Another short one today
as taxa nesting at the transition from marsupial to placental are shown together, perhaps for the first time, to scale (Fig 1). Currently late-surviving Early Eocene Oodectes is the last common ancestor of all placentals in the large reptile tree (LRT, 2320 taxa).
Placentals go back to the Early Jurassic based on coeval placental taxa.
Figure 1. The marsupials closest to placentals in the LRT along with the placentals closest to marsupials, to scale. Oodectes is currently the last common ancestor of all placentals. Only the LRT recovers this. Note the number of molars in the three basal placentals.
Oodectes was nested
as the last common ancestor of all placentals back in 2022.
Figure 2. Monodelphis sorex full scale @ 72dpi. Hello, great grandpa!
Deserving extra attention today: Monodelphis sorex
The scientific name Monodelphis is from the Greek meaning “single womb” sorex is Latin meaning “shrew”. Most marsupials have a dual uterus distinct from placentals and not scored in the LRT. Females lack a pouch and possess more mammae than any other mammal – 25-27, arranged with five central nipples and the remainder arranged in lateral lines. Lacking a pouch is relevant here, as placentals also lack a pouch. Newborns never have to compete for nipples. There are four for each of the 6-8 youngsters born to each birth sequence. The species is terrestrial and crepuscular = twilight active.
“Some species in Monodelphis are apparently semelparous, with few individuals living past their first reproduction. Others may produce up to four litters per year.” (Nowak, 1997). “Because the pouch of females is not well developed in this genus, it is likely that the young must cling to the nipple until they are large enough to ride on the back of their mother.”
References
Nowak R 1997. Monodelphis: Short-tailed opossums. Walker’s Mammals of the World, On-Line at marsupialia.didelphidae.monodelphis
wiki/Southern_red-sided_opossum – Monodelphis sorex
wiki/Placentalia
wiki/Marsupial
The Pterosaur Heresies 