What Is a Therapsid? The Animals Halfway to Being Mammals
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Between the sprawling sail-backed reptile-like animals of the early Permian and the first true mammals sits a group that acquired mammalian features one at a time over roughly eighty million years. The fossil record of that acquisition is unusually complete and it makes an abstract process concrete.
What changed and in what order
The transition involved a long list of changes and they did not happen together, which is the most instructive thing about it. Limb posture shifted from sprawling towards the sides to being held beneath the body, improving efficiency of movement and allowing breathing and running simultaneously. Teeth differentiated into incisors, canines and cheek teeth with distinct functions, replacing rows of similar pegs. A secondary palate developed, separating the nasal passage from the mouth so that an animal could breathe while chewing, which is a prerequisite for prolonged food processing. Jaw musculature enlarged and the dentary bone expanded to dominate the lower jaw while the other jaw bones shrank. Ribs became restricted to the chest, implying a diaphragm and therefore a different breathing mechanism. Turbinate bones in the nose, whose traces appear in some specimens, imply warming and moistening of inhaled air and therefore an elevated metabolic rate. Each of those appears at a different point in the sequence.
The evidence for hair and warmth
Soft tissue does not fossilise readily and several lines of evidence bear on when these animals became warm-blooded and furry:
- •Foramina in the snout of several cynodonts, which are openings for nerves and blood vessels of the kind that supply whiskers in living mammals
- •Fossilised burrows containing therapsid remains, since burrowing implies behaviour and physiology suited to an insulated environment
- •Bone microstructure showing rapid sustained growth of the kind associated with high metabolic rates rather than the interrupted growth of most reptiles
- •Ridges in the nasal cavity supporting turbinates, which conserve heat and moisture and are found only in animals maintaining a high body temperature
- •Coprolites containing hair, which have been reported from the late Permian and would place hair before the appearance of mammals
- •Isotopic evidence from teeth bearing on body temperature, which is technically demanding and has produced results consistent with elevated metabolism in some groups
The groups within it
The assemblage is diverse and several branches are worth distinguishing. Dinocephalians were large-bodied early forms, some with thickened skulls suggesting head-to-head contests. Anomodonts, including the dicynodonts, were enormously successful herbivores with a beak and a pair of tusks, ranging from burrow-dwelling animals to species the size of large cattle, and they were among the most abundant land vertebrates for a long period. Gorgonopsians were sabre-toothed predators of the late Permian and among the first vertebrates to develop that tooth arrangement, which subsequently evolved independently several more times. Therocephalians occupied various predatory roles. Cynodonts, appearing late in the Permian, are the group containing mammals and show the transitional features most completely, including small burrowing forms that survived the end-Permian extinction and diversified in the Triassic. That survival is the point at which the whole subsequent history of mammals hangs on a small number of unremarkable animals.
Why the record is so good
The transition is documented better than most because of a combination of circumstances. The relevant rocks in the Karoo Basin of South Africa are extensive, continuously deposited across the critical interval, and have been collected intensively for over a century, producing an enormous sample with reasonable stratigraphic control. Comparable sequences in Russia and elsewhere supplement it. The changes involved are skeletal and therefore fossilise. And the sequence has been tested repeatedly as new specimens have been described, which has revised details without overturning the overall picture. That makes it a standard example when the completeness of the fossil record is questioned, since the objection that transitional forms are missing is hard to sustain against a sequence in which the jaw joint can be watched moving bone by bone across dozens of genera. The remaining gaps concern soft tissue, behaviour and the timing of physiological changes, which is where the evidence is indirect and the arguments continue.
The takeaway
Mammalian features appeared one at a time across roughly eighty million years rather than together, with upright limbs, differentiated teeth, a secondary palate and an expanded dentary bone all arriving at different points. Whisker foramina, burrows, rapid bone growth and nasal turbinate ridges bear on when warmth and hair appeared. The Karoo Basin sequence documents the whole transition well enough to watch a jaw joint move bone by bone.