|Kangaroo Joey inside the pouch|
Photo by Geoff Shaw, Zoology, Melbourne, Australia CC BY-SA 3.0)
They sampled data from the literature and examined specimens in the Hill and Hubrecht Collections at Museum für Naturkunde in Berlin. (In published sources they found examination of photographs and drawings more reliable than verbal descriptions.) There is a huge data set in the Supporting Information.
|Altricial and precocial neonates exemplified by the|
mouse (above) and guinea pig
A reconstruction was attempted of the neonate of the therian ancestor. This suggested it was anatomically more like extant placentals than extant marsupials. In contrast, the preweaning period was very long compared to intrauterine gestation and thus infancy in this ancestor was more marsupial-like. The relative timing of eye opening was between that of placental and marsupial mammals. Gestation lasted 31 days in this hypothetical ancestor. It was reduced to 21 days in the marsupial lineage while increasing fourfold in the placental lineage.