SVP abstracts – Earliest avemetatarsalian?

Patellos et al. 2019 brings us
news of the earliest archosaur in the lineage of birds (rather than crocs).

Okay. That’s already wrong. In the large reptile tree (LRT, 1592 taxa) only crocs and dinos make up the Archosauria. Nesbitt et al. does not understand that hypothesis of interrelationships due to taxon exclusion and poor scoring going back to Nesbitt 2011. The purported clade, ‘Avemetatarsalia’ (= Ornithodira) was invalidated by the LRT.

From their abstract:
“Understanding of the evolution of the earliest avemetatarsalian (bird-line) archosaurs and the morphology of the hypothetical common ancestor of Archosauria is hampered by a poor fossil record.”
Incorrect. The common ancestor of Archosauria has been identified in the LRT as the PVL 4597 specimen wrongly attributed to Gracilisuchus. After that: Turfanosuchus (Fig. 1).

Figure 2. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Figure 1. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Patellos et al. 2019 continue:
“The earliest-diverging avemetatarsalians known, such as Teleocrater, are separated from the earliest diverging pseudosuchian (crocodylian-line) archosaurs, and the closest outgroups of Archosauria by a clear morphological gap.”
The LRT invalidates the traditional clade, ‘Pseudosuchia.’ Crocodylian-line archosaurs are Crocodylomorphs, distinct from bird-line archosaurs, dinosaurs. Remember, these authors consider the lepidosaurian pterosaurs to be closely related to dinosaurs, a theory with as much evidence as tail-dragging dinosaurs.
“Here we describe a potential early-diverging avemetatarsalian from the Middle Triassic (~ 230 Ma) “Basal Isalo II” beds of Madagascar, which appears to bridge these gaps. This new taxon is represented by a well-preserved partial skeleton including articulated cervical
vertebrae with articulated osteoderms; a scapulocoracoid; a partial femur; isolated trunk, sacral, and caudal vertebrae; and an ilium.”
“Noteworthy features of the neck region include: anteroposteriorly elongated vertebrae with laterally expanded dorsal ends of the neural spines, and an articulated set of osteoderms dorsal to the vertebrae. The cervical osteoderms, three pairs per vertebra, arranged in paramedian row, and bear tapering anterior processes.” 
“Potential synapomorphies of this specimen with avemetatarsalians include: femur with an incipient anterior trochanter, 1st sacral vertebra with a dorsoventrally expanded sacral rib, and ilium possessing a notch on the articulation surface with the ischium. This combination of features places the new taxon represented by this specimen at the base of Avemetatarsalia, outside aphanosaurs + dinosaurs, but this position is poorly supported.”
The best known members of the invalid Aphanosauria include Yarasuchus and Teleocrater (Fig. 2), taxa nested with a long line of non-Aphanosauria by the LRT between Rauisuchia and Archosauria.

Figure 3. Yarasuchus, Qianosuchus and Turfanosuchus nest together in Nesbitt et al. 2017 after rescoring.

Figure 2. Yarasuchus, Qianosuchus and Turfanosuchus nest together in Nesbitt et al. 2017 after rescoring.

Patellos et al. 2019 continue:
“More broadly, this new specimen indicates that cervical osteoderms were present in the earliest avemetatarsalians and were soon lost in the lineage.”
There’s no need for such phylogenetic gymnastics in the LRT.
“The generally plesiomorphic morphology of the new taxon also underscores the difficulty of identifying early avemetatarsalians from incomplete skeletons. Presence of an early diverging avemetatarsalian together with a lagerpetid and silesaurid in the “Basal Isalo II” beds of Madagascar documents the co-occurrence of multiple avemetatarsalian subgroups in Gondwana during the Triassic.”
They wish. The LRT resolves all such problems with high resolution. Blame S. Nesbitt for relying on his own poorly scored cladogram, inventing the ‘Aphanosauria’ and supporting the ‘Avemetatarsalia.’ Blame M. Benton for inventing the clade ‘Avemetatarsalia’.
Don’t trust those clades. Don’t trust the LRT. Run your own tests so you’ll know. In science this is the first, last and best option to resolve all such disagreements.

References
Patellos E et al. 2019. A new reptile from the ?Middle Triassic of Madagascar may represent the earliest-diverging avemetatarsalian (Archosauria). Journal of Vertebrate Paleontology abstracts.

