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RE: Additional material on the Sinornthosaurus venom report
I just made this long comment on the blog page that Tom Holtz cited (slight
formatting changes here):
>From the paper:
"Interestingly, much of the effective erupted length of the teeth is
composed of the tooth root. The erupted portion of the largest maxillary
tooth in the type specimen of S. millenii (IVPP V12811) measures 12 mm long
and occupies the seventh alveolus. There is a distinct groove on the labial
side running from the base of the root to the tip. The tooth crown is not
really as elongated as it appears because of a hyper-erupted tooth root, and
the tooth sockets are not especially deep."
This tends to support Dr Holtz's suggestion that the teeth have slipped
partly out of their alveoli; the authors' claim that "the tooth sockets are
not especially deep" requires quantification before the unusual condition of
deeply exposed roots (in life) should be inferred.
The roots of teeth in archosaurs are hollow, and the cavity tapers upward
inside the crown to some extent, varying through the cycle of tooth
replacement as new mineral layers are deposited on the inner surface. Since
the Sinornithosaurus fossils are strongly compressed on a bedding plane of
fine-grained sediment, individual bones have undergone plastic deformation.
This lateral compression of hollow teeth can be expected to produce lateral
grooves; I'd predict them in any small archosaur with this kind of
preservation, as an artifact. However, such grooves would not be expected to
reach the tip of the tooth, and despite the statement in the paper they
actually don't seem to do so in this fossil. Also not mentioned is that
grooves are also present on the lingual surface (anterior dentary teeth in
fig. 2), equally compatible with compression artifact but not supporting the
idea that the lateral groove is specialized for venom conduction. The
grooves look broad and shallow enough to be artifacts, rather than having
distinct margins formed by enamel ridges.
Also, there's nothing in lizards or snakes analogous to the subfenestral
fossa of Sinornithosaurus; it looks more like evidence of pneumatization, or
possibly a salt gland (or maybe a specialized sensory structure like the pit
organ of crotalid snakes), than a venom gland. The posterior orientation of
the fossa canal (supposedly carrying the venom duct to the tooth bases) is
exactly in the wrong direction for that function, so it would need to make a
hairpin bend and then run forward for the full length of the fossa. We can't
expect perfect design (cf. recurrent laryngeal nerve), but the venom duct
hypothesis doesn't predict or explain this feature very well.
Snakes that prey on birds tend to have very long, slender and weakly
recurved anterior teeth on both the maxilla and dentary; the dentary teeth
here are practically isodont, while the largest uppers are in the middle of
the jaw, not the front. (The authors refer to the upper dentition as
'heterodont', but that's the wrong term because they vary in size but hardly
in morphology; they should have written 'anisodont'.) Even if the teeth
really were as long in life as they appear in the fossil, it's typical
dromaeosaur dentition and no analogy suggests a specialized diet compared to
other small theropods.
So I guess I'm not convinced by the paper (even that it should have been
published at this stage without further tests), but there are lots of
testable ideas in there.
Dr John D. Scanlon, FCD
Riversleigh Fossil Centre, Outback at Isa
"Get this $%#@* python off me!", said Tom laocoonically.