Summary, for those in a hurry:
Newham E et al. 2020 attempt to understand metabolic levels (endothermy) for basal mammals and pre-mammals. Unfortunately, the paper suffers from a traditional invalid cladogram in which the monotreme, Sinodelphys (Fig. 1), is portrayed as a marsupial and the marsupials, Morganucodon and Hadrocodium are portrayed as pre-mammals among other issues based on cherry-picking taxa (= taxon exclusion).
From the Newham et al. text:
“To estimate mammaliaform lifespans, we used cementochronology. This well-established technique, which counts annual growth increments in tooth-root cementum, has been used to record lifespans in extant mammals with >70 species aged using this technique. Despite this potential, cementochronology has not previously been attempted for fossil mammals older than the Pleistocene (2.6 Ma).”
“Only the short-beaked echidna Tachyglossus aculeatus, a monotreme with long lifespan and low metabolic rate, exceeds the distance above the mammalian mean for Kuehneotherium, but not for Morganucodon.”
(pre-mammal according to Newham et al., mammal according to LRT) has the following characters, according to Newham et al.:
- “Single replacement of milk teeth, suggests maternal feeding while toothless via mammary gland
- Improved olfaction and tactile sensitivity suggestive of nocturnality
- Mandibular depth suggests determinate growth”
(pre-mammal according to Newham et al., mammal according to LRT)
- “Encephalization quotient equivalent to extant mammals.”
(gliding pre-mammal, Multituberculata, according to Newham et al., gliding mammal (Glires, Carpolestidae) according to LRT).
(marsupial according to Newham et al., basal monotreme according to the LRT).
More from Newham et al. 2020:
“Despite considerable advances in knowledge of the anatomy, ecology and evolution of early mammals, far less is known about their physiology.”
Just the opposite. Newham et al. know more about their physiology than their evolution and interrelationships in the LRT.
“Evidence is contradictory concerning the timing and fossil groups in which mammalian endothermy arose.”
First fix the phylogeny.
“To determine the state of metabolic evolution in two of the earliest stem-mammals, the Early Jurassic Morganucodon and Kuehneotherium, we use separate proxies for basal and maximum metabolic rate. Here we report, using synchrotron X-ray tomographic imaging of incremental tooth cementum, that they had maximum lifespans considerably longer than comparably sized living mammals, but similar to those of reptiles, and so they likely had reptilian-level basal metabolic rates.”
Understood, but probably not the best way to say this, since mammals are reptiles in the LRT. Birds + dinosaurs and pterosaurs + fenestrasaurs likely also had high metabolic rates.
“Measurements of femoral nutrient foramina show Morganucodon had blood flow rates intermediate between living mammals and reptiles, suggesting maximum metabolic rates increased evolutionarily before basal metabolic rates.”
Morganucodon is a basal marsupial. The platypus also has an active mammalian-style lifestyle, but was not tested. Let’s not cherry-pick taxa.
Ornithorhynchus (platypus) metabolism: “Metabolism was 8% less than that of marsupials in general, and 35% lower than that of eutherian mammals.” (Grant and Dawson 1978, not cited in Newham et al. 2020). Note: that’s a bigger jump just between marsupials and placentals.
More from Newham et al. 2020:
“Stem mammals lacked the elevated endothermic metabolism of living mammals, highlighting the mosaic nature of mammalian physiological evolution.”
First fix the phylogeny. Nothing proceeds without a valid cladogram.
Grant TR and Dawson TJ 1978. Temperature Regulation in the Platypus, Ornithorhynchus anatinus: Production and Loss of Metabolic Heat in Air and Water. Physiological Zoology 51(4):1–6. https://www.journals.uchicago.edu/doi/abs/10.1086/physzool.51.4.30160956
Newham E et al. (19 co-authors) 2020. Reptile-like physiology in Early Jurassic stem-mammals. Nature Communications 11, Article number: 5121