Eosinopteryx – part 3 – to scale

Sometimes it just helps
to see taxa to scale with possible sisters (Eosinopteryx in this case, Fig. 1). Smaller than Anchiornis, Eosinopteryx also had a shorter snout, deeper mandible, more robust postorbital, longer torso, shorter tail, larger chevrons, more robust clavicle, coracoid and scapula, shorter forelimb, smaller deltopectoral crest and shorter pubis.

Figure 1. Eosinopteryx and kin to two scales.

Huaxiangnathus shared with Eosinopteryx a short coracoid, but little else is a closer match than the similarly built Anchiornis. Godefroit et al. (2013) reported, “The straight and closely aligned ulna-radius of Eosinopteryx also means that pronation/supination of the manus with respect to the upper arm would have been limited; combined with the absence of a bony sternum and weakly developed proximal humerus, these attributes suggest that Eosinopteryx had little or no ability to oscillate the arms to produce a wing beat.”

Funny that they didn’t even mention the short coracoid.
The locked down elongate coracoid is a hallmark of flapping tetrapods (pterosaurs and birds) and an elongate clavicle does the same thing in bats.

According to the Sinkkonen illustration (Fig. 1), the radius and ulna are likewise essentially straight in Anchiornis, which is the plesiomorphic condition, as shown by Huaxiagnathus and other theropods. The ulna is barely bowed in Archaeopteryx and more greatly bowed in subsequent flapping taxa, including oviraptorids (by convergence?). Xiaotingia (Fig. 2), the outgroup to Anchiornis + Eosinopteryx, also has a short rostrum, but also a greatly bowed anterbrachium and a locked-down coracoid. I suspect a change in tree topology may be warranted or else we’ll learn something here about reversals.

I have asked for the matrix.

Figure 2. Xiaotingia the outgroup to Anchiornis + Eosinopteryx + other Troodontidae. Red arrow points to bowed antebrachium. DGS enabled identification of the coracoids, which are elongated here. This is a flapping theropod. Inset shows previous tracing that did not identify two coracoids.

The bowed antebrachium
produces a parallelogram in living birds that serves to automatically extend and fold the manus bearing the outer flight feathers with flexion/extension of the elbow. Prior to this, muscle power would have to extend and bend the wrist, independent of the flexion/extension of the elbow.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Godefroit P, Demuynck H, Dyke G, Hu D, Escuillié FO and Claeys P. 2013. Reduced plumage and flight ability of a new Jurassic paravian theropod from China. Nature Communications 4: 1394. doi:10.1038/ncomms2389

wiki/Eosinopteryx

Eosinopteryx – part 2 – Better Resolution = Better Reconstruction

Yesterday we looked at Eosinopteryx (Godefroit et al. 2013, Middle-Late Jurassic, Tiaojishan Formation) and discussed a possible new nesting site (Fig. 1) based on a lack of included short-coracoid taxa preceding Archaeopteryx to compare it with. By comparison, Cosesaurus has a “flapping”-type coracoid and it has much less wing tissue trailing its front limbs. So the long, locked-down coracoid in bird predecessors was among the last traits to evolve, post-dating the appearance of elongated forelimb feathers.

Figure 1. Supplementary information from Godefroit et al. (2013) showing their nesting of Eosinopteryx with Anchiornis, but the tree lacks several short-coracoid taxa that might provide more parsimonious nesting sites.

Addendum: The analysis of Godefroit et al. (2013) was based on and provided only a segment of an earlier analysis that DID include these more primitive taxa. Thus my doubt is reduced considerably as all pertinent taxa were included. The skull on DGS

Not sure if a skull reconstruction was provided by Godefroit et al. (2013) or not. (not) I’d still like to see the paper. (I have the paper now. ) Even so, in order to create yesterday’s reconstruction I applied DGS (digital graphic segregation) to the skull (Fig. 2). I don’t know birds as well as dinosaurs and did not attempt a palate reconstruction. If I made any mistakes, please send me an email. (Figure 2 is an update based on higher resolution images of an earlier posted figure.)

Figure 2. The skull of Eosinopteryx in situ (above), traced using DGS (middle), and as presented by Godefroit et al. 2013). The red bone is the quadrate sticking through the mandibular fenestra, which was purportedly missing. See Figure 3 for reconstruction and bone identification. While my tracing appears to be chaotic, every bone was traced on a separate and segregated layer.

Above
The skull of Eosinopteryx traced using Photoshop, a process known as DGS or Digital Graphic Segregation. For followers of this blog, these updated images reflect the importance of high resolution data in using DGS. Much like the Hale telescope, greater resolution enables the identification of finer lines and bones. This demonstrates that its not the mechanics of the technique so much, as the intimate knowledge long months of study provides when employed, and higher resolution really helps. (Higher resolution image did provide improved data.)

Figure 3. The skull of Eosinopteryx after tracing in higher resolution (1200 dpi). Here more bones and teeth were correctly traced and identified. A mandibular fenestra is present (contra Godefroit et al. 2013). It just had a quadrate stuck through it. Frontals overlap due to convexity. Even with these improvements, any errors should be brought to my attention for repair.

The premaxilla includes at least three elongated teeth (here seen from the inside). The saddle-shaped nasal was broken into at least four pieces during crushing. The maxilla is decayed beyond the normal fenestration. Here (Fig. 3) I reconstruct it conventionally. There is no elongated posterior lacrimal process and vestigial anterior process as Godefroit et al. (2013) reported. Rather the lacrimal is similar to that of other theropods. They did not provide a reconstruction. An earlier reconstruction of mine misplaced the quadrate articulation, which is repaired here. It also did not recognize the posterior mandible, which in situ is separated from the articular area. But here (Fig. 3) this has been repaired, thanks to theropod expert, M. Mortimer, for pointing this out. It was also overlooked by Godefroit et al. (2013, Fig. 2).

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Godefroit P, Demuynck H, Dyke G, Hu D, Escuillié FO and Claeys P. 2013. Reduced plumage and flight ability of a new Jurassic paravian theropod from China. Nature Communications 4: 1394. doi:10.1038/ncomms2389
Paul GS 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press 320 pp.

wiki/Eosinopteryx

Eosinopteryx – part 1 – Feathers, but no flapping

Eosinopteryx brevipenna (Godefroit et al. 2013, Middle-Late Jurassic, Tiaojishan Formation) is represented by a new complete skeleton. It was a feathered theropod dinosaur about 30 cm long. The forelimb feathers were quite long (Fig. 1), but the tail feathers were not.

