Sinornithosaurus venom

[Mark Pauline <markpauline@rocketmail.com>]

The Gong et al. PNAS paper "The birdlike raptor Sinornithosaurus was venomous" 
is weakened by unsupported speculation.  The authors could have strengthened 
the paper by quantifying their judgments, making more extensive comparisons 
with other taxa, and addressing obvious alternatives to their interpretations.

1) Type of venom
The authors' statement that "The poison of
Sinornithosaurus may have been similar in properties to rear-fanged
snakes and helodermid lizards in that it did not kill the envenomated
animal quickly but rather placed it into a rapid state of shock" seems
to be wholly unsubstantiated speculation. The toxicity of the venom, if any, is 
completely unknown, and no lines of evidence are brought to bear here.

2) Tooth length
The authors write that "The anterior maxillary teeth are so long and fang-like 
that the animal appears to be saber-toothed." yet they make no effort to 
provide measurements of the teeth relative to tooth row length or any other 
metric, and they provide no comparison to other taxa. They provide only one 
measurement, the length of the largest maxillary tooth in  IVPP V12811, which 
is provided as "12mm long". This does not seem out of the range of tooth 
proportions for tetrapods in general.

Moreover, they note "Interestingly, much of the effective erupted length of the 
teeth is
 composed of the tooth root." The type skull is also semi - disarticulated. 
These seem to strongly suggest that the teeth have fallen out of their sockets, 
yet this possibility is not addressed.

3) The inference of venomousness
The authors cite Folinsbee et al. (2007) in stating "Pocketing within the 
maxilla in conjunction with grooved fangs is considered well supported evidence 
for
venom delivery systems in fossil taxa" But, in fact, Folinsbee et al. suggest 
that the hypothesis of venomousness is only "fairly well supported", in two 
extinct species, one therocephalian with hollow, tubular, teeth, and a soricine 
with
 strong similarities to Solenodon. They urge caution in assigning venomousness 
to extinct species. In particular they point out that grooved teeth are widely 
distributed in non - venomous species.

4) Maxillary pitting
The authors describe a new feature, a "subfenestral fossa", marked by a pitted 
area of the maxilla, ventral to the anterior antorbital fenestra. They do not 
mention that this pitting is much more extensive, and that there may be no 
distinction in the amount of pitting across the entire antorbital fossa. Xu&Wu 
(2001) describe "a number of pits and ridges on the anterolateral surface of 
the antorbital fossa" (see Fig. 4B), but they do not reconstruct any distinct 
subfenestral fossa. Moreover, some observers believe that this pitting is an 
artifact of preparation, where the extremely thin outermost layer of bone has 
been damaged in
 several specimens (Mark Norell, personal communication).

5) Plucking specializations
The authors also suggest that the premaxillary teeth functioned in plucking the 
feathers off of Sinornithosaurus' prey, and that the maxillary teeth are 
exceptionally long in order to penetrate the feathers. In neither case do they 
offer examples of other animals that have specialized teeth for preying on 
birds or processing feathers. Indeed, one might speculate that the premaxillary 
teeth of Sinornithosaurus functioned to preen the animal's OWN feathers, rather 
than pluck off the feathers of its prey, and one would have made an equally 
strong inference, with exactly no support. This tendency of the authors to 
assign functions to characters without support seems particularly unscientific.

Overall, the paper seems to be more an attempt to paint a colorful functional 
scenario, rather than to make a rigorous scientific study of the material. It 
is weakened by overstatement and speculation.