The first solid evidence for endothermy in Triassic synapsids came from detailed studies of therocephalians and cynodonts. Watson (1913), in describing the skull and endocranium of the Middle Triassic cynodont Diademodon, noted its mammalian-type brain, ethmoturbinals in the nasal cavity, and concluded (p. 228): “Many of the differences between a mammal and a reptile – the soft skin, the increased body-temperature, the hair and all it implies, the more perfect joints in the limbs – are directly connected with increased activity and precision of movement, and these in turn are dependent on cerebellar improvement. The importance of the Diademodon brain, ear, and nose lies in the evidence which they afford that this change was actually taking place in the Therapsids, and that it is to all appearances a very gradual one and may to a large extent have preceded the development of a mammalian structure.”
He later (Watson, 1931) interpreted the numerous fine pits on the snouts of Late Permian and Triassic synapsids (therocephalians and cynodonts) as evidence for vibrissae, sensory whiskers, and hence for hair more generally. He noted the very mammalian-like limbs, vertebral column, brain, and presumed sensory organs of the snout in the Early Triassic therocephalian Ericiolacerta, and wrote (Watson, 1931, p. 1200): “The general outer surface of the maxilla of Ericiolacerta is exceptionally richly provided with small foramina, whose existence goes to show that the skin of the face was sensory or required a large blood-supply, a condition only understandable if it were muscular. Thus any possible interpretation of these foramina in the maxilla of Ericiolacerta leads to the conclusion that the animals' skin was mammal-like, in that it possessed specially developed sense-organs (?hairs) and was movable and muscular.” Such heavily perforated snouts have been noted in many other Triassic cynodonts (Fig. 5A).
Brink (1957, p. 86) expanded on Watson's evidence, noting that “[v]ibrissae are obviously specialized hair [so] that ordinary hair must have been present even in earlier forms.” He suggested hair had evolved to shield these synapsids from absorbing too much heat from the sun, appropriate perhaps during a time that was as hot and arid as the earliest Triassic, but also to retain heat (insulation) at cooler times. Further, Brink (1957, p. 87) speculated that early synapsids had sweat glands associated with their hair, as well as mammary glands, which are usually interpreted as modified sweat glands.
The story of the early origins of synapsid hair has taken an interesting twist more recently. Smith and Botha-Brink (2011) and Bajdek et al. (2016) independently reported fossil finds of possible hairs in coprolites from South Africa and Russia respectively. These finds suggest the origin of hair during the Late Permian. In future, it will be helpful to determine whether specimens such as these show conclusive evidence of hair characters, such as melanosomes embedded in the structures. Melanosomes, the capsules that contain the pigment melanin in modern birds and mammals, could also give suggestions of the colours of the hair of early synapsids.
A recent study has suggested a much later origin for hair. In their investigation of synapsid brains, Benoit et al. (2017) argued that the presence of a true infraorbital canal in derived cynodonts suggests that they had a mobile rhinarium and whiskers. These authors used evidence of pleiotropic genetic linkage, through the homeogene Msx2 in mice, between the ossification of the parietal fontanelle and development of the cerebellum with the development of mammary glands and hair, to suggest that the first two characters seen in derived cynodonts confirms the presence of mammary glands and hair at least by that point in the Middle Triassic, before the origin of mammaliaforms. Some current researchers are less convinced by Watson's arguments about snout vessels and vibrissae and indicate that hair evolved in the early Late Triassic (Botha-Brink et al., 2018). However, the evidence from cranial nerves provides a conservative estimate, and the coprolites give tantalising evidence for the origin of hair in the Late Permian.
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