Morganucodon and Kuehneotherium are mammals, not stem-mammals

Newham et al. 2019 report,
“Surprisingly long lifespans and low femoral blood flow suggest reptile-like physiology in key Early Jurassic stem-mammals.

“There is uncertainty regarding the timing and fossil 5 species in which mammalian endothermy arose, with few studies of stem-mammals on key aspects of endothermy such as basal or maximum metabolic rates, or placing them in the context of living vertebrate metabolic ranges. Synchrotron X-ray imaging of incremental tooth cementum shows two Early Jurassic stem-mammals, Morganucodon and Kuehneotherium, had lifespans (a basal metabolic rate 10 proxy) considerably longer than comparably sized living mammals, but similar to reptiles, and that Morganucodon had femoral blood flow rates (a maximum metabolic rate proxy) intermediate between living mammals and reptiles. This shows maximum metabolic rates increased evolutionarily before basal rates, and that contrary to previous suggestions of a Triassic origin, Early Jurassic stem-mammals lacked the endothermic metabolism of living mammals.”

That conclusion would be true
if their cladogram was correct. Unfortunatley, it was not.

Figure 1. Subset of the LRT focusing on Basal Mammalia including Creodonta.

Figure 1. Subset of the LRT from 2018 focusing on Basal Mammalia including Morganucodon and Kuehneotherium.

According to
the large reptile tree (LRT, 1579 taxa; subset Fig. 1), Kuehneotherium (Fig. 2) is a basal protothere mammal (= monotreme) in the lineage of echidnas and platypuses. Morganucodon is a very basal metathere mammal (= marsupial). The Virginia opossum, Didelphis, is the most closely related extant taxon in the LRT.

Figure 1. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Figure 2. Brasilodon compared to Kuehneotherium, Akidolestes and Ornithorhynchus, the living platypus.

Here’s a data point of interest:
Newham et al. report, “Only the short-beaked echidna Tachyglossus aculeatus, a monotreme with long lifespan and low metabolic rate, exceeds the Kuehneotherium, but not Morganucodon, distance above the mammalian mean.” And THAT is reflected in the LRT. I also note the platypus, Ornithorhynchus, is not mentioned in the text, only in the citations. Same with Didelphis.

So what does that do to the results?
Seems like the Newham et al. study is suffering from taxon exclusion and an invalid traditional understanding of basal mammal interrelations. Unfortunately Professor MJ Benton is a co-author, infamous for taxon exclusion and guiding his students and any protégé to do the same.

Please tell Elis Newham et al.
to add the platypus and opossum to their study and get back to us! Don’t let this work become another waste of time due to taxon exclusion.

Newham E et al. (19 co-authors) 2019. Reptile-like physiology in Early Jurassic stem-mammals. bioRxiv preprint

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