What traits separate phytodinosaurs from theropods?

Yesterday we looked at the origin of dinosaurs in the context of and contra the recent Baron et al. 2017 paper. Today we’ll look at the basal split between basal phytodinosaurs, like Eodromaeus (Figs. 1, 2), with the closely related basal theropods, like Tawa (Fig. 1).

Figure 1. The theropod Tawa compared to the closely related phytodinosaur, Eodromaeus.

Figure 1. The theropod Tawa compared to the closely related smaller phytodinosaur, Eodromaeus.

Placed side-by-side to scale
Tawa and Eodromaeus are similar overall, though the plant-eaters were initially smaller. The details (below) demonstrate the initial steps toward herbivory that characterize the Phytodinosauria, distinct from the Theropoda and basal Dinosauria from which they evolved (contra Baron et al. 2017).

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

How do basal phytodinosaurs differ from the basal theropods?
Here’s the LRT list:

  1. lateral rostral shape: convex and smoothly curved (also in ancestral Herrerasaurus and Gracilisuchus);
  2. premaxilla/maxilla angle 25–45º;
  3. naris shape in lateral view almost round (not longer than tall or taller than long);
  4. postfrontal has no contact with the upper temporal fenestra;
  5. opisthotic oriented laterally without posttemporal fenestrae;
  6. palatal teeth (only on basalmost taxa);
  7. maxillary tooth depth ≤ 2x width in lateral view;
  8. last maxillary tooth at mid orbit (also in Herrerasaurus);
  9. olecranon process present (convergent in Buriolestes);
  10. metacarpals 2 and 3 align with m1.1 (except Eodromaeus);
  11. acetabulum laterally oriented (no ventral deflection, as in basal theropods);
  12. femoral head with neck and offset (appears later in theropods);
  13. penultimate manual phalanges not the longest in each series;
  14. loss of pubic boot (likely plesiomorphic because outgroups to Herrerasaurus do not have a pubic boot).

Summary and significance
Compared to the closely related theropod Tawa, the overall similar phytodinosaur Eodromaeus had a taller rounder rostrum, shorter teeth, a higher coronoid process, a longer dorsal region with more robust dorsal vertebrae, reduced gastralia, a more robust pectoral girdle and forelimb with shorter, less raptorial fingers, a deeper pubis and ischium with more robust hind limbs. The shorter teeth and larger belly together with the more robust limbs and back are traits seen in a wide variety of herbivores, even if only transitional at this early stage.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

Basal dinosaurs: what makes them dinosaurs?

Earlier we looked at Baron et al. 2017, who heretically allied ornithischians with theropods to create a new clade ‘Ornithoscelida.’ After a week of study and several tests, I came away with the realization that the Baron et al. tree topology did not depend on taxon exclusion, based on comparable results from subsets of the large reptile tree (LRT). Reducing the list of LRT taxa to match the Baron et al. taxon list did not change the dinosaur tree topology. Rather scoring inaccuracies were the problem, and that goes back to the original matrix presented by Nesbitt 2011, which Baron et al. continued using. You might remember, that’s the study that nests pterosaurs with parasuchians with no intervening taxa, among many other odd nestings. We’ll detail those inaccuracies later this week.

One of the reasons
Baron et al. allied theropods with ornithisichians is because they nested two phytodinosaurs, Eodromaeus (Fig. 1) and Eoraptor, as basalmost theropods. So when they thought they were comparing the ‘theropod’ Eoraptor with the ornithischian, Heterodontosaurus (their figure 2), they were actually comparing two phytodinosaurs. So, of course they shared a long list of characters. To really test their assertion, Baron et al. should have tested Heterodontosaurus against a real basal theropod, like Tawa (Fig. 2). (Tomorrow we’ll compare Tawa with Eodromaeus.)

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Figure 1. Eodromaeus reconstructed. We will look at this taxon in more detail tomorrow.

