Friday, February 7, 2014

Caudal Rods (1)

Let's turn to the topic of "caudal rods" in the long-tailed pterosaurs and the dromaeosaurs:
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. The fully articulated and three-dimensionally preserved caudal series of some dromaeosaurid specimens show that individually these caudal rods were flexible, not rigid as previously thought. However, examination of the arrangement of the caudal rods in cross-section indicates that the combined effect of multiple caudal rods did provide substantial rigidity in the dorsoventral, but not in the lateral, plane. The results of digital muscle reconstructions confirm that dromaeosaurids and rhamphorhynchids also shared greatly reduced caudofemoral muscles in the anterior tail region. The striking similarities between the tails of dromaeosaurids and rhamphorhynchids suggest that both evolved under similar behavioral and biomechanical pressures. Combined with recent discoveries of primitive deinonychosaurs that phylogenetically bracket the evolution of dromaeosaurid caudal rods between two arboreal gliding/flying forms, these results are evidence that the unique caudal morphologies of dromaeosaurids and rhamphorhynchids were both adaptations for an aerial lifestyle.
Here is a link to the full pdf.

At the start, the tail skeleton of Deinonychus appears normal. Just past the hips, the neural spines, caudal ribs, and chevrons all have a typical shape and they all project to a respectable extent. But, as the tail progresses towards the tip (and it doesn’t take long) things start to get weird. The neural spines, caudal ribs, and chevrons all shrink in, with the former two disappearing entirely . . . and then come the caudal rods. Both the vertebrae and chevrons abruptly develop pairs of elongated rods of bone that project towards the hips. These rods are slender, but very long (the longest easily overlap seven other sequential vertebrae), and they split, each becoming two still thinner rods. Together, rods of the vertebrae form a quiver that virtually encapsulates the dorsal (upper) portion of the tail, and together the rods of the chevrons do the same to the ventral (lower) portion.
The tail of Deinonychus and its raptor relatives is bizarre, but it is not (as Professor Ostrom himself realized) unique. Among all known vertebrates, a similar tail anatomy has evolved in one other group. . . and now we come to why I have been allowed to spend so much time discussing dinosaurs on what is supposed to be a blog about pterosaurs.
While later and more advanced pterosaurs (like Pterodactylus) only had short, stubby tails, early pterosaurs had long ones. The caudal skeletons of these long-tailed pterosaurs (with the exceptions of the dimorphodontids and very primitive forms) are strikingly similar to that of Deinonychus. In the case of long-tailed pterosaurs, the function of the caudal rods has always seemed obvious. As flying animals, increased rigidity would have helped a tail to serve as a stabilizer or as a rudder.
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 aVelociraptor 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).
In Professor Ostrum’s description of Deinonychus, he expressed his interest in considering this striking example of convergent evolution in a later study. Regrettably, however, he never got around to it -- after all, he soon had a revolution on his hands.
However in 2010, Scott Persons, a graduate student from the University of Alberta proposed that Tyrannosaurus's speed may have been enhanced by strong tail muscles.[108] He found that theropods such as T rex had certain muscle arrangements that are different from modern day birds and mammals but with some similarities to modern reptiles.[109]
Cal King comment:
Archaeopteryx has a large number of bird-like characters, besides feathers, that are not found in any dinsoaur. These include a reversed hallux (first toe), crocodilie teeth (the same type found recently in a mutant chicken embryo), birdlike skull, movable quadrate, and the socket for the arm bone in the shoulder is pointed outward (in dinosaurs they are pointed downward because the legs of dinosaurs are tucked underneath the body) and retroverted pubis (not found in theropods but are found in ornithischian dinosaurs, which are not considered close relatives of birds). Archaeopteryx and all other birds also lack the caudofemoralis muscle, which is found in dinosaurs and crocodilians. The caudofemoralis muscle is attached to the femur or the thigh bone and to the tail, as the name suggests, so that the tail moves when the legs move. Instead, Archaeopteryx has suprapubic muscles, which connect the tail to the pubes but not to the leg bones, a quite different arrangement than the caudofemoralis muscle in theropod dinosaurs.

Another Cal King comment:
We do know that Archaeopteryx and Caudipteryx lack the M. caudofemoralis. That makes sense if Archaeopteryx is a flyer and Caudipteryx is a secondarily flightless bird.
Early birds like Archaeopteryx and Microraptor had long tails and hindlimb wings to help generate additional lift. A long tail with feathers can generate more lift if it is light weight than if it is heavy and muscular. Since a flyer does not need a muscular tail to balance it as it runs, losing the M. caudofemoralis is not maladaptive but adaptive in a primarily arboreal and volant animal. Even though Caudipteryx is most likely a flightless animal, it still retains the ancestral condition of the loss of the M. caudofemoralis. As Louis Dollo points out, evolution is irreversible. Caudipteryx cannot simply re-evolve the lost M. caudofemoralis. OTOH, if Caudipteryx was really a dinosaur that did not have a flying ancestor, then it makes no sense for it to lose such an adaptive feature as the M. caudofemoralis.

I will post more material on this topic in the next post.

No comments:

Post a Comment