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Re: Rép : Post–K-Pg Radiation of Placentals -- open access!!!

David Marjanovic <david.marjanovic@gmx.at> wrote:

> Of all these (plus *Ichthyoconodon*, known from teeth similar to
> *Argentoconodon* and the teeth of *Volaticotherium*), only the arboreality
> of *Henkelotherium* seems to be homologous to that of Theria.

Hmmm... I'm not convinced it's that straightforward.  The "arboreal"
characters shared by _Henkelotherium_ and the first therians might
actually be "scansorial" characters; and these same "scansorial"
characters were not necessarily associated with tree-climbing.

_Henkelotherium_ was a small mammal (~ 7cm long) that has been likened
to tree-shrews (Scandentia) in its inferred behavior.  Some
tree-shrews spend most of their time on the ground, and so are not
strictly speaking arboreal (so neither "tree" nor "shrew"!), although
they are excellent climbers.  Most small mammals, because of their
small size, are adapted to clambering over uneven terrain.  This
necessitates "climbing" behavior - even if these mammals spend most of
their time on ground.

So the postcrania of these small mammals tend to have a lot of
flexibility, including a wide range of motion (excursion) at the
joints.  It's my understanding that the presence of "scansorial"
characters in _Henkelotherium_ probably relates to its small size,
whereas the length and morphology of the tail is more indicative of
true arboreality.  Basal eutherians and basal metatherians show these
scansorial characters.  They also show additional characters in the
pectoral girdle, forelimb, and hindlimb that indicate arboreality.

(BTW, none of these scansorial/arboreal characters are evident in
maniraptorans close to birds.  Aside from small body size, that is.)

> I should also
> have mentioned Spalacotheroidea, likewise thought to have been arboreal and
> in the same general region of the tree.

However, other taxa close to crown Theria appear to have been
terrestrial, such as _Vincelestes_ and _Zhangheotherium_.  So even
though _Henkelotherium_ was likely scansorial/arboreal, it's unclear
if arboreality is primitive for the Dryolestida+Theria clade.  There
were quite a few lineages, and some fairly impressive ecological
diversity, between Dryolestida and crown Theria.  This includes the
newly recognized clade Meridiolestida (_Mesungulatum_, _Cronopio_,
_Necrolestes_, etc).