Kittycat5
Bluelighter
Here is a paper suggesting why their is little morphological differences between living and extinct monotremes. Read below for the authors conclusions.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2234122/#!po=39.4737
Monotremes have left only a meager fossil record, but what is known at present is consistent with the view that soon after their divergence, in or before the Early Cretaceous, monotremes settled into rates of molecular and morphological evolution and speciation far slower than in the living clades of therian mammals. Even the monotreme metabolic rates and ventilation rates are much slower than in therian mammals of similar body mass, and their body temperature is lower as well (81). In what measure and to what degree these various rates are coupled or were independently evolving phenomena remains to be determined.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2234122/#!po=39.4737
Monotremes have left only a meager fossil record, but what is known at present is consistent with the view that soon after their divergence, in or before the Early Cretaceous, monotremes settled into rates of molecular and morphological evolution and speciation far slower than in the living clades of therian mammals. Even the monotreme metabolic rates and ventilation rates are much slower than in therian mammals of similar body mass, and their body temperature is lower as well (81). In what measure and to what degree these various rates are coupled or were independently evolving phenomena remains to be determined.