The Protodinosauria and the Origin of the Dinosauria

The large reptile tree recovers the following taxa at the base of the Archosauria leading toward the Dinosauria (Fig. 1). This is an update of prior posts on dinosaur origins with PVL 4597 moving from closer to Trialestes to between Gracilisuchus and Lewisuchus.

  1. Gracilisuchus
  2. PVL 4597 (the Tucuman specimen attributed to Gracilisuchus)
  3. Lewisuchus
  4. Pseudhesperosuchus, Carnufex and Junggarsuchus
  5. Trialestes
  6. and finally, the basal dinosaur, Herrerasaurus.

Pterosaurs and Lagerpeton nest elsewhere. They are not part of the dino lineage. Marasuchus, which often nests outside the Dinosauria in other trees, nests with a few other odd theropods here.

Figure 1. Origin of the dinosaurs and protodinosaurs. Here Gracilisuchus, at the base of the Crocodylomorpha and Archosauria, is basal to the PVL 4597 specimen attributed to Gracilisuchus, Lewisuchus and Pseudhesperosuchus, taxa leading to Herrerasaurus at the base of the Dinosauria.

Figure 1. Origin of the dinosaurs and protodinosaurs. Here Gracilisuchus, at the base of the Crocodylomorpha and Archosauria, is basal to the PVL 4597 specimen attributed to Gracilisuchus, Lewisuchus and Pseudhesperosuchus, taxa leading to Herrerasaurus at the base of the Dinosauria. Click to enlarge.

Here (Fig.1) crocs and dinos have a last common ancestor
close to Gracilisuchus, probably in the Middle Triassic. Both started small and bipedal. Crocs had a wider skull. Dinos had a narrower skull. The reduction of the calcaneal tuber occurred in parallel. The tuber redeveloped in extant crocs.

Prior to these taxa
are larger forms, including Decuriasuchus and the basal poposaur, Turfanosuchus. So once again, phylogenetic miniaturization is key to the origin of both crocs and dinos (together, the Archosauria).

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 5. Family tree of the Archosauria and basal Dinosauria. Bootstrap scores are shown.

Figure 3. Family tree of the Archosauria and basal Dinosauria. Bootstrap scores are shown.

 

 

 

 

Wikipedia reports, Paleontologists think that Eoraptor (Fig. 2) resembles the common ancestor of all dinosaurs;[ if this is true, its traits suggest that the first dinosaurs were small, bipedal predators. The discovery of primitive, dinosaur-like ornithodirans such as Marasuchus and Lagerpeton in Argentinian Middle Triassic strata supports this view; analysis of recovered fossils suggests that these animals were indeed small, bipedal predators. Dinosaurs may have appeared as early as 243 million years ago, as evidenced by remains of the genus Nyasasaurus from that period, though known fossils of these animals are too fragmentary to tell if they are dinosaurs or very close dinosaurian relatives.”

Too bad they are so tentative at Wikipedia when the large reptile tree lays it out pretty clearly. The purported and popular clade, “Ornithodira,” is, of course, not supported by the large reptile tree.

References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Bittencourt JS, Arcucci AB, Maricano CA and Langer MC 2014. Osteology of the Middle Triassic archosaur Lewisuchus admixtus Romer (Chañares Formation, Argentina) its inclusivity, and relationships amongst early dinosauromorphs. Journal of Systematic Palaeontology. Published online: 31 Mar 201. DOI:10.1080/14772019.2013.878758
Bonaparte JF 1982. Classification of the Thecodontia. Geobios Mem. Spec. 6, 99-112
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
Clark JM et al. 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20 (4): 683–704. doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull. Nature 430:1021-1024.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Nesbitt SJ. et al. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-8
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Reig, OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchusadmixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13

wiki/Gracilisuchus
wiki/Lewisuchus
wiki/Pseudhesperosuchus
wiki/Trialestes

The Origin of Dinosaurs as told by The Smithsonian, Wiki, etc.

Many of the biggest dino museums in the world
have produced their version of the origin of dinosaurs. Here’s what they have to say online:

Smithsonian – National Museum of Natural History
“The earliest dinosaurs were probably carnivorous, bipedal animals less than two meters long and weighing about 10 kilograms. From these small beginnings evolved thousands of different dinosaurs species.”