Along the way, 
I was able to better understand what was going on at the  base of the Dinosauria and at the basal Theropod/Phytodinosaur split. For instance, as noted earlier, Herrerasaurus (Fig. 3) has a smaller number of cervicals than other dinos do, matching the number found in ancestral basal crocodylomorpha, like Lewisuchus, not silesaurids, like Silesaurus.

Figure 2. Tawa, a basal theropod. Note how closely it resembles the phytodinosaur, Eodromaeus. That's because they are closely related as basalmost dinosaurs.

Figure 2. Tawa, a basal theropod. Note how closely it resembles the phytodinosaur, Eodromaeus. That’s because they are closely related as basalmost dinosaurs.

How do basal dinosaurs differ from the basal croc ancestors?
Here’s the latest list:

  1. skull shorter than cervical series (not Herrerasaurus):
  2. skull width less than 1.2x height;
  3. posterolateral premaxilla narrower than naris (not Herrerasaurus);
  4. quadrate curls posterodorsally (not Herrerasaurus);
  5. dentary tip straight or descends (does not rise);
  6. deep canine maxillary teeth absent (not Herrerasaurus);
  7. nine or more cervicals (not Herrerasaurus);
  8. cervical ribs slender, parallel centra (Herrerasaurus unknown);
  9. 3–4 sacral ribs (not Herrerasaurus, which has 2);
  10. mid-caudal centra 3x longer than tall where known (not Herrerasaurus);
  11. clavicles poorly ossified or absent (reappear in later taxa);
  12. interclavicle poorly ossified or absent;
  13. manual unguals long with penultimate phalanges longer than proximal phalanges;
  14. tibia not shorter than femur (not Herrerasaurus);
  15. fourth trochanter sharp (also in the proximal dinosaur outgroup taxon PVL 4597);
  16. tibia not < 2x ilium length (not Herrerasaurus);
  17. advanced and simple metatarsal joint without a calcaneal tuber (also in Lewisuchus, convergent in Silesaurus);
  18. compact metatarsus;
  19. longest metatarsal: #3;
  20. metatarsal #1 < 75% of metatarsal #3 length;
  21. metatarsals 2–4 ≥ tibia (not Herrerasaurus);
  22. proximal metatarsals 1 and/or 5 reduced in diameter.

Summary and significance
Arising from the morphology of basal bipedal crocodylomorphs, many of these basal dinosaur traits document a longer, leaner bipedal morphology with an increasingly robust fulcrum (pelvis and sacrum), along with longer distal hind limb elements with more reliance on the middle digits for faster locomotion. By contrast the proximal outgroup clade, Junggarsuchus and kin, documents a return to quadrupedalism terminating with Trialestes and its long forelimbs. The increasingly elongate radiale and ulnare of the Junggarsuchus clade turn out to be convergent, rather than homologous, with those of other bipedal crocodylomorphs, like Terrestrisuchus and Hesperosuchus, a clade that also transitioned toward a quadrupedal configuration, as seen in extant crocodylomorphs. We’ll take a closer look at this clade and this transition later this week.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

Figure 1. Lewisuchus cervical/dorsal transition at top photo and the same for Herrerasaurus drawings, including a foreword shift of the pectoral girdle in a 2-frame GIF movie. The cervical ribs are imagined.

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.

 

Estimating dino/croc divergence times: Turner et al. 2017

This might have been yet another case
of scientists TRUSTING authority (= the work of others) rather than TESTING competing phylogenetic analyses. In this case, however, two of the three authors in Turner, Pritchard and Matze 2017 relied on their own flawed (= serious taxon exclusion problems) phylogenetic analyses and for good measure they threw in a third flawed (= more taxon exclusion problems) analysis (Nesbitt 2011) that we examined and reexamined in an 11-part series.

In any case, since none of the trees
in the new Turner et al. study  stand up to scrutiny (= do not agree with one another, do not produce gradual accumulations of traits in derived taxa and depend on long ghost lineaages), everything Turner et al. (2017) did afterwards has no credibility and no utility. So sadly, the entire paper is a waste of their time. Metaphorically, they built their house on sand.

