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segnosaurs & sloths

Tom Holtz writes:

> Not to say that I support the idea of an arboreal ancestor of the
> therizinosauroids (aka segnosaurians), but it IS interesting that,
> of all mammals, ground sloths seem to be the best analogue for these
> strange dinosaurs.  And ground sloths, after all, were simply the
> giant, herbivorous, secondarily-terrestrial descendants of small,
> once-insectivorous, then-herbivorous, arboreal forms...

Heh, heh! You quick, Kemo Sabe...

Arboreality could explain why segnosaurians are essentially absent from the
fossil record prior to the evolution of large, ground-dwelling forms in the
Early and Late Cretaceous. Unfortunately, there are also dozens of other
reasons. Footprint experts: Are trackways known in which the hind-foot
impressions are tetradactyl with long, very narrow unguals? These would be
good segnosaurian trackway candidates. Nessov (1995) noted "segnosaur"
trackways as early as Late Jurassic in central Asia, but he didn't figure
them, but I haven't as yet had a chance to really dive into the paper and
read about them in detail.