Wikipedia
“Dinosaurs evolved within a single lineage of archosaurs 232-234 Ma (million years ago) in the Ladinian age, the latter part of the middle Triassic. Dinosauria  is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus.

“The process leading up to the Dinosauromorpha and the first true dinosaurs can be followed through fossils of the early Archosaurs such as the Proterosuchidae, Erythrosuchidae and Euparkeria which have fossils dating back to 250 Ma, through mid-Triassic archosaurs such as Ticinosuchus 232-236 Ma. Crocodiles are also descendants of mid-Triassic archosaurs.

“Dinosaurs can be defined as the last common ancestor of birds (Saurischia) and Triceratops (Ornithischia) and all the descendants of that ancestor. With that definition, the pterosaurs* and several species of archosaurs narrowly miss out on being classified as dinosaurs. Archosaur genera that also narrowly miss out on being classified as dinosaurs include Schleromochlus 220-225 Ma, Lagerpeton* 230-232 Ma and Marasuchus* 230-232 Ma.

“The first known dinosaurs were bipedal predators that were 1-2 metres (3.3-6.5 ft) long. Spondylosoma may or may not be a dinosaur; the fossils (all postcranial) are tentatively dated at 235-242 Ma.

“The earliest confirmed dinosaur fossils include saurischian (‘lizard-hipped’) dinosaurs Nyasasaurus 243 Ma, Saturnalia 225-232 Ma, Herrerasaurus 220-230 Ma, Staurikosaurus possibly 225-230 Ma, Eoraptor 220-230 Ma and Alwalkeria 220-230 Ma. Saturnalia may be a basal saurischian or a prosauropod. The others are basal saurischians.”

* these are false nestings according to the tree topology of the large reptile tree.

University of Bristol
“Those archosaurs most closely related to the dinosaurs are forms such as Marasuchus. The detailed evolutionary relationships are still debated, but by the late Triassic, several early theropods are known, as the dinosaurs rapidly diversified. These dinosaurs, such as Eoraptor, Coelophysis and Herrerasaurus were all carnivores, and, despite their diversity, were quite rare at this time.”

Natural History Museum of Los Angeles County
“The ancestry of dinosaurs can be traced back some 230 million years ago to the Late Triassic. All dinosaurs belong to a group of reptiles called archosaurs-a group that also includes crocodiles and a variety of Mesozoic reptiles (pterodactyls and others) that are often misinterpreted as dinosaurs. The anatomical characteristics of both the earliest known dinosaurs and their archosaurian relatives suggest that the common ancestor of all dinosaurs was a small bipedal predator, which had forelimbs shorter than hind limbs. This ancestor was probably similar to the 235-million-year-old Lagosuchus from Argentina, pictured below.

“From the most primitive Triassic forms to the most advanced ones of the latest Cretaceous, all dinosaurs share defining traits that distinguish them from their closest archosaurian relatives. Among these innovations, the femur (or upper leg bone) developed a distinct head for a tied attachment into a hollow hip socket. These and other changes resulted in a hind limb that was tucked directly underneath the body, providing upright, pillar-like support of the body and also enhancing locomotive abilities. The changes that led to the erect posture of dinosaurs from the sprawling posture of their reptilian predecessors had a profound effect on the evolutionary success of these animals. These transformations may have also been coupled with the evolution of a higher metabolism (a step towards warm bloodedness) that endowed them with a greater capacity for sustained activities such as running.”

Genesis park genesispark.com
“The Bible states that on the fifth day of creation God created great sea monsters and flying creatures. This would have included the great swimming and flying reptiles (like the plesiosaur and pterosaur creatures mentioned at our Genesis Park website). On the sixth day God created the land animals, which would have included all of the dinosaur kinds (Genesis 1:20-25).”

YouTube
Brief Lecture on the Origin of Dinosaurs – one commenter correctly noted, “This doesnt (sp) explain the actual origin of dinasaurs (sp) like the title states.”

The origin and evolution of dinosaurs Paul Sereno
Annual Review of Earth and Planetary Sciences
Vol. 25: 435-489 (Volume publication date May 1997)

“Phylogenetic studies and new fossil evidence have yielded fundamental insights into the pattern and timing of dinosaur evolution and the emergence of functionally modern birds. The dinosaurian radiation began in the Middle Triassic, significantly predating the global dominance of dinosaurs by the end of the period. The phylogenetic history of ornithischian and saurischian dinosaurs reveals evolutionary trends such as increasing body size. Adaptations to herbivory in dinosaurs were not tightly correlated with marked floral replacements. Dinosaurian biogeography during the era of continental breakup principally involved dispersal and regional extinction.”