The skull of PVL 4597 joins its post-crania in the LRT

The PhD thesis of Agustina Lecuona 2013
on the several specimens attributed to the Middle Triassic Gracilisuchus (Fig. 2) is online (PDF). It includes the previously unpublished skull of PVL 4597 (Figs. 1, 2), which the large reptile tree (LRT, 1592 taxa) nests apart from Gracilisuchus, as the last common ancestor of all archosaurs (crocs + dinos only) with or without its skull. We reviewed Gracilisuchus yesterday, so this addition to the LRT is timely.

Figure 1. The skull of PVL 4597 in several views from the 2013 PhD thesis of A. Leucona. Colors added.

Figure 1. The skull of PVL 4597 in several views from the 2013 PhD thesis of A. Lecuona. Colors added.

The differences between PVL 4597 and Gracilisuchus are few (Fig. 2).
So, it is not a surprise that Lecuona considered them congeneric.

However,
the differences are fewer between PVL 4597 and its ancestor, Turfanosuchus (Fig. 4), and  its descendant, Herrerasaurus, the last common ancestor of dinosaurs traditionally and in the LRT. 19 additional steps are added when PVL 4597 is forced to nest with Gracilisuchus in the LRT.

FIgure 2. Comparing PVL 4597 to Gracilisuchus. Despite their many similarities, these two do not nest together in the LRT.

FIgure 2. Comparing PVL 4597 to Gracilisuchus. Despite their many similarities, these two do not nest together in the LRT. Taxon exclusion is the issue with the PhD dissertation and the use of an invalidated analysis from Nesbitt 2011.

Basal members of large clades
are sisters to basal members of sister clades (Fig. 3). We compare those taxa with one another, ignoring the more derived members.

Figure 3. Subset of the LRT focusing on basal archosaurs and their immediate ancestors.

Figure 3. Subset of the LRT focusing on basal archosaurs and their immediate ancestors.

Here (Fig. 4) are the skulls of
Turfanosuchus and Herrerasaurus, taxa closer to PVL 4597 than PVL 4597 is to Gracilisuchus is in the LRT. The long-awaited skull confirms the nesting of the post-crania.

Figure 2. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Figure 4. Skull of Turfanosuchus compared to Herrerasaurus, the basalmost dinosaur.

Without PVL 4597, 
the LRT still nests Turfanosuchus and basal bipedal crocs close to the base of the Dinosauria, contra the results of other studies that generally do not include those taxa.

Unfortunately,
Lecuona’s PhD thesis employed a borrowed and flawed cladogram on which she mistakenly trusted in: Nesbitt 2011. Even though Lecuona’s revised cladogram includes the basal bipedal crocs (which nest at derived nodes in her thesis), earlier we dismantled Nesbitt 2011 in a 7-part series ending here. Rescored Nesbitt 2011 resembles the LRT.


References
Lecuona A 2013. Anatomía y relaciones filogenéticas de Gracilisuchus stipanicorum y sus implicancias en el origen de Crocodylomorpha. PhD thesis. PDF
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

wiki/Gracilisuchus

SVP 2018: Junggarsuchus µCT scans

Ruebenstahl and Clark 2018
pull new data from the basalmost crocodylomorph, Junggarsuchus (Fig. 1) using µCT scans. They consider it a sphenosuchian nesting uneasily deep within Crocodylomorpha. I hope they test it with basal bipedal crocs listed in the large reptile tree (LRT, 1315 taxa, subset Fig. 3). PVL 4597 is a LRT sister that will provide clues to the hind quarters of Junggarsuchus, currently missing.

Figure 8. The CAPPA specimen of Buriolestes compared to the more primitive Junggarsuchus, basal to the other branch of archosaurs, the crocs.

Figure 1 The CAPPA specimen of Buriolestes (a dinosaur) compared to the more primitive Junggarsuchus, basal to the other branch of archosaurs, the crocs.

The authors report, “In addition to braincase characters, we also identify a unique morphology in the palate and pterygoid of Junggarsuchus, which, although similar to the condition in other sphenosuchians, has several aspects that are unlike anything reported in any ‘sphenosuchians’, including a far reaching anterior process of the pterygoid.” A similar pterygoid is found in the basal dinosaur, Herrerasaurus (Fig. 2) and the poposaur, Silesaurus.

Figure 1. The basalmost dinosaur, Herrerasaurus. Note the palate and the long pterygoid.

Figure 2. The basalmost dinosaur, Herrerasaurus. Note the palate and the long pterygoid.

Ruebenstahl and Clark 2018
provide no indication that they understand the close relationship of Junggarsuchus to basal dinosaurs, basal poposaurs and Decuriasuchus. If the authors only see croc interrelationships it’s time to add taxa.

Figure 2. Subset of the LRT focusing on Crocodylomorpha (basal Archosauria) including Armadillosuchus.

