Hypuronector, many questions, one (long) answer.

[Silvio Renesto <renesto@mailserver.unimi.it>]

Dear Listmembers,
        since I received quite a number of request to explain further my views, 
I
thought to send the reply to the whole list, not only because I'm lazy :-)
, but also because it concerns Mesozoic reptiles so it is too much off
topic....and you can always push the "trash" button without reading.
Feedbacks either positive and negative, all equally welcome (Gregory
Bateson wrote: Science DOES NOT EXPLAIN anything, science EXPLORES -
emphasis is mine - so I do not pretend to be right and I warmly wish no one
will be flamed against me).

IN MY HUMBLE AND SUBJECTIVE OPINION , Hypuronector skeleton shares all the
drepanosaurids feature that suggest arboreal adaptations (most of what I
write here has already been published in my 1994 and 2000 papers on
Megalancosaurus; in the latter paper I mentioned aside but did not quote
directly  Hypuronector because it was not yet published):

1) The skeleton is gracile (thin ribs, slender limbs etc.).
2) The limbs are too long for an animal that swims mainly by undulation of
the tail. 
3) On the other hand they are too gracile and with proportions much
different with respect to those reptiles which swim propelled by their limbs.
4)few phalanges are preserved in Hypuronector, however, they reflect the
usual drepanosaurid pattern: elongate distal phalanges (and possibly narrow
sharp claws?), this is not common in aquatic reptiles, but it is common in
many scansorial vertebrates.
5) The typical drepanosaurid  high and "very" narrow scapular blade looks
like that of chameleons, Icarosaurus, Longisquama, and, to a lesser extent,
bird-like dinosaurs and birds, all not suspected to be aquatic.
6) This shape of the scapula is convergent with the aforementioned taxa in
which it is a functional answer to the need of good brachiation and
grasping at preys or tree twigs.
7) going to the pelvic girdle, again it is a replica of that of Chamaeleo
(that one of Longisquama is unknown, what a pity!!!)  in having a high,
narrow and forward directed iliac blade. Without going too much into
details it is related to trunk rising off the substrate ...
8) the anterior half of the rib cage has "normal" ribs, in the posterior
half the ribs become fused to the neural arches, why? I do not really have
an answer, but perhaps it may be a stiffening device for the long trunk,
useful when the animal bridged from a tree twig to another one (gap bridging).
9) do you need anything more?

Now let's go to the tail:
embarassing tail ... Hmm, in all known drepanosaurids, the tail is
preserved raised at its base, then curled ventrally (to the author of that
nice Megalancosaurus  drawing  in the website mentioned yesterday:
VENTRALLY curled please, otherwise it looks like an impossible Walt Disney
chameleon). The zygapophyses posteriorly to the fourth-fifth caudal
vertebra embrace half of the preceeding vertebra forbidding any lateral
movement.
A similar pattern is reported also in Hypuronector and if you look at fig.
7 B of Hypuronector paper, tell me if you see a raised tail with ventral
curling or not. You see it at a lesser extent in Fig. 6 because the tail
has been detached, the body twisted and there are small "faults" crossing
it. But if you look carefully at the hip and proximal tail again you see an
angle.  Just to finish with, the very very long hemapophyses slant so
posteriorly that contributed in stiffening the tail, didn't them?

So what was that damned tail for?
In other drepanosaurids the distal portion of the tail is definitely a
grasping, clinging device: in at least two genera it ends with a hooked
spine and in another one in a prehensile chameleon-like curled end.
But the middle portion is always leaf-like, even if at a lesser extent than
in Hypuronector.

In my poster for the Pterosaur (but in the subtitle they mentioned all
flying/gliding reptiles) Symposium in Toulouse (published in "Strata")  I
suggested two possibilities (=wild guesses, speculations, unsubstantiated
hypotheses, fool dreams, call them as you prefer) for the expanded
leaf-like portion:
1) display,  such a  tail would have been quite visible if brightly
coloured, like frills of extant reptiles and the "wings" of some Draco
species.
2) rudder, IF (and it is advisable to repeat: IF) some  drepanosaurids were
able to glide in a gliding squirrel fashion (the smaller genera, surely not
Drepanosaurus).
It has to be kept in mind that the long leaf-like tail did not provide lift
 because it was LATERALLY compressed, and if it was kept straight it would
have forbidden gliding because the animal almost surely pitched due to the
posterior placement of the center of weight, but with the tail well raised
and the (stiff?) neck extended, the center of gravity may have been shifted
forward, thus MAYBE that gliding MIGHT have been possible. I do not stick
to this, I am just guessing. 
Hypuronector lacks the clinging device but as a display (or rudder) was
well equipped....

Thanks to all who survived until here without trashing my message.

                                Silvio Renesto

 






-

"Who looks for Knowledge increases himself every day
who looks for Wisdom loses himself every day"   
                                        (Lao Tzu)

Dr. Silvio Renesto
Dipartimento di Scienze della Terra
Università degli Studi di Milano
via Mangiagalli 34
I 20133 Milano
Italy

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