More PVL 4597 post-crania

Lecuona et al. 2017 
redescribe the post-crania of the basal archosaur and basal Crocodylomorph (in the large reptile tree = LRT), Gracilisuchus. They used six specimens and recovered them as basal Suchia (= aetosaurs, rauisuchians and crocodylomorphs). The LRT does not recover these three monophyletic clades in one larger monophyletic clade. So the LRT does not support the ‘Suchia’ as a clade.

Figure 1. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT.

Figure 1. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT. The pelvis of Herrerasaurus is shown on frame 2 of this simple animation. 

Unfortunately
Lecuona et al. excluded from their analysis several taxa that nest close to Gracilisuchus in the large reptile tree. These include Saltopus and Scleromochlus, which nest as sisters to Gracilisuchus in the LRT. Lewisuchus is mentioned in the text and combined as a chimaera with Pseudolagosuchus, but I don’t see the combo in the published trees. Their cladogram includes several suprageneric taxa (always to be avoided) including ‘Pterosauromorpha’ (= pterosaurs + Scleromochlus, not recovered as a clade in the LRT as a clade and how would one score such an internally varied taxon??).

Lecuona et al. nest their purported Gracilisuchus specimens together.
By contrast the LRT nests PVL 4597 at the base of the Dinosauria.

Fortunately 
the authors provide more data on the provisional dinosaur outgroup taxon, PVL 4597, so far based on hind limb traits only (Lecuona and Desojo 2011. The preserved skull in the specimen has not yet been published.

As in the LRT
the authors also find a close relationship between Turfanosuchus and Gracilisuchus. Unlike the LRT, they nest Yonghesuchus between them. The LRT nests it as a sister to Dromicosuchus.

It’s not common for a specimen to be published in bits and pieces
The original pelvis data came out 6 years ago. The new data for PVL 4597 (Fig. 1) still lacks the skull, which will be published in the future. I have written to Dr. Lecuona encouraging an expansion of the taxon list. I also hope the suprageneric taxa will be broken up into lists of genera.

References
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.
Lecuona A, Desojo JB and Pol D 2017.
New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position. Zoological Journal of the Linnean Society zlx011 1–40.

The forgotten clade: the REAL proximal ancestors to Dinosauria

Ignored by Baron et al. 2017, and everybody else
the Junggarsuchus clade (including Pseudhesperosuchus, Carnufex and Trialestes in order of increasing quadrupedality, Figs. 1–4) nests as the proximal ancestors to Herrerasaurus (Fig. 1) and the rest of the Dinosauria (Fig. 5) in the large reptile tree (LRT). That cladogram tests a wider gamut of taxa in greater detail than any other reptile cladogram ever published, attempting to not overlook anything. The Junggarsuchia is a sister clade to the Crocodylomorpha with both arising from a taxon near Lewisuchus (Fig. 1). Traditional paleontology (see Wikipedia) nests this largely ignored clade with the sphenosuchian crocodylomorphs (Fig. 4)… and for two good reasons!

Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.

Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.

One: Paleontologists never seem to include Dinosauria
in their smaller gamut croc analyses because they’re looking at crocs!~. So once again, taxon exclusion is holding some workers back from seeing ‘the big picture’. ReptileEvolution.com and the blog you are currently reading is all about examining ‘the big picture.’

Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur.

Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur, closely related to Junggarsuchus.

Two: Junggarsuchians ALSO have elongate proximal wrist bones
Elongate proximal carpals are found in both sphenosuchian crocs and derived members of the Junggarsuchus clade. Paleontolgists wrongly assumed such odd wrist bones were homologous. It’s an easy mistake to make. However, the LRT makes clear that intervening taxa, including Junggarsuchus, do not have elongate wrist bones.

Among taxa that preserve the manus,
(Fig. 3) it is Junggarsuchus that nests closest to Herrerasaurus and the Dinosauria.

Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.

Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.

Like the basal members of the Crocodylomorpha
the Junggarsuchus clade (the Prodinosauria here) transition from bipedal basal members to quadrupedal derived members, with the longest forelimbs belonging to the most derived member, Trialestes (Fig. 3). Distinct from the others and contra the original interpretation, I think Trialestes may have had a larger ulnare than radiale, to match its larger ulna.

Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.

Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.