American Museum of Natural History
Strangely, they don’t have an online account of dinosaur origins.

ReptileEvolution.com
Meet a long list of the best known taxa preceding dinos, the advent of dinos and see their family tree here and here. More specifics here (Fig. 1).

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

The Dinosaur Heresies NYTimes Book Review from 1986

the_dinosaur_heresies200Now almost 30 years old, here’s something you might like to read (perhaps again?).
This is the NY Times book review of Dr. Robert Bakker’s ‘The Dinosaur Heresies’ from 1986. You can read the complete original here. I went to the prophesies below and marked them with a [+] or a [-] for those supported today or not and for those that are still questionable: [?].

BOOKS OF THE TIMES;
Dinosaur Mysteries
By MICHIKO KAKUTANI
Published: November 8, 1986

THE DINOSAUR HERESIES. New Theories Unlocking the Mystery of the Dinosuars and Their Extinction. By Robert T. Bakker. Illustrated. 481 pages. William Morrow & Company. $19.95.

Mr. [not Dr.?] Bakker has a quirky, free-floating imagination, and in the course of this book – which is generously illustrated with his own charming sketches – he raises many offbeat questions: Were changes in dinosaur eating patterns responsible for the evolution of flowering plants? [+] Did pink pterodactyls exist? [?] What sort of lips did dinosaurs have? [+] Could a human being beat a tyrannosaurus at arm wrestling? [?]

Mr. Bakker, the adjunct curator at the University Museum in Boulder, Colo., has published many papers in the field of vertebrate paleontology, and his book stands as an informative layman’s introduction to the wonderful world of dinosaurs while at the same time making an impassioned case for his own – sometimes heretical – views on their endurance and extinction. ”I’d be disappointed,” he writes, ”if this book didn’t make some people angry”; and given the often fiercely polarized world of vertebrate paleontology, he’s unlikely to be let down.

As Mr. Bakker sees it, dinosaurs have been given a bad rap over the years as ”failures in the evolutionary test of time” – portrayed as small-brained, cold-blooded sluggards who couldn’t ”cope with competition from the smaller, smarter, livelier mammals.” Such portraits, he suggests, are unfair as well as scientifically inaccurate: in the first place, dinosaurs dominated history for 130 million years [+] – a remarkably long period of time that attests to a decided ability to survive (the human species, in contrast, has only been around for 100,000 years). And while Mr. Bakker acknowledges that dinosaurs were probably not brilliant thinkers [+], he makes a persuasive argument for their physiological adaptability and their prodigious energy [+] – he even speculates that tyrannosaurus could gallop about at speeds approaching 45 miles an hour.  [-] 

Much of ”The Dinosaur Heresies,” in fact, revolves around the question of whether the animals were cold-blooded (and more closely related to reptiles) or, as Mr. Bakker contends, warm-blooded (and more closely related to mammals and birds) [+]. While he occasionally stops to summarize opposing viewpoints, he is less interested in presenting an objective overview of the field than in mustering evidence to support his own theories.

He argues that gizzards and large digestive tracts in [some] dinosaurs would have compensated for their weak teeth [+], enabling them to eat high quantities of land plants, necessary to support a high metabolic rate. He argues that birds and pterodactyls – both of which would have had to evolve high-pressure hearts and lungs before flight could have been achieved  [+] – descended from dinosaurs  [+] [-], and that it’s not unlikely that these ancestor dinosaurs were already equipped for high metabolism [+]. He argues that the dinosaurs’ ”adaptations for sex and intimidation” – horns, head-butting armor and all manner of bony frills -suggest that they led active, aggressive lives, uncharacteristic of lethargic, cold-blooded animals  [+]. He argues that the growth rate of dinosaurs more closely resembles that of mammals than reptiles [+]. And, finally, he argues that dinosaurs’ porous bone tissue indicates the sort of high blood-flow rate usually associated with warm-blooded creatures [?].

