Sunday, 2 May 2010

Were Deinonychosaurians and Avialians descended from an aboreal, sickle toed, four winged ancestor?

Above; Pedopenna is a typical example of a primitive Parave. It has four "wings", with long feathers on the hind and forelimbs. Uniquely this animal comes from the mind Jurassic of China and as a result lived at an earlier time than Archaeopteryx. Pedopenna was more primitive than later Paraves. It had a less developed "killing claw" and the feathers were symmetrical unlike the aerofoil asymmetrical feathers of other Paraves.

All early members of the Paraves share the features described in the title. Paraves as a group includes the raptors or Deinonychosaurs such as the Troodontids and Dromaeosaurs, and the Avialians which are birds and any other dinosaurs which have a closer common ancestry with birds than Deinonychosaurs.

Above; the skeleton of Anchiornis reconstructed. Troodontids had shorter arms and longer legs than other Paraves.

One of the earliest members of the Troodontidae is Anchiornis, a small animal from the late Jurassic of China's remarkable Liaoning province. Here, in a lake filled valley below a volcano, various animals, choked by ash, were washed into these lakes and buried in a sterile volcanic tomb. Because these creatures were covered over so quickly soft tissues were preserved. Anchiornis is covered in very advanced feathers on both its forelimbs and hind limbs. These "wings" would have probably allowed it to glide from tree to tree like a modern day gliding squirrel. The tail is also shortened and stiffened to enable more precise maneuverability amongst the branches and its arms are long for gripping on to branches. The pubis is pointed vertically as opposed to foreword in most meat eating dinosaurs, a step towards the bird like condition of a backward pointing pubis. The braincase and eyes are also enlarged, giving Anchiornis an almost bird like intelligence about it. In fact Troodontids are hailed as being the most intelligent dinosaurs that have ever lived on our planet, with brain relative to body ratios far surpassing other animals of their time.

Above; the advanced ground dwelling Troodontid Troodon from North America illustrates the general trend followed by Deinonychosaurian Paraves towards a predatory ground dewlling existence.

The retractable sickle shaped claw on the second toe was almost certainly present in Anchiornis as in other Troodontids, but was not as large as the classic raptorial claws of creatures like Velociraptor and was probably being used to help it to grip onto branches rather than to capture prey. The relatively long hind legs of Anchiornis made it unusual amongst basal Paraves and is a unique Troodontid condition that was probably the group's first step towards living a grounded existence as its descendants did. Other similar Troodontids have been found, but Anchiornis is the most unique in the respect that its melanosomes or pigment cells were preserved in its feathers, making its colouration deductible. Like modern birds Anchiornis clearly used its feathers for display; it possessed a huge brown crest of feathers on its head.

Above; Jinfengopteryx was once mistaken for a bird, but the long legs and short arms suggest it is a Troodontid.

Indications of a more bird like diet in Troodontids are seen in the seedy stomach contents of Jinfengopteryx. For a while this little Cretaceous Chinese dinosaur was mistaken for a bird, but recent analysis confirms its Troodontid position. The presence of seeds as part of this animals diet suggests that other Troodontids may not have been as carnivorous as was previously thought and probably lived much like bears, taking every opportunity to provide themselves with the energy to maintain a large bird like brain by becoming omnivorous.

Above; the foot of the advanced ground dwelling Dromaeosaurid Deinonychus. The inner toe has the large curved claw of its climbing ancestors, which in Dromaeosaurs has become greatly enlarged.

The Dromaeosaurs were the only group of Paraves to enlarge their second toe claw to the extent that they had, using it to its full potential as seen in the movie Jurassic Park being used to slash prey apart, but these gory portrayals are unfortunately fiction, not science. Rather than being used to climb trees as their ancestors did the Dromaeosaurs climbed their victims. The angulation of the curvature of their "killing claw" (160 degrees) matches that of a climbing animal rather than a ground dwelling slasher like a cassowary, and rather than being sharp sided as a slicing implement would be, the the claw has a blunt edge, but sharp tip. All this suggests that the Dromaeosaurian "killing claw" was a gripping claw rather than a slashing claw, a vestigial left over from their arboreal ancestors that was now used to grip on to and grab prey.

Above; a typical early arboreal Dromaeosaur from China; Microraptor.

Microraptor, an early Dromaeosaur from Liaoning was basically identical to Anchiornis in every way except perhaps its slightly shorter legs; Microraptor was probably more well adapted for an arboreal existence than Anchiornis because of this feature. Wind tunnel analysis of Microraptor models suggest that its winged hind legs played an important role in Microraptor's gliding cycle. The legs would swing foreword orienting the front wings upwards to give one final burst of lift before landing.

Above; the skeleton of the Madagascan flying Dromaeosaur, Rahonavis. Darker bones are speculative, but the lighter yellow bones represent the parts that were recovered from the fossil site.

An Uelangine Dromaeosaur from the late Cretaceous of Madagascar called Rahonavis is particularly interesting because it seems to have been capable of powered flight. The arm bones are much stronger than those of Archaeopteryx and are adorned with quill knobs for feather attachment. Rahonavis was clearly just as good as, if not better at flying than Archaeopteryx; the first "bird". It is ironic to think that a creature more closely related to Jurassic Park's Velociraptor was a better flier than some of the earliest birds. If Rahonavis is really a Dromaeosaur then it is possible that flight had evolved in Dinosaurs before Dromaeosaurs split off from the main bird family line and are therefore secondarily flightless animals. The gliding stage of Dromaeosaurs like Microraptor may not have been a step towards flight, but a step away from it. By 65 million years ago at the very end of the dinosaurs reign, all Dromaeosaurs were flightless ground dwelling carnivores rather than bird like climbers (except Rahonavis).

Above; the skeleton of Archaeopteryx is almost identical to that of the primitive Dromaeosaurs and Troodontids.

Archaeopteryx from the late Jurassic of Germany is commonly placed in the Avialian group with birds. Although this may not be true it possesses no features with either specifically Dromaeosaurs or Troodontids that aren't shared by all three, but does possess features shared with Deinonychosaurs (Troodonts and Dromaeosaurs) that are not present in birds: Archaeopteryx has a stiffened tail and a vertically oriented pubis. The boot of the pubis in all Paraves extends towards the ischium and in true birds fuses with the ischium completely. More recently discovered similarities between Dromaeosaurs, Troodontids and Archaeopteryx include lengthened feathers on the hind limbs and a retractable second toe. Although it lacks a large claw as in Deinonychosaurs one of the roller joints on this toe is extended far dorsally, allowing it to be flexed backwards. It is likely that Archaeopteryx did not really need this feature and that it was vestigial, birds certainly do not need it due to their array of foot tendons and this suggests a closer relationship with Avialians that Deinonychosaurs. On the other hand Archaeopteryx may have been evolving towards the sickle clawed condition seen in Deinonychosaurs. This feature was certainly useful as a climbing aid and by keeping the second toe off the ground and free of wear the first stage towards maintaining a "killing claw" was complete. If this is true then Archaeopteryx was more primitive than both Avialians and Deinonychosaurs.

I will try to explain the opposition to the theory of Deinonychosaurs and Avialians sharing a common volant ancestor in a later post, but for now my work is done. (The evolutionary relationships of Mahakala are hard to get your head around on a Sunday afternoon.)

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