of young modem elephants of a similar size, all ratios involving the tibia of another M. exilis and one Elephas falconeri exceed those for any Recent animals. If neonatal size is small, characteristically adult body proportions can appear in small individuals. Both genetic and nutritional factors may contribute to size reduction and enhance variability in the dwarfs.