On the other hand,
when you start with a study that provides a gradual accumulation of derived traits in all derived taxa, and minimizes the effect of taxon exclusion, like the large reptile tree (LRT (949 taxa) then you’ve metaphorically built your house on solid ground. And it’s much simpler to pinpoint the dino/croc divergence time because you are provided with a last common ancestor for these sister clades: Gracilisuchus (Figs. 1, 2). Crocs and dinos are sister taxa. None of the studies used by Turner et al. (Pritchard et al. 2015, Nesbit 2011, Turner 2015) recovered that tested relationship.

Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor.

Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor.

So when did dinos and crocs diverge?
Let’s look a the three most recent taxa both clades share in common in reverse chronological and phylogenetic order:

  1. Gracilisuchus = 230 mya.
  2. Turfanosuchus = 235 mya.
  3. Decuriasuchus = 240 mya.

So that narrows the divergence time pretty well…

And how did Turner et al. do?
They report,“The average ghost lineage for the group as sampled is 31 million years.” Their conclusion states no firm date or date range. Rather, their whole paper appears to be a long story on how they tested this that and the other without getting around to their headline topic. And without nailing down a last common ancestor or a croc/dino divergence time.

Figure 2. Basal crocs. Decuriasuchus and Gracilisuchus are found in both croc and dino lineages.

Figure 2. Basal crocs. Decuriasuchus and Gracilisuchus are found in both croc and dino lineages.

All the other taxa
and all the other testing performed by Turner et al. were for nought.

For more information
on any of the taxa employed by Turner et al, just look them up at ReptileEvolution.com.

References
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History;352:1–292.
Pritchard AC, Turner AH, Nesbitt SJ, Irmis RB and Smith ND 2015. Late Triassic tanystropheids (Reptilia, Archosauromorpha) from Northern New Mexico (Petrified Forest Member, Chinle Formation) and the biogeography, functional morphology, and evolution of Tanystropheidae. Journal of Vertebrate Paleontology. ;e911186.
Turner AH 2015. A Review of Shamosuchus and Paralligator (Crocodyliformes, Neosuchia) from the Cretaceous of Asia. PLoS ONE. 2015;10(2):e0118116. doi: 10.1371/journal.pone.0118116. pmid:25714338
Turner AH, Pritchard AC and Matzke NJ 2017. Empirical and Bayesian approaches to fossil-only divergence times: A study across three reptile clades. PLoS ONE 12(2): e0169885. doi:10.1371/journal.pone.0169885

 

Go back far enough in dinosaur ancestry and you come to: Heleosaurus

With our never-ending fascination with dinosaurs
it’s interesting to list some of the taxa in their deep, deep!, deep!! ancestry. One such ancestor is Heleosaurus (Fig. 1; Broom 1907; Middle Permian ~270 mya, ~30 cm snout to vent length), the first known basal prodiapsid, the clade the includes diapsids (sans lepidosaurs, which are unrelated but share the same skull topology by convergence).

Figure 1. Heleosaurus is closer to the main lineage of dinosaurs. It retained canine fangs.

Figure 1. Heleosaurus is close to the main lineage of dinosaurs. It retained canine fangs. Note the squamosal distinct from the quadratojugal, as in Nikkasaurus. Also note the continuing lacrimal contact with the naris, as in Protorothyris.

But first
I want to discuss a derived Heleosaurus cousin, Nikkasaurus (Ivahnenko 2000; Fig. 2), also one of the most basal prodiapsids.

It is only by coincidence
that Ivahnenko labeled Nikkasaurus one of his ‘Dinomorpha,’ a clade name ignored by other authors. Wikipedia considers Nikkasaurus one of the Therapsida and possibly a relic of a more ancient stage of therapsid development. Like Heleosaurus, Nikkasaurus had a single synapsid-like lateral temporal fenestra. Only their nesting outside of that clade and basal to the clade Diapsida in the LRT tell us what they really are. Most of the time, as you know, we can tell what a taxon is simply by looking at it. In this case, as in only a few others, we cannot do so readily.

