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Ditto for squirrels.
> But there was next to nil quartz sand in the Solnhofen carbonate
> was mostly a limey mud. Carbonates, especially as mud, would have
> Mohs hardness.
True enough, but it also provides less purchase for movement,
and so promotes slipping and the like. [Lime mud is notoriously
slippery]. This might 'compensate' for the lack of hardness
(if Archi even spent much time on such a treacherous surface).
> This is probably true for Sinornis' lineage. However, almost all
> Mesozoic birds (Icthyornis, Hesperornis, Patagopteryx, the
> Enatornithoformes, etc.) are demonstrably nonarboreal.
These are all very specialized forms, anatomically speaking,
so they are hardly likely to be characteristic of the earliest
[The fossil record of birds is deomnstrably worse than of any
other boney organism, excpet perhaps small theropods].
> Thus, Sinornis shows
> that birds were capable of evolving arboreal forms, but is
> distant from the common ancestor of modern perchers/tree dwellers.
Probably, but so were the aquatic forms you listed!
The peace of God be with you.