Let’s not forget
PVL 4597 (Fig. 6) which was mistakenly considered a specimen of Gracilisuchus by (Lecuona and Desojo 2011), but under phylogenetic analysis in the LRT, still nests as the proximal outgroup to Herrerasaurus. It is tiny specimen, supporting the hypothesis of phylogenetic miniaturization at clade origin. And it retains a small proximally oriented calcaneal tuber, as found in other Junggarsuchians.

Figure 1. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).

Figure 5. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).

We looked at
phylogenetic miniaturization at the origin of several pterosaur clades. Well, it happens here too, at the base of the Dinosauria (Fig. 1) with PVL 4597 (Fig. 6), easily overlooked, easily mistaken for something else.

One should not ‘choose’ outgroup taxa
based on paradigm, tradition, guessing, convenience or opinion. Rather outgroup taxa should ‘choose themselves’ based on rigorous testing of a large gamut of outgroup candidates in phylogenetic analysis. To minimize selection bias, the LRT provides 858 outgroup taxa the opportunity to nest close to dinosaurs.

Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.

Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.

 

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.
Bonaparte JF 1969. 
Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Butler RJ. et al. 2014. New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation. BMC Evol. Biol. 14, 128.
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.
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 2011. The early evolution of archosaurs: relationship and the origin ofmajor clades. Bull. Amer. Mus. Nat. Hist. 352, 1–292.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
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.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.
Zanno LE, Drymala S, Nesbitt SJ and Schneider VP 2015. Early Crocodylomorph increases top tier predator diversity during rise of dinosaurs. Scientific Reports 5:9276 DOI: 10.1038/srep09276.

wiki/Pseudhesperosuchus
wiki/Junggarsuchus
wiki/Carnufex
wiki/Herrerasaurus
wiki/Sanjuansaurus

 

The cervical/dorsal transition in Herrerasaurus

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 in the drawing. See text for details. Drawings from Novas and Sereno 1994.

In their report on the basal dinosaur Herrerasaurus,
Sereno and Novas 1993 reported, “The cervical column (Fig. 1) was preserved in articulation with the skull. The anterior cervical vertebrae are better preserved than the posterior cervical vertebrae, and nearly all the ribs are lacking.”

“Because the cervical-dorsal transition in vertebrae or ribs is not preserved, we regard the first ten presacral vertebrae as cervical vertebrae, based on the condition in other basal dinosaurians.”

Despite their assessment, 
Novas and Sereno appear to have reconstructed Herrerasaurus with no more than six or possibly seven cervicals (Fig. 1, original).  With phylogenetic scoring at issue, a deeper look was warranted.

Novas and Sereno 1993 considered Herrerasaurus
a member of the ‘Ornithodira’ thus related to pterosaurs and Lagerpeton. That hypothesis is not supported by the present study.

By contrast,
in the large reptile tree (LRT) Herrerasaurus arises from another list of taxa, including Junggarsuchus, Pseudhesperosuchus, LewisuchusTurfanosuchus and further distantly Decuriasuchus. Bittencourt et al. 2014 identify seven cervicals in Lewisuchus (Fig. 1) with the eighth having ribs descending into the torso. Seven is a number common to crocodylomorpha* and Lewisuchus nests at its base. Turfanosuchus and Decuriasuchus each have eight cervicals, a plesiomorphic number going back at least to basal archosauriformes, like Proterosuchus and to basal diapsids, like Petrolacosaurus.

Based on available data,
Herrerasaurus had but seven cervicals as a basal dinosaur. Based on data from Tawa, Marasuchus, Eodromaeus and Eoraptor, all slightly more derived basal dinosaurs had 9 or 10.

* Among Crocodylomorpha, Scleromochlus has six cervicals, Gracilisuchus, Litargoschus and Terrerstrisuchus have eight, as do modern crocs and their relatives by convergence. Intervening taxa often have seven.

References
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
Nesbitt SJ. et al. 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464(7285):95-8
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto 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
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.

wiki/Lewisuchus
wiki/Herrerasaurus

 

New paper on stem archosauromorpha: Foth et al. 2016

When Foth et al. 2016 report,
“Here, we analyse the cranial disparity of late Permian to Early Jurassic archosauromorphs and make comparisons between non-archosaurian archosauromorphs and archosaurs (including Pseudosuchia and Ornithodira) on the basis of two-dimensional geometric morphometrics.” we are immediately ready for a bogus report based on the antiquated inclusion of the clades listed above.

