Sunday, March 9, 2014

Caudal Rods (2)
In the tails of dromaeosaurid dinosaurs and rhamphorhynchid pterosaurs, elongate osteological rods extend anteriorly from the chevrons and the prezygapophyses. These caudal rods are positioned in parallel and are stacked dorsoventrally.
And caudal muscles:
Remember the quickly reduced neural spines, caudal ribs, and chevrons? Those all indicate that the caudal muscles of both dromaeosaurids and pterosaurs were substantially reduced.
The basal paraves Scansoriopteryx also had caudal rods.

That is very strong evidence for the pterosaur to bird theory.


The tails of pterosaurs and dromaeosaurids are so similar that, in the fossil-forging black-markets of China, the tail of one is often used to “complete” a partial skeleton of the other. Skeletal image of Rhamphorhynchus courtesy of Scott Hartman (

For comparison:


Fourth trochanter on femur:

The fourth trochanter is a shared characteristic common to archosaurs. It is a knob-like feature on the posterior-medial side of the middle of the femur shaft that serves as a muscle attachment, mainly for the Musculus caudofemoralis longus, the main retractor tail muscle that pulls the thighbone to the rear.

Also, a large process on the shaft of the [archosuar] femur, the fourth trochanter, served as the attachment point for major tail muscles, the caudofemoralis group of thigh retracting muscles.

It is now also possible to think a step further and consider the muscles of the tail. Let’s first try to do that in very general qualitative terms. Remember the quickly reduced neural spines, caudal ribs, and chevrons? Those all indicate that the caudal muscles of both dromaeosaurids and pterosaurs were substantially reduced.
To help consider the problem quantitatively, a technique I used was to create digital models of the tail skeleton of a Velociraptor and a Rhamphorhynchus (a pterosaur) and to sculpt the corresponding muscles over the skeletal models. The results of this modeling concur with the qualitative inference. In particular, raptors and pterosaurs were found to have very weak caudofemoral muscles (indeed, some pterosaurs may not have had caudofemoral muscles at all).

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the [paravian] primitive fourth trochanter present in archosaurs, dinosaurs and theropods was much reduced

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