Platypus evolution
The oldest known member of the monotremes is Teinolophus trusleri and it was discovered in Victoria, Australia and described in 1992 from a very small partial jaw (Rich et al. 2013). It is approximately 123 million years old.
Below is a high resolution X-ray computed tomography and morphology of this specimen indicates that Teinolophos lies within the crown clade Monotremata and is a basal platypus (Rowe et al 2008).
References
Rich et al. (2013). Alcheringa 4(40): 475-501.
T. Rowe, T.H. Rich, P. Vickers-Rich, M. Springer and M.O. Woodburne (2008). Proceedings of the National Academy of Sciences 105:1238-1242.
This image is from the specimen of a left dentary, was collected from the Wonthaggi Formation, Bunarong Marine Park, Victoria, Australia. It was made available to The University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Thomas Rich of Museum Victoria. Funding for scanning and image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin. Copyright from Dr. Timothy Rowe, Dr. Thomas H. Rich, Dr. Patricia Vickers-Rich, Dr. Mark Springer, and Dr. Michael O. Woodburne, 2008, "Teinolophos trusleri" (On-line), Digital Morphology. Accessed September 21, 2021 at http://digimorph.org/specimens/Teinolophos_trusleri/216575/.