On the question of the dinosaurs’ demise, Mr. Bakker sides with those paleontologists who discount new theories of mass extinction caused by some sort of cosmic catastrophe – he cites evidence suggesting the extinctions occurred not during a single ”doomsday” period but over tens of thousands of years [+] [-] [?]. In his view, the development of new sorts of dinosaurs and other animals, combined with changes in the physical and genetic environment, gradually led to their doom [+] [-].

On a side note:
I liked Dr. Bakker’s quote about making some people angry with his novel ideas based on overlooked data.

On another side note:
like our antiquated notions about dinosaurs from over 30 years ago, pterosaurs today have been given a bad rap. They are still portrayed as ungainly quadrupeds, bound by membranes that tied their legs together and tied their wings to their ankles (along with a long list of other false paradigms). The data deniers, unfortunately, are still out there, thinking that if they just turn a blind eye toward certain data and hypotheses they will go away.

As everyone knows,
this blog, Pterosaur Heresies, was intended to approach data with the same verve and testing of false traditions that Dr. Bakker demonstrated.

 

 

Deinonynchus foot – dromaeosaur track

Updated June 19, 2015 with the addition of the Velociraptor foot.

Deinonychus is the bigger cousin to Velociraptor. Here (Fig. 1) both feet are laid out flat (in vivo it would have been digitigrade) in dorsal view compared to a dromaeosaur track, distinctive for its 2-1/2 toe impressions.

Figure 1. Deinonychus right foot in dorsal view compared to a dromaeosaur track, not to scale. Just added on the right, a Velociraptor foot. Note the differences.

Figure 1. Deinonychus right foot in dorsal view compared to a dromaeosaur track, not to scale. Just added on the right, a Velociraptor foot. Note the differences.

The reduced fourth toe of Velociraptor (or elongated third toe) is the major difference between the two. The trackmaker appears to be midway between the two.

Yesterday was a very big day for visitors.
Thank you Jurassic World on your opening day!

Haplocheirus: a basal alvarezsaroid and more…

Earlier we looked at two Cretaceous alvarezsauroid theropod dinosaurs that likely clung to trees while resting. Today we’ll look at a basal alvarezsauroid theropod dinosaur from the Jurassic with bigger claws and more primitive traits…

Figure 1. Haplocheirus sollers traced from several photos. This specimen is 10 million years older than Archaeopteryx and tens of million years older than dromaeosaurs and alvarezsarids.

Figure 1. Haplocheirus sollers traced from several photos. This specimen is 15 million years older than Archaeopteryx and tens of million years older than dromaeosaurs and alvarezsarids. Click to enlarge. Note the robust pedal digit 2 and manual digit 1.Haplocheirus sollers (Choiniere et al. 2010 Late Jurassic, 150 mya, 2m long) is a a theropod dinosaur from the Jurassic that nests at the base of the alvarezsaurids (including Mononykus and Shuvuuia) and also basal to the Cretaceous dromaeosaurids (including Velociraptor), ~and~ basal to Jurassic proto-birds (including Aurornis, Fig. 2).

Figure 1. Theropods in the large reptile tree. Haplocheirus is highlighted in yellow.

Figure 1. Theropods in the large reptile tree. Haplocheirus is highlighted in yellow.

Despite the obvious similarities
to Velociraptor, Haplocheirus is treated only as a basal alvarezsauroid in Wikipedia. Certainly manual digit 1 is more robust in Haplocheirus, as in alvarezsaurids. But just as certainly, pedal digit 2 is more robust with a ginglymoid (pulley-shaped) joint (Fig. 1), as in dromaeosaurids (deinonychosaurs).

Phylogenetic bracketing indicates that Haplocheirus was covered with primitive feathers. The present tree (Fig. 2) is similar to that of other prior dinosaur cladograms.

The large reptile tree now includes 550 taxa.

References
Choiniere JN, Xu X, Clark JM, Forster CA, Guo Y, Han F 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science 327 (5965): 571–574.

Archaeornithura: a basal modern bird from 130 mya

Figure 1. Archaeornithura meemannae DGS tracing over aligned plate and counter plate (left). Tracing without fossil at right. Lateral view of post cervical skeleton (with black body outline).

Figure 1. Archaeornithura meemannae DGS tracing over aligned plate and counter plate (left). Tracing without fossil at right. Lateral view of post cervical skeleton (with black body outline). Click to enlarge. Different from a chicken or sparrow: 1. smaller sternum, 2. more gracile ventral pelvis, 3. longer tail and 4. unfused manus bones. The broad and robust sacrum is similar to modern birds.

