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Re: Featherless _Mononykus_? (long essay)



Ron Orenstein recently wrote:
------------------------------------------------

>On Wed, 24 Apr 1996 Stang1996@aol.com wrote:
>
>> Whatever for?  Is there some reason not to put feathers on _Mononykus_?
>>
>Yes! The best reason there is - good science. No feathers have been
>discovered with this fossil.  :-)
>
>Byron

Not really.  After all the vast majority of undisputed fossil birds also
lack feather imprints, and yet I would argue that it would be unscientific
to portray them WITHOUT feathers as they clearly are members of a clade
whose earliest known forms were feathered and for which no definitely
unfeathered members are known.  If Mononykus is viewed as being a member of
this clade it should be portrayed with feathers, as that would be a
parsimonious prediction given its cladistic placement.  If it is NOT placed
within the clade that is another matter, and I would certainly agree with
your statement as it applies to forms like (say) ornithomimids or
oviraptorids (though I am partial to Greg Paul's feathered restoratons of
these critters - and who knows, maybe they had'em after all!)
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And Nick Longrich
wrote:------------------------------------------------------

        No muscles, no intestines, no skin, no ligaments or tendons or
horn sheaths for the claws, either, for Mononykos. We infer these from
what evidence we do have of fossil and living animals.
         We can be confident that the common
ancestor of archaeopterygians and modern birds bore feathers; if Mononykos
fits inside that clade, it should have feathers as well. If it didn't the
question is still open because we have no idea where feathers originated.
        What is the general consensus? Is Mononykos still considered a
secondarily flightless animal by most? What is the evidence for avian
kinship, beyond the reversed pubes and keeled breastbone?
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Okay, here we go. I am not the first to come up with this >>earthshaking<<
notion, but: {{{{_Mononykus_ is NOT a "bird"}}}}. That is, it is NOT a member
of the clade that includes _Archaeopteryx_ and modern birds and their most
recent common ancestor. This clade is usually called Avialae, when it's not
being called Aves. At least, based on presently published information, it is
very, very unlikely to be a bird.

One major problem is the TAIL. In the tails of _Archaeopteryx_ and all known
later birds, the haemal arches, also known as the chevon bones, are greatly
reduced or absent (so are the tails, for that matter!). This is most
certainly a flight adaptation, since it lightens the tail (by removing bone
mass) and improves its aerodynamic characteristics. It also surely has much
to do with the modifications of the caudifemoral musculature from reptilian
to avian as described by Gatesy in a number of recent publications. But in
_Mononykus_--as restored by the authors in the original description--the
haemal arch series remains prominent, and the tail is very long. In fact, the
anatomy of the tail is similar to that of ornithomimid tails.

Unfortunately, in the holotype only the first caudal vertebra is
preserved(!). I presume that the authors have restored the tail from caudal
vertebrae preserved in one or more of the several known referred specimens of
_Mononykus_; otherwise, I have no idea how they came to restore the tail the
way they did. The recent study of _Mononykus_ by Perle et al. (1994) says
nothing about why the tail was restored long and with haemal arches, but I'm
taking their restoration at face value. Evidently, the authors do not
consider the tail particularly important, either anatomically or in
elucidating phylogeny. But once lost (as in avialan birds), it would be
virtually impossible for an entire haemal-arch series, as depicted in
restorations of _Mononykus_, to re-evolve in its original form.

Another problem is the arctometatarsalian foot of _Mononykus_. This feature,
in which the upper portion of the middle metatarsal bone (metatarsal III) is
"pinched" between the metatarsals on either side (metatarsals II an IV), does
not occur in oviraptorids, dromaeosaurids, _Archaeopteryx_, or later birds.
In those "pre-avialan" and avialan dinosaurs, metatarsal III remains
unpinched all the way up the foot to the ankle. The arctometatarsalian foot
occurs in tyrannosaurids, ornithomimids, troodontids, and avimimids. This
suggests that _Mononykus_ was more closely related to one or more of those
groups than to the lineage that led to avialan birds. Perle et al., however,
nevertheless assert that the arctometatarsalian foot of _Mononykus_ arose
convergently with the former theropods, all by its lonesome.

The straplike scapula with abruptly expanding acromion process and the
semicircular coracoid are typically theropod-like in their anatomy, with a
downwardly directed glenoid. In _Archaeopteryx_, the glenoid has a decidely
laterally directed component, and in all volant birds the glenoid is directed
upwardly. If _Mononykus_ were an avialan, the downwardly directed glenoid and
the whole configuration of the pectoral girdle would constitute a series of
major reversals in forelimb anatomy.

