SICKLE-CLAWED DINOSAURS

[darren.naish@port.ac.uk]

On the subject of which dinosaurs were equipped with pedal sickle 
claws, T. Mike Keesey wrote..

> There were three radiations that we know of. _Noasaurus_, an 
> abelisaur, had a large "sickle claw" that was different in 
> morphology from the other two radiations. Troodontids were once 
> grouped with dromaeosaurids (a.k.a. "'raptors") because of their 
> similar "sickle claws", although they are now generally recognized 
> as a separate lineage, closerr to ornithomimosaurs (a.k.a. "ostrich 
> mimics"). Primitive paravians, including dromaeosaurids and 
> _Archaeopteryx_, as well as the enantiornithine _Vorona_ and the 
> unnamed "flying 'raptor" of Madagascar, also show the trait, 
> although various advanced paravian lineages (including modern birds) 
> have lost it. 

_Noasaurus_, so far as I know, has never been regarded as an 
abelisaurid - nevetheless it is related to them and considered part 
of a more inclusive clade called Abelisauroidea by Holtz, Novas, 
Bonaparte and others.

The Holtz paradigm has led to wide acceptance of the idea that 
troodontids are most closely related to ornithomimosaurs because both 
share an arctometatarsalian pes and an inflated parasphenoid capsule, 
among a few other things. The two groups are Holtz' Bullatosauria. 
But the Holtz paradigm is not necessarily correct, and other workers 
- most notably Hans Sues - support a troodont-dromaeosaurid 
relationship as a result of newer analyses. The idea that 
Deinonychosauria could be reinstated is therefore possible, but 
debatable.

I was unaware that _Vorona_ may have been sickle-clawed. Are you sure 
about that? AFAIK, the type and only known specimen is represented 
only by a tibiotarsus, and you can only tell if digit II was 
sickle-clawed if, surprise surprise, you have bones from digit II. 
Maybe my memory is at fault and digit II is known for _Vorona_.

Finally, sickle-claws have evolved more times than the three 
radiations you cite: they are also seen in members of the 
seriema-phorusrhacoid clade. Some fossil phorusrhacoids, like Miocene 
_Psilopterus_, has what can be considered a sickle-claw, as do some 
living seriemas. There are two living seriemas, _Chunga_ and 
_Seriema_: I forget which has the sickle claw. 

It is tempting, then, to regard this as a primitive feature of the 
clade which has been inherited by both later phorusrhacoids, and by 
later cariamids (seriemas). However, last time I looked at photos of 
one of the best preserved early phorusrhacoids, _Aenigmaornis sapei_  
from Messel (mid Eocene) (I may have spelt the genus name wrong: note 
also that the specific name is in honour of SAPE, The Society for 
Avian Paleontology), I saw no sickle-claw. This _suggests_ that it 
evolved in parallel in some phorusrhacoids and some seriemas, in 
which case it appeared in this clade at least twice. 

Also, how do you properly define a sickle claw? Cassowaries, as 
everyone on the list should now be aware, have an elongate, very 
sharp claw that they use as a slashing/stabbing weapon - it is not 
kept raised off the ground, and is not sickle-shaped. Meanwhile, a 
number of birds of prey (true raptors) have an enlarged, 
sickle-shaped claw on digit II. In falconids this is sometimes kept 
raised when the bird is perched.

In many theropods, the ungual of digit II is often the biggest of all 
pedal unguals. Hence theropods may be predisposed for the evolution 
of raptorial claws on that digit.

"It's kind of a.. Mulder-Scully sort of thing"

DARREN NAISH
darren.naish@port.ac.uk