Paravian? or Preavian?
We’ve been looking for a feathered theropod without elongated coracoids to precede Archaeopteryx. We also need this taxon to be not pre-oviraptorid or pre-alvarezsaurid. The authors argue, with a very extensive phylogenetic analysis, that this is a troodontid resembling Anchiornis, with less extensive feathers on the hind limbs and tail. Anchiornis greatly resembled Archaeopteryx and is, therefore, closely related. Of that, there is no doubt.

Why There is Doubt
I have not created a competing analysis. Checking out Greg Paul’s figure of Anchiornis (Paul 2010), I note his Anchiornis has the short torso and elongated coracoid also seen in Archaeopteryx, troodontids and deinonychosaurs.

Figure 1. Click to enlarge. Eosinopteryx reconstructed in lateral view. Soft tissue impressions preserved on the fossil are represented here in gray. Note the small size of the coracoid (yellow) and its curved lower rim, which indicates this specimen was a pre-flapping dinosaur. Pedal digit 2 was not modified as a “killing” claw. Elements figured with DGS.

What sets Eosinopteryx apart from these?
A short coracoid with a broad curved ventral rim - Therefore Eosinopteryx did not flap and was not descended from flappers. We haven’t seen a terrestrial descendant of Archaeopteryx yet without elongated coracoids. For more on this, compare Huaxiagnathus (with its short coracoid) to Velociraptor, (with its long, tall coracoid). Otherwise these two greatly resemble one another, with the former lacking sternal plates, a retroverted pubis and caudal rods. These traits are also lacking in Eosinopteryx.

A relatively smaller skull – Much smaller than in Anchiornis.

A relatively longer torso – Much longer than in Anchiornis.

A relatively shorter pubis – Much shorter than in Anchiornis.

All these traits are primitive for theropods.

Unfortunately, 
Huaxiagnathus was not included in the analysis of Godefroit et al. (2013). Neither were oviraptorids or alvarezsaurids. Eosinopteryx needs to be compared to these missing basal taxa along with the other taxa they previously tested. Once that’s done, let’s see if the topology of the tree doesn’t shift Eosinopteryx down below (more primitive than) Archaeopteryx. 

Addendum: The analysis of Godefroit et al. (2013) was based on and provided only a segment of an earlier analysis that DID include these more primitive taxa. Thus my doubt is reduced somewhat as all pertinent taxa were included.  Even so, I wonder why these two “sisters” don’t look more alike.

If anyone has details on why Godefroit et al. 2013 said the “bone structure would have limited its ability to flap its wings,” I’d like to see it.

Interesting that this birdy topic just came up a few days ago with Mahakala. Reminds me to be careful what I wish for.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Godefroit P, Demuynck H, Dyke G, Hu D, Escuillié FO and Claeys P. 2013. Reduced plumage and flight ability of a new Jurassic paravian theropod from China. Nature Communications 4: 1394. doi:10.1038/ncomms2389
Paul GS 2010. The Princeton Field Guide to Dinosaurs. Princeton University Press 320 pp.

wiki/Eosinopteryx

Mahakala – Size Evolution Preceding??? Avian Flight

Today we’ll look at a flightless bird/dromaeosaurid: Mahakala.
While the paper by Turner et al. (2007), ““A basal dromaeosaurid and size evolution preceding avian flight” (pdf). was ostensibly about the origin of flight in birds, it actually says nothing about taxa preceding Archaeopteryx and neither does their included cladogram. Mahakala (Fig. 1, the new basal dromaeosaurid, as everyone knows, is Late Cretaceous and demonstrates the reduction of the wings FOLLOWING Archaeopteryx, its Late Jurassic predecessor. Lots of birds reduce the wings when they are no longer necessary for flight. Makhala is only one more. If you want to read more about the taxa preceding Archaeopteryx, look here.

Fig. 1. Artist’s reconstruction of Mahakala omnogovae, a two-foot-long dinosaur unearthed in the Gobi Desert. © Frank Ippolito This is a good-looking representation of an incomplete fossil.

Some thoughts from Xu Xing in NatGeo 

Xu said a combination of birds’ ability to fly and to evolve quickly might have helped them survive.

“Birds mature within one year, and that gives them the means to adapt very rapidly to big changes in the environment,” he said.

My research indicates some birds take more than one year to mature (ostrich = 3-4 years, crow=2 years). Others take less. Reporters may have reported what they wanted to, perhaps misquoting Xu.

Some thoughts from Alan Turner in NatGeo|

“Paleontologists have long thought that miniaturization occurred in the earliest birds, which then facilitated the origin of flight,” said Alan Turner, lead author on the study and a graduate student at the American Museum of Natural History and Columbia University in New York. “Now, the evidence shows that this decrease in body size occurred well before the origin of birds and that the dinosaurian ancestors of birds were, in a sense, pre-adapted for flight.” Not so. Where’s the logic here? Large wings in the Jurassic clearly preceded the small wings of Mahakala of the Late Cretaceous. And, at least some of the more birdy taxa that followed Archaeopteryx were much, much smaller, as we looked at here.

Although paleontologists have shown that birds evolved from bipedal carnivorous dinosaurs known as theropods, fossil evidence of miniaturization and other characteristics leading to flight have been sparse. Not really… just not studied much. As in pterosaur precursors, everyone is looking for what they think they should find and ignoring what evidence is already out there.

Now Mahakala is providing the first signs of some of these early evolutionary steps. In particular, while other dinosaurs of the Cretaceous Period were evolving in favor of increased body size, Mahakala represented a progressive step towards miniaturization of body forms that would be necessary for feathered dinosaurs to eventually take flight. Not really. Mahakala is about the same size as Archaeopteryx.

“Flight isn’t an easy thing, because you are, in effect, countering the force of gravity,” said Turner. “Being really small appears to be a necessary first step. Other groups that evolved flight, such as pterosaurs and bats, all evolved from small ancestors.” Really? And those ancestors are…????? (Really, I know pterosaurs and bats evolved from small ancestors, see bats here and pterosaurs here. But those specimens are not acknowledged by traditional paleontologists. So, I’m wondering which taxa Turner was referring to? Or hoping for?)

Traditionally it’s been thought that the earliest birds were the first theropods to become really small. With the discovery of Mahakala we were able to show that this miniaturization occurred much earlier.” Ahem. You mean extended much later to the Late Cretaceous. Who was editing/proofing this copy??

 Mahakala shows that dinosaur size decreased progressively as they evolved toward birds. While this is something that has long been expected, Mahakala provides the first empirical evidence of this phenomenon. Again, the charts included in the article say otherwise, both chronologically and phylogenetically.