A recent paper by Wang et al. (2015) brings us the earliest bird of modern aspect, one from the modern Ornithomorpha clade (all living birds), Archaeornitura (Fig. 1). It lived 130 mya in the Early Cretaceous. Two specimens were found. Both had rich feather preservation with primary wing feathers long enough for flight.

Primitive, yet modern
The sternum is small. The ventral pelvis is gracile. The sacrum is large and robust, but not fused together and not fused to the ilium. At least one specimen appears to have retained a long set of tail bones. The coracoids were long and firmly attached to a large sternum, though not nearly as large as in modern flying birds. The fingers were reduced, but unfused. The skull was not well preserved in either specimen. About nine not-very-long cervicals were present. Not much, if any, of a pubic boot in Archaeornithura, but all sister tested sister taxa have one.

Compare the skeleton
of Archaeornithura (Fig. 1) to that of Gallus the chicken (Fig. 2). The chicken, like most modern birds, has a larger, deeper sternum, a larger deeper ventral pelvis, fused fingers and more cervicals.

Figure 1. Gallus the chicken is representative of modern birds. Note the large size of the sternum and ventral pelvis, the fused manual bones, the extended cervical series and the reduced tail

Figure 1. Gallus the chicken is representative of modern birds. Note the large size of the sternum and ventral pelvis, the fused manual bones, the extended cervical series and the reduced tail

So, in pterosaurs and their predecessors 
the pectoral and pelvic girdles came first, the wings developed later. In birds, the wings came first, the pectoral and pelvic girdles took a while to develop.

References
Wang M et al. (7 other authors) 2015. The oldest record of ornithuromorpha from the early cretaceous of China. 6:6987 DOI: 10.1038/ncomms7987

Yi qi and Occam’s razor

Yesterday and earlier we looked at the new dino-bird, Yi qi. This is the third and final note.

Figure 1. Identification errors (in red) on the original Yi qi diagram from Xu et al. 2015.

Figure 1. Identification errors (in red) on the original Yi qi diagram from Xu et al. 2015. LSE = left styliform element. RSE = right styliform element.

To reiterate: 
In Xu et al. (2015) the authors noted a new long bony element in the fore limb of Yi qi (Fig. 1) never before seen on any bird, dinosaur, or archosaur. They labeled the new bone a ‘styliform element’ in accord with similar elements in flying squirrels. Chemical analysis determined that the new bone was indeed bone. This long bone, as long as (or in the opinion of the authors, longer than) any other forelimb element appeared to be attached at one end to the wrist, and at the other end, to nothing else. In certain flying squirrels, the styliform extends laterally to expand the width of the gliding membrane. Xu et al. also compared the unfamiliar bone to similar bones in gliding marsupials, bats and pterosaurs (the pteroid), all of which arise from different parts of the anatomy, frame extradermal membranes and are employed during aerial exercises.

Xu et al. wrote:
“The most striking feature of Yi is the presence of an anomalous, slightly curved, distally tapered, rod-like structure whose length considerably exceeds that of the ulna, associated with each wrist and apparently extending from the ulnar side of the carpus. However, the rod-like bone of the forelimb of Yi is morphologically unlike any normal theropod skeletal element. Indeed, no equivalent of the rod-like bone is known in any other dinosaur even outside Theropoda,” So Yi qi had potentially analogous structures among other tetrapods, but not among birds or other dinosaurs.

The big question is:
was the new bone indeed longer than the ulna? If you trace the elements as they are (Figs. 1, 2), the new bone is not longer than the ulna.

yi-qi-recon1000b

Figure 2. Yi qi tracing of the in situ specimen using DGS method and bones rearranged, also using the DGS method, to form a standing and flying Yi qi specimen. Note the lack of a styliform element, here identified as a left radius and right ulna.

It’s easy to make errors here
The right radius is incomplete and splintered (Figs. 1-3) so that it looks like a radius and ulna. The distal right radius is lost beyond the edge of the matrix. The right so-called ‘styliform element’ extends beyond the elbow and was presumed to also extend beyond the end of the matrix. However, taken ‘as is’ the two so-called ‘styliform elements’ are actually similar in length to each other and similar to the virtually complete left ulna.

