As a total amateur, I am just wondering if the very early mammals had hedgehog-like quills rather than hair. Am I correct?

Since birds developed from reptiles, feathers must have developed from scales. Between these two, there were quills giving better warmth than scales. But hedgehogs also have quills so did those quills develop entirely independantly from bird quills or did quill bearing reptiles give rise to both?

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- A) What was like the feature that eventually evolved into hair in mammals?

- B) Did mammal quills evolved independently from bird quills?

- C) Did quill bearing reptiles give raise to both (mammals and birds)?.

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It is a very complicated matter, since the origins of feathers and hair date back to extinct animals, and it is very difficult to find remains of these traits (especially hair) in fossils. I have very little knowledge on this matter, however there is an interesting paper of Danielle Dhouailly, called A new scenario for the evolutionary origin of hair, feather, and avian scales (http://www.ncbi.nlm.nih.gov/pmc/...) that seems to throw clarify a lot about it.

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Mammals, birds and reptiles are all amniota*.  It is very well documented that birds derived from reptiles (sauropsida), while the clade that gave origin to mammals (synapsida) split before the aparition of reptiles. In other words, mammals don't come from reptiles:

Source of the image: Evolution | Kusumi LabKusumi Lab

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Following the footprints of the type of keratin present in the the skin of mammals and birds, he suggests that mammal hair comes from "hypothetical glandular integument of the first amniotes, which may have presented similarities with common day terrestrial amphibians", whileas feathers "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."

I.e, hair neither comes from reptile scales nor from feathers. Also, feathers don't come from reptile scales. So the answer to question B) is yes, mammals' quills evolved independently from birds quills, and the answer to question C) is no, quill bearing reptiles didn't give rise to them both.

But...what did hair look like when it appeared for the first time?

When did hair arise?. It seems that Pelycosaurs lacked scales, and propbably also all basal therapsids.  Accordingly with wikipedia "The hairs of the fur in modern animals are all connected to nerves, and so the fur also serves as a transmitter for sensory input. Fur could have evolved from sensory hair (whiskers). The signals from this sensory apparatus is interpreted in the neocortex, a chapter of the brain that expanded markedly in animals like Morganucodon and Hadrocodium.

The more advanced therapsids could have had a combination of naked skin, whiskers, and scutes. A full pelage likely did not evolve until the therapsid-mammal transition."

Hair

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.

So the answer to question A) is probably -and only probably- like whiskers.

Interesting readings:

http://www.ncbi.nlm.nih.gov/pmc/...

Hair

and for not shy readers the discussion about the first paper posted in archosaurheresies is interesting  (The origin of feathers and hair (part 2: hair) ). But, caution, the blog reflects the opinions of his author who clearly opposes to many of the views of paleontologists. So you have to read it with an analytical eye.