But what about….?
Now, I realize the Early Cretaceous tritosaur lizard, Huehuecuetzpalli, chronologically succeeded Triassic fenestrasaurs and pterosaurs. However, phylogenetically a sister of Huehuecuetzpalli preceded Triassic fenestrasaurs and pterosaurs. Makhala does not phylogenetically precede Archaeopteryx, according to the published charts, which show  no pre-Archaeopteryx taxa. That’s just not right when other studies have found a series of taxa with a gradually accumulating list of bird traits.

What we know

Figure 2. Click to enlarge. The origin of birds cladogram. Large clades are colorized. Red arrow points to the place on the chart where we will someday place the closest known ancestors of Archaeopteryx.

At least this cladogram (Fig. 2) shows some Triassic and Jurassic taxa (not that that’s required). In any case, we’re still looking for the precursor to Archaeopteryx that is not a closer precursor to oviraptorids, alvarezsaurids, etc. One of the keys to figuring out flightless or flapless taxa that came before or after is the design of the coracoid, as we looked at earlier with the precursors to pterosaurs that were flapping long before they were flying. With a locked down coracoid flapping can progress. With a sliding coracoid (the plesiomorphic condition), not so much.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Turner AH, Pol D, Clarke, JA, Erickson GM and Norell M 2007. “A basal dromaeosaurid and size evolution preceding avian flight” (pdf). Science 317 (5843): 1378–1381. doi:10.1126/science.1144066. PMID 17823350.

Microraptor leg feathers and the evolution of bird flight

A recent abstract by Habib et al. 2012 hypothesized that “Microraptoran dinosaurs may have experienced intrinsic difficulties with pitch control because they retained a trunk of typical dromaeosaurid proportions, as opposed to the shortened trunk of ornithurine birds. 

Figure 1. Microraptor with feather impressions.

“Specimens of Microraptor gui show that a fan of feathers existed near the terminus of the tail. This would be sufficient to correct for small deviations of the center of gravity from the center of lift. The tail could not have provided significant control in yaw or roll, but the forewings and hindwings would have been well suited to providing those control functions.

Figure 2. Microraptor standing. Here you have wings, a horizontal stabilizer on the tail and vertical stabilizers on the hind legs. With such small wings and inexperience as a pilot, you need all the control surfaces you can get!

“We suggest that a new and more compelling general model for the evolution of flight in paravians and early birds is emerging. Early in the evolution of theropod flight, major flight control functions were relatively evenly distributed between the forewings and the auxiliary control surfaces – namely, the hindwings and tail. This allowed the comparatively robust hindlimbs and tail of paravians to carry much of the mechanical loading associated with tight maneuvers, launching, and landing.  In more derived members of the avian line, most control function shifted to the forewings, though primary launch power continued to be provided by the hindlimbs. This model explains how animals such as Microraptor could fly in cluttered environments with small pectoral muscle fractions and gracile forelimbs.”

I did not see the presentation by Habib et al. 2012. Funny that the abstract title (see below) focuses on the tail, when the hind limb feathers are what everyone is chatting about. Their presentation made some news here at Findognews.blogspot.com. The opening paragraph states, “A rethink of four-winged dinosaurs suggests that the much-debated hind wings stayed tucked under the body until deployed in the air for tight turns to dodge branches or chase prey.” I suppose this was the gist of the talk.

ScientificAmerican.com included this statement by Habib, “A combination of pitch control by the tail, roll generation by the ‘hindwings’ and multi-purpose control by the main wings would have madeMicroraptor a highly maneuverable animal.” Seems more than reasonable.

The blogspot went on with history of the hind leg feathers, “The first reconstruction showed the small dinosaur gliding in the air with all four limbs extended outward. A later proposal lowered the hind-limb feathers for a Wright-Brothers biplane of wings. Both arrangements have drawn criticism. In a simpler solution, the dinosaur could have kept its hind limbs under its body much of the time until needed for banking in a turn.”

Figure 3. Early illustration of Microraptor sprawling like a flying lizard, not a flying dinosaur.

Yes, that first sprawling Microraptor illustration (Fig. 3) had everyone aching, as it essentially popped those dinosaurian right angle femora out of their sockets.

Kevin Padian commented about the abstract in the blogspot, “Powered flight and gliding downward have developed in quite different evolutionary branches.” Maneuverability is certainly important to both, but he does not see gliding as an evolutionary baby step on a path toward powered flight.”

Dr. Padian jumped on the gliding half of the hypothesis because he observed that the presentations focused on the effect of the hindlimb on a gliding animal. From the Scientific American blogspot, “He questioned why the team’s model would focus on gliding parameters when the forelimb shape was consistent with flapping, not gliding, and the hindlimb would have generated so much drag.” 

Not sure about hind feather drag. Cylinders create lots of drag. Bare legs are cylinders. Feathered legs turn those cylinders into tear drop shapes, which minimize drag while providing large surfaces that can be turned into the line of flight in order to increase drag, redirect the airstream and maneuver, the way airplanes do. That’s why wheels on airplanes are often given “pants.” That’s why vertical stabilizers are shaped like they are.

The coracoids of Microraptor were shaped and immobilized in the manner or birds and fenestrasaurs (including pterosaurs), which are ideal for for flapping. Maybe not great flapping, but you got start somewhere.

Figure 4. Longisquama in lateral view, dorsal view and closeup of the skull. Like Microraptor, Longisquama glided/flew with similarly-sized wings both fore and aft, and here with a much longer torso, ideal for leaping. Note the stem-like coracoids, a sure sign of a flapper.

The distribution of flight control systems across all four limbs (and tail) find an interesting parallel in Longisquama (Fig. 4), a sister to the Pterosauria, in which the forelimb wings and hind wing uroptagia formed four wings. Here again, the forelimbs could have provided thrust while in the air. In the air the hind limbs could have acted like vertical stabilizers, or, when extended laterally, could have provided lift, but not flapping.

While the key reason for developing hind leg flight surfaces might have been for maneuverability, the most important moment in that maneuverability would have been a two-point landing following a positive pitch flare, bringing the airspeed down to zero while still maintaining control. I think Habib et al. (2012) are right on the money. There are analogous reptiles demonstrating the same sort of evolution (Longisquama) and it all makes sense in every way. If they didn’t talk enough about flapping, that’s a minor point.

And those extra feathers ain’t a bad secondary sexual character either.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Habib M, Hall J, Hone DW and Chiappe LM 2012. Aerodynamics of the tail in Microraptor and the evolution of theropod flight control. Journal of Vertebrate Paleontology Abstracts, p. 105. 

wiki/Microraptor

 

Little Early Birds – The Tiny Descendants of Archaeopteryx

Archaeopteryx, one of the most primitive birds, was among the smallest of all adult dinosaurs. But what followed Archaeopteryx was even smaller (Fig. 1).

