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Re: Dromaeosaurid tails like rhamphorhynchid tails from flight use



Matthew Martyniuk <martyniuk@gmail.com> wrote:

> I'd be careful assigning such significance to Mahakala. It is at least
> 75 Ma removed from the common ancestor of all dromaeosaurs. So while
> Mahakala may be among the most basal dromaeosaurs, that doesn't
> necessarily mean it's the most primitive.


I agree with Augusto on this one.  The features of _Mahakala_ are more
important than time after divergence.  The absence of elongate
chevrons and prezygapophyses is recovered as a primitive trait in
_Mahakala_, retained for at least 75 million years.  But those 75
million years allowed the line leading to _Mahakala_ to acquire
autapomorphies (used to diagnose the genus).  Short forelimbs have
been interpreted as autapomorphic for _Mahakala_; but it is possible
that this character is also primitive for Dromaeosauridae (and
Deinonychosauria), although currently this is not the most
parsimonious hypothesis.


At first blush, the hypothesis that the first deinonychosaurs were
arboreal creatures that glided (or even flew) is intuitively
attractive.  There is certainly no shortage of proponents of this view
(e.g., a fellow named Gregory S. Paul).  However, if you drill a
little deeper, this scenario is not as straightforward as it first
seems.  I suspect the first deinonychosaurs could indeed climb and
glide.  (Therefore short-armed _Mahakala_ and _Tianyuraptor_ would
both be secondarily "non-climbing" and "non-gliding".)  But the
evidence for true arboreality* for any non-avialan maniraptoran is
slim, at best.  I severely doubt that _Archaeopteryx_ or _Microraptor_
glided from tree-to-tree.

* Arboreal = rarely spends time on the ground, and forages and
shelters in the trees.


Among maniraptorans, elongated prezygapophyses and chevrons appear to
be limited to microraptorines and eudromaeosaurs.  These caudal rods
might be related to aerial locomotion (especially gliding).  But IMHO
other hypotheses are worthy of consideration.


> Monotremes are the most
> basal living mammals, but that doesn't mean the ancestral mammal was
> platypus-like.


True.  Nevertheless, the platypus retains many characters that are
primitive for crown mammals (egg-laying being chief among them).  It's
these characters that identify the monotreme lineage (Prototheria) as
basal to marsupials and placentals.  The platypus-like morphology
(including those characters associated with an aquatic habitus)
emerged after this split; although the platypus-like morphology is
possibly primitive for crown Monotremata.


> It's always possible that such odd (for a dromaeosaur)
> features as very small forelimbs and un-stiffened tails in Mahakala
> were due to some novel ecological niche (burrowing?).


Yes, this is possible.  _Mahakala_ might be an oddball, and its short
forelimbs and un-stiffened-tail might have been acquired *after* it
diverged from other dromaeosaurids.  But this is not the most
parsimonious hypothesis at present.  Phylogenetic analyses thus far
have recovered _Mahakala_ at the very base of the Dromaeosauridae,
with its caudal morphology interpreted as primitive for the clade.


_Mahakala_ is certainly an interesting critter though.






Cheers

Tim