Dhouailly 2009 reports: “The synapsid lineage, which separated from the amniote taxa in the Pennsylvanian about 310 million years ago, may have evolved a glandular rather than a scaled integument, with a thin alpha-keratinized layer adorned with alpha-keratinized bumps. Those bumps may have even presented some cysteine-rich alpha-keratins, precursors of the hair-type keratins. In addition, the first synapsids may have developed both a lipid barrier outside the epidermis, similar to current amphibians living in xeric habitats, and some lipid complex with the alpha-keratins of the stratum corneum as in current mammals as a means to strengthen the barrier against water loss of the integument.”
So reptilian scales were never part of the mammal legacy — just naked glandular skin.
A dense coat of fur is found in all basal extant mammals, even those that lay eggs. Thus the origin of hair is to be found in the common ancestor of all living mammals, perhaps among therapsid-grade synapsids (Thrinaxodon – Chiniquodon), or, more conservatively, perhaps right at the origin of early Jurassic mammals.
Dhouailly 2009 reports: “No intermediate form has ever been found between scales and hairs, resulting in only a few proposals of how mammalian hairs may have evolved from scales. These proposals were based on the development of sensory bristles in the hinge scale region of reptiles.” Unfortunately basal reptiles and therapsids did not have scales (see below).
The traditional cynodont whisker hypothsis
Foramina (tiny holes) on the faces of basal gorgonopsians, therocephalians and cynodonts have been interpreted as providing passages for nerves and blood vessels supplying movable skin (subcutaneous muscles) and sensory vibrissae (whiskers). This would represent the first appearance of hair only to be followed by more and more hair spreading around the body. This essentially duplicates the new hypothesis on feather origin by Persons and Currie (2015, see that discussion tomorrow).
Unfortunately for this hypothesis,
the basal lizard, Tupinambis has similar rostral foramina, yet it lacks sensory vibrissae (Bennett and Ruben 1986).
An alternate mammal hair genesis hypothesis
Given that pelycosaurs and Estemmenosuchus were naked and had no hair, the origin of mammal-type hair must have occurred closer to mammals. On their way to evolving into mammals, taxa like Pachygenelus and Megazostrodon became progressively smaller in a rather common process known as phylogenetic miniaturization (the opposite of Cope’s Rule).
Due to their increased surface/volume ratio, smaller animals find it more difficult to internally thermoregulate because their insides are closer to their outsides. Having insulating fur when tiny would be helpful. That’s the traditional hypothesis for mammal hair genesis in tiny taxa, like Megazostrodon. Unfortunately the insulation hypothesis gives no reason for the first appearance of tiny sprigs of precursor hair, not yet plentiful enough to trap air (for insulation). Nor does it take into account that the smallest of all basal mammals, their newborns, are hairless.
Dhouailly 2009 reports: “Hairs [may have] evolved from sebaceous glands, with the hairshaft serving as a wick to draw the product of the gland to the skin surface, strengthening the barrier against water loss.”
The curb-feeler hypothesis
As others have noted, individual hairs provide tactile feedback. Those are especially useful to nocturnal and burrowing animals.
Naked mole rats provide a good analogy. Like therapsids, naked mole rats burrow, adjust their internal temperature to ambient temperatures, AND they have only a few whisker-like hairs that crisscross the body to form a sensitive array that helps them navigate in the dark. We know that certain small cynodonts were also burrowers. That’s where we find them. We don’t know if they had whisker-like hairs that crisscrossed their body. Only the bones are preserved.
In this way,
individual hairs would have been like curb-feelers (Fig. 2), small wires that make a noise whenever a 1950s era automobile approaches a curb. Thus provided, basal mammals could have avoided multiple abrasions while running through their tunnels using their own curb feelers.
if that’s how hair started, once provided with the ability to grow hair, simply growing more hair would have provided incremental opportunities to spend more and more time outside of the burrow. Hair insulated mammals not only from ambient temperature, but from the environment at large, including the approach of winged insects like flies and mosquitoes. Note that those insects that finally developed the ability to burrow past the hair barrier, fleas, lost their wings in order to do so.
learned in dark tunnels could be readily transferred to leaf litter in the open air at night (all the while avoiding the predatory gaze of hungry Jurassic dinosaurs).
The “scaly tail” of mammals,
like the opossum (Fig. 2), is actually, a criss-cross series of epidermal folds interspersed with hairs, not homologous with the scale of any other animal (Dhouailly 2009).
The surprising origin of mammary glands
Dhouailly 2009 reports: “The mammary gland apparently derives from an ancestral sweat or sebaceous gland that was associated with hair follicles, an association which is retained in living monotremes, and transiently in living marsupials. The original function of the mammary gland precursor may not have been feeding the young, but as a means to provide moisture to the eggs.”
Tomorrow: dinosaur feathers.
Bennett AF and Ruben JA 1986. The metabolic thermoregulartory status of therapsids. In The Ecology and Biology of Mammal-like reptiles (Hottom, Roth and Roth eds) 207-218. Smithsonian Institution Press, Washington DC
Chudinov PK 1970. Skin covering of therapsids [in Russian] In: Data on the evolution of terrestrial vertebrates (Flerov ed.) pp.45-50 Moscow: Nauka.
Dhouailly D 2009. A new scenario for the evolutionary origin of hair, feather, and avian scales. Journal of Anatomy 214:587-606.
Persons WC4 and Currie PF 2015. Bristles before down: A new perspective on the functional origin of feathers.Evolution (advance online publication) DOI: 10.1111/evo.12634