|Tailless tenrec (Tenrec ecaudatus) with litter|
Photo (C) by H. Schütz. From Goodman, Benstead and Schütz
The Natural History of Madagascar
Small litter size is nonetheless typical for mammals and is currently in focus because of a remarkable fossil from the Early Jurassic (Hoffman & Rowe). This comprised a clutch of at least 38 perinates and the presumed mother in the genus Kayentatherium.
The mammals derive from the synapsids and Kayentatherium is on a non-mammalian side branch called tritylodontids. The trees in Prothero (previous post) suggest they diverged from the branch leading to mammals way back in the Triassic. So the finding does not exactly pinpoint when a reduction in litter size started to happen.
The abstract claims that 38 is "well outside the range of litter sizes documented in recent mammals." Except one might add in the tailless tenrec.