Histological and life history data for small-bodied mammals from: Multituberculate mammals show evidence of a life history strategy similar to that of placentals, not marsupials

The remarkable evolutionary success of placental mammals has been partly
attributed to their reproductive strategy of prolonged gestation and
birthing of relatively precocial, quickly weaned neonates. Although this
strategy was conventionally considered derived relative to that of
marsupials with highly altricial neonates and long lactation periods,
mounting evidence has challenged this view. Until now, the fossil record
has been relatively silent on this debate, but here we find that
proportions of different bone tissue microstructures in the femoral
cortices of small extant marsupials and placentals correlate with length
of lactation period, allowing us to apply this histological correlate of
reproductive strategies to Late Cretaceous and Paleocene members of
Multituberculata, an extinct mammalian clade that is phylogenetically
stemward of Theria. Multituberculate bone histology closely resembles that
of placentals, suggesting that they had similar life history strategies.
That a stem-therian clade exhibits evidence of placental-like life
histories supports the hypothesis that intense maternal-fetal contact
characteristic of placentals is ancestral for therians. Alternatively,
multituberculates and placentals may have independently evolved prolonged
gestation and abbreviated lactation periods. Our results challenge the
hypothesis that the rise of placental mammals was driven by unique life
history innovations, and shed new light on early mammalian
diversification.