Tuesday, March 25, 2014

Body size and forelimb length

Bird-like characteristics are found in basal Paraves. They are not found in dinosaurs.
That is because basal paraves are not descended from dinosaurs. They are descended from pterosaurs.

Let's look at body size and forelimb length.
Notice that the changes appear for the first time at the origin of Paraves (not earlier).

Mark Puttick and colleagues investigated the rates of evolution of the two key characteristics that preceded flight: body size and forelimb length.  In order to fly, hulking meat-eating dinosaurs had to shrink in size and grow much longer arms to support their feathered wings. 
"We were really surprised to discover that the key size shifts happened at the same time, at the origin of Paraves," said Mr Puttick of Bristol's School of Earth Sciences.  "This was at least 20 million years before the first bird, the famous Archaeopteryx, and it shows that flight in birds arose through several evolutionary steps." 
Being small and light is important for a flyer, and it now seems a whole group of dozens of little dinosaurs were lightweight and had wings of one sort or another. Most were gliders or parachutists, spreading their feathered wings, but not flapping them.                                               
'High rates of evolution preceded the origin of birds' by Puttick, M.N., Thomas, G.H., and Benton, M.J. in Evolution: DOI: 10.1111/evo.12363
The origin of birds (Aves) is one of the great evolutionary transitions. Fossils show that many unique morphological features of modern birds, such as feathers, reduction in body size, and the semilunate carpal, long preceded the origin of clade Aves, but some may be unique to Aves, such as relative elongation of the forelimb. We study the evolution of body size and forelimb length across the phylogeny of coelurosaurian theropods and Mesozoic Aves. Using recently developed phylogenetic comparative methods, we find an increase in rates of body size and body size dependent forelimb evolution leading to small body size relative to forelimb length in Paraves, the wider clade comprising Aves and Deinonychosauria. The high evolutionary rates arose primarily from a reduction in body size, as there were no increased rates of forelimb evolution. In line with a recent study, we find evidence that Aves appear to have a unique relationship between body size and forelimb dimensions. Traits associated with Aves evolved before their origin, at high rates, and support the notion that numerous lineages of paravians were experimenting with different modes of flight through the Late Jurassic and Early Cretaceous.

Note that in the following two references the researchers are working within the dino to bird paradigm. To make the evidence fit the theory they are also forced to claim improbable rates of evolution.

Mike Lee et al

Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds

Recent discoveries have highlighted the dramatic evolutionary transformation of massive, ground-dwelling theropod dinosaurs into light, volant birds. Here, we apply Bayesian approaches (originally developed for inferring geographic spread and rates of molecular evolution in viruses) in a different context: to infer size changes and rates of anatomical innovation (across up to 1549 skeletal characters) in fossils. These approaches identify two drivers underlying the dinosaur-bird transition. The theropod lineage directly ancestral to birds undergoes sustained miniaturization across 50 million years and at least 12 consecutive branches (internodes) and evolves skeletal adaptations four times faster than other dinosaurs. The distinct, prolonged phase of miniaturization along the bird stem would have facilitated the evolution of many novelties associated with small body size, such as reorientation of body mass, increased aerial ability, and paedomorphic skulls with reduced snouts but enlarged eyes and brains.



Stephen Brusatte et al
Gradual Assembly of Avian Body Plan Culminated in Rapid Rates of Evolution across the Dinosaur-Bird Transition
The evolution of birds from theropod dinosaurs was one of the great evolutionary transitions in the history of life [ 1–22 ]. The macroevolutionary tempo and mode of this transition is poorly studied, which is surprising because it may offer key insight into major questions in evolutionary biology, particularly whether the origins of evolutionary novelties or new ecological opportunities are associated with unusually elevated “bursts” of evolution [ 23, 24 ]. We present a comprehensive phylogeny placing birds within the context of theropod evolution and quantify rates of morphological evolution and changes in overall morphological disparity across the dinosaur-bird transition. Birds evolved significantly faster than other theropods, but they are indistinguishable from their closest relatives in morphospace. Our results demonstrate that the rise of birds was a complex process: birds are a continuum of millions of years of theropod evolution, and there was no great jump between nonbirds and birds in morphospace, but once the avian body plan was gradually assembled, birds experienced an early burst of rapid anatomical evolution. This suggests that high rates of morphological evolution after the development of a novel body plan may be a common feature of macroevolution, as first hypothesized by G.G. Simpson more than 60 years ago [ 25 ]. 

Monday, March 17, 2014

Harry Govier Seeley - Pterosaurs to Birds

The idea that birds are descended from pterosaurs has an interesting history. The idea was proposed by Harry Govier Seeley. But note the negative reaction he received. Not much has changed in that respect.