Figure 1. Archaeopteryx and several other basal birds. Here Archaeopteryx is a relative giant. This is an older illustration, predating all of the recent and now, not so recent, finds from China. The wings, sternum and tail are derived in the smaller birds. That’s the smallest Archaeopteryx, the Eichstätt specimen. 

Size reduction in early birds
Early maturation produces smaller adult birds. They cycle through life and reproduce quickly, before reaching the size of their parents and grandparents. And if their chicks mature even more quickly they will have smaller hips to produce smaller eggs. Smaller eggs produce smaller chicks that reproduce quickly. On and on, well, you get the idea.

The wings lost their individual fingers in these early birds. The tail shortened, developing a pygostyle (fused tail bones). The rostrum was shorter. The orbit was relatively larger. The sternum deepened, developing toward its modern shape. These little guys were smaller, lighter and stronger flyers. They were probably very hard for a predator to catch. They could lay eggs in otherwise inaccessible places. They may have had a higher, more bird-like metabolism, with greater needs for food, and more rapid reactions. They weighed only a fraction of Archaeopteryx, so the little birds could jump and fly easier.

Evolution happens more quickly in small taxa because they grow and reproduce at a faster rate. More generations appear in less time. By flying, birds can access more environments, from trees to seashores, which also has selective effects.

Parallels in pterosaurs
Earlier we talked about various pterosaur lines that shrank and evolved into new forms prior to ultimately producing larger forms. However, pterosaur ancestors, like Cosesaurus, were not larger than early pterosaurs. They did not develop wings through serial size reduction.

Parallels in bats
Today there are megabats and microbats. The most primitive bats were small. Bat ancestors, within the single genus Protictis, demonstrate the size reduction seen in birds during the development of wings.

Juveniles vs. Adults
Gobipteryx was considered an embryo. If so, it would have grown to the size of Archaeopteryx. Not sure about the possible juvenile status of the others. Since birds grew up so quickly and are all about the same size, the odds are the rest represent adults.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Dames W 1884. Ueber Archaeopteryx. Palaeontologische Abhandlungen, 2 (3):119-198.
Heller F 1959. Ein dritter Archaeopteryx Fund aus den Solnhofener Plattenkalken von Langenaltheim/Mfr. Erlanger Geologische Abhandlungen, 31: 1-25; Erlangen
von Meyer H 1861. Archaeopteryx litographica (Vogel-Feder) und Pterodactylus von Solenhofen. Neues Jahrbuch für Mineralogie, Geognosie, Geologie und Petrefakten-Kunde. 1861: 678–679.
Owen R 1863. On the Archaeopteryx von Meyer, with a description of the fossil remains of a long-tailed species from the lithographic stone of Solnhofen. Philosophical Transactions of the Royal Society, London 153: 33-47.
Paul G 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Johns Hopkins University Press. 460 pp.
Wellnhofer P 1974. Das fünfte Skelettexemplar von Archaeopteryx. Palaeontographica Abt. A Vol. 147 S: 168-216.

wiki/Archaeopteryx

Archaeopteryx in the Nude

Figure 1. Archaeopteryx sans feathers. Here is the raw dinosaur beneath all the bird accouterment, perched in full scale on the balcony of my apartment. Made of wood and wire, with a little plumbers putty for claws and a glass eye from the taxidermy catalog.

Archaeopteryx, perhaps the most fascinating of all dinosaurs.
Here, seen as never before, stripped of its feathers is the ‘mother of all birds’.  I sculpted this several years back when I was doing such things and its been sitting on my shelf ever since. Not much I can say about Archaeopteryx that hasn’t been said before. It’s bird. It’s a dinosaur. It’s both!

Nice change of pace from all the finger-pointing  and nay-saying of the past few days.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

wiki/Archaeopteryx

Dr. Ellenberger and his Petite Cosesaurus – part 2: post-cranial

Yesterday we looked at Dr. Paul Ellenberger’s long-time interest in the tiny Mid-Triassic reptile, Cosesaurus aviceps, with a focus on the skull. He thought of his little Cosesaurus as an important transitional taxon, a bird precursor living several tens of millions of years earlier than Archaeopteryx.

Figure 1. Cosesaurus insitu, image rotated 180 degrees from the image presented yesterday to give the illusion of the specimen elevated above the matrix (the brain assumes the light comes from above). The original is actually depressed beneath the matrix surface, as shown earlier.

New observations and phylogenetic analysis nest Cosesaurus within the Tritosauria at the base of the Fenestrasauria (Peters 2000b) leading toward the Pterosauria. Derived from a sister to the basal lizard, Huehuecuetzpalli, Cosesaurus is the Archaeopteryx of the Pterosauria. Cosesaurus could run on two legs (Peters 2000a, Fig. 12, based on a perfect match to Rotodactylus tracks) and flap its fiber-trailed forelimbs (Peters 2009), but it could not fly. That would come several million years later.

Post-Crania
Having covered several cranial traits earlier, today we’ll look at the post-cranial traits observed by Ellenberger (1978, 1993, Fig. 1), many reinterpreted by Peters (2000, 2009, Fig. 12) both correctly and incorrectly and here corrected again.

Figure 1. From Ellenberger 1993. Overall view of Cosesaurus aviceps in standing, bird-like pose, with hind limbs oriented too parasagittal for the shape of the proximal femur. See Figure 12 (below) for a new interpretation.

The Cervicals, Dorsals and Sacrals
Ellenberger (1993) correctly interpreted the eight cervicals as individually longer than the dorsals and provided with elongated ribs (Fig. 1). The dorsals were procoelous and relatively short as a set. More than two sacrals were present. Unfortunately Ellenberger interpreted the largest sacral transverse process as an ischium (Fig. 7). There are no similarly-shaped ischia in candidate sisters. The actual sacral transverse processes were identified by Peters (2000, Fig. 9).

Figure 2. The tail of Cosesaurus as interpreted by Ellenberger (1993). One uropatagium is in blue. The jellyfish is in orange.

The Tail
The tail was attenuated after the first ten caudals, which were provided with elongated transverse processes (Fig. 2). Thus the caudofemoral muscles were still important femoral abductors. The distal centra were three times longer than their depth. The chevrons did not descend as in most reptiles, but remained parallel to the centra, as in Archaeopteryx (and ignored by Ellenberger, as in Sharovipteryx and pterosaurs). Ellenberger (1993) noted a valid break in the tail of Cosesaurus, in which the tail was rotated 90 degrees on either side of the break, but that break is not reflected in the extended soft imprint.

