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.

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!

When Paleontologists Disagree: Who Do You Trust?

Gregrory Paul
(2002, figure 3.1; Fig. 1) published various illustrations of the skull of several specimens of the basal bird, Archaeopteryx, as restored by several paleontologists. You’ll note that these restorations are not identical. Paul also notes the inconsistencies bone-by-bone in his caption and he provides his own restorations in an effort to set the record straight. His book, Dinosaurs of the Air, is chock full of original art and text, well-researched and referenced.

Figure 1. Images from Paul 2002, figure 3.1) but flipped, rotated  and reordered.

Figure 1. Images from Paul 2002, figure 3.1) but flipped, rotated and reordered with permission.

For our purposes
it is enough to know that interpretations of paleontological data do differ from one professional worker to another. Every individual sees or traces something slightly different, depending on their level of precision and the experience they bring to the tracing. Many paleontologists, as we have seen, are not concerned with precision. It’s just not a priority.

Fossils are not preserved as blueprints.
Rather they have to be traced (choose your own method) then interpreted, skewed and rotated to produce straight dorsal and lateral views, as shown above, so they can be understood, compared and shared. Drawings are always done by hand (or mouse) and therefore unconscious bias, laziness, the effort to show something novel, and other human foibles always creep in, hopefully for the benefit of the reader whenever chaos is clarified.

So who do you trust?
For several decades most of us have trusted Gregory Paul over and above more than a few paleontologist illustrators with PhDs.

Wikipedia reports, “Paul helped pioneer the “new look” of dinosaurs in the 1970s. Through a series of dynamic ink drawings and oil paintings he was among the first professional artists to depict them as active, warm-blooded and – in the case of the small ones – feathered. Many later dinosaur illustrations are a reflection of his anatomical insights or even a direct imitation of his style. The fact that he worked closely with paleontologists, did his own independent paleontological research and created a series of skeletal restorations of all sufficiently known dinosaurs, lead many to regard his images as a sort of scientific standard to be followed.This tendency is stimulated by his habit of constantly redrawing older work to let it reflect the latest finds and theories.

“Paul lacks a formal degree in paleontology, but has participated in numerous field expeditions and has authored or co-authored over 30 scientific papers and over 40 popular science articles.”

Paul suggests that, “‘each paleoartist should produce their own skeletal restorations based on original research. This would include using photos of the skeleton, or an illustrated technical paper on the particular taxon.’ Paul has earned wide acclaim for this method.” 

Others have not earned wide acclaim by following Paul’s methods.

Darren Naish wrote
here: “Paul’s massive influence mostly comes from the fact that his reconstructions have always been based on an underlying, apparently empirical effort to depict anatomy. In an ideal world, all attempts to reconstruct fossil animals would proceed this way. Paul argued that one should strive to produce multi-view skeletal reconstructions of fossil archosaurs, and that a good understanding of the overlying musculature should result in a reconstructed form that – bar integument – is essentially that of the living animal. [there was] a time when certain palaeontologists decried the reconstructing of small dinosaurs as feathery or furry as wholly unscientific and as evidence of the obviously inferior intellect and experience of Paul and his artist colleagues, but look where we are now.”

Naish continues, “People like reading what Paul writes because he is well known for being controversial; for promoting new and sometimes daring and weird ideas about dinosaur evolution, biology, locomotion and behaviour. This is the person who was arguing for feathered theropods and fuzzy ornithischians before such ideas went mainstream, argued for the evolution of widespread secondary flightlessness across maniraptoran theropods, and worked to emphasise the idea that watching live dinosaurs would be like watching modern animals on the Serengeti – there would be dust in the air, interspecies conflicts, occasional herbivory in predators and occasional carnivory in herbivores.”

Naish further continues, “Some researchers say that the very raison d’etre of this work – Paul’s body of high-fidelity skeletal reconstructions – is problematic, with the underlying reconstructive process being subjective, prone to bias and misinterpretation, and far more artistic than Paul makes clear. Producing skeletal reconstructions of this sort involves extrapolation, interpretation and a degree of guesswork, so perhaps it would be helpful – and this is not a specific criticism of Paul, but one that could be directed at all technical skeletal reconstructions – if the reconstructions were framed as hypotheses where some (SOME, not all) of the details are open to alternative interpretations.”

On the downside,
quadrupedal plateosaur poses championed by Paul were disputed by Mallison 2010.

The beauty of Science is this:
everything that is presented, by Greg Paul, yours truly, or anyone else, can be tested by repeating the process yourself. Do the work and reap the rewards of discovery, confirmation and refutation. Paul saw things other experts had missed. He used his artistic talents to convey and share his insights so that others could appreciate dinosaurs as he saw them.