Foth et al. 2016 set up their study
based on traditional phylogenies, not the large reptile tree [my comments follow]:

  1. “Living birds and crocodylians, as well as their extinct relatives including pterosaurs and non-avian dinosaurs, comprise the extraordinarily diverse and successful crown clade Archosauria.” [pterosaurs are lepidosaurs]
  2. “non-archosaurian archosauromorphs (i.e. taxa on the stem lineage leading towards archosaurs) formed a species rich component of Triassic ecosystems (>90 valid species) and achieved high morphological diversity, including highly specialized herbivores (Azendohsaurus, rhynchosaurs), large apex predators (erythrosuchids), marine predators with extremely elongated necks (tanystropheids), armoured crocodile-like forms (dosewellids, proterochampsids ), and possibly even turtles).” [Azendosaurus, rhynchosaurs, tanystropheids and turtles are all lepidosauromorphs].

The Foth et al. cladogram includes the following taxa
that have nesting problems:

  1. Tanystropheidae [should be in Tritosauria, Lepidosauria]
  2. Allokotosauria (a new paraphyletic ‘clade’ by Nesbitt et al. 2015 nesting between Protorosaurus and Prolacerta) – Pamelaria [Protorosauria], Azendohsaurus, Trilophosaurus [Rhynchocephalia] Teraterpeton [this is actually an archosauriform sister to Diandongosuchus.]
  3. Rhynchosauria [should be in Rhynchocephalia, Lepidosauria]
  4. Pterosauria [should be in Tritosauria, Lepidosauria]
  5. and the archosauriforms could use a lot of work! It’s all mixed up in there.

The rest of the paper
discusses the large amount of  cranial disparity in this clade. No wonder there is so much cranial disparity, they have thrown in so many unrelated taxa!!! As a referee I would have sent this manuscript back to the authors. The sister taxa do not demonstrate a gradual accumulation of character traits. They really need to expand their taxon list. They are missing SO many transitional taxa.

By contrast
there is not so much cranial disparity in the archosauriform subset of the LRT because they are more closely related to each other. In fact, the differences between sisters have been minimalized by taxon inclusion, creating the microevolution between taxa that even Creationists support.

References
Foth C, Ezcurra MD, Sookias RB, Brusatte SL and Butler RJ 2016. Unappreciated diversification of stem archosaurs during the Middle Triassic predated the dominance of dinosaurs. BMC Evolutionary Biology, 2016, Volume 16, Number 1, Page 1 online here.

Nesbitt SJ, Flynn JJ, Pritchard AC, Parrish MJ, Ranivoharimanana L and Wyss AR 2015. Postcranial osteology of Azendohsaurus madagaskarensis (?Middle to Upper Triassic, Isalo Group, Madagascar) and its systematic position among stem archosaur reptiles. Bulletin of the American Museum of Natural History. 398: 1–126.

The Archosauria according to the U of Maryland website

The University of Maryland website on the Rise of the Dinosauria includes the following cladogram (Fig. 1) which pretty much follows paleo traditions. Note the proximal position of pterosaurs to ‘Dinosauromorpha’ and the distant position of crocodylomorphs, which makes room for many intervening taxa to be considered archosaurs (= birds + crocs).

Figure 1. The Archosauria according to the University of Maryland. Here pterosaurs are close to dinosaurs.

Figure 1. The Archosauria according to the University of Maryland. Here pterosaurs are close to dinosaurs. Click to enlarge.

By contrast
in the large reptile tree, pterosaurs nest far from dinosaurs and crocs nest alongside them. So there are no intervening taxa between dinosaurs and crocs (Fig. 2). And there are no odd nesting partners here, like pterosaurs nesting with taxa with small hands and tiny fingers and no toe 5, etc. etc

Figure 2. Same cladogram rearranged to more closely match the large reptile tree. Note how, even at this scale, the gradual evolution of dinosaur traits is not interrupted by the odd morphology of pterosaurs. And how the basal bipedal crocs nest close to the basal bipedal dinos. Click to enlarge. 

Figure 2. Same cladogram rearranged to more closely match the large reptile tree. Note how, even at this scale, the gradual evolution of dinosaur traits is not interrupted by the odd morphology of pterosaurs. And how the basal bipedal crocs nest close to the basal bipedal dinos. This tree is missing SO many taxa, it puts the reader into the position of having to believe the relationships, not observe them. Click to enlarge.

