During the Middle to Late Triassic
- Mammals developed fur/hair.
- Aetosaurs developed plates and horns beyond the earlier paired dorsal scutes.
- Crocodylomorphs developed large scales beyond the earlier paired dorsal scutes.
- Dinosaurs lost those paired scutes and developed placodes and quills. Ultimately these became scales and feathers.
- Turtles developed hard scales over a carapace and plastron.
- Lepidosaurs developed small scales.
- Pterosaurs and their Late Triassic sisters developed pycnofibers
All of these developed on the soft, naked skin
(think of a plucked chicken) that was a universal covering for Carboniferous and Permian tetrapods (Early forms retained large ventral scales inherited from finned ancestors, but these were lost by the Permian).
All of these extradermal structures have one thing in common.
They separated and/or protected the animal’s naked skin from the environment, one way or another. They developed by convergence. Dhouailly 2009 and other workers discussed the chemical similarities of the keratin found in these dermal structures.
The question is:
What was different about the Triassic environment that was not present in earlier Carboniferous and Permian environments? We can eliminate heat, cold, UV rays, rain, aridity, etc. as possible reasons for the development of insulator structures because those factors had always been present. So what was new in the Triassic that affected all terrestrial tetrapods?
Flies and their biting, piercing kin.
“The earliest definitive flies known from the mid-Triassic of France, approximately 230 Ma (Krzemiski and Krzeminska, 2003)” according to Blagoderov, Grimaldi and Fraser 2007. The order Diptera (flies, mosquitos and kin) tend to land on large tetrapods for food, blood, etc. Scales, scutes, hair, feathers, etc. all separate flying insects from the naked skin of Triassic terrestrial tetrapods. Williams et al. 2006 even found mosquito repellents in frog skin. It is notable that, except for armored placodonts and mosasaurus (derived varanid lepidosaurs), aquatic and marine tetrapods also had naked skin with the thalattosaur, Vancleavea, a notable sermi-terrestrial exception. Is that because they had aquatic antecedents in the Triassic that were never affected by flying insects?
It’s not just the insect bite that drives this evolution,
it’s the appearance of new vectors for the rapid spread of disease that drives this evolution.
Interesting coincidence.
If this is not the case, this will take further study.
Figure 1. Lacertulus, a basal squamate from the Late Permian
Carroll and Thompson 1982 report
on the Late Permian lepidosaur, Lacertulus (Fig. 1), “No scales dermal or epidermal are evident in the specimen.”
From the Dhouailly 2009 abstract:
“I suggest that the alpha-keratinized hairs from living synapsids may have evolved from the hypothetical glandular integument of the first amniotes, which may have presented similarities with common day terrestrial amphibians.
Concerning feathers, they may have evolved independently of squamate scales, each originating from the hypothetical roughened beta-keratinized integument of the first sauropsids. The avian overlapping scales, which cover the feet in some bird species, may have developed later in evolution, being secondarily derived from feathers.” Not realized by Dhouailly, the purported clade ‘Sauropsida’ is paraphyletic and a junior synonym for Amniota and Reptilia in the LRT.
Earlier we looked at the first appearances
of hair, quills, pycnofibers and hard scales in a three-part series here, here and here.
Exceptionally, humans are terrestrial tetrapods
that have lost most of their hair, more or less returning to the primitive naked state. And yes, flies and mosquitos do bother humans. It is the price we pay for the benefits of naked skin. Clothing helps provide a barrier.
Remember:
Just because an idea is proposed and a hypothesis is advanced doesn’t make it so. In science ideas have to be confirmed or refuted following their first appearance. If anyone has data concerning scales or other dermal structures in Carboniferous or Permian taxa, please make us aware of those.
References
Blagoderov V, Grimaldi D and Fraser NC 2007. How Time Flies for Flies: Diverse Diptera from the Triassic of Virginia and Early Radiation of the Order. American Museum Novitates 3572:1-39. DOI: 10.1206/0003-0082(2007)509[1:HTFFFD]2.0.CO;2
Carroll RL and Thompson P 1982.A bipedal lizardlike reptile from the Karroo. Journal of Palaeontology 56:1-10.
Dhouailly D 2009. A new scenario for the evolutionary origin of hair, feather, and avian scales Journal of Anatomy 214(4): 587–606. doi: 10.1111/j.1469-7580.2008.01041.x
Krzeminnski, W., and E. Krzeminska. 2003. Triassic Diptera: descriptions, revisions and phylogenetic relations. Acta Zoologica Cracoviensia (suppl.) 46: 153–184.
Maderson PFA and Alibardi L 2000. The Development of the Sauropsid Integument: A Contribution to the Problem of the Origin and Evolution of Feathers. American Zoologist 40:513–529.
Rohdendorf BB, Oldroyd H and Ball GE 1974. The Historical Development of Diptera. The University of Alberta Press, Edmonton, Canada. ISBN 0-88864-003-X.
Williams CR, Smith BPC, Best SM and Tyler MJ 2006. Mosquito repellents in frog skin. Biol Lett. 2006 Jun 22; 2(2): 242–245. doi: 10.1098/rsbl.2006.0448
The Pterosaur Heresies 
Doesn’t Ascendonanus (latter half of the early Permian) have scales? Or am I misremembering?
Good question! Overlooked until you brought it up. So thank you, Neil. The authors of that paper report, “Preliminary observations show a regular scale pattern…” The eyelids are scaled (their Fig. 17). The entire ventral surface of the torso and tail includes transverse scales that continues to cover the limbs in elongate scales (their Fig. 20). We’ve seen transverse ventral scalation in the earliest amniotes. So that is carried through here. The question is, other than the eyelids, and maybe the limbs, are other parts of the dorsal surface scaled? I don’t see the evidence for it yet. Let’s keep on this subject to see where the evidence takes us.