Now we all see dinos his way.

Thanks to Gregory Paul
for permission to publish his work (Fig. 1). Thanks also for his many novel insights, his devotion to precision, and for sharing with all of us a new paradigm for how dinosaurs looked, behaved and interacted.

References
Mallison, H. 2010. The digital Plateosaurus II: An assessment of the range of motion of the limbs and vertebral column and of previous reconstructions using a digital skeletal mount. Acta Palaeontologica Polonica 55, 433-458.
Paul G 2002. Dinosaurs of the Air: The Evolution and Loss of Flight in Dinosaurs and Birds. Johns Hopkins University Press, Baltimore. 406 pp.

wiki/Gregory Paul

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.

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

Notes on Limusaurus (Dinosauria, Theropoda) and its odd little hand

Figure 1. Limusaurus in situ. The associated croc has been painted red here.

Figure 1. Limusaurus in situ. The associated croc has been painted red here. Their marriage was legal in only 16 states.  : )

Limusaurus inextricabilis (Xu et al. 2009; earliest Late Jurassic, Oxfordian; 1.7m in est. length; IVPP V 15923; Figs. 1, 2) is an herbivorous theropod with very tiny arms and hands in the lineage of proto-birds. Coincidentally, Limusaurus was buried next to the skeleton of a tiny Jurassic croc (in red above). Pol and Rauhut (2012) nested Limusaurus with Elaphrosaurus and Spinotropheus. Those taxa have not been added yet to the large reptile tree where Limusaurus nests between Sinocalliopteryx and Aurornis. Limusaurus shares a ventral pelvis, but not long arms and large hands with these bird-like taxa. Like birds, Limusaurus has a pair of sternae. The coracoids appear to be transitional between disc-like and stem-like.

Figure 2. Limusaurus reconstructed. Both hands are shown and colorized. Digit 1= purple. Digit 2= pink. Digit 3 = Green. Digit 0=pale yellow. Digit 0 goes back to basal tetrapods, like Ichthyostega, and only appear here due to the vestigial nature of the manus in which it matured at an embryologically immature state compared to sister taxa.

Figure 2. Limusaurus reconstructed. Both hands are shown and colorized. Digit 1= purple. Digit 2= pink. Digit 3 = Green. Digit 0=pale yellow. Digit 0 goes back to basal tetrapods, like Ichthyostega, and only appear here due to the vestigial nature of the manus in which it matured at an embryologically immature state compared to sister taxa.

Limusaurus is notable for two main reasons:

  1. It is an herbivorous theropod from the earliest Late Jurassic
  2. It has four fingers when all sister taxa have three. This fact has added credence and confusion to the chick embryo digit identification issue we looked at earlier here

It’s well worth looking at both sides of this extra finger issue:
From the Xu et al. abstract: “Theropods have traditionally been assumed to have lost manual digits from the lateral side inward, which differs from the bilateral reduction pattern seen in other tetrapod groups. This unusual reduction pattern is clearly present in basal theropods, and has also been inferred in non-avian tetanurans based on identification of their three digits as the medial ones of the hand (I-II-III). This contradicts the many developmental studies indicating II-III-IV identities for the three manual digits of the only extant tetanurans, the birds. Here we report a new basal ceratosaur from the Oxfordian stage of the Jurassic period of China (156–161 million years ago), representing the first known Asian ceratosaur and the only known beaked, herbivorous Jurassic theropod. Most significantly, this taxon possesses a strongly reduced manual digit I, documenting a complex pattern of digital reduction within the Theropoda. Comparisons among theropod hands show that the three manual digits of basal tetanurans are similar in many metacarpal features to digits II-III-IV, but in phalangeal features to digits I-II-III, of more basal theropods. Given II-III-IV identities in avians, the simplest interpretation is that these identities were shared by all tetanurans. The transition to tetanurans involved complex changes in the hand including a shift in digit identities, with ceratosaurs displaying an intermediate condition.”

There were a long list of authors (see below) that agreed with the above, some of whom have come to our attention earlier for publishing various errors, along with their otherwise excellent work.

Unfortunately the Xu et al. abstract ignores the fact that basal tetrapods, like Acanthostega, had an additional digit medial to digit #1 (we’ll call this digit #0). Therefore digit #0 is part of the phylogenetic history of all tetrapods, despite the fact that it is almost never expressed in tetrapod adults, only embryos. Digit #0 is expressed in Limusaurus, in which the hand is a small vestige relative to those of sister taxa. Like other vestiges the hand of Limusaurus did not continue to develop normally as a hatchling and into adulthood. Rather the hand retained a shape found at a certain embryological stage, a stage that included digit #0.  That’s why the big metacarpal (in purple) has the morphology of metacarpal 1 in sister taxa (Fig. 3) and metacarpal #0 continues the shape of metacarpal #1 in cross section. In most tetrapods digit #0 is fused to metacarpal #0 or otherwise disappears before birth or hatching.