Figure 1. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Figure 2. Nikkasaurus, one of the most primitive prodiapsids, direct but ancient ancestors of dinosaurs.

Nikkasaurus tatarinovi (Ivahnenko 2000) Middle Permian was a tiny basal prodiapsid with a large orbit. It retained a large quadratojugal. The fossil is missing the squamosal. Others mistakenly considered the quadratojugal the squamosal, as in therapsids. That’s an easy mistake to make. Compare this bone to the QJ in Heleosaurus (Fig. 1), another prodiapsid. Nikkasaurus has small sharp teeth and no canine fang. Nikkasaurus is a sister to Mycterosaurus. They both share a large orbit and fairly long snout. What appears to be a retroarticular process may be something else awaiting inspection in the actual fossil. Based on all other data points, I don’t trust that post-dentary data. It doesn’t match the in situ figure.

Distinct from other prodiapsids,
the Nikkasaurus, Mycerosaurus and Mesenosaurus maxilla extended dorsally, overlapping the lacrimal and contacting the nasal, as it does in Dimetrodon and basal therapsids like Hipposaurus and Stenocybus. This trait tends to be homoplastic / convergent in all derived taxa, but the timing differs in separate clades.

Figure 1. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it.

Figure 2. Nikkasaurus and what little is known of its postcrania. Above, in situ. Below, tentative reconstruction. If anyone has a picture of the fossil itself, please send it. Note the posterior mandible mismatch in the purported retroarticular process. I suspect the process is not there.

And finally we come back to Heleosaurus.
Slightly closer to the lineage of dinosaurs is the slightly more basal prodiapsid, Heleosaurus (Fig. 2), which retained canine fangs, had a more typical posterior mandible and retained a lacrimal / naris contact. This naris trait was retained by Petrolacosaurus, Eudibamus, Spinoaequalis and other basal diapsids (archosauromorpha with both upper and lateral temporal fenestra ). The maxilla did not rise again to cut off lacrimal contact with the naris in the ancestry of dinosaurs until the small Youngina specimens huddled together, SAM K 7710 and every more derived taxon thereafter, up to and including dinos.

References
Broom R 1907. On some new fossil reptiles from the Karroo beds of Victoria West, South Africa. Transactions of the South African Philosophical Society 18:31–42.
Ivahnenko MF 2000. 
Cranial morphology and evolution of Permian Dinomorpha (Eotherapsida) of eastern Europe. Paleontological Journal 42(9):859-995. DOI: 10.1134/S0031030108090013

Lagerpeton: not the first of its kind, but the last of its kind

Quick note
I updated the reconstruction and nesting of Colobomycter, which you can see here.

Traditional paleontologists
consider Lagerpeton (Fig. 1, Romer 1971) a basal dinosauromorph, thus the first of its kind (ancestral to dinosaurs).

In contrast
Lagerpeton nests as a terminal taxon in the large reptile tree, leaving no known descendants. Here (Fig. 1) convergent evolution has created a bipedal chanaresuchid, derived from Tropidosuchus that has similar pedal proportions to the second specimen attributed to Tropidosuchus.

Figure 3. The closest kin of Tropidosuchus are the much larger Chanaresuchus (matching Nesbitt 2011) and the smaller Lagerpeton.

Figure 1. The closest kin of Tropidosuchus are the much larger Chanaresuchus (matching Nesbitt 2011) and the smaller Lagerpeton.

According to
Wikipedia, seven fossil specimens have so far been attributed to L. chanarensis. They don’t add up to much more than a hind limb and pelvic girdle.

  1. UPLR 06 (holotype) – articulated right hindlimb
  2. PVL 4619 – articulated pelvis with sacrum, partial right and complete left hindlimbs
  3. PVL 4625 – left pelvis with left femur and articulated vertebral column (dorsal, sacral and anterior caudal vertebrae
  4. PVL 5000 – proximal end of left femur
  5. MCZ 4121 – complete left, and partial right, femur.