Seeley was also an authority of pterosaurs, and in 1901 published a popular book on the subject, Dragons of the Air. In it, he gave an overview of animal flight, reptiles, the discovery of pterosaurs and pterosaur skeletal structure. He initially believed that birds descended from pterosaurs, but under intense criticism from his peers, backed off this assertion and argued that they shared common ancestry. "It would therefore appear from the vital community of structures with Birds, that Pterodactyles and Birds are two parallel groups, which may be regarded as ancient divergent forks of the same branch of animal life," he wrote. 

His popular book on Pterosaurs, Dragons of the Air (1901) found that birds and pterosaurs are closely parallel. His belief that they had a common origin has been proved, for both are archosaurs, just not as close as he thought.
I suggest that pterosaurs and birds are as close as Seeley thought. 

Ornithodesmus (meaning "bird link") is a genus of small, deinonychosaurian dinosaur from the Isle of Wight in England, dating to about 125 million years ago. The name was originally assigned [by Harry Govier Seeley] to a bird-like sacrum (a series of vertebrae fused to the hip bones), initially believed to come from a pterosaur. More complete pterosaur remains were later assigned to Ornithodesmus, until recently a detailed analysis determined that the original specimen in fact came from a small theropod, specifically a dromaeosaur. All pterosaurian material previously assigned to this genus has been renamed Istiodactylus.

By the 1840s, however, there was little doubt that Cuvier had been correct, and some naturalists were very impressed by resemblances between the skeletons of the flying fiends [pterosaurs] and birds. As Richard Owen stated in an 1874 monograph of Mesozoic fossil reptiles:
Every bone in the Bird was antecedently present in the framework of the Pterodactyle; the resemblance of that portion directly subservient to flight is closer in the naked one to that in the feathered flier than it is to the forelimb of the terrestrial or aquatic reptile.
Just like Owen, Seeley saw no way to “evolve an ostrich out of an Iguanodon,” but Huxley turned the argument from convergence against his opponents. The traits supposedly shared between birds and pterosaurs had to do with flight, and given that both lineages had become adapted to flying, common traits in their skeletons were to be expected. The diagnostic traits in the hips, legs, and feet of dinosaurs, on the other hand,were found in all birds, not just ground-dwelling ones. This meant that these characters marked a true family relationship and not just a shared way of life.

Saturday, March 15, 2014

Hyposphene-hypantrum articulations

Dinosaurs had hyposphene-hypantrum articulations. Basal Paraves did not.  


The hyposphene-hypantrum articulation is an accessory joint found in the vertebrae of several fossil reptiles of the group Archosauromorpha. It consists of a process on the backside of the vertebrae, the hyposphene, that fits in a depression in the front side of the next vertebrae, the hypantrum. Hyposphene-hypantrum articulations occur in the dorsal vertebrae and sometimes also in the posteriormost cervical and anteriormost caudal vertebrae.[1]
Hyposphene-hypantrum articulations were present in the derived and birdlike dromaeosaurid Rahonavis, but are lost [not present] in modern day's birds, probably due to their highly modified vertebrae.[4]
Early Dinosauromorphs (early ancestors of dinosaurs) like MarasuchusLagosuchus and Euparkeria as well as ornithischian dinosaurs lack hyposphene-hypantrum articulations. Because these articulations are absent in both saurischian ancestors and all non-saurischian dinosaurs, they are considered a synapomorphy (a distinctive feature) of the Saurischia, as proposed by Gauthier (1986).[4] Hyposphene-hypantrum articulations are found in all the basal members of the Saurischia.[5] However, they became lost in several saurischian lineages. They were present in the derived and birdlike dromaeosaurid Rahonavis, but are lost in modern day's birds, probably due to their highly modified vertebrae.[4] Within the Sauropodomorpha, they were present in prosauropods and most sauropods, but became independently lost in two cretaceous sauropod lineages, the Titanosauria and the Rebbachisauridae.[1][3]


Avialan characters of scansoriopterygids include: Hyposphene-hypantrum articulations in trunk vertebrae absent (according to Senter).

Chiappe (2001) united the Pygostylia in possessing four unambiguous synapomorphies. The trait that gives the group its name is the presence of a pygostyle. Next is the absence of a hyposphene-hypantrum. Next is a retroverted pubis separated from the main axis of the sacrum by an angle of 45 to 65 degrees. Last is a bulbous medial condyle of the tibiotarsus.

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 (www.skeletaldrawing.com).

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).


Page 440
the [paravian] primitive fourth trochanter present in archosaurs, dinosaurs and theropods was much reduced