There is a clinging to tradition at the U of Maryland
that needs to be revisited. If students need to regurgitate these antiquated hypotheses in order to get a good grade, then what does that teach them at the university level?

Take a look at those key traits (in red) above (Fig. 1).

  1. Elongate pubes and ischia: also found in basal bipedal crocs and prodinosaurs, like the PVL 4597 specimen. Also in poposaurs, like Poposaurus an Turfanosuchus.
  2. Parasagittal stance and hinge-like ankle joint: also found basal bipedal crocs, like Scleromochlus and Terrestrisuchus. Sure pterosaurs have such a stance and ankle, but so do fenestrasaurs (tritosaur lepidosaurs) like Sharovipteryx.
  3. Ellongate tibiae and metatarsi; loss of bony armor: again, basal bipedal crocs and fenestrasaurs.
  4. The lower traits are synapomorphies.

Students,
put your thinking caps on. Ask the hard questions. Do the experiments yourself. This is Science. Don’t be satisfied with answers that don’t make sense and can’t be validated up and down the entire cladogram.

The large reptile tree does not use suprageneric taxa, as shown above. Only species- and specimen-based taxa are included there. All taxa demonstrate a gradual accumulation of derived traits. All subsets retain the tree topology. The tree has grown from 200+ taxa to 674 taxa with the same 228 characters lumping and splitting them to full resolution.

Plus pterosaurs and plus basal therapsids drive this taxon list into the 900s.

 

 

The Origin of Dinosaurs x2 (2010) revisited

Several years ago
the top vertebrate paleontologists in the world (Brusatte et al. 2010) reported on the origin of dinosaurs. Coincidentally Langer et al. (2010) wrote a similar report.  It is now 6 years later. Let’s see how well those report have held up as they compare to the current data (2016) in the large reptile tree.

From the Brusatte et al. introduction
“During the past 25 years, numerous new fossils, reinterpretations of long-forgotten specimens, and numerical analyses have significantly revised our understanding of this major macroevolutionary event, which is one of the most profound and important evolutionary radiations in the history of life.”

What has stood the test of time:

  1. Dinosaurs are archosaurs: birds+crocs and last common ancestor
  2. Archosaurs are diapsid reptiles = Eudibamis, Petrolacosaurus and all their descendants.
  3. Dinosaurs are: “Triceratops horridus, Passer domesticus, and all descendants of their most recent common ancestor.” Or alternatively: ““the least inclusive clade containing Megalosaurus and Iguanodon.” Thus dinosaurs are monophyletic.
  4. The suite of traits common to dinosaurs include: 1) upright and fully erect posture [shared with basal crocs and dinosauromorphs]; 2) an enlarged deltopectoral crest on the humerus [shared with Trialestes]; 3) a “specialized” hand; 4) a perforated acetabulum (hip socket) [which may close]; 5) a well-developed fourth trochanter on the femur; 6) a lesser trochanter on the femur; 7) and a simple hinge ankle joint with proximal tarsals fixed immovably to the tibia and fibula [shared with basal crocs and dinosauromorphs].
  5. Dinosaurs likely originated during the Middle Triassic. They are diverse at the earliest Late Triassic.
  6. Herrerasaurus and Eoraptor are some of the most complete specimens of any early dinosaur.
  7. Langer: Herrerasaurs are basal to the Ornithischia-Saurischia dichotomy, but the actual dichotomy is Theropoda/Phytodinosauria
  8. Langer: The oldest dinosaurs include Herrerasaurus, Eoraptor, Staurikosaurus, Saturnalia and Panphagia all from the Carnian (early Late Triassic). These are also among the most primitive dinosaurs. Missing from this list is Barberenasuchus, also Carnian, not commonly considered a dinosaur, but nests as a sister to Eodromaeus.