Most of the cited "avian" and "avialan" features of _Mononykus_ are
characters that involve reduction or loss, such as the moderately keeled
sternum (keel recuded in prominence), reduction of the fibula to a splint and
loss of the distal articulation of the fibula with the tarsus, and so forth.
Many other cited features are unique to _Mononykus_ and thus supply no
phyletic information. Features in which the derived state involves a
reduction or a loss of a character primitively present or prominent can occur
repeatedly in otherwise unrelated lineages, simply as convergent adaptations
to similar lifestyles. Likewise the opisthopubic pelvis, which arose
independently in at least two dinosaurian groups (Phytodinosauria and
Maniraptora), could as well have arisen independently in _Mononykus_. (And
incidentally, the pelvis of the holotype is very incomplete, rendering the
authors' reconstruction as opisthopubic suspect. Again, I take the
reconstruction at face value.) But _reacquiring_ the haemal arches, which
would have had to happen were _Mononykus_ descended within the clade Avialae,
represents a profound reversal in which a feature _lost_ is regained in
precisely its original form. There is no evidence that such a reversal has
_ever_ occurred in vertebrate evolution. To me, the existence of the
non-degenerate haemal arches in the tail of _Mononykus_ strongly outweighs
the birdlike (but easily duplicated through convergent evolution) characters
cited in the original description.

As I mentioned before, the holotype specimen includes only a single caudal
vertebra, from which the long tail and its haemal arches have somehow been
inferred. If these features were not present in the living animal, then WHAT
ARE THEY DOING IN THE RECONSTRUCTION?

It remains possible that the common ancestor of _Archaeopteryx_ and modern
birds primitively retained a series of well-developed haemal arches, and that
archaeopterygids and modern birds lost their haemal arches (and, in the case
of modern birds, most of their tails as well) convergently. This circumstance
would allow _Mononychus_ to retain a position in Avialae between
archaeopterygid and modern birds, provided that _Mononykus_ also
independently acquired arctometatarsalian feet like those of certain
non-avialan theropods, and retained (or reacquired) a primitively
theropod-like pectoral girdle. Not very likely, I'm afraid.

There is little doubt that _Mononykus_ was some kind of cursorial dino-bird.
But Perle et al. generally downplay the dinosaurian features of the
animal--too numerous and anatomically detailed to cover in this short
note--and overemphasize its supposed birdlike features in their joint paper,
presumably to support their contention that _Mononykus_ was an avialan more
advanced than _Archaeopteryx_. But I contend, contrarily, that the
postcranial anatomy that they themselves present strongly supports placement
of _Mononykus_ well below Avialae in the dinosaur-bird cladogram. As
paleornithologist Larry Martin might have said in his recent presentation on
_Mononykus_ (SVP 1995), its anatomy screams "Dinosaur!" Let me quote from his
abstract (JVP volume 15 supplement to #3): "None [emphasize NONE] of the
features that have been described as avian' in _Mononykus_ are diagnostic
for birds or even closely similar to the avian conditions with which they
have been compared."

How does removal of _Mononykus_ from Avialae affect the existence of
feathers? Not very much. There is little doubt that feathers had a fairly
long evolutionary history prior to their appearance, "fully fledged," in
_Archaepteryx_. _Mononykus_ is, after all, phyletically not very distant from
Avialae and could well have inherited such plumage as depicted in its recent
life restorations. So could a lot of other small Cretaceous theropods (such
as _Avimimus_--which shares a number of features with _Mononykus_ and could
well be closely related; compare the hind limb bones of the two genera, for
example).

And Greg Paul
wrote:----------------------------------------------------------

Mononykus is extremely avian, far more so than Archaeopteryx (I have access
to data that proves this). Feathers are entirely plausible for the creature.
------------------------------------------------------------------------------


I cannot imagine "data" in paleontology as ever "proving" anything. Proofs
occur only in mathematics. No collection of anatomical features can ever
conclusively prove or disprove a phyletic hypothesis. Paleontological data at
best can only affect the likelihood of an evolutionary event. (For an extreme
example, it is entirely possible that every species of extant bird acquired
all of its anatomical features [wings, wishbones, feet, you name it]
independently of and convergently with all the other extant bird species. But
just how LIKELY is this to have occurred?) Absolute proof--no. But perhaps
Greg would care to tabulate the data he mentioned, so that we could modify
our own conclusions about the _likelihood_ that _Mononykus_ belongs to the
clade Avialae.

In researching the new section on Mesozoic avian taxa for _Mesozoic
Meanderings_ #2 (third printing), I took a good look at what we know of
_Archaeopteryx_, dromaeosaurids, and primitive avialan birds. For many years
I was convinced, largely by the arguments of others, that _Archaeopteryx_ and
dromaeosaurids were more closely related to each other than either was
related to the later birds. But having looked over some of the recent papers
on dinosaur-bird relationships, I've changed my mind. _Archaeopteryx_ is much
more advanced toward the avialan condition in many important and major areas
of the skeleton, including the skull, the pectoral girdle, and the vertebral
column, than were any of the known dromaeosaurids. I am still firmly
convinced that dromaeosaurids and all other theropods descended from small
arboreal forms representing various stages in the evolution of flight, but
the stage that gave rise to the dromaeosaurids was considerably less
avialan-advanced than the one including _Archaeopteryx_. Dromaeosaurids were
by no means giant, flightless archaeopterygids, just giant, flightless
dino-birds of a more primitive grade.

My current best guess as to the phyletic position of _Mononykus_ is in a
sister group to _Avimimus_. This clade itself arose within Maniraptora,
likely as a sister group to the clade Bullatosauria (Troodontidae plus
Ornithomimidae), sometime before the divergences of the dromaeosaurids
(suborder Deinonychosauria) and oviraptorids (suborder Oviraptorosauria) from
the lineage joining the common ancestor of Dinosauria to Avialae.