Figure 3. Subset of the LRT focusing on Crocodylomorpha (basal Archosauria) including Armadillosuchus.

References
Ruebenstahl AA and Clark JM 2018. Junggarsuchus sloani: a transitional ‘sphenosuchian’ and the evolution of the crocodilian skull.” SVP abstracts.

The joy of finding mistakes: fewer stem dinosaurs

Finding mistakes is what I hope to do every day
in my own work, as well as that of others. Each time that happens, the data set improves. Lumping and splitting improves. The hypothetical topology of the large reptile tree (LRT, 1036 taxa) gets closer to echoing the topology of Nature itself. Science is a process of winnowing through the data and finding earlier mistakes.

Figure 1. Revision to the LRT with a focus on the Archosauria. Here taxa with a long carpus all nest within the Crocodylomorpha, following traditional thinking. Dinosaur outgroups are reduced. PVL 4597 is still the basalmost archosaur.

Figure 1. Revision to the LRT with a focus on the Archosauria. Here taxa with a long carpus all nest within the Crocodylomorpha, following traditional thinking. Dinosaur outgroups are reduced. PVL 4597 is still the basalmost archosaur.

Today
I discovered some scoring errors among former ‘stem dinosaurs’ that turned them into basal crocodylomorphs. That’s a small shift and it involved turning some ‘absent’ scores in pedal digit 5 to ‘unknown’. It’s noteworthy that some related taxa have two tiny phalanges on pedal digit 5. A related taxon, Gracilisuchu, was illustrated by Romer (1972, Fig. 3) as a combination or chimaera of separate specimens, something I just today realized and rescored. One of those specimens is the so-called Tucuman specimen (PVL 4597, Fig 1), which nests apart from the Gracilisuchus holotype (Fig. 2) 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.

Figure 2. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT. That fibula flange turns out to be another important trait. 

The corrected results
resolve the long proximal carpal issue in crocodylomorphs very neatly. Now, as in traditional thinking, that trait is restricted to only the crocodylomorphs and it gives us a basalmost taxon with the trait, Junggarsuchus. You might think, and it would be reasonable to do so, that phylogenetic bracketing permitted the addition of a long carpus and long coracoids with more confidence to taxa that don’t preserve this, like Gracilisuchus and Saltopus. But another related basal crocodylomorph, Scleromochlus, has small round coracoids, evidently a reversal. The carpal length is not clearly documented in Scleromochlus (Fig. 4).

Gracilisuchus

Figure 3. A basal archosaur with a very similar nasal bone, Gracilisuchus. Note pedal digit 5 here. This is how Romer 1972 illustrated it. The actual data is shown in figure 2, the Tucuman specimen, PVL 4597. The coracoid is not known in the holotype. 

Despite the short round coracoids of Scleromochlu
and its apparently short carpals, enough traits remain to nest it as a basal crocodylomorph, following the rules of maximum parsimony.

Figure 1. Scleromochlus forequarters. The yellow area shows the hand enlarged in situ. The size of the Scleromochlus hand makes it the last possible sister to pterosaurs, famous for their very large hands.

Figure 4. Scleromochlus forequarters. The yellow area shows the hand enlarged in situ. Large carpals do not appear to be present and the coracoids are not elongated. 

On a more personal note
I found out my art and a short bio were included in a paleoart website:
http://paleoartistry.webs.com while looking for information on friend and paleoartist, Mark Hallett, (wikipage here) whose website is down and I worried about his health. No worries. Mark just let his website lapse.

The author of the paleoartistry page
had both kind words and controversy for me:
“After David Peters’ excellent paintings in Giants, and A Gallery of Dinosaurs and Other Early Reptiles, as well as his own calendar, it seemed he was on his way to becoming one of the most reliable paleoartists of the 1990s, if not of all time. However, very controversial theories on reconstructing pterosaurs led to some harsh critiques obscuring Peters’ artistic brilliance.” 

That’s okay.
“Very controversial” does not mean completely bonkers (or am I reading too little into this?). It just means it inspires a lot of chatter. Or… it could mean that the author of the post follows the invalidated observations of Elgin, Hone and Frey 2010, which are the traditional views (Unwin and Bakhurina 1994), still used in David Attenborough films. If so, that would be a shame. Science is usually black and white – is or isn’t, because you can observe and test (Fig. 5) and all tests, if done the same, should turn out the same.

And you don’t toss out data
that doesn’t agree with your preconception, like Elgin, Hone and Frey did. In reality, my “very controversial reconstructions” remain the only ones built with DGS, not freehand guesswork or crude cartoonish tracings (as in Elgin, Hone and Frey 2010). The membranes (brachiopatagia and uropatagia) were documented in precise detail in Peters 2002, 2009 and here online.