What has not stood the test of time:

  1. Archosaurs (crocs + dinos alone) no longer include pterosaurs
  2. Diapsids no longer include lizards, snakes, rhynchocephalians (including rhynchosaurs and trilophosaurs) and pterosaurs. Those have a diapsid skull by convergence.
  3. Arizonasaurus is no longer an archosaur since crocs and birds had a more recent common ancestor, a sister to Gracilisuchus.
  4. The clades Crurotarsi (= Pseudosuchia) and Avemetatarsalia (= Ornithodira, Ornithosuchia) are now junior synonyms for older nomenclature based on their inclusion sets (Archosauriformes and Reptilia respectively).
  5. Pterosaurs no longer nest with archosaurs, but with lepidosaurs, in a new clade known as the Tritosauria nesting between basal rhynchocephalians and basal protosquamates.
  6. Lagerpeton is not a dinosauromorph, but a sister to Tropidosuchus.
  7. Marausuchus is does not nest outside the Dinosauria, but as a basal theropod.
  8. Sacisaurus, Silesaurus and Asilisaurus are not the immediate sisters of dinosaurs. Rather they now nest with poposaurs, the proximal outgroup to the Archosauria (crocs + dinos only).
  9. Overlooked by Brusatte et al., Lewisuchus, Zupaysaurus, Pseudhesperosuchus, Trialestes, and their kin are the now the immediate sisters of dinosaur, the true dinosauromorphs.
  10. Some manner of feathers now diagnose the Dinosauria, which primitively had naked (not scaly) skin, like a plucked chicken.
  11. Herrerasaurus and Eoraptor are no longer incerta sedis but the most basal dinosaur and one of the basal phytodinosaurs respectively.
  12. Zupaysaurus no longer nests as a theropod, but a dinosauromorph
  13. Berberosaurus no longer nests as a theropod, but as the basalmost phytodinosaur
  14. Ornithischia no longer branch off first from Saurischia, but are derived from basal phytodinosaurs. Sauropodomorpha are sisters to basal Ornithisichia with Daemonosaurus and Chilesaurus at the base.
  15. Langer: Eusaurischia (Sauropodomorpha + Theropoda) is a junior synonym for Dinosauria
  16. Langer: Silsauridae (all taxa closer to Silesaurus than to Marasuchus + Heterodontosaurus) is a junior synonym for Poposauria, if kept monophyletic.
  17. Langer: the basal-most dinosaurs were not probably omnivorous,
  18. Langer: herrerasaurs were not theropods
  19. Langer: there is no Onithischia-Saurischia dichotomy. Saurischia is a junior synonym  for Dinosauria.
  20. Langer: Agnophitys is a dinosaur sister to Marasuchus.
  21. Langer: Putative dinosaur Saltopus is a basal archosaur close to Gracilisuchus.
Figure 1. Click to enlarge. Subset of the large reptile tree focusing on the Archosauria (crocs + dinos). Sharp-eyed observers will find minor changes here.

Figure 1. Click to enlarge. Subset of the large reptile tree focusing on the Archosauria (crocs + dinos). Sharp-eyed observers will find minor changes here.

Staurikosaurus
Langer et al. (2010) mentioned Staurikosaurus (Colbert 1970) as the first consensual early dinosaur to be collected. Here it nests as a basal theropod, basal to a clade of theropods that is often overlooked that includes Marasuchus, Procompsognathus and Segisaurus. Yes, Staurikosaurus has but two sacral vertebrae. So do other clade members.

Guaibasaurus
Langer et al. (2010) also mentioned Guaibasaurus (Bonaparte et al., 1999) who reported, “The mesotarsal condition and the outline of the distal section of tibia indicate the saurischian nature of this new form, but the almost unreduced medial wall of the acetabular portion of ilium shows an unrecorded primitive condition within the cited group. Several features suggesting affinities with both the Prosauropoda and Theropoda, imply that Guaibasaurus candelariensis may belong to the ancestral group for both of them.” The large reptile tree nests Guaibasaurus as a basal theropod and as the sister to Marasuchus + Procompsognathus, not far from Staurikosaurus. 

The Novas (1992) dinosaur definition
According to Langer et al., Novas (1992b) provided the first phylogenetic definition of Dinosauria as ‘‘the common ancestor of Herrerasauridae and Saurischia + Ornithischia, and all of its descendants’’. The addition of herrerasaurs does not change the current tree (Fig. 1). Padian & May (1993) explicitly restricted the use of Dinosauria to the clade composed of Saurischia and Ornithischia, exclusive of ‘‘Herrerasaurus and its allies’’. But Novas has priority. Moreover, the last common ancestor of Saurischia and Ornithischia is currently a herrerasaur. The diagnosis of the Dinosauria has seen some changes over the years. Many are traits that are not covered by the large reptile tree. Please check out the references below for lists and histories of those lists.

