Basal paraves had a partially closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Pterosaurs had a completely closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Dinosaurs had a completely open acetabulum that did not allow them to abduct (splay, sprawl) their legs.
Pterosaurs had a completely closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Dinosaurs had a completely open acetabulum that did not allow them to abduct (splay, sprawl) their legs.
Dinosaur to bird theorists focus on whether the acetabulum had a hole in it or not. From that point of view birds and dinosaurs were somewhat similar in that they both had a hole in the acetabulum. (Although one had a partially closed acetabulum, the other a completely open acetabulum).
But that overlooks the important issue which is whether the acetabulum was cup-like or not. From that point of view dinosaurs are not like birds. While pterosaurs in that respect are like birds.
This is significant because dinosaurs could not splay their legs, but basal paraves and pterosaurs could
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PTEROSAUR
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The hip joint of pterosaurs is more mobile and profoundly different from that of theropods but is reminiscent of that of mammals, especially of humans, allowing a wide range of adduction and abduction in the vertical plane
The femoral component [of the Anhanguera pterosaur femur] is a well defined spherical head which is distinctly separated from the shaft by a narrow non-articular neck at an obtuse angle of 160 degrees.
The head forms a ball and socket joint with the close-fitting, shallow and imperforate acetabulum.
In this orientation, there is no overhanging acetabular rim to support the femur in a vertical position.
http://link.springer.com/article/10.1007%2Fs10336-014-1098-9
ReplyDeleteJurassic archosaur is a non-dinosaurian bird
Stephen A. Czerkas, Alan Feduccia
Unlike theropod dinosaurs, invariably exhibiting a
completely perforated and open acetabulum, Scansoriopteryx
has a partially closed acetabulum, and no sign of a
supra-acetabular shelf or an antitrochanter. Along with the
mostly enclosed acetabulum indicated by the surface texture
of the bone within the hip socket, the proximally
oriented head of the femur is functionally concordant with
a closed or partially closed acetabulum and with sprawling
hindlimbs.
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Do you have anything to contribute on the subject of the acetabulum?
Deletehttp://www.ucmp.berkeley.edu/diapsids/dinomm.html
ReplyDeleteQuote:
One important dinosaurian synapomorphy is the perforate [completely open] acetabulum, simply a "hip bone" (actually three connected bones, together called the pelvis) with a hole in the center where the head of the femur ("thigh bone") sits. This construction of the hip joint makes an erect stance (hindlimbs located directly beneath the body) necessary — like most mammals, but unlike other reptiles which have a less erect and more sprawling posture. Dinosaurs are unique among all tetrapods in having this perforate [completely open] acetabulum.