Click to animate. This is the Vienna specimen of Pterodactylus, which preserves twin uropatagia behind the knees.

Figure 5. Click to animate. This is the Vienna specimen of Pterodactylus, which preserves twin uropatagia behind the knees.

References
Elgin RA, Hone DWE and Frey E 2011. The extent of the pterosaur flight membrane. Acta Palaeontologica Polonica 56 (1), 2011: 99-111. doi: 10.4202/app.2009.0145
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Romer AS 1972. 
The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.
Unwin DM and Bakhurina NN 1994. Sordes pilosus and the nature of the pterosaur flight apparatus. Nature 371: 62-64.

wiki/Gracilisuchus
paleoartistry.webs.com/1980s.htm

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

 

Return of the short-face Gracilisuchus MCZ 4116

Earlier we looked at the MCZ 4116 specimen attributed to Gracilisuchus (Fig. 1).

Figure 1. MCZ 4116 a short-faced Gracilisuchus compared to the holotype with a longer face.

Figure 1. MCZ 4116 a short-faced Gracilisuchus compared to the holotype with a longer face. These two nest as sister taxa at the base of the Archosauria.

 

Gracilisuchus (Romer 1972) 
nests at the base of the Archosauria in the large reptile tree. Scleromochlus and Saltopus are sister taxa. So are these short-faced specimens (Fig. 1), MCZ 4116 and 4117 (Brinkman 1981). That short rostrum looks juvenile, but note these specimens are not smaller than the holotype (Fig.1). Romer and Parrish (year?) restored the snout tip with a Gracilisuchus-like big round nasal and a very short, transverse premaxilla. As an option, I just followed existing contours and added a premaxilla similar in length to the holotype.

Could this be a juvenile of a much larger adult?

Figure 1. Gracilisuchus revised with new, slightly longer legs  and more precise feet after tracing the holotype. Romer added a pelvis and tail that are erased here.

Figure 1. Gracilisuchus revised with the subtraction of limbs and tail.  Romer added limbs and a tail that are erased here.

References
Brinkman D 1981. The origin of the crocodiloid tarsi and the interrelationships of thecodontian archosaurs. Breviora 464: 1–23.
Romer AS 1972. 
The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.

New clade of enigmatic early archosaurs? No.

Updated one day after publication. The taxa come from the Supp. Data, most not shown in the greatly simplified chronological cladogram.

Recently, Butler et al. (2014)
recovered a “new clade of enigmatic early archosaurs” comprised of Yonghesuchus, Gracilisuchus and Turfanosuchus.

Figure 1. Do these taxa nest in a single clade? No. Turfanosuchus, Gracilisuchus and Yonghesuchus. Each nests more closely with other taxa.

Figure 1. Do these taxa nest in a single clade? No. Turfanosuchus, Gracilisuchus and Yonghesuchus. Each nests more closely with other taxa. Yonghesuchus nests with the crocodylomorph Dromicosuchus. Gracilisuchus nests with Saltopus as a much more basal crocodylomorph. Turfanosuchus nests at the base of the Poposauridae.

Unfortunately they added the unrelated Mesosuchus (lepidosaur), Vancleavea (thalattosaur) and two pterosaurs (lepidosaurs).

And they did not add the true sisters of Gracilisuchus (Pseudhesperosuchus, Decuriasuchus, Lewisuchus, Saltopus, the SMNS 12591 specimen and Scleromochlus).

Red Flags
In the Butler et al. (2014) tree the following purported sister taxa are all “odd bedfellows” that do not look like one another.

  1. Prolacerta is derived from Mesosuchus (and presumably the rhynchosaurs)
  2. Euparkeria is derived from Tropidosuchus and Chanaresuchus.
  3. Tropidosuchus and Chanaresuchus are derived from Vancleavea.
  4. Vancleavea is derived from Erythrosuchus.
  5. Pterosaurs are derived from parasuchians!!!!!!
  6. Lagerpeton is derived from pterosaurs.
  7. Ornithosuchia is derived from Lewisuchus.
  8. Theropoda is derived from Ornithischia.
  9. Ornithosuchia is a sister to Pterosauria, also derived from Parasuchia.
  10. Revueltosaurus is a sister to the Aetosauria and derived from Ornithosuchia
  11. The new Gracilisuchus clade is derived from Revueltosaurus.
  12. Poposaurus and the poposaurs are derived from Qianosuchus, Xilosuchus and Arizonasaurus
  13. Prestosuchus and the Rauisuchidae is derived from Ticinosuchus.
  14. Hesperosuchus and the Crocodylomorpha are derived from Rauisuchidae.