The Tail – Soft Tissues
The tail of the Cosesaurus fossil was surrounded by a broad matrix area that sloped gently downward  (Fig. 2). The area beyond the attenuated caudals was subdivided by a series of regularly spaced lines. Under his bird bias, Ellenberger (1993) considered such data an indication of elongated tail feathers (rectrices) with feather shafts. That would have preceded the development of theropod feathers by several tens of millions of years. Unfortunately, no Cosesaurus sister taxa have any sort of similar structure. If just the narrow, hair-like shafts are valid then the tail of Cosesaurus had keratinous hairs along its length. In certain pterosaurs these hairs at the tail tip coalesced to form a tail vane. This, of course, is an attempt to explain away the broader shape surrounding the tail. This could be the effect of water and granular matrix interacting with the tail hairs. Or this broad shape could be a valid structure because it appears to be continuous with the uropatagium coming off the left femur (Figs. 2, 11). Hard to figure.

Figure 3. The pectoral region of Cosesaurus as interpreted by Ellenberger (1993). Light blue = coracoids. Yellow = unossified sternum and ossified ventral keel rim. Purple = clavicles. Green = scapulae.

The Pectoral Girdle
Ellenberger (1993) identified the strap-like scapulae more or less correctly (Fig. 3), but not quite long enough (Fig. 5). Birds and pterosaurs both have a strap-like scapula. Ellenberger considered the broad oval ventral plate a pair of giant coracoids fused medially (Fig. 3). Peters (2000) mistakenly followed this interpretation, but recently reinterpreted the plate as an anteriorly migrated sternum (Fig. 5). The prominent quadrant-shaped stem Ellenberger identified as a sternal keel (Fig. 3) turned out to be a very pterosaur-like coracoid stem (Fig. 5). The problem was: there are no other known taxa that have a pectoral girdle close to the Ellenberger (1993) restoration/reconstruction. However, there are three other taxa with the current Peters restoration/reconstruction: Sharovipteryx, Longsiquama and pterosaurs. Cosesaurus demonstrated the genesis of the sternal complex and the stem-like coracoid.

Figure 4. The pectoral girdle of Cosesaurus as restored by Ellenberger (1993). Light blue = coracoids. Yellow = unossified sternum with ossified keel. Purple = clavicles. Green = scapulae.

The Sternum
Ellenberger (1993) considered the sternum of Cosesaurus to be unossified (Figs. 3, 4), supporting that otherwise disconnected and ossified ventral keel. That was an invention created due to fulfill his bird-bias. What Ellenberger (1993) considered a giant ventral coracoid, is now identified as a sternum (Fig. 5) having migrated forward to a position it occupies in Longisquama and pterosaurs, up against the clavicle and the transverse processes of the interclavicle creating a sternal complex.

Figure 5. New interpretation of the pectoral elements of Cosesaurus. Light blue = coracoids. Yellow = unossified sternum. Purple = clavicles. Pink = clavicles. Green = scapulae. Red = interclavicle.

The Clavicles
Ellenberger (1993, Fig. 3) correctly traced the clavicles of Cosesaurus as transversely oriented and straight overlapping medially and rimming the anterior interclavicle and coincident sternum. Then he restored the clavicles (Fig. 4) as disconnected from the other elements and V-shaped like a deep wishbone-shaped furcula (fused clavicles) following the restored V-shaped of his coracoids. Jurassic birds, like Archaeopteryx, did not attain such a shape in the clavicle. In Cosesaurus, Longisquama and pterosaurs the clavicles indeed rim the anterior interclavicle/sternum complex as shown in situ (Fig. 5) and reconstructed (Fig. 12). They are coplanar with the sternum.

Scapulae
Ellenberger (1993) correctly identified the scapulae. Peters (2000a) did not. While attempting to follow the example of a sister, Macrocnemus, I considered the scapulae to be disarticulated ribs and other elements to be the disc-like scapulae. Sanz and Lopez-Martinez (1984) made the same mistake. The day I could finally “see” my error was a good day for enlightenment. I could almost hear the Moody Blues.

The Humerus
Ellenberger (1993) illustrated the humerus (Figs. 3, 4) as essentially straight, and it was, with a slightly expanded proximal and distal end.

Figure 6. The forelimb of Cosesaurus, a pigeon, Archaeopteryx and a Tern from Ellenberger (1993). Ellenberger flippled the hand in order to more closely match the digit lengths in birds. No such flipping is necessary in comparisons to pterosaurs. The two "cartilage" ovals identified by Ellenberger (1993) are homologs to the pteroid and pre-axial carpal in pterosaurs (Peters 2009). The general lack of carpal ossification is a trait shared with sister taxa.

The Radius and Ulna
The radius and ulna in Cosesaurus were straight, parallel and closely appressed to each other (Fig. 1), unlike birds, just like pterosaurs (Fig. 6). The forearm was becoming increasingly restricted in pronation and supination due to the straighter shapes of the radius and ulna. Ellenberger (1993) reported fibers emerging from the posterior ulna (Fig. 1). These are clear in his photos and I confirmed them when I visited the fossil (Peters 2009). Due to his bird bias, Ellenberger (1993) considered these to be feather precursors. Finding closer connections with pterosaurs in phylogenetic analysis (Peters 2000), Peters (2009) considered these to be aktinofibril precursors, the fibers that support the wing and uropatagium in pterosaurs and their kin.

The Hands
Ellenberger (1993) labeled the fingers correctly several times, but when restoring Cosesaurus he flipped the hands, making #2 the longest finger (Fig. 6). This created a more bird-like hand. Unflipped the hands of Cosesaurus resemble those of fenestrasaur and tritosaur sister taxa with shorter medial metacarpals and fingers, except digit 5, which is very short, on its way to becoming a vestige, as seen in Huehuecuetzpalli and the wings of Sharovipteryx, Longisquama and pterosaurs.

The Wrist
Ellenberger found two spots on the medial wrist that he ascribed to cartilage. Peters (2009) identified those as migrated centralia, now homologous with the pteroid and preaxial carpal. The other carpals were poorly ossified, as in the closest Cosesaurus sister taxa, including the basal lizard, Huehuecuetzpalli.

Figure 7. The insitu pelvis of Cosesaurus as interpreted by Ellenberger 1993. Green = Retroverted pubis with prepubic process. Yellow = ilium. Orange = Ischium.

The Pelvis
Attempting to find homologs for the retroverted pubis of birds, Ellenberger (1993) considered a displaced prepubis (Fig. 9) and a drifted gastralium to be the retroverted pubis of birds (Figs. 7-8). Never mind that the pubis of Archaeopteryx was not so retroverted or attenuated. Never mind that no other sisters had a similar pelvis. Ellenberger (1993) described elongated ilia, which is essentially correct (but perhaps over elongated). Ellenberger (1993) considered a displaced sacral transverse process to be an ischium, overlooking the actual coosified ischium + pubis (Fig. 9) and completely overlooking the pterosaur-like prepubis, probably because it was not expected in a bird ancestor. Once I recognized one prepubis in plain sight on the ilium, the other, tucked beneath the femur (Fig. 9), was easier. Unfortunately a manuscript describing these revelations and synapomorphies was rejected. Hence this website.

