Speculation on the origin of infrared sensors and implications for palaeontology
The fossil record shows that hair anatomy developed very early possibly in mammaliaforms in the Triassic pre-200 Ma. The infrared interpretation of guard hair opens the possibility that these small, shrew-like animals evolved infrared sensitivity to defend against warm-blooded predators. That encouraged hair to develop as the main body covering in mammals. Their main predators endothermic theropods needed to hide their body heat and downy feathers are by far the most efficient body covering to cloak infrared emission from warm parts.
Figure 1 Mammaliaforms and coelurosaurian theropods from the late Triassic may have had the same infrared arms race as rodents and owls do today
The infrared sensor interpretation of guard hair in the fur of small animals has an impact on palaeontology. The house mouse and European rabbit have guard hairs that are virtually identical and it seems that this template is shared with most rodents and lagomorphs despite a split believed to be circa 87 Ma. This suggests that hair anatomy is very slow to evolve presumably because the details need to maintain a mathematical rigour. The hairs of shrews and antechinus are radically different suggesting a far more distant split and a very early emergence of infrared sensing in ancestral mammals.
Infrared sensors in the skin of early mammaliaforms probably started with heat-sensitive cells and later went on to develop keratin structures to concentrate the infrared signal and improve directionality. At each stage the structure needs to be mathematically perfect and this is the reason an existing hair cannot be repurposed. It is also not possible to evolve a sensor beneath fur so guard hair was probably a component in the earliest fur and it may be significant that in mice embryos guard hair is the first hair to grow. The fossil trail for fur and hair points to an early emergence. The oldest hair specimen trapped in amber was from 100 Ma (from France) reported by Vullo, 2010. The authors conclude that modern cuticular features were already present in the early Cretaceous. The oldest fossil hair with identifiable modern hair structure is a 125 Ma rat-like animal (Spinolestes xenarthrosus) reported by Martin, 2015. Fossil guard hair has been identified in Megaconus and Castorocauda from 161 Ma (Qiang Ji, 2006). Fur is believed to have evolved in small, shrew-like mammaliaforms from the late Triassic and has been inherited and sustained ever since. The markedly different guard hair structures of marsupials and rodents also hints at the inheritance of infrared sensors from ancestors prior to the Therian split (190-165 Ma). The emergence of infrared sensitivity in mammaliaforms in the late Triassic appears to parallel the adoption of downy feathers in Coelurosaurian theropods, animals that very likely preyed on them. Feathers are far more efficient than fur at blocking infrared emission and one feather would shield a large area of skin (consider the thick coat of fur required by mammals to achieve the same blocking). This is a progressive arms race where improved infrared sensitivity in mammaliaforms parallels improved infrared concealment in endothermic theropods. B. H. Michaels depiction of a typical Coelurosaurian theropod, Zuolong salleei, dated at 160 Ma shows the warm areas of the body covered in feathery down exactly as a modern barn owl. Owls have almost perfect infrared concealment (Figure 2).
Figure 2 The emergence of infrared sensitivity in mammaliaforms may have forced Coelurosaurian theropods to evolve downy feathers to hide their body heat just as owls do today
Summary of timelines
Martin T, Marugán-Lobón J, Vullo R, Martín-Abad H, Luo Z, Buscalioni A. D, “A Cretaceous eutriconodont and integument evolution in early mammals”, Nature, 2015; 526 (7573): 380 DOI:10.1038/nature14905
Vullo R, Girard V, Azar D, Neradeau D, “Mammalian hairs in early Cretaceous amber”, Naturwissenschaten, July 2010, Volume 97, Issue 7, pp 683-687
Qiang Ji et al, “A swimming Mammaliaform from the Middle Jurassic and ecomorphological diversification of early mammals”, Science 311:1123-27, 2006
Updated April 10th 2020