Brusatte et al.
found Early Triassic footprints they attributed to lagosuchids. In reality the ichnites were closer to Rotodactylus tracks, which match the feet of fenestrasaurs, like Cosesaurus through pterosaurs.

In the large reptile tree
archosauriformes split at their origin, shortly after Youngina (AMNH 5561) and Youngoides (UC 1528) into two clades. The larger specimens start with Proterosuchus and radiate into choristoderes, parasuchians, doswellians and chanaresuchians terminating with Lagerpeton and its sister, Tropidosuchus (Fig. 1). The other branch starts with Euparkeria and extends to crocs, dinos and birds.

So,
Lagerpeton is not a close relative of dinosaurs, but convergent in several regards. The odd feet and pelves give them away as distinctly different from dinosaurs. Even so paleontologists continue clinging to this hypothesis. Better dino ancestors can be found here.

References
Arcucci A 1986. New materials and reinterpretation of Lagerpeton chanarensis Romer (Thecodontia, Lagerpetonidae nov.) from the Middle Triassic of La Rioja, Argentina. Ameghiniana 23(3-4):233-242. online pdf
Brusatte SL, Niedźwiedzki G, Butler RJ 2011. “Footprints pull origin and diversification of dinosaur stem lineage deep into Early Triassic.”Proceedings of the Royal Society B: Biological Sciences 278 (1708): 1107–1113. doi:10.1098/rspb.2010.1746PMC 3049033PMID 20926435.
Romer AS 1971 The Chanares (Argentina) Triassic reptile fauna X. Two new but incompletely known long-limbed pseudosuchians: Brevoria, n. 378, p. 1-10.
Sereno PC and Arcucci AB 1993. Dinosaurian precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis. Journal of Vertebrate Paleontology, 13, 385–399.

wiki/Lagerpeton

News on the Origin of Dinosaurs: Megapnosaurus and Zupaysaurus

Coelophysis bauri and the former Syntarsus rhodesiensis
(= Coelophysis rhodesiensis, Coelophysis kayentakatae, Megapnosaurus kayentakatae)
 have been considered congeneric. Unfortunately, the large reptile tree (subset Fig. 1) did not recover that relationship based on data below.

Figure 6. Proto-dinosaurs including Zupaysaurus and Megapnosaurus.

Figure 1. Proto-dinosaurs including Zupaysaurus and Megapnosaurus. They do not nest with the theropod dinosaur Coelophysis.

According to Wikipedia
on the topic of its name, “It was formerly called Syntarsus, but that name was already taken by a beetle, and was subsequently given the name Megapnosaurus by Ivie, Ślipiński & Węgrzynowicz, in 2001, though many subsequent studies have classified it in the genus Coelophysis.”

According to Wikipedia
on the topic of Coelophysis rhodesiensis, “Syntarsus” rhodesiensis was first described by Raath (1969) and assigned to Podokesauridae.[6] The taxon “Podokesauridae”, was abandoned since its type specimen was destroyed in a fire and can no longer be compared to new finds. Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.”

Figure 1. The palates of Coelophyis and Megapnosaurus (illustrated by Cope 1989) together with the palates of Lewisuchus and Pseudhesperosuchus in phylogenetic order based on the large reptile tree. Note the gradual evolution of the elements here and the certainty that Megapnosaurus is not congeneric with Coelophysis. The palates of Lewisuchus and Pseudhesperosuchus are evidently only partially preserved. These line drawings are the only data currently available here. 

Figure 2. The palates of Coelophyis and Megapnosaurus (illustrated by Cope 1989) together with the palates of Lewisuchus and Pseudhesperosuchus in phylogenetic order based on the large reptile tree. Note the gradual evolution of the elements here and the certainty that Megapnosaurus is not congeneric with Coelophysis. The palates of Lewisuchus and Pseudhesperosuchus are evidently only partially preserved. These line drawings are the only data currently available here.

