[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
Re: Noasaurids strike back
Jaime Headden wrote-
> I would like to point out a few mechanical problems with the idea that the
> *Noasaurus* phalanges could hyperflex, as Mickey Mortimer appears to
> consider they can. The non-ungual phalanx in *Noasaurus,* PVL 4061, lacks
> the ventral "heel" of the proximal articular surface, as in
> dromaeosaurids, some troodontids, and of course, the one and only phalanx
> known for *Ligabueino* (MACN-N 42); instead, this phalanx in *Noasaurus*
> possesses almost _no_ heel but a strip of bone that inserted between the
> ginglymoidal condyles of the preceeding phalanx, and had a prominent
> rounded articulation for fitting over the dorsal and ventral portions of
> that phalanx -- any rotation at that joint would have been relatively
> limited and probably only as much as it would take to walk with ... were
> it pedal in nature.
Bonaparte (1980) notes the distal articular surface allows a wide range of
flexion/extension, so at least one phalanx seems capable of this.
Interestingly, Carrano et al. (2002) note the manual phalanges of
Masiakasaurus possess distal articular surfaces suggestive of
hyperextension, and asymmetrical proximal concavities separated by dorsal
and ventral prongs, the ventral ones being far more prominant. This agrees
with Noasaurus' non-ungual phalanx. If one compares Masiakasaurus' pedal
phalanx II-2 however, differences are noted between it and Noasaurus. The
proximal end is stated to have two ventral heels, and a median dorsal
extension is present. There are no described differences between phalanges
III-2, IV-2 and IV-3 in Masiakasaurus and Noasaurus' phalanx. No non-ungual
phalanges are illustrated in Masiakasaurus however, making more detailed
All of Velocisaurus' phalanges are longer except IV-2, IV-3 and IV-4. Only
the last two have as pronounced of a dorsal concavity in lateral view, and
all phalanges except perhaps IV-2 have much larger ligament pits. Only
phalanges IV-1 and IV-2 have ligament pits shifted dorsally like Noasaurus,
and only III-3 has the distal articular surface elongated distodorsally like
the latter. All have a broad lower heel/lip and prominant pointed upper
one, quite unlike Noasaurus. Based on these comparisons, Noasaurus'
non-ungual phalanx tentatively seems more likely to be a manual phalanx.
> The ungual, unlike those in dromaeosaurids or
> troodontids, possesses _very_ distinct upper and lower lips on the
> proximal surface that virtually locks it onto the preceeding phalanx,
> which by shape would appear to be the one previously described.
> Hyperflexion could not occur in *Noasaurus* if the _only known_ digit was
> the digit supposed to do the hyperflexion.
As for the ungual, it's different from both Masiakasaurus' manual and pedal
unguals. It differs from the manual unguals in having a more concave
articular surface, no flexor tubercle, a distally placed flexor pit, a more
distally placed proximodorsal 'corner', and a deeper more curved morphology.
It differs from the pedal unguals in having a more prominant proximoventral
lip, being longer and more curved, as well as more lateromedially
compressed. Other features are like the pedal unguals though, such as the
proximodorsal morphology, virtual absence of a flexor tubercle, and keeled
ventral surface. In addition, Carrano et al. note the unillustrated pedal
ungual II(?) is more lateromedially compressed than the illustrated ungual
III(?) or IV(?). So I find Noasaurus' preserved ungual more likely to be
pedal. I agree with you observation regarding its apparent inability to
hyperextend, though I don't see why it has to belong to the same digit as
the other phalanx. Indeed, my comparisons suggest it doesn't.
> Also by extension, the second
> metatarsal of hyperflexing theropods usually has an expanded distal
> condyle that exposes further on the dorsal surface than it does in other
> theropods, to allow the digit to "freeze" in the hyperflexed position,
> with a deeper dorsal "pit" or extensor depression than in non-hyperflexing
> theropods or in the other metatarsals, to be exact; these features are
> lacking in the known metatarsal of *Noasaurus,* which may be the fourth as
> well as the second, and which exposes the non-ginglymoidal distal condyle
> further flexorly/ventrally than extensorly/dorsally.
The remains of Masiakasaurus show Noasaurus' metatarsal is the second, and
the presence/absence of an extensor pit is unillustrated and undescribed to
my knowledge. But I agree the distal articular surface morphology argues
against hyperextension of its digit.
> Other evidence offered is the apparently more primitive *Ligabueino,*
> which has a single known phalanx that has a distinct set of grooves in the
> proximal end that would show a tight articulation with the preceeding
> phalanx, and a longer ventral extension of the proximal articular surface
> than the dorsal, leading Mickey Mortimer to characterize this as a
> "proximoventral heel." In this way, the characterization is fitting given
> the nature of the distal end of the phalanx in which the articular
> condyles are more prominent above the dorsal half than below the ventral
> half of the phalanx is split longitudinally and parallel to the ventral
> surface. Unlike hyperflexing dromaeosaurids or troodontids, however, the
> lateral ligamental pits are no located further dorsally than at the
> mid-point (they are midway between the dorsal and ventral surfaces).
> Another feature of the ligament pits is that in dromaeosaurids and some
> troodontids, at least, these face dorsally and the dorsal surface of the
> phalanx is much narrower than the ventral surface by less than 80%, but
> this feature is unknown in *Ligabueino* based on the only published
> illustrations, of which the description is non-enlightening. Whithout a
> metatarsal or other phalanges (or for that matter, articulation) this
> phalanx is largely uninterpretable, and using a large ungual-phalanx ratio
> as in *Noasaurus* to support referral of the digit to the foot, or even
> referring the phalanx in *Ligabueino* to the foot, would seem highly
> suspect at this point. Rather, this phalanx should be treated as an
> anomalous phalanx and not neccessarily applying to the pes, nor does the
> data for it being hyperflexing very strong, though likely based on the
> total data.
I feel you're being FAR too cautious here. Yeah, the ligament pit isn't
displaced dorsally as in deinonychosaurs and Rahonavis, but that doesn't
mean it couldn't hyperflex. The absence of any data on how dorsally the
pits face is completely irrelevent. Until a manual phalanx is discovered
with such an elongate proximoventral heel, I see no reason to doubt its