Complete Primate Skeleton from the Middle Eocene of Messel in Germany: Morphology and Paleobiology
Stage of tooth eruption indicates that when it died Darwinius was about 9–10 months old (gray bar) on a Saimiri developmental scale. Growth as a proportion of 36-month adult size is plotted by age in months, with developmental events added for the interval from birth to 36 months along the bottom. Saimiri is born with deciduous incisors erupted and quickly completes the deciduous dentition. Note that adult brain weight is achieved rapidly in development; adult foot length, head and body length, and femur length are achieved more slowly; and adult body weight is achieved very slowly. Using Saimiri as a model, we estimate that Darwinius died at about 60% of its projected adult body weight, 80–85% of its projected adult body and limb length, and 98% of its projected adult brain weight. By this model, Darwinius is projected to add 66% (1/0.60) to its weight, 20% to its body and limb length (averaging 1/0.80 and 1/0.85), and 2% to its brain weight (1/0.98). Sources: tooth development, weight, head-body, and foot lengths are from the same captive growth study ; other studies give brain weight , femur length , and long-bone maturation .