Triassic origin of scales, scutes, hair, etc. as biting fly barriers?

During the Middle to Late Triassic

  1. Mammals developed fur/hair.
  2. Aetosaurs developed plates and horns beyond the earlier paired dorsal scutes.
  3. Crocodylomorphs developed large scales beyond the earlier paired dorsal scutes.
  4. Dinosaurs lost those paired scutes and developed placodes and quills. Ultimately these became scales and feathers.
  5. Turtles developed hard scales over a carapace and plastron.
  6. Lepidosaurs developed small scales.
  7. 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

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

Thinking about the Crocodylomorpha…

Short one today.
The following GIF animation of 4 frames (5 seconds each) is a subset of the large reptile tree (LRT, 1280 taxa) focusing on the Crocodylomorpha and showing:

  1. Clades within Crocodylomorpha
  2. Extent of bipedality
  3. Extent of dorsal scutes
  4. Extent of elongate ulnare and radiale (synapomorphy of the clade)
FIgure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

FIgure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades. There are more taxa at the origin of this clade, fewer extant taxa here. When traits are unknown (lost due to taphonomy), colors are estimated.

Distinct from all prior smaller studies,
here the Crocodylomorpha is the sister taxon to the Dinosauria. Together these alone comprise the Archosauria. They arise from a sister to the Poposauria and these arise from basalmost Rauisuchia, a sister to Vjushkovia (not Postosuchus). The traditional clade ‘Pseudosuchia‘ is invalid (= not recovered as a monophyletic clade) under this hypothesis/cladogram.

Taxon exclusion
misinformed earlier studies of the Crocodylomorpha. Those earlier studies did not include enough pertinent taxa to recover the present hypothesis of relationships. Most prior studies included irrelevant taxa (members of the Pterosauria), which nest not with archosaurs, but within Lepidosauria.

Figure 2. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocritta. Dorsal scutes are shown above the dorsal vertebrae.

Figure 2. The genesis of the Archosauria embodied in PVL 4597 to scale with a modern archosaur, Cyanocritta. Dorsal scutes are shown above the dorsal vertebrae.

Phylogenetic miniaturization
occurred at the origin of the Archosauria, with duck-sized PVL 4597 (Fig. 2) at the origin of the Archosauria. Thereafter dinosaurs lost their dorsal scutes and gained feather placodes.

References
https://en.wikipedia.org/wiki/Crocodylomorpha

Saltopus preserves early archosaur skin and scales

Basal Crocodylomorpha

Figure 1. Basal Crocodylomorpha, including Gracilisuchus, Saltopus, Scleromochlus and Terrestrisuchus

The basal archosaur
(crocs + dinos) Saltopus (von Huene 1910; Late Triassic; ~210 mya, ±60 cm long; Figs. 1–3) poorly preserves bones, but also preserves some scaly skin.

Figure 2. Saltopus skin and scales surrounding the right pelvis.

Figure 2. Saltopus skin and scales surrounding the right pelvis. Not all bones nor all scales are traced here. Benton and Walker 2011 reported no evidence for osteoderms. The bones are hard to delineate and segregate from scales because here they are covered with fossilized desiccated skin. Photo from Benton and Walker 2011 who trace the femoral head extending beneath the pelvis. 

Saltopus nests well within
the Crocodylomorpha and, along with Scleromochlus (Fig. 1), present examples of basal archosaur skin and scales. Even more basal, Gracilisuchus (Fig. 1) had dorsal scutes derived from ancestors going back to the Early Triassic Euparkeria.

Figure 2. Saltopus pelvis latex peel from Benton and Walker 2011. They found two large sacrals. Using DGS I found four sacrals, the same length as the dorsals and causals. Sister taxa have four sacrals.

Figure 2. Saltopus pelvis latex peel from Benton and Walker 2011. They found two large sacrals. Using DGS I found four sacrals, the same length as the dorsals and causals. Sister taxa have four sacrals.

Gracilisuchus and basal dinosaurs
had only two sacral vertebrae, but basal bipedal crocs, like Scleromochlus, double that number. Benton and Walker 2011 traced two sacrals in a latex cast of the sacral area, but each sacral was twice as long as proximal dorsals and causals. Here four sacrals are tentatively identified in the latex peel, all about as long as proximal non-sacral vertebrae.

Dinosaur skin
can be scaly, or naked with feathers, or a combination of the two. Dinosaur scales may be different than croc or lizard scales in that at least some dinosaur scales, like those on the metatarsus of theropods appear to be derived from former feathers.

References
Benton MJ and Walker AD 2011. Saltopus, a dinosauriform from the Upper Triassic of Scotland. Earth and Environmental Science Transactions of the Royal Society of Edinburgh: 101 (Special Issue 3-4):285-299. DOI:10.1017/S1755691011020081
von Huene FR 1910. Ein primitiver Dinosaurier aus der mittleren Trias von Elgin. Geol. Pal. Abh. n. s., 8:315-322.

wiki/Saltopus