What does the large reptile tree diagnose dinosaurs?
The following suite of traits are found in basal dinosaurs and not their proximal outgroups, Trialestes, the Pseudhesperosuchus clade. However many of these traits are found elsewhere on the tree. And many traits are lost in more derived dinos.

  1. Naris opening lateral
  2. Parietal skull table weakly constructed
  3. Mandible tip straight (neither upturned nor down)
  4. Interclavicle poorly ossified or absent
  5. Coracoid shape disc-like, even if fused (elongate or strap shape in outgroup)
  6. Radiale and ulnare not elongated (as in outgroup)
  7. Manus with long penultimate phalanxes and raptorial claws
  8. Femoral head interned and sub rectangular (reversed in the Marasuchus clade).
  9. Longest metatarsal: 3
  10. Proximal metatarsals: 1 and 5 reduced

Bipedality
has long been touted as a key dinosaurian trait, but dinosaurs evolved from basal bipedal crocodylomorphs, like Gracilisuchus and Scleromochlus. Interesting that Scleromochlus has been often associated with unrelated pterosaurs. Pterosaur removal sets things a little straighter in the retelling of the dinosaur ancestry story. Scleromochlus has not often been touted as a dinosaur ancestor, but by virtue of its false association with pterosaurs in various cladograms, it has always been there.

The long coracoids and proximal carpals of basal bipedal crocs
have set them apart from consideration as possible dino ancestors. But if you just let the software do its job, then you’ll recover nestings that indicate the elongate coracoids and proximal carpals became reduced to shorter, more primitive conditions in basal dinos.

Traits found in dinosaurs exclusive of Herrerasaurus:

  1. Feathers (not on the matrix, but worth mentioning)
  2. Skull shorter than cervicals
  3. Cranium convex
  4. Naris opening
  5. Maxilla ventral margin straight
  6. Jugal qj process straight
  7. Quadrate curls posterodorsally
  8. Jaw joint aligned with ventral maxilla
  9. Canine maxillary teeth not present
  10. Nine or more cervical vertebrae
  11. Some caudal vertebrae 3x longer than tall
  12. Tibia not shorter than femur
  13. Metatarsus not shorter than half the tibia
  14. Phalanges on metatarsal 5: 0 (reversed in higher clades)

Then if wanted to
you could simply list all the traits of Herrerasaurus, the basalmost dinosaur, knowing full well that Herrerasaurus itself is derived from the first, as yet undiscovered, dinosaurs.

References
Bonaparte JF, Ferigolo J and Ribeiro M 1999. A new early Late Triassic saurischian dinosaur from Rio Grande do Sol state, Brazil” (PDF). Proceedings of the Second Gondwanan Dinosaur Symposium, National Science Museum Monographs 15: 89–109.
Brusatte SL, Nesbitt SJ, Irmis RB, Butler RG, Benton MJ and Norell MA 2010.
The origin and early radiation of dinosaurs. Earth-Science Reviews 101 (2010) 68–10.
Colbert EH 1970. A Saurischian dinosaur from the Triassic of Brazil. American Museum Novitates 2405; 1-39
Langer MC. Ezcurra MD, BittencourtJS, Novas FE 2010. The origin and early evolution of dinosaurs. Biological Review 85, 55–110.

Crocodylomorpha and their place in the Archosauria

Among living archosaurs
there remain only birds and crocs. By definition, the last common ancestor of birds and crocs and all of its descendants are also archosaurs. By that reckoning only crocodylomorphs and dinos (including birds) are archosaurs according to the large reptile tree. That tree recovers a dino/croc split shortly after or including Gracilisuchus, but not the separate clade arising from Gracilisuchus that includes Saltopus and its cousin Scleromochlus  (Fig. 1).

Figure 2. Mariliasuchus nests at the base of a derived clade of narrow-skulled crocs.

Figure 1. Mariliasuchus nests at the base of a derived clade of narrow-skulled crocs. PVL 4597 and Lewisuchus are pro to-dinosaurs. All archosaurs are derived from a sister to Turfanosuchus +  the poposaurs and beyond that, Decuriasuchus.

Earlier crocodylomorpha cladograms
like the one in Wikipedia (refs therein, Fig. 2) do not mention dinosaurs or protodinosaurs. Instead they branch crocodyomorphans off from Postosuchus-like rauisuchians.

Figure 2. Crocodylomorpha according to Wikipedia.