Taphonomic churning
Evidently the elements were churned prior to burial with medial digits of the left hand now lateral and the right foot high kicking like a Rockette. Thus Padian’s comment, “Yi qi’s body is not preserved below the ribcage, so reconstructions of the pelvis, hindlimbs and tail must be conjectured from what is known of other scansoriopterygids,” is incorrect or an oversight (see Figs. 1, 2).

Figure 3. Right ulna (former styliform process) of Yi traced using DGS techniques. See Figure 4 for comparison to Epidendrosaurus.

Figure 3. Right ulna (former styliform process, amber color here) of Yi traced using DGS techniques. See Figure 5 for comparison to Epidendrosaurus. The newly identified ulna was flipped prior to burial so that the thicker proximal end is now closer to the lost wrist. The radius (pink and purple) is cracked lengthwise with toothy ridges marking the break. This gave the impression of a radius and ulna, but no intact radius and ulna have tooth-like cracks and ridges. This is a shattered hollow bone. 

On the left forelimb,
the left so-called ‘radius’ (Figs. 1, 4) is barely present. The left so-called ‘styliform element’ gives the erroneous impression of continuing beneath the left humerus. It does not.

Figure 3. Closeup of the former 'styliform element' here identified as a radius in Yi qi.

Figure 4. Closeup of the former ‘styliform element’ here identified as a left radius in Yi qi.

The authors never considered the possibility
that the ulna and radius could have been splintered during crushing prior to burial. Both forearm elements are as hollow as long bones from other dino-birds. And they have been subject to torsion and churning prior to burial. In this scenario the purported left ‘radius’ (Fig. 4) is simply a splinter of the ulna, separated during axial torsion. Similarly, the purported right ‘ulna’ is a splinter of the right radius (Fig. 3).

In this hypothesis
the curved right ‘styliform element’ is in reality an ulna and the straighter left ‘styliform element’ is a radius. As preserved the new radius and ulna are about the same length and similar in diameter to their counterparts. In birds and sister taxa, like Epidendrosaurus (Fig. 5), the radius is typically straighter than the ulna. And the ulna tapers distally.

In the Xu et al. 2015 Supplemental Data,
the ‘styliform element’ lengths each have an asterisk associated with them: 133.5* and 91.3* mm. The ulna likewise has an asterisk measurement, 88.5* mm. The length of the radius was not estimated or shown. The authors note: “* indicates estimated value; the preserved length of the right styliform element is 91.3 mm, but taphonomic information and morphological comparisons between the right and left styliform elements lead us to estimate that the total length of the styliform element is 133.5 mm.” In this case, they overestimated. Perhaps a more precise tracing would have been helpful.

Occam’s razor
As everyone knows, “The principle states that among competing hypotheses that predict equally well, the one with the fewest assumptions should be selected.” Xu et al. struggled with the identity of the odd long bones in Yi qi. Unfortunately they did not select the identity with the fewest assumptions (and autapomorphies). Instead they made headlines around the globe and more than a few eyebrows rise with the invention of a new styliform element, which is really just a misidentified common fore arm element.

In this case
the left ‘styliform element’ articulates with the wrist because the left radius likewise articulates with the wrist. That the distal end no longer articulates with the elbow can be ascribed to taphonomic torsion prior to burial.

The right radius still articulates with the humerus, but the right ulna has flipped lengthwise, such that the narrow distal end (Fig. 4) is now proximal, far behind the elbow. While more difficult to visualize how this may have happened, this bone flip also must be the product of taphonomic churning prior to burial. Perhaps it goes along with the high kick of the foot. Comparisons to the articulated specimen of Epidendrosaurus (Fig. 4) are instructive here. The ulna shapes are virtually identical.

Figure 4. Right ulna of Yi (former 'styliform element') compared to right ulna of Epidendrosaurus.

Figure 5. Right ulna of Yi (former ‘styliform element’) compared to right ulna of Epidendrosaurus.

References
Padian K. 2015. Paleontology: Dinosaur up in the air. Nature (2015) doi:10.1038/nature14392
Xu X, Zheng X-T, Sullivan C, Wang X-L, Xing l, Wang Y, Zhang X-M, O’Connor JK, Zhang F-C and Pan Y-H 2015.
 A bizarre Jurassic maniraptoran theropod with preserved evidence of membranous wings.Nature (advance online publication)
doi:10.1038/nature14423