Did monotremes begin in water?

Hand et al 2024 studied a Cretaceous platypus humerus
(Fig 1) and determined that it was an aquatic taxon.

That makes sense.

According to publicity in Discover Magazine:
“Scientists may have gotten the evolutionary origins of some of the strangest animals on the planet backward. They’ve long thought that monotremes — egg-laying mammals that include the platypus and hedgehog-like creatures known as echidnas — originated on land. The platypus took the occasional aquatic foray, making it semi-aquatic, while echidnas stayed out of the water, the thinking went.

But new analysis of a bone found 30 years ago may mean that theory requires a revision. A single humerus, originally discovered at Dinosaur Cove in southeastern Australia, indicates that modern monotremes arose from water-dwelling ancestors, according to a report in the journal PNAS.”

The problem in Hand et al 2024 is taxon exclusion.
The Hand et al cladogram (Fig 1) mixed up monotremes with placentals  with marsupials and pre-mammals. The problem is they borrowed a cladogram from Huttenlocker et al 2013, which we looked at earlier here.

Huttenlocker et al were describing the extinct echidna Cifelliodon, which they nested with the stylinodontid placental Vintana and those two with multituberculates.

Figure 1. Cladogram from Hand et al 2024. Colors added here. Yellow = placentalia2 taxa. Aqua = monotremes. Pink = marsupials. White = pre-mammals in the LRT. ” data-image-caption=”

Figure 1. Cladogram from Hand et al 2024. Colors added here. Yellow = placentalia2 taxa. Aqua = monotremes. Pink = marsupials. White = pre-mammals in the LRT.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg?w=216″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg?w=584″ class=”size-full wp-image-92642″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg” alt=”Figure 1. Cladogram from Hand et al 2024. Colors added here. Yellow = placentalia2 taxa. Aqua = monotremes. Pink = marsupials. White = pre-mammals in the LRT. ” width=”584″ height=”812″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg?w=584&h=812 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg?w=108&h=150 108w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg?w=216&h=300 216w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/05/hand.monotreme.cladogram588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />

Figure 1. Cladogram from Hand et al 2024. Colors added here. Yellow = placentalia2 taxa. Aqua = monotremes. Pink = marsupials. White = pre-mammals in the LRT.

Oddly
Cifeillidon did not make it into the Hand et al taxon list borrowed from Huttenlocker et al and it is not mentioned in the text, even though it is a monotreme in the LRT.

The problem in Hand et al 2024 is taxon exclusion. They don’t realize that Eutheria = placentals evolved 3x in Mammalia, and that multituberculates are one of those times, which they would learn by adding taxa.

References
Hand SI et al (more than 10 co-authors) 2024. Bone microstructure supports a Mesozoic origin for a semiaquatic burrowing lifestyle in monotremes (Mammalia). 122 (19) e2413569122 https://doi.org/10.1073/pnas.2413569122

Huttenlocker AK et al (4 co-authors 2018)., Late-surviving stem mammal links the lowermost Cretaceous of North America and Gondwana. Nature 558, 109–112.

Publicity
https://www.discovermagazine.com/the-sciences/evolutionary-origins-of-the-strange-platypus-and-echidna-found-at-dinosaur

Cifelliodon: new echidna ancestor from the Early Cretaceous of Utah