Colbert (1989) illustrated the two palates together
of Coelophysis and Megapnosaurus (Fig. 2) and they sure do not look alike. I wondered about this discrepancy in two supposedly close sister taxa. I finally found a solution when I added Megapnosaurus and Zupaysaurus to the large reptile tree. They both nested between the basal proto-dinosaur Lewisuchus and the clade of basal pro to-dinosaurs that includes Pseudhesperosuchus.

Note in the palates gradual evolution in
the coming together and fusion of the vomers, the development of the anterior and posterior embayments of the palatine, the enlargement of the cheek articulation of the ectopterygoid and the anterior angle of the transverse process of the pterygoid, among other evolutionary advances here, along with individual variations.

Megapnosaurus_skull588

Figure 3. Megapnosaurus kayentakatae does not nest with Coelophysis, but with Zupaysaurus, between Lewisuchus and the proto-dinosaurs in the Pseudhesperosuchus clade.

Megapnosaurus rhodesiensis (kayentakatae) (Raath 1969, renamed by Ivie, Ślipiński & Węgrzynowicz, in 2001, Fig. 3, early Jurassic, 3m length) 30 individuals were found in a fossil bed. According to Wikipedia, “Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.”

This is what happens with taxon exclusion.

Figure 3. Zupaysaurus nests with Megapsnosaurus as a proto-dinosaur.

Figure 4 Zupaysaurus nests with Megapsnosaurus as a proto-dinosaur.

Zupaysaurus rougirez (Arcucci and Coria 2003, Latest Triassic to Earliest Jurassic,  PULR-076, up to 4m long, 45cm skull length, Fig. 4) nests as a late-surviving pre-dinosaur (dinosauromorph) with Megapnosaurus between Lewisuchus and the Pseudhesperosuchus clade. Everyone else considers it a theropod, but my guess is they have not tested against these candidate taxa. Traditional paleontologists are still stuck on Lagerpeton. Here are the verified dinosaur precursors to scale (Fig. 5).

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. 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.

And here’s the skull of Coelophysis for comparison (Fig. 6).

Figure 4. Coelophysis skull for comparison.

Figure 6. Coelophysis skull for comparison.

Just found this, so this was added to the post less than 24 hours later.
Down 2000 compared Coelophysis bauri with Syntarsus rhodesiensis and concluded that both were “remarkably similar morphologically.” He reported, “Although I have not been able to personally study fossils of Syntarsus rhodesiensis, I am confidant that, except for possible mis-articulation of the skull roof, the drawings prepared for Raath’s dissertation accurately represent this taxon. Raath’s illustrations, in almost every case, depict the Ghost Ranch theropod Coelophysis bauri more exactly than do the drawings and descriptions in Colbert’s (1989) monograph (which I glean data from). C. bauri and S. rhodesiensis…differ only in minor details such as neck length, proximal and distal hind limb proportions and proximal caudal anatomy. These differences do not justify a generic separation. The genus Coelophysis has priority over the genus Syntarsus.”

Take a look for yourself
at figures 3 and 6. They appear to be distinct to my eye. What do you think? Note that Downs does not mention the distinct palate. I’d like to see more postcranial data on Megapnosaurus. It’s out there. If you have it, send it. A shorter neck than in Coelophysis is to be expected if Megapnosaurus is a sister to Lewisuchus. There is no indication that Downs compared his taxa to Lewisuchus or the Pseudhesperosuchus clade, nor is there any indication of phylogenetic analysis.

