The origin of placental mammal life histories

After the end-Cretaceous extinction, placental mammals quickly diversified 1 , occupied key ecological niches 2 , 3 and increased in size 4 , 5 , but this last was not true of other therians 6 . The uniquely extended gestation of placental young 7 may have factored into their success and size increase 8 , but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping 9 , 10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30–75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals. Using palaeohistology and geochemistry, the placental-like life history of a pantodont species 62 million years of age is determined.