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Re: Differences between *Vancleavea* and thalattosaurs
All well and good Jaime.
Let's take your points one at a time:
thecodont dentition - also in Miodentosaurus Askeptosaurus
interdental plates -- also in Miodentosaurus Askeptosaurus
lack of post-axial intercentra - also in Miodentosaurus Askeptosaurus
femur with medially inflected head - from Nesbitt et al. 2009: "The proximal
head is poorly defined, being expanded but continuous with the shaft."
sigmoidal femoral shaft - also in Miodentosaurus Askeptosaurus, but to your
point, maybe not so much.
lack of intertrochanteric fossa on femur - also in Miodentosaurus Askeptosaurus
presence of osteoderms - a-ha! got me... except the outgroup includes
placodonts, so there you go.
Check out the literature. Miodontosaurus in particular.
While it is sometimes important to protect the status quo, some things just
On Dec 5, 2009, at 2:35 PM, Jaime Headden wrote:
> In addition to all this, following Parker and Barton (the OTHER paper on
> *Vancleavea campi*), *V. campi* also possesses thecodont dentition, separate
> and distinct interdental plates (also contingent on but elaborate to the
> presence of thecodonty), and (P&B, p.13):
> "Unambiguous synapomorphies supporting the inclusion of *Vancleavea campi*
> into Archosauriformes include the lack of post-axial intercentra, a femur
> with a medially inflected head, a sigmoidal femoral shaft, the lack of a
> distinct intertrochanteric fossa on the proximoventral surface of the femur,
> and the presence of osteoderms."
> Despite *V. campi* being _another_ apparently aquatically derived taxon
> (such that it SHOULD resemble things like pachypleurosaurs, mesosaurs,
> nothosaurs, metriorhynchoid crocs, etc.) *V. campi* is distinguihsed from
> them by a host of features that ensures that it is not, in fact, a
> nonarchosauriform diapsid. That is at least based so far on the two cladistic
> analyses to included it with the new data (Parker and Barton, 20009 and
> Nesbitt et al., 2009).
> Parker, W. G. and B. J. Barton. 2009. New information on the Upper Triassic
> archosauriform *Vancleavea campi* based on new material from the Chinle
> Formation of Arizona. _Paleontologica Electronica_ 11(3)[14A]:20p.
> Jaime A. Headden
> "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)
> "Human beings, who are almost unique in having the ability to learn
> from the experience of others, are also remarkable for their apparent
> disinclination to do so." --- Douglas Adams (Last Chance to See)
> "Ever since man first left his cave and met a stranger with a
> different language and a new way of looking at things, the human race
> has had a dream: to kill him, so we don't have to learn his language or
> his new way of looking at things." --- Zapp Brannigan (Beast With a Billion
>> Date: Sat, 5 Dec 2009 14:48:12 +0100
>> From: email@example.com
>> To: firstname.lastname@example.org
>> Subject: Differences between *Vancleavea* and thalattosaurs
>> Judging *Vancleavea* from the illustrations here:
>> - Thalattosaurs retain the supratemporal. *Vancleavea* lacks it.
>> - Thalattosaurs have an open lower temporal arch (the caudal process of
>> the jugal doesn't reach very far back, so the lower temporal fenestra is
>> open ventrally); I'm not even sure if they have a quadratojugal at all
>> (none is in any case preserved in *Miodentosaurus*). It's closed in
>> *Vancleavea*, the qj is large, and the caudal process of the j is very
>> broad dorsoventrally, much unlike the very delicate T-shaped jugals of
>> - The maxilla contacts the naris in thalattosaurs. In *V.*, they are
>> separated by a huge caudodorsal process of the premaxilla.
>> - Thalattosaurs have a very large pineal foramen. *V.* lacks any trace
>> of it.
>> - Thalattosaurs retain caudal ribs -- real free mobile ribs on the first
>> few tail vertebrae. Can't see such a thing in the low-resolution
>> reconstruction of *V.*.
>> *V.* shares all these character states with all other archosauriforms,
>> except for the thin jugal of... maniraptoriforms or something.
>> Both, however, have a single bone called the "postorbital" in *V.* and
>> the "postfrontal" in thalattosaurs... The quadrate looks similar, too,
>> differing from the pillar dinosaurs have, but I'm not familiar with
>> enough diapsid quadrates to tell what that means.
>> - In thalattosaurs the rostral margin of the orbit is formed by the
>> maxilla. In *Vancleavea*, the prefrontal does that, and (perhaps because
>> the orbit is so remarkably small) it even contacts the jugal, excluding
>> the maxilla from the orbit altogether. Or is the "prefrontal" of *V.*
>> actually the lacrimal, which is altogether missing in thalattosaurs (or
>> at least *Miodentosaurus*)?
>> - *V.* has very long hemal spines. Thalattosaurs never.
>> - *V.* has considerably smaller limbs, especially hands & feet, than
>> Finally, *V.* is much smaller than *Miodentosaurus*. This means that
>> things like the relatively much smaller orbit of *V.* (complete with
>> taller jugal and maxilla) or its relatively much shorter extension of
>> the jaw joints caudal to the occiput cannot be size-related -- the
>> opposite relationship would be expected.
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