References
Arcucci AB and Rodolfo AC 2003. A new Triassic carnivorous dinosaur from Argentina. Ameghiniana 40(2):217-228.
Cope ED 1889. On a new genus of Triassic Dinosauria. American Naturalist 23: 626
Late Triassic Norian.
Colbert E 1989. The Triassic Dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57: 160.
Downs A 2000. Coelophysis bauri and Syntarsus rhodesiensis compared, with comments on the preparation and preservation of fossils from the Ghost Ranch Coelophysis quarry. In:Lucas, S.G.; Heckert, A.B. (eds.). “Dinosaurs of New Mexico”. New Mexico Museum of Natural History Bulletin 17: 33–37.
Raath  MA 1969. A new Coelurosaurian dinosaur from the Forest Sandstone of Rhodesia. Arnoldia Rhodesia. 4 (28): 1-25.
Raath MA 1977. The Anatomy of the Triassic Theropod Syntarsus rhodesiensis (Saurischia: Podokesauridae) and a Consideration of Its Biology. Department of Zoology and Entomology, Rhodes University, Salisbury, Rhodesia 1-233.
Rowe T 1989. A new species of the theropod dinosaur Syntarsus from the Early Jurassic Kayenta Formation of Arizona. Journal of Vertebrate Paleontology. 9, 125-136.

wiki/Zupaysaurus
wiki/Coelophysis_rhodesiensis

SVP 23 – a new archosaur close to Junggarsuchus

Sullivan et al. 2015
describe a sister of Junggarsuchus sloani (Fig. 1) which is a basal archosaur and a basal dinosauriform, not far from crocs and a sister to Pseudhesperosuchus.

Figure 1. Junggarsuchus colorized. Once thought to be the crocodylomorph closest to crocodylformes, it now nests as a pre-dinosaur.

Figure 1. Junggarsuchus colorized. Once thought to be the crocodylomorph closest to crocodylformes, it now nests as a pre-dinosaur.

From the abstract
“The Middle-Late Jurassic Daohugou (or Yanliao) Biota and Early Cretaceous Jehol Biota are successive assemblages from northeast China. Nno crocodylomorph has been reported from either assemblage. However, a basal (i.e., non-crocodyliform) crocodylomorph was recently recovered from the Daohugou Biota site of Mutoudeng, Hebei Province. The meter-long skeleton shows crocodylomorph synapomorphies, including elongated carpals, and lacks crocodyliform traits such as an expanded coracoid process. The specimen shares key features with the Chinese basal crocodylomorph Junggarsuchus sloani, including vertebral hypapophyses and a slender, rotated metacarpal I. However, the Mutoudeng specimen differs from J. sloani in having shorter distal forelimb segments, and in lacking enlarged anterior maxillary teeth. Phylogenetic analysis recovers this specimen and J. sloani as sister taxa slightly outside Crocodyliformes.

J. sloani is known from one individual lacking the hindquarters, but the Mutoudeng specimen preserves two surprising features in this region: the iliac preacetabular process is very long, and the middle and posterior parts of the tail are entirely sheathed in osteoderms. Such extensive caudal armor is common in basal crocodyliforms but rare in more basal crocodylomorphs. The elongated preacetabular process likely gave parts of the iliotibialis and puboischiofemoralis internus musculature a near-horizontal orientation, allowing them to act as strong femoral protractors. This arguably supports prevailing views of basal crocodylomorphs as terrestrial cursors, but soft tissues preserved with the skeleton unexpectedly challenge the conventional wisdom. Scaly skin is associated with the superimposed feet, and with the right hand, forming sheets of small (< 1 mm) polygons defined by dark lines. In the right hand, patches of skin are visible between digits IV and V, extending distally to the second phalanges of the two digits. The presence of skin in this area implies the hand was webbed, which in turn suggests the Mutoudeng crocodylomorph frequented wet environments and was probably at least semi-aquatic.”

Just a few clues here
as to what will be presented, but more taxa in the ancestry of dinosaurs, or crocs, even if they are not recognized as such by Sullivan et al., are always welcome.

References
Sullivan C et al. 2015. A new basal crocodylomorph with unexpected skeletal and soft-tisse features from the Middle Late Jurassic Daohugou biota of Northeast China. Journal of Vertebrate Paleontology abstracts.