Figure 3. The manus of several theropods including Limusaurus. Here digit 1 is purple, digit 2 is pink and digit 3 is green. Note the presence of digit 0 in this vestigial hand, a holdover from basal tetrapods that has not been correctly identified by Xu et al. and others.

Figure 3. The manus of several theropods including Limusaurus. Here digit 1 is purple, digit 2 is pink and digit 3 is green. Note the presence of digit 0 in this vestigial hand, a holdover from basal tetrapods that has not been correctly identified by Xu et al. and others. Click to enlarge.

That’s why otherwise you only see digit #0 in embryos. Thus there is no “phase shift” of digit identity. There is only loss, fusion or absorption of digit #0, a factor missed by earlier workers.

References
Xu X, Clark JM. Mo J, Choiniere J, Forster CA, Erickson GM, Hone DWE, Sullivan C, Eberth DA, Nesbitt S, Zhao Q, Hernandez R, Jia C-K, Han F-L. and Guo Y 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature, 459(18): 940–944.

Chilesaurus, new dinosaur: not so ‘enigmatic’ after all…

Chilesaurus diegosuarezi (Novas et al. 2015; Late Jurassic, 150 mya, Fig. 1) is the current media darling. Described as an ‘enigmatic’ and ‘bizarre’ theropod, Chilesaurus was nested  with Velociraptor, Tawa and kin, and Elaphrosaurus using various prior cladograms in the supplementary data. So that’s an issue (no internal agreement).

Several articulated specimens are known at distinct ontogenetic stages.

Unfortunately taxon exclusion raises its ugly head again…
The large reptile tree nests Chilesaurus outside of the Theropoda, near the base of the Phytodinosauria, at the base of the Ornithischia and at the base of the clade that also includes Daemonosaurus and Jeholosaurus (Fig. 1), two taxa that were unfortunately ignored by the Novas et al. study. Hate to see that happen yet again.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Figure 1. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria to scale.

Figure 2. Look familiar? Here are the pelves of Jeholosaurus and Chilesaurus compared.  As discussed earlier, this is how the ornithischian pelvis evolved from that of Eoraptor and basal saurorpodomorpha.

Figure 2. Look familiar? Here are the pelves of Jeholosaurus and Chilesaurus compared. As discussed earlier, this is how the ornithischian pelvis evolved from that of Eoraptor and basal saurorpodomorpha..

Folks,
Chilesaurus is not bizarre.

It is simply a descendant from an unknown Late Triassic transitional taxon at the base of the Ornithischia, a hypothesis overlooked by Novas et al. Chilesaurus is not a theropod, but a phytodinosaur (Fig. 3). The fact that fossils of Chilesaurus were found much later than the original split is not a cause for concern. That happens all the time.

Pelvis  changes
Along with Jeholosaurus, Chilesaurus demonstrates the changes that were happening to the dinosaurian pelvis at the genesis of the ornithischian pelvis. As a plant eater, Chilesaurus and kin were expanding their gut volume to digest less digestible plant matter.

Manus and tooth changes
The hands of Chilesaurus are not as primitive and plesiomorphic as might be hoped, but then Chilesaurus is a descendent of that Late Triassic transitional taxon, appearing tens of millions of years after the split. Things evolve! While the teeth remain large and robust, Chilesaurus had flat teeth, rather than the pointed ones of its Triassic sister, Daemonosaurus or its Cretaceous sister, Jeholosaurus.

Figure 3. Cladogram of basal dinosaurs. Note that Chilesaurus nests near the base of the Phytodinosauria and at the base of the Ornithischia, both far from the Theropoda.

Figure 3. Cladogram of basal dinosaurs. Note that Chilesaurus nests near the base of the Phytodinosauria and at the base of the Ornithischia, both far from the Theropoda.

Clues
to the largely missing post-crania of Daemonosaurus are provided by its sister Chilesaurus.

Now let’s talk about the PR barrage
This is where the science reporters separate themselves from the scientists. All those who reported without testing the results of Novas et al. … you may have to do some backtracking.

Theropod database
See M.Mortimer’s take on Chilesaurus here. Mortimer found a raft of miscodings in the original paper by Novas et al.

References
Novas FE, Salgado, Suárez LM, Agnolín FL, Ezcurra MND, Chimento NSR.,de la Cruz R, Isasi MP, Vargas AO, Rubilar-Rogers D. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature. doi:10.1038/nature14307