Say it ain’t so!
As you can see, many of these relationships don’t make sense. Sister taxa share very few traits with one another (pterosauria and parasuchia, is the worst such example). Many relationships are upside down with basal taxa, like theropods, derived from derived taxa, like ornithischia. (M. Mortimer also had this problem a few years ago).

What is needed is a large reptile tree in which basal taxa are basal to derived taxa and all sisters look alike (share most traits). In the large reptile tree, sister taxa look quite a bit like one another. The authors should have cast a critical eye on these results, which are very similar to those of Nesbitt (2011), who also recovered many strange bedfellows.

If I had proposed that pterosaurs arose from parasuchians,
the ridicule would be endless and justified, as it is here. Taxon exclusion seems to be the culprit again, along with the tradition of using previously published matrices, even those riddled with Red Flags and strange bedfellows.

In the large reptile tree, Gracilisuchus nests with the SMNS 12591 specimen, Saltopus and Scleromochlus at the base of the Archosauria. Turfanosuchus nests at the base of the Poposauridae, between Decuriasuchus and the base of the Archosauria, not far from GracilisuchusYonghesuchus, nests between Dromicosuchus and Protosuchus.

And, because this is Science, you can repeat these experiments to see for yourself which taxa share more traits — that make sense.

References
Butler et al. 2014. New clade of enigmatic early archosaurs yields insights into early  pseudosuchian phylogeny and the biogeography of the archosaur radiation. BMC Evolutionary Biology  14:128. doi:10.1186/1471-2148-14-128
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.

Is Nyasasaurus the oldest dinosaur?

Nesbitt et al. (2014) report, “Here, we describe Nyasasaurus parringtoni gen. et sp. nov., which is identified as either the earliest known member of, or the sister–taxon to, Dinosauria. Nyasasaurus possesses a unique combination of dinosaur character states and an elevated growth rate similar to that of definitive early dinosaurs.”

Nesbitt et al. (2012) did not provide a reconstruction. The specimen is only known from a few Middle Triassic (Anisian) vertebrae and a proximal humerus. Here (Fig. 1) is a reconstruction that seems to fit pretty well based on the holotype and referred specimens of Nyasasaurus placed onto the bauplan of a large and slightly larger (for the cervicals) specimen of Turfanosuchus, a basal archosaur at the base of the Poposauridae (now nesting basal to archosaurs in the large reptile tree). This was, what you might call, an early ‘false start’ mimicking the actual rise of the Dinosauria.

Figure 1. Click to enlarge. Nyasasaurus bones placed on an enlargement of Turfanosuchus, a middle Triassic basal archosaur, not a dinosaur. Dinos and crocs all started out as tiny bipeds.

Figure 1. Click to enlarge. Nyasasaurus bones placed on an enlargement of Turfanosuchus, a big. middle Triassic basal archosaur, not a dinosaur. Dinos and crocs all started out as tiny bipeds, also derived from Turfanosuchus, but smaller.

Nesbitt et al. ran their analysis and found Nyasasaurus to nest between 1) Lewisuchus and dinosaurs; 2) basal to Ornithischia; or 3) as the sister to Dilophosaurus, a Jurassic dinosaur. I didn’t run an analysis as my characters would not resolve relationships based on so few parts.

The large reptile tree finds two small specimens, Trialestes and an unnamed specimen incorrectly referred by Lecuona and Desojo (2011) to Gracilisuchus (PVL 1259) at the base of the Dinosauria. The latter is as old as Nyasasaurus, fulfilling chronological predictions.

Nesbitt et al. note a ventrally elongate deltopectoral crest on the humerus, but that assumes a short humerus. The crest is not so elongate if the humerus is a little longer.

Nesbitt et al. note three sacral vertebrae, but basal dinos don’t have three sacrals, only two. Turfanosuchus also has only two, but look at the size difference! Poposaurus, a sister taxon, has five sacrals. So Nyasasaurus is something else. Nesbitt et al. note hyposphene–hypantrum intervertebral articulations in the pre sacral vertebrae. Sorry, not much about that in Turfanosuchus data. In Turfanosuchus, as in Nyasaurus, the cervical vertebrae are laterally concave.

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.
Nesbitt SJ, Barrett PM, Werning S, Sidor CA and Charig AJ (posthumously) 2012. The oldest dinosaur? A Middle Triassic dinosauriform from Tanzania. Biology Letters 9: 20120949.