A review of monotreme (Monotremata) evolution

Abstract

Advances in dating and systematics have prompted a revision of
monotreme evolution to refine the timing of adaptative trends
affecting body size and craniodental morphology. The oldest known
monotreme, Teinolophos trusleri, is restricted to uppermost Barremian
deposits of the Strzelecki Group in Victoria, Australia. Its body mass
is estimated at ~40 g, making it the smallest known monotreme.
Teinolophos trusleri likely possessed an electro-sensitive and/or
mechano-sensitive ‘bill’ or ‘beak’, which we suggest evolved for
insectivory in seasonally dark Early Cretaceous polar forests. During
the early Albian–mid-Cenomanian, monotremes diversified in Australia
and evolved body masses greater than 4 kg, becoming amongst the
largest Mesozoic mammals. A gap of 35 million years subsequently
separates the youngest Mesozoic monotremes from the oldest Cenozoic
monotreme, Monotrematum sudamericanum, which is a Paleocene stem
ornithorhynchid from southern South America. We also hypothesize that
tachyglossids originated in Melanesia, perhaps on the emergent
Vogelkop landmass, and then dispersed to Australia during the
Pliocene-Pleistocene. Finally, we present a classification of
Monotremata to include five families—Teinolophidae fam. nov.,
Kollikodontidae, Steropodontidae, Ornithorhynchidae, and
Tachyglossidae. We also propose a new genus, Murrayglossus gen. nov.
for a gigantic Pleistocene echidna from southwestern Western
Australia.

Open access:
https://www.tandfonline.com/doi/full/10.1080/03115518.2022.2025900