Triassic and Early Jurassic mammal metabolism

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).

Figure 3. Skull and forelimbs of Sinodelphys in situ. Arrow shows the displacement of the entire hand that otherwise appears to be lost beyond the matrix. How fortuitous!

Figure 3. Skull and forelimbs of Sinodelphys in situ. Arrow shows the displacement of the entire hand that otherwise appears to be lost beyond the matrix. How fortuitous!

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.” 

Kuehneotherium is a monotreme in the LRT. Morganucodon is a marsupial in the LRT. The keywords: “Ornithorhynchus” and “platypus” are not found in the text.

Morganucodon
(pre-mammal according to Newham et al., mammal according to LRT) has the following characters, according to Newham et al.:

  1. “Single replacement of milk teeth, suggests maternal feeding while toothless via mammary gland
  2. Improved olfaction and tactile sensitivity suggestive of nocturnality
  3. Mandibular depth suggests determinate growth”

Hadrocodium
(pre-mammal according to Newham et al., mammal according to LRT)

  1. “Encephalization quotient equivalent to extant mammals.”

Arboroharamiya
(gliding pre-mammal, Multituberculata,  according to Newham et al., gliding mammal (Glires, Carpolestidae) according to LRT).

Castorocauda
(proximal mammal outgroup found with full fur pelage according to Newham et al., but a sister to the much more basal cynodonts, Probainognathus and Chiniquodon in the LRT).

Sinodelphys
(marsupial according to Newham et al., basal monotreme according to the LRT).

Figure 4. Subset of the LRT cladogram of basal Mammalia. Note the traditional clade Metatheria is a grade with new names proposed here.

Figure 4. Subset of the LRT cladogram of basal Mammalia. Note the traditional clade Metatheria is a grade with new names proposed here.

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.

Figure 1. Subset of the LRT focusing on basal placentals, including multituberculates.

Figure 3. Subset of the LRT focusing on basal placentals, including multituberculates.

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.


References
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
https://www.nature.com/articles/s41467-020-18898-4

News
https://www.bristol.ac.uk/news/2020/october/ancient-tiny-teeth.html

https://phys.org/news/2020-10-ancient-tiny-teeth-reveal-mammals.html

https://www.sciencetimes.com/articles/27672/20201012/200-million-year-old-first-mammals-lived-reptiles.htm

Large vs. tiny vascular channels in tetrapods

Huttenlocker and Farmer 2016
have tied red blood cell (RBC) size to vascular diameter in the bones of fossilized tetrapods in part to determine when greater aerobic capacity evolved… something we actually already know based on stance, extant homologs, etc. Their conclusions support tradition and are broadly applicable (Fig. 1). This appears to be a lot of work with a large resulting dataset.

The Huttenlocker and Farmer abstract reports
“We find that several fossilizable aspects of bone microstructure, including the sizes of vascular and lacunar (cellular) spaces, provide useful indicators of RBC size in tetrapods.”

Figure 1. from Huttenlocker and Farmer 2016, white while labels added.

Figure 1. from Huttenlocker and Farmer 2016, white while labels added. The family tree shown here is not recovered in the LRT.

The Huttenlocker and Farmer graphic
show that turtles are about like amphibians in vascular and RBC size. (Fig. 1). It also indicates that, compared to extant mammals, cynodonts in the earliest Triassic sometimes had smaller vascular channels and RBCs, sometimes had similar RBCs and sometimes had larger RBCs. However, the chart also indicates that the pigeon has larger RBCs than a duck. Alligators and crocodiles are different from one another and two congeneric lizards are different from one another in RBC size, but not by much.

Results:
The shrew and only one of the rabbits appear to have the smallest RBCs, based on the figure 1 graphic. Based on the figure 2 graphic, the pigeon has more of the smallest vascular diameters.

Figure 2. Chart from Huttenlocker and Farmer detailing the vascular sizes recovered from their observations. Red lines and type were added by me. Gray area was also added by me.

Figure 2. Above: Vascular channels in an amphibian (left) and mammal (right). Below: Chart from Huttenlocker and Farmer detailing the vascular sizes recovered from their observations. Red lines and type were added by me. Gray area was also added by me. Red = mammals. Violet = birds. Blue = reptiles. Green = amphibians. 

A few questions arise:

  1. Bone cross-sections in some warm-blooded mammals are similar to some cold-blooded reptiles if the largest channels are not considered. Does this mean anything?
  2. White blood cells are often much larger than red blood cells. How does that work in the smaller channels?
  3. How does ontogeny, especially degree of development at birth, affect RBC size and metabolism? Perhaps that’s the next step in this study.
  4. How does body size alone correlate with RBC size?
  5. How does relative level of activity alone correlate with RBC size? Perhaps adding sloths and hummingbirds would be appropriate. Perhaps sedentary vs, athletic humans, too.
  6. Do active (warm-blooded) fish and sea turtles have smaller RBC sizes than passive fish and desert turtles?

Interesting study.

References
Huttenlocker AK and Farmer CG 2016. Bone microvaculature tracks red blood cells zip diminution in Triassic mammal and dinosaur forerunners. Current Biology online here.