Figure 8. The pelvis of Cosesaurus as reconstructed by Ellenberger (1993). Green = Retroverted pubis with prepubic process. Yellow = ilium. Orange = Ischium.

Figure 9. The tiny pelvis and robust sacrum of Cosesaurus with most pelvic and sacral elements, including the prepubes, re-identified.

Figure 10. The pes of Cosesaurus according to Ellenberger (1993). Centrale in pink. Distal tarsal 4 in yellow. Ellenberger considered the pes fully webbed.

The Hind Limb
Ellenberger (1993) correctly interpreted the femur without any sort of head or neck, as in basal reptiles including lizards. Unfortunately he reconstructed the femur as a parasagittal element (Fig. 1), as in birds, rather than a sprawling one, as in the basal pterosaur, MPUM 6009.

The tibia and fibula were both straight and closely appressed to one another with the fibula less than half the diameter of the tibia, as in birds and pterosaurs. The length of the tibia/fibula becomes longer than the femur in derived theropods in the bird lineage, and in Sharovipteryx and higher taxa, in the pterosaur lineage.

The Tarsus and Pes
Ellenberger (1993) correctly interpreted the pes of Cosesaurus with an astragalus, calcaneum, centrale and distal tarsal 4 as the four largest tarsal elements. He also found tiny distal tarsals 1-3. Such a tarsus is a synapomorphy Cosesaurus shared with Tanystropheus, Sharovipteryx and pterosaurs. In addition, all these taxa had a very short metatarsal 5 and a hyper-elongated phalanx 5.1, which Ellenberger (1993) correctly identified, but did not make the pterosaur connection and no bird or bird ancestor has such a toe. One would have to go back to Proterosuchus to find a bird ancestor with digit 4 longer than 3. And certain armored aetosaurs develop a longer fourth toe. This is the trait that first drew me toward this taxon as a possible sister to pterosaurs. No gracile archosaur or archosauriform had such a toe.

Uropatagia
Ellenberger photographed and noted soft tissues emanating from the left femur and tibia (Fig. 11). He considered these to be possible feather precursors. These impressions also greatly resembled the fiber-embedded uropatagia of sister taxa, Sharovipteryx and pterosaurs. Note the fibers anterior to the knee in figure 11. Cosesaurus was much decorated in such soft tissue, and this makes phylogenetic sense as a precursor to the party queen of the Triassic, Longisquama.

Figure 11. The uropatagium following the left hind limb of Cosesaurus, photographed by Ellenberger (1993).

In Summary
Despite intense study, Ellenberger (1993) invented some insightful and strange structures in Cosesaurus due to his strong bird-bias. These have been both credited and criticized. As a suite and from head to toe, Cosesaurus shares more traits with the tritosaurs, Huehuecuetzpalli through pterosaurs. The changes in the pectoral and pelvic region distinguish Cosesaurus from Macrocnemus and nest it with similarly endowed reptiles, including Sharovipteryx, Longsiquama and pterosaurs.

Figure 12. Current interpretation of Cosesaurus.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Ostrom JH 1969. Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana. Peabody Museum of Natural History Bulletin 30: 1–165.
Peabody FE 1948.  Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah.  University of California Publications, Bulletin of the  Department of Geological Sciences 27: 295-468.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330.
Peters D 2011. A Catalog of Pterosaur Pedes for Trackmaker Identification Ichnos 18(2):114-141.
Sanz JL and López-Martinez N 1984. The prolacertid lepidosaurian Cosesaurus aviceps Ellenberger & Villalta, a claimed ‘protoavian’ from the Middle Triassic of Spain. Géobios 17: 747-753.

Dr. Ellenberger and his Petite Cosesaurus – part 1: Cranial Traits

Dr. Paul Ellenberger (pronounced “El-len-ber-zhay”) spent a large part of his life attempting to link a tiny Mid-Triassic fossil reptile, Cosesaurus aviceps, to birds. He considered it a precursor to Archaeopteryx in the years just following the publication of Ostrom (1969) on Deinonychus. Ellenberger published two small papers (Ellenberger and de Villalta 1974, Ellenberger 1978) and a very large (664 pp.) unpublished tome (Ellenberger 1993) on this little reptile perpetually entwined with an amorphous jellyfish. No one has spent more time studying Cosesaurus than Ellenberger. No one has put more effort into describing it and photographing it from every angle in the most precise detail.

Figure 1. Cosesaurus aviceps at close to actual size. The blob next to it is a jelly fish. No actual bones are preserved. Cosesaurus is nothing but a deep impression faithfully preserving every aspect of its skeleton down to the finest soft tissue details. The tail is especially deep, which created the impression, when transferred to 2-D, of emanating feathers. Tomorrow the same image will be presented but flipped 180 degrees.

Even so…
Ellenberger (1993) got many things wrong. He had a mistaken preconception and that biased his observations. It can happen. I’ve seen it happen to the best paleontologists out there. Following tradition is easy, but it leads to problems. Testing tradition is good science. Distrusting the validity of autapomorphies is key. Phylogenetic analysis trumps all.

The Power of Pet Ideas
Ellenberger’s (1993) bird hypotheses were never taken seriously or supported by other writers in the literature. Nevertheless, Ellenberger created a body of data leading to an interest in the taxon that launched Cosesaurus in a new direction for me. It never occurred to Ellenberger to link Cosesaurus to Sharovipteryx, Longisquama and pterosaurs. I raised the subject with him after seeing Cosesaurus in Barcelona and while staying with Paul for a day or two at his home in Montpellier, France. Ellenberger didn’t like the idea (because it didn’t support his bird hypothesis), and he didn’t want to discuss it.

Giving Credit
Well, we’re going to explore Dr. Ellenberger’s view of this little predecessor taxon. The point of this report is to give credit where credit is due and to shine a light on any mistakes.

Figure 1. From Ellenberger 1993. Overall view of Cosesaurus aviceps in standing pose, lateral and dorsal views. Note the bird-like restoration, a little too erect with hands flipped to more closely match the hands of birds. Other problems in the pectoral and pelvic regions will be discussed in part 2 of this blog tomorrow.

Ellenberger’s Reconstruction of Cosesaurus
Ellenberger saw Cosesaurus as a bird precursor, therefore he saw it as a digitigrade narrow-gauge biped. These are all true. Matching footprints (Rotodactylus) are evidence (Peters 2000). Despite being a footprint expert, Ellenberger (1993) did not consider a match of Rotodactylus to Cosesaurus. He did not produce an illustration with a bent-back pedal digit 5, which would have completed the match (Fig. 5).