Figure 2. Crocodylomorpha according to Wikipedia. The outgroup, CM 73372, was referred to Postosuchus, considered an outgroup by the authors of Wikipedia. In the large reptile tree Postosuchus is a terminal rauisuchian without known descendants.

On the same note,
earlier archosaur cladograms did not nest crocs and dinos together.
No, they put several more taxa between them (Fig. 3). The Wiki authors did not want to take sides in a 40-year-old debate, so they showed all the work by four widely cited paleontologists (Fig. 3).

Figure 3. Click to enlarge. Shown here are trees from Gauthier 1986, Sereno 1991, Benton 1999, and Nesbitt 2011, all from Wikipedia's article on archosaurs.

Figure 3. Click to enlarge. Shown here are trees from Gauthier 1986, Sereno 1991, Benton 1999, and Nesbitt 2011, all from Wikipedia’s article on archosaurs. As you can see, the inclusion set varies a bit between these authors. They all like to nest crocs with rauisuchians (both trending toward bipedal). Sometimes aetosaurs get a mention. Sometimes they don’t. Pterosaurs appear here by default in all four studies, based on traditional thinking. In each of these tree pterosaurs and parasuchians both share a common ancestor (which I hope you’ll agree is odd).

Shown above are trees from Gauthier 1986, Sereno 1991, Benton 1999, and Nesbitt 2011, all from Wikipedia’s article on archosaurs (refs therein). As you can see, the inclusion set varies a bit between these authors, as they do not all agree on what constitutes an archosaur or an archosauriform. They all like to nest crocs with rauisuchians (both trending toward bipedal). Sometimes aetosaurs get a mention. Sometimes they don’t. Pterosaurs appear here by default in all four studies, based on traditional thinking that cannot be supported by a demonstrated gradual accumulation of pterosaurian traits (but you CAN see such a gradual accumulation here. In each of these trees pterosaurs and parasuchians both share a recent common ancestor (which I hope you’ll agree is odd).

With the same issues
Naish (2001) wrote “Fossils explained 34/Crocodilians.” I only do this because some of these outmoded ideas are still floating around today (see above) and are still taken as gospel. The Naish article is online and uneditable so it also represents currently available hypotheses.

Overall, this is a good article
written and illustrated by a much younger Darren Naish (15 years ago). It introduces various members of the Crocodylomorpha in text and illustration and sets them into their family tree with a short list of derived traits.

However
when Naish sets up the Archosauria and the Crocodylomorpha early in the chapter. he reports on traditional thinking (see above), now falsified by the large reptile tree. Here are a few examples:

“However, numerous soft and hard-tissue features show that crocodilians are more closely related to birds than to lizards, snakes and turtles, and, together with pterosaurs, dinosaurs and other groups, birds and crocodiles are part of the reptilian subgroup called the Archosauria.” Pterosaurs are not in this clade as we learned here.

“Some fossilcrocodilians display the bony opening between the nostril and orbit, called the antorbital fenestra, that is unique to the Archosauria (‘ruling reptiles’).” Not unique as we learned here.

“Other unique archosaurian features, including the latero-sphenoid bone in the braincase and the fourth trochanter, a prominent muscle attachment site on the posteromedial surface of the femur, are evident in crocodilians.” Turtles and derived snakes both have a laterosphenoid, though neither is homologous to that of archosaurs.

“Crocodilians are not descendants of dinosaurs. While both groups do belong to the Archosauria, they are from fundamentally different lineages.” Not true. They both share a single common ancestor close to Gracilisuchus apart from all other fossil archosauriforms as shown here (Fig. 1).

If you want to excuse these trees
by saying ‘sure pterosaurs and parasuchians have a last common ancestor in deep time, we just haven’t found them yet,’ then you’ll have to compete with the cladogram of the large reptile tree that does not have to imagine such deep time ancestors, but provides contemporaneous sisters that actually look like sisters,

See this is why
I see the world of paleo as a little bit topsy turvy (trees have too many mismatched sisters), and wishy-washy (no one in academia wants to show what’s wrong with prior trees and champion a corrected tree.)

If you want to see
the taxa at the base of the archosauria, with one branch leading toward crocs and the other toward dinos, look here and here.

References
Bhullar B-AS and Bever GS 2009. An Archosaur-Like Laterosphenoid in Early Turtles (Reptilia: Pantestudines) Breviora Number 518 :1-11.
Naish D 2001. Fossils explained 34/Crocodilians. Geology Today 17(2):71-76.