Basal bipedal crocs reviewed, with a focus on Barberenasuchus

Leardi, Yáñezc and Pol 2020 bring us
their thoughts on new and previously discovered South American crocodylomorphs. In the large reptile tree (LRT, 1737+ taxa; subset Fig. x) only Crocodylomorpha + Dinosauria comprise the Archosauria. The Poposauria (= Turfanosuchus and kin;  Fig. 1) is the proximal outgroup.

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

Figure 1. The origin of dinosaurs in the LRT to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.  Note the phylogenetic miniaturization at the origin of Archosauria (Crocs + Dinos).

From the Leardi, Yáñezc and Pol Abstract
“Crocodylomorpha is a clade that has its origins during the Late Triassic and attained a global distribution early in their radiation.In this contribution we review the crocodylomorph Triassic record in South America by analyzing three units that have yielded fossils of the clade: the Santa María Supersequence in Brazil; and, the Ischigualasto and Los Colorados formations in Argentina.”

Good start!

Figure 3. Barberenasuchus to scale with sister taxa, Herrerasaurus, Eoraptor, Lewisuchus and Trialestes and Junggarsuchus, but without the autapomorphies of its sister Herrerasaurus. At present Barberenasuchus is the basalmost dinosaur. Note the difference in the nasal between the dinosaurs and protodinosaurs.

Figure 1. Barberenasuchus to scale with sister taxa, Herrerasaurus, Eoraptor, Lewisuchus and Trialestes and Junggarsuchus, but without the autapomorphies of its sister Herrerasaurus. At present Barberenasuchus is the basalmost dinosaur. Note the difference in the nasal between the dinosaurs and protodinosaurs.

Continuing from the abstract
“Our review does not support previous assignments of the taxon Barberenasuchus (Fig. 1) from the Santa María Supersequence as a non-crocodyliform crocodylomorph, as it displays traits that are absent in all known crocodylomorphs and are present in other earlier branching archosaurs.”

The LRT agrees, but what Barberenasuchus isn’t isn’t the same as what it is (Fig. 2). Adding taxa gradually and ultimately nests all taxa.

Figure 2. Subset of the LRT focusing on the Phytodinosauria.

Figure 2. Subset of the LRT focusing on the Phytodinosauria.

More specifically,
the LRT nests Barbarenasuchus with Eodromaeus (Fig. 3) within the base of the Phytodinosauria (Fig. 2). Only Buriolestes is more primitive in this plant-eating clade of the Dinosauria.

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. Note the relatively small head of this plant eater.

Continuing from the abstract
“The Los Colorados Formation has a diverse crocodylomorph record being represented by a non-crocodyliform crocodylomorph (Psedhesperosuchus [Fig 1]) and two crocodyliforms (Hemiprotosuchus and Coloradisuchus).”

We looked at these two yesterday.

In the LRT, the Triassic is a little too early
for crocodyliforms. Adding taxa moves Hemiprotosuchus to the base of the Aetosauria. Coloradisuchus nests among basal bipedal crocodylomorpha, as we learned earlier. These are indeed found in the Triassic.

Continuing from the abstract
“Here we present a putative new non-crocodyliform crocodylomorph taxon from Los Colorados Formation. When compared with other crocodylomorph bearing formations around Pangea, the Ischigualasto Formation bears similarities with the crocodylomorphs assemblages of North America due to the presence of early branching crocodylomorphs (Trialestes) including “large-bodied” taxa. The Los Colorados Formation reveals a transitional composition corresponding to Norian and Early Jurassic assemblages of Pangea, as it shares the presence of basal crocodyliforms (i.e., protosuchids) typical of Early Jurassic units (e.g., Upper Elliot) and basal non-crocodyliform crocodylomorphs, widely present in Norian assemblages.”

Still waiting for data on this unnamed taxon. Meanwhile, let’s get back to Barberenasuchus (Figs. 1, 2).

Figure x. Subset of the LRT focusing on Euarchosauriformes and Crocodylomorpha.

Figure x. Subset of the LRT focusing on Euarchosauriformes and Crocodylomorpha.

When you add taxa, as done in the LRT,
Barberenasuchus brasiliensis (Mattar 1987, Middle Triassic) nests as a basal phytodinosaur. Barberenasuchus has shorter teeth and a larger orbit that more primitive carnivorous taxa. The skull is more gracile and smaller in size, as in other basal phytodinosoaurs.

