RE: Claw function in Deinonychus

[John Scanlon <riversleigh@outbackatisa.com.au>]

Hi folks,

A few days ago I posted the following on Google group
sci.bio.paleontology, which apparently terminated discussion of the
subject there. It might be of more interest here:

(Oct 17, 1:11 pm)

John Harshman wrote: 
[snip] 
>... It sounds fairly convincing to me. Two questions: 
> 1. Did they compare the claw to those of raptorial birds? All I see 
> mentioned are tree-climbers. 
> 2. And why not consider tree-climbing itself as a use for the great 
> claw, rather than prey-climbing? It seems clear that at least a few 
> dromaeosaurs were arborial. How about a tree-nesting Deinonychus? 

A few observations: 
The idea of dromaeosaurs climbing the bodies of their prey has 
certainly been suggested before, possibly first by Ostrom but then in, 
for example, Bob Bakker's 'Raptor Red'. 

I've never been convinced that 'disembowelling' was a plausible 
function for such a strongly curved claw on a very flexible digit.  The 
ostrich and cassowary are widely reputed to be able to disembowel 
humans, dogs, lions etc. with a front kick, and though I do not have 
details of any documented cases this seems worth investigating (just 
annoy your local ratite with a pig carcass on a stick?). Ratite claws 
are not especially sharp-tipped, thick and strong but relatively weakly 
curved; but the middle claw in cassowaries is quite variable in length 
(fairly ordinary and emu-like in the Australian species, much longer in 
some New Guinea animals) suggesting it may be evolving specialised 
functions within the extant genus.  Has Feduccia written any reviews on 
claw geometry and disembowelment in non-dinosaurian birds? 

As Manning et al conclude, a strongly curved claw with an 
elliptical cross-section is not an effective cutting tool; and there 
seems to be no evidence from ungual morphology to override the 
inference from the EPB (extant phylogenetic bracket, i.e. parsimonious 
character reconstruction based on living animals). To reconstruct the 
claw with a ventral cutting ridge would thus be an extrapolation that 
would have to based on either extant analogues (of which there are 
apparently none) or a biomechanical argument for the claw being 
modified in other ways for disembowelling (probably ruled out by the 
strong curvature). 

Tree-climbing, why not?  Likely to be more important than prey-climbing 
most of the time, I would think, since most maniraptors were relatively 
small and likely to be generalist insectivore-carnivores like varanid 
lizards. If the claw morphology evolved in association with arboreal 
foraging (for insects or small vertebrates), nocturnal perching, or 
juvenile escape behaviour in small generalists, its use in predation 
would be an 'exaptation' that might have been critical in allowing 
specialisation on large prey (which may or may not have actually 
occurred). 

There may now be enough different species of maniraptorans known from 
foot bones to map out some morphological trajectories and compare with 
limb proportions and dentition, to see if mega-carnivory or potential 
arborealism is the better explanation for the origin of the big claw. 
Thesis topic for someone?
 
-----------------------------------------------
Dr John D. Scanlon
Palaeontologist, 
Riversleigh Fossils Centre, Outback at Isa
19 Marian Street / PO Box 1094
Mount Isa  QLD  4825
AUSTRALIA
Ph:   07 4749 1555
Fax: 07 4743 6296
Email: riversleigh@outbackatisa.com.au