Figure 2. The brain of Cosesaurus and the binocular vision reported by Ellenberger (1993). Note the elongated antorbital fenestra above the maxilla and below the prefrontal/nasal. The naris is indicated by two short lines here, better viewed in figure 3 (lower of the two skulls).

The Brain of Cosesaurus
No one but Ellenberger (1993) bothered to document the cranial capacity of Cosesaurus. Ellenberger applied reverse geometry to re-inflate the crushed skull of Cosesaurus to determine its likely dimensions in 3-D. Of course, he hoped to show that the brain of Cosesaurus had enlarged to bird-like proportions. It had also enlarged to pterosaur-like proportions. This was no ordinary reptile.

Figure 3. The skull of Cosesaurus in two lighting conditions with the antorbital fenestra and dorsal/cranial fibers/frill visible in the upper photo. From Ellenberger (1993). Also note the premaxilla crest in front of the orbit.

The Antorbital Fenestra
Ellenberger (1993) reported an antorbital fenestra in Cosesaurus and his images (Fig. 3) confirm that. I also confirm that, having seen the fossil in Barcelona.

By contrast, Sanz and Lopez-Martinez (1984) said there was no antorbital fenestra and considered Cosesaurus a juvenile Macrocnemus (Fig. 4). They also missed dozens of other traits that distinguish Cosesaurus from Macrocnemus (Fig. 5). They illustrated Cosesaurus in an inaccurate cartoonish fashion virtually identical to a cartoon Macrocnemus without any distinguishing traits other than a shortened rostrum, not realizing that in this clade hatchlings are virtually identical to adults. Altogether the Sanz and Lopez-Martinez (1984) report can be considered dated, biased and bogus because they didn’t put the effort in that was needed to trump earlier data by Ellenberger.

The same can be said of the Senter (2003) dissertation that reported no antorbital fenestra, even though he illustrated one, again in cartoonish fashion. I don’t understand how scientists can be so blinded by paradigm and bias that they cannot report the presence of an antorbital fenestra in Cosesaurus (Fig. 3). Unfortunately others (Evans 1988, Hone and Benton 2008) used the bogus data in phylogenetic analysis, preferring those simplified drawings to the precision of Elleberger (1978, 1993) and Peters (2000) or their own examinations(!)

Figure 4. Cosesaurus illustrated as a juvenile Macrocnemus by Sanz and Lopez-Martinez (1984).

Binocular Vision
Ellenberger determined that the large eyes of Cosesaurus poised over the small rostrum probably delivered 50 degrees of overlapping vision. That seems reasonable and sets Cosesaurus apart from Macrocnemus.

The Teeth 
Ellenberger reported the upper and lower three posterior teeth in the jaws of Cosesaurus were different that the others: broader and less pointed. These were precursors to the multicusped teeth found in derived fenestrasaurs.

The Naris
Ellenberger reported a slit-like naris in Cosesaurus, displaced from the snout tip. Such a naris is also found in all descendants of a sister to Huehuecuetzpalli, including pterosaurs and tanystropheids.

The Jaw Tip
Ellenberger considered the extended jaw tips to be beak precursors. The skull was also longer than the tooth row in the more primitive lizard, Huehuecuetzpalli. In more derived fenestrasaurs teeth protruded from the anterior jaws.

Figure 5. Current interpretation of Cosesaurus. Click to enlarge. Post-crania will be presented tomorrow.

Jugal
Ellenberger (1993) correctly illustrated a jugal with a new quadratojugal process in Cosesaurus.

Occiput
Ellenberger (1993) reported the occiput leaned posteriorly, which would have been appropriate for a reptile standing erect on hind limbs, whether bird ancestor or pterosaur ancestor.

Palate
Ellenberger illustrated as much of the palate as was visible and it was essentially correct and similar to that of pterosaurs, as reported earlier.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Ellenberger P and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
Ellenberger P 1978. L’Origine des Oiseaux. Historique et méthodes nouvelles. Les problémes des Archaeornithes. La venue au jour de Cosesaurus aviceps (Muschelkalk supérieur) in Aspects Modernes des Recherches sur l’Evolution. In Bons, J. (ed.) Compt Ren. Coll. Montpellier12-16 Sept. 1977. Vol. 1. Montpellier, Mém. Trav. Ecole Prat. Hautes Etudes, De l’Institut de Montpellier 4: 89-117.
Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Evans SE 1988. The early history and relationships of the Diapsida. Pp. 221–260 in: Benton, M. J. (ed.) The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Syst Assoc Sp Vol No. 35A. Clarendon Press, Oxford.
Gauthier JA 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Science 8: 1–55.
Ostrom JH 1969. Osteology of Deinonychus antirrhopus, an unusual theropod from the Lower Cretaceous of Montana. Peabody Museum of Natural History Bulletin 30: 1–165.
Peabody FE 1948.  Reptile and amphibian trackways from the Lower Triassic Moenkopi formation of Arizona and Utah.  University of California Publications, Bulletin of the  Department of Geological Sciences 27: 295-468.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330
Sanz JL and López-Martinez N 1984. The prolacertid lepidosaurian Cosesaurus aviceps Ellenberger & Villalta, a claimed ‘protoavian’ from the Middle Triassic of Spain. Géobios 17: 747-753.

The Origin of Archaeopteryx – Illustrated

An Old Debate, Now Clearly Settled
The origin of birds has been long debated, but the debate has been over for awhile. Birds arose from theropod dinosaurs and recent finds from China have presented us with a wide variety of feathered theropods — all part of the big evolutionary bush that ultimately begat the modern birds that fill our skies, waddle across Antarctica and sprint across the plains of the southern continents.

There have been several phylogenetic analyses of bird origins and diversification. Today’s blog simplifies and focuses the process by eliminating most of the side branches, like the tyrannosaurids, the alvarezsaurids, etc.

The Old View
In the old days paleontologists hauled out Compsognathus, Archaeopteryx and Gallus (the chicken). These were all essentially correct and told the right story, but now we know more of the details.

Figure 1. Old yet essentially correct view of bird evolution featuring Compsognathus, Archaeopteryx and Gallus, the chicken.

The Current View
The current view of bird evolution (below) gets a little more detailed.

Figure 2. Theropods to birds step by step. Purple taxa should not have been included. They are not related to dinosaurs. Yellow highlights refer to taxa shown and discussed below.

The Current View and The List of 23 Increasingly Bird-Like Characters
The current view of bird relations (above) nests many more taxa along the lineage of pre-bird theropods. Each one adds at least one more bird-like character to the list. The only serious problems are pterosaurs and Lagerpeton (in purple) are not related to dinosaurs.