Traditionally, and according to Wikipedia
“Barberenasuchus is an extinct genus of an archosauriform. Its phylogenetic position within Archosauriformes is uncertain; the author of its description classified it as a sphenosuchid crocodylomorph, while Kischlat (2000) considered it to be a rauisuchian. Irmis, Nesbitt and Sues (2013) stated that they “could not find any crocodylomorph character states preserved in the holotype specimen”.

Adding taxa makes the position of all taxa, including Barberenasuchus, in the LRT ever more certain.


References
Irmis RB, Nesbitt SJ and Sues H-D 2013.Early Crocodylomorpha. In Nesbitt SJ. Desojo JB and Irmis RB (eds.). Anatomy, phylogeny and palaeobiology of early archosaurs and their kin. The Geological Society of London. pp. 275–302. doi:10.1144/SP379.24
Kischlat E-E 2000.
 Tecodôncios: a aurora dos arcossáurios no Triássico. In Holz, M.; De Ros, L.F. (eds.). Paleontologia do Rio Grande do Sul. Porto Alegre: CIGO/UFRGS. pp. 273–316.
Leardi JM, Yáñezc I and Pol  D 2020. South American Crocodylomorphs (Archosauria; Crocodylomorpha): A review of the early fossil record in the continent and its relevance on understanding the origins of the clade. Journal of South American Earth Sciences. https://doi.org/10.1016/j.jsames.2020.102780
Mattar LCB 1987. Descrição osteólogica do crânio e segunda vértebrata cervical de Barberenasuchus brasiliensis Mattar, 1987 (Reptilia, Thecodontia) do Mesotriássico do Rio Grande do Sul, Brasil. Anais, Academia Brasileira de Ciências, 61: 319–333.

wiki/Eodromaeus
wiki/Barberenasuchus

A new ornithischian, Changmiania, enters the LRT

Meet a new fossorial fossil dinosaur,
perfectly preserved in its own burrow.

Yang et al. 2020 bring us
self-buried specimens of Changmiania liaoningensis (Yixian, Early Creteceous; Figs. 1–3).

Figure 1. Changmiania in situ and illustrated in situ from Yang et al. 2020.

Figure 1. Changmiania in situ and illustrated in situ from Yang et al. 2020.

The authors nest this ornithischian
as the basalmost member of the clade Ornithopoda (= duckbills, etc.; Fig. y).

Figure y. Cladogram of the Ornithischia from Yang et al. 2020. Colors added. Green= related taxa in the LRT. Yellow = taxa share with the LRT.

Figure y. Cladogram of the Ornithischia from Yang et al. 2020. Colors added. Green= related taxa in the LRT. Yellow = taxa share with the LRT. Compare to figure x which includes more outgroup taxa to polarize the basal taxa.

By contrast,
the large reptile tree (LRT, 1733+ taxa; subset Fig. x) nests Changmiania (Figs. 1–3) with Kulindadromeus (Fig. 4) and Heterodontosaurus (Fig. 5). That’s several nodes apart from the clade Ornithopoda in the LRT (Fig. x).

Figure x. Subset of the LRT focusing on Ornithischia. This cladogram differs considerably from that published in Yang et al. 2020.

Figure x. Subset of the LRT focusing on Ornithischia. This cladogram differs considerably from that published in Yang et al. 2020. Here Haya is a basal ornithischian. In Yang et al. Haya is deep within the Ornithopoda.

Digging (fossorial behavior)
traits found in Changmiania proposed by the authors include:

  1. fused premaxillae
  2. nasal laterally expanded overhanging the maxillas
  3. shortened neck formed by only six cervical vertebrae;
  4. neural spines of the sacral vertebrae completely fused together, forming a craniocaudally-elongated continuous bar;
  5. fused scapulocoracoid with prominent scapular spine;
  6. and paired ilia symmetrically inclined dorsomedially, partially
    covering the sacrum in dorsal view.
Figure 2. Changmiania skull in dorsal view.

Figure 2. Changmiania skull in dorsal view from Yang et al. 2020. Colors added here.

Figure 3. Changmiania skull in lateral view from Yang et al. 2020. Colors added here.

Figure 3. Changmiania skull in lateral view from Yang et al. 2020. Colors added here.

Several ornithischian taxa
(Orodromeus, Oryctodromeus, and Zephyrosaurus) have also demonstrated (or suggested by phylogenetic bracketing) fossorial behavior. Heterodontosaurus (Figs. 5, 6) shares traits #1, 2 and 3. Heterodontosaurus has longer fingers #2 and #3 (for digging?).

