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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
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
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
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.