Synapsida: From slow to fast

Amniote diversity:

Note: In HONR219D, we use "reptile" strictly in a phylogenetic sense. Maybe the ecologies of many fossil synapsids were distinctly "reptilian," but we don't call them reptiles to avoid confusion and especially avoid the old-fashioned term "mammal-like reptile."

Pantelosaurus by D. Bogdanov from Wikipedia
Synapsida: All organisms more closely related to Mammalia than to Sauropsida.

The interval from the Late Carboniferous to Early Triassic saw synapsids dominating the world's land faunas. A great diversity of synapsids lived, and inaugurated three major trends in synapsid evol.

Some Paleozoic synapsids, briefly noted:

By the Middle Jurassic, any synapsid you saw would be difficult to distinguish from a mammal, and many were proper early mammals. We take them up next.

Pair of Diictodon (small dicynodonts) who died in their burrow Show Me South Africa

But first, if time permits, a detour: What are the biological implications of the snuggling dicynodonts?

Constraining the history of endothermy; Endothermy (warm-bloodedness) is considered a characteristic feature of mammals. What exactly is it and from how did it arise?

Fowler's toad - a bradymetabolic poikilothermic ectotherm
When we say that an animal is cold-blooded, we mean:

Because of how these characteristics are distributed among living vertebrates, we think of tachymetabolic and homeothermic as going together, but that is an oversimplification. How is homeothermy achieved?

Of course this begs the question: Where does the ability quickly to metabolize glucose come from?

Red-browed finch - a tachymetabolic homeothermic endotherm
When we say a creature is warm-blooded, we mean:

Thus, we are really talking about three separate capabilities:

Each of these concepts has its own set of terms:

Fundamentally, these abilities are determined by separate and distinct functions of the body, however there are some connections between them. As a result we are used to - bradymetabolic poikilothermic ectotherms (right above) and tachymetabolic homeothermic endotherm (right), but there do exist:

Hoverfly from Wikipedia
Energy pathways: There are two general ways in which a body cell can convert glucose into useful energy: Vertebrates "prefer" to respire aerobically, but can add on anaerobic respiration during intense activity. Lactic acid builds to where the exercise must cease, then aerobic metabolism is used to clean up the lactic acid - i.e. the creature pays off its oxygen debt.

Resting metabolic rate: The base rate at which an organism metabolizes glucose while at rest. In endothermic homeotherms like mammals, this can be roughly ten times what it is in ectothermic creatures.

Aerobic scope: The factor to which an organism can elevate its activity levels over the resting state without slipping into anaerobic respiration. For most mammals, that is 10 - 20. The aerobic champs are insects like hoverflies, some of which have aerobic scopes approaching 300.

How is this achieved. The body cells of animals are not that different in their ability to metabolize. Where animals differ is in the arrangement of their plumbing. For an animal to maintain a high metabolic rate, its cells must be well-supplied with:

Glucose plumbing: To make more glucose available to its cells, an animal must:

Oxygen plumbing: To make more oxygen available to its cells, an animal must:

The connection to temperature: Metabolic reactions are mediated by protein catalysts that are sensitive to temperature:

Thus, whenever a creature requires rapid metabolism, it also needs to keep its body within a narrow temperature range. This is easy for sea creatures, but a real challenge for land animals that inhabit environments with variable temperature ranges.

Eastern painted turtle soaking up the sun
Where to get the heat?

Duck-billed platypus cooling off
How to get rid of excess heat?

Thus, adaptations for maintaining constant body temperature and adaptations for rapid metabolism tend to be mutually reinforcing.

Animals that have mastered that challenge of dumping excess heat may evolve integumentary systems like fur or feathers to limit the loss of metabolic heat through the skin.

Somewhere between the first synapsid and the first mammal, the transition from ectothermy to endothermy occurred. Some speculation:

    Diadectes by D. Bogdanov from Wikipedia
  1. Dry skin: Frogs and salamanders have little ability to elevate their body temperatures above ambient levels, even by ectothermic means, because of their need to keep their skin moist to facilitate cutaneous breathing. This subjects them to evaporative cooling any time they come into open air. In contrast, even "cold blooded" amniotes can maintain an elevated body temperature while active because their skin is dry and impermeable. Achieving this was the first step in terrestrial thermoregulation. When did it occur? Once indication: ectothermic herbivores must maintain high body temperatures to facilitate digestion. The presence of large herbivorous diadectomorphs and basal synapsids suggests that the last common ancestor of diadectomorphs and amniotes had progressed that far.

  2. Cooling fins: Larger animals with lower surface area/volume ratios heat up and cool down too slowly. To accelerate morning activity and remain active at midday, heat exchange surfaces evolved, including the "sails" of sphenacodontines like Dimetrodon. (Of course, this structure could be used for display purposes, also.) But note: The presence of the sail indicates that Dimetrodon lacked the means to either heat or cool itself internally.

    Biarmosuchus by D. Bogdanov from Wikipedia
  3. Posture: For Dimetrodon, there was little difference between standing tall and sprawling flat. They didn't have to invest much energy in "standing up" before taking a step. Therapsids were different. With relatively short torsos, longer legs, and postures that brought the feet close to being underneath the body when they stood up, These were creatures that had to make a significant investment of energy in "standing up" before walking. One expects that, having made that investment, they would engage in longer bouts of walking around than their basal synapsid ancestors. That requires the energy we associate with tachymetabolism. How elevated was their body temperature when they were active or at rest? Good question.

    Thrinaxodon by Kemp from Palaeos
  4. Ventilation: When did the muscular diaphragm appear? One indication is the separation of the torso into distinct thoracic and abdominal regions. We begin to see the differentiation of dorsal vertebrae and ribs reflecting this at the base of Cynodontia. What about evaporative cooling mechanisms? Only mammals have fully ossified nasal turbinates, however non-mammalian cynodonts and a few of their close relatives have ridges in their nasal cavities that seem to mark the attachment points for cartilagenous turbinates.

    Brasilitherium by Smokeybjb from Wikipedia
  5. Fur: Alas, we depend on rare fossil occurrences. We know this much:
    • Sail-back sphenacodontines would absolutely not have had it.
    • Cynodonts similar to Morganucodon very close to the ancestry of mammals definitely had proper fur. (E.G.: Castorocauda fossil and reconstruction.)
    Anything more is reckless speculation. Did fur originate as an insulating body coat, or did it first appear as whiskers in burrowing therapsids?

    The overall picture is that non-therapsid synapsids were ectothermic in the style of lizards. A general increase in metabolic energy seems to have been achieved among therapsids, enabling more prolonged activity. Outward signs of ectothermic heat exchange with the environment are absent. Indications of advanced respiratory features are associated with cynodonts. Mammalian-style endothermy had been achieved before the origin of proper Mammalia.