1. Mesotarsal ankle – the Ornithodira (includes pterosaurs)
2. Functional tridactyl foot – Lagerpeton 
3. Fully perforated acetabulum – Herrerasaurus
4. Loss of digit 5 on hand – Tawa
5. Postaxial vertebral pneumaticity – Tawa
6. Pedal digit 1 loses contact with ankle – Coelophysis
7. Furcula – Coelophysis
8. Maxillary fenestra – Spinosaurus
9. Strap-like scapula – Allosaurus
10. Dorsal astragalus tall/broad – Allosaurus
11. Expanded pneumatic ectopterygoid -Tyrannosaurus
12. Three tympanic systems in ear region – Tyrannosaurus
13. Promaxillary fenestra – Tyrannosaurus
14. Fused semilunate carpal – Alvarezsauria
15. Enlarged sternum - Alvarezsauria
16. Ossified sternal ribs - Therizinosauria* + Oviraptosauria* + Dromaeosauridae + Troodontidae
17. Shortened tail – Dromaeosauridae + Troodontidae + Archaeopteryx
18. Subdivided ulna - (not sure what this means, unless these are quill locations)  Dromaeosauridae + Troodontidae + Archaeopteryx
19. Retroverted pubes – Dromaeosauridae + Troodontidae + Archaeopteryx
20. Asymmetric flight feathers – Dromaeosauridae + Troodontidae + Archaeopteryx
21. Sickle claw on foot - Dromaeosauridae + Troodontidae
22. Forelimb longer than hindlimb – Archaeopteryx (doesn’t appear to be true)
23. Pygostyle – Jeholornis and higher birds

* Derived taxa had a shortened tail (#17) and retroverted pubis (#19)

But wait there’s more…

Figure 3. Sample taxa in the lineage of birds sans all the cousins and offshoots. From top to bottom: Tawa, Juravenator, Sinocalliopteryx, Archaeopteryx, Cathayornis, Sinornis plus enlarged skulls.

The Revised View and List of Characters
This simplified view of bird relations notes that pterosaurs and Lagerpeton were not related to dinosaurs and birds. It also takes the view that dromaeosaurids and oviraptorids were probably derived from Archaeopteryx due to the shared trait of an elongated coracoid (analysis not done yet). Oviraptorids appear to have reshifted the pubis forward (but that’s for another blog). Here’s a new list of bird characters as they appeared in the above taxa (Fig. 3). Tyrannosaurids and Alvarezsaurids, among others, were skipped because, although their stage of evolution did add characters, in both cases the forelimbs became reduced, representing offshoots that did not ultimately evolve into birds.

Here, in this simplified and focused account, Tawa, Juravenator and Sinocalliopteryx precede Archaeopteryx. Sinornis and Cathayornis succeed Archaeopteryx. The following list of 29 characters are offered to replace the list of 23 (above).

1. Mesotarsal ankle - Gracilisuchus at the base of the Archosauria
2. Functional tridactyl foot - Trialestes (but note sauropods had a functionally pentadactyl foot, so reversals are possible)
3. Fully perforated acetabulum - Herrerasaurus

4. Loss of digit 5 on hand - Tawa
5. Postaxial vertebral pneumaticity - Tawa
6. Strap-like scapula - Tawa

7. Finger 3 shorter than 2. - Juravenator
8. Loss of digit 4 on hand – Juravenator
9. Pedal digit 1 loses contact with ankle - Juravenator
10. Furcula (fused clavicles) - Juravenator
11. Maxillary fenestra - Juravenator
12. Pubis rotated beneath anterior ilium – Juravenator 

13. Dorsal astragalus tall/broad - Sinocalliopteryx
14. Promaxillary fenestra - Sinocalliopteryx
15. Fused semilunate carpal - Sinocalliopteryx
16. Ossified sternal ribs - Sinocalliopteryx
17. Protofeathers – Sinocalliopteryx
18. Smaller teeth – Sinocalliopteryx 

19. Pubes beneath or behind acetabulum - Archaeopteryx
20 Asymmetric flight feathers - Archaeopteryx
21. Forelimb nearly as long as hindlimb - Archaeopteryx
22. Elongated coracoid locked onto sternum – Archaeopteryx
23. Reduced cervical ribs – Archaeopteryx
24. Chevrons parallel centra – Archaeopteryx
25. Toes beneath shoulder glenoid – Archaeopteryx
26. Anterior skull half the height of posterior skull – Archaeopteryx
27. Pedal digit 1 retroverted for perching – Archaeopteryx 

28. Sickle toe claw – Dromaeosaurids and troodontids (neoflightless birds)
29. Pygostyle - Cathayornis and higher birds, including oviraptorids by convergence
30. Toothless – Certain higher birds, including oviraptorids by convergence

The elongated and immobile coracoids of dromaeosaurids and oviraptorids indicate they were secondarily flightless, following a sister to Archaeopteryx. Alvarezasaurids and therizinosaurids appear to have been derived from sisters to Juravenator or Sinocalliopteryx (among listed taxa) and developed their bird-like characters (short tail, retroverted pubis) by convergence.

Sinocalliopteryx was an Early Cretaceous and too large to be in the lineage of Archaeopteryx, but an earlier, smaller sister would have been a suitable candidate. The development of longer forelimbs and longer coracoids was initiated here. Along with protofeathers, some early flapping may have accompanied running.

Archaeopteryx had larger forelimbs and relatively smaller hips, indicating a transition to forelimb locomotion, which included vigorous flapping assisted by flight feathers. The center of balance had shifted forward to the shoulder glenoid, which is where it is located in flying birds (and bats and pterosaurs). This was accomplished by shortening the torso. The neck could be pulled back further and the forelimbs were elongated and adorned with larger feathers.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Chiappe LM 2009. “Downsized Dinosaurs: The Evolutionary Transition to Modern Birds”. Evolution: Education and Outreach: 248–256.
Heilmann G 1926. The Origin of Birds. London: Witherby. 208 pp
Ji Q and Ji S-A 1996. On the discovery of the earliest bird fossil in China and the origin of birds (PDF). Chinese Geology 233: 30–33.
von Meyer CEH 1861. Archaeopteryx lithographica (Vogel-Feder) undPterodactylus von Solnhofen (in German). Neues Jahrbuch für Mineralogie, Geologie und Paläontologie 1861: 678–679.
Ostrom JH 1973. The ancestry of birds. Nature 242 (5393): 136–136.Bibcode 1973Natur.242..136O. doi:10.1038/242136a0.
Paul GS 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Baltimore: Johns Hopkins University Press. p. 472p

wiki/Origin_of_birds