Figure 1. Kulindadromaeus, a sister to Heterodontosaurus with proto-feathers. Images from and traced from Godefroit et al. 2014. Since theropods and heterodontosaurs both had something like feathers, if they were the same kind of feathers, their last common ancestor had feathers. That last common ancestor was a herrerasaur or its proximal predecessor. Note the Godefroit et al. skull does not match their description but has a standard maxilla ascending process. See color overlays for correct ed interpretation.

Figure 4. Kulindadromaeus, a sister to Heterodontosaurus with proto-feathers. Images from and traced from Godefroit et al. 2014. Since theropods and heterodontosaurs both had something like feathers, if they were the same kind of feathers, their last common ancestor had feathers. That last common ancestor was a herrerasaur or its proximal predecessor. Note the Godefroit et al. skull does not match their description but has a standard maxilla ascending process. See color examples for correct ed interpretation. Click to enlarge.

Figure 7. Heterodontosaurus skull. Note the fused premaxillae, overhanging nasals and pmx/mx notch for a lower fang.

Figure 5. Heterodontosaurus skull. Note the fused premaxillae, overhanging nasals and pmx/mx notch for a lower fang. The general layout of the skull is very much like that of Changmiania. See figures 2 and 3.

Figure 1. Heterodontosaurus with feather quills arising from the lower back, sacrum and proximal tail.Figure 1. Heterodontosaurus with feather quills arising from the lower back, sacrum and proximal tail.

Figure 6. Heterodontosaurus with feather quills arising from the lower back, sacrum and proximal tail.

It’s worth noting
that many extant birds dig burrows, too. They use their bills to peck and their feet to sweep. Progress is often slow. Here’s an online article discussing birds that use and dig burrows.

Figure 7. Changmiania, both specimens in situ.

Figure 7. Changmiania, both specimens in situ.

Here’s a YouTube video
of a belted kingfisher working on its own burrow. Much is not shown, but the feet are back-kicking out the rubble, presumably produced by the pecking of the strong beak at the back wall of the burrow.

References
Yang Y, Wu W, Dieudonne P-E and Godefroit P 2020. A new basal ornithopod dinosaur from the Lower Cretaceous of China. PeerJ 8:e9832 DOI 10.7717/peerj.9832

Changmiania publicity:
naturalsciences.be/en/news/item/19274

Saturnalia skull parts!

Bronzati, Müller and Langer 2019 bring us
additional skull data for the basal sauropodomorph, Saturnalia tupiniquim (Fig. 1).

FIgure 1. GIF movie of Saturnalia skull as originally restored and using phylogenetic bracketing to restore a longer rostrum and teeth only anterior to the orbit.

FIgure 1. GIF movie of Saturnalia skull as originally restored and using phylogenetic bracketing to restore a longer rostrum and teeth only anterior to the orbit.

Saturnalia tupiniquim (Langer et al. 1999) Carnian, Late Triassic period, ~225 mya, 1.5 m in length, was one of the oldest true dinosaurs yet found. It was basal to the clade Prosauropoda, 

Figure 1. Grallator illustration from Li et al. 2019 with two basal phytodinosaur possible sisters to the track maker, Pampadromaeus and Saturnalia.

Figure 2. Grallator illustration from Li et al. 2019 with two basal phytodinosaur possible sisters to the track maker, Pampadromaeus and Saturnalia.

The skull was recently described (Bronzati, Müller and Langer 2019). It had a large orbit, like Pantydraco. More cervicals were present and each one was elongated, creating a much longer neck. The scapula was narrow in the middle. The forelimbs were more robust with a large deltopectoral crest on the humerus. The hind limbs were more robust. The calcaneum did not have such a large tuber.

Figure 2. Subset of the LRT focusing on the Phytodinosauria.

Figure 3. Subset of the LRT focusing on the Phytodinosauria.

Adding scores to Saturnalia
provided an opportunity to review scores for other phytodinosaurs in the large reptile tree (LRT, 1568 taxa). These changes resulted in small modifications to the tree topography and higher Bootstrap scores (Fig. 2). Basal phytodinosaurs still give rise to the clades Sauropodomorpha and Ornithischia.


References
Bronzati M, Müller RT, Langer MC 2019. Skull remains of the dinosaur Saturnalia tupiniquim (Late Triassic, Brazil): With comments on the early evolution of sauropodomorph feeding behaviour. PLoS ONE 14(9): e0221387. https://doi.org/ 10.1371/journal.pone.0221387
Langer MC, Abdala F, Richter M, and Benton M. 1999. A sauropodomorph dinosaur from the Upper Triassic (Carnian) of southern Brazil. Comptes Rendus de l’Académie des Sciences, 329: 511-517.
Langer MC 2003. The pelvic and hind limb anatomy of the stem-sauropodomorph Saturnalia tupiniquim (Late Triassic, Brazil). PaleoBios, 23(2): 1-30.

wiki/Saturnalia

Pampadromaeus in exquisite detail

Langer et al. 2019
bring us new data on the basal phytodinosaur, Pampadromaeus (Cabriera et al. 2011; Fig. 1). Pampadromaeus was a small (1m length) biped with a generalized basal dinosaur morphology.

The authors produced a cladogram
(Fig. 2) focusing on Pampadromaeus and kin. Due to taxon exclusion, they considered Pampadromaeus a basal sauropodomorpha. The LRT (subset Fig. 3) nests it as the Late Triassic last common ancestor (LCA) of Sauropodomopha + Ornithischia.

Figure 2. Basal ornithischia and Pampadroameus, a sister to their common ancestor. Daemonosaurus likely resembled Pampadromaeus, with its long neck.

Figure 1. Basal ornithischia and Pampadroameus, a sister to their common ancestor. Daemonosaurus likely resembled Pampadromaeus, with its long neck.

Unfortunately,
Langer et al. are using an old taxon list that is missing many taxa. In the large reptile tree (1406 taxa, LRT, subset Fig. 3) Marasuchus is a basal theropod, not a dinosaur outgroup. Silesaurus is an outgroup poposaur (a dinosaur-mimic), not a sister to Ornithischia. Ornithischia is a dinosaur in-group, nesting with basal phytodinosaurs and sauropodomorphs. Crocodylomorpha (not included in Langer et al.) is the outgroup for the Dinosauria in the LRT.

Figure 2. Cladogram from xx 2019, colors added. Marasuchus is not an outgroup to the Dinosauria. In the LRT it nests as a basal theropod.

Figure 2. Cladogram from xx 2019, colors added. Marasuchus is not an outgroup to the Dinosauria. In the LRT it nests as a basal theropod.

The differences between the tree topologies
in the Langer et al. cladogram and the LRT appear to result largely from the choice of outgroup. In the LRT the outgroups are not chosen, but are recovered from a list of 1400+ taxa. Oddly, some taxa in the LRT are not included in the Langer et al. study. These include Leyesaurus, Barberenasuchus, Eodromaeus, and one specimen of Buriolestes. The LRT includes no suprageneric taxa, like Ornithischia (Chilesaurus, Daemonosaurus, Jeholosaurus and their descendants).

Figure 4. Subset of the LRT focusing on the Phytodinosauria. Three sauropods are added here.

Figure 3. Subset of the LRT focusing on the Phytodinosauria. Three sauropods are added here.

Experiment with outgroups
If the taxon list for the LRT is reduced to more or less match that of Langer et al. 2019, AND Marasuchus is chosen as the outgroup, the result more closely approaches the Langer et al. tree topology (Fig. 4).

Figure 3. Matching the taxon list to that of xx 2019 and choosing Marasuchus for the outgroup results in this cladogram of basal dinosaur relationships.

Figure 4. Matching the taxon list to that of xx 2019 and choosing Marasuchus for the outgroup results in this cladogram of basal dinosaur relationships using LRT data and taxa.

Adding Euparkeria to this list
(Fig. 4) to create a more distant outgroup for  all included taxa moves Silesaurus outside the Dinosauria and moves Marasuchus and Guabisaurus into the Dinosauria. Basically this is what the LRT recovers with a taxon list similar to Langer et al. 2019.

Figure 5. Adding Euparkeria to figure 4 results in this tree where Silesaurus nests outside the Dinosauria, as it does in the LRT. Theropods nest together, as in the LRT. So do phytodinosaurs.

Figure 5. Adding Euparkeria to figure 4 results in this tree where Silesaurus nests outside the Dinosauria, as it does in the LRT. Theropods nest together, as in the LRT. So do phytodinosaurs.

Pampadromaeus barberenai (Cabriera et al. 2011) is a new dinosaur from the Late Triassic of Brazil. It was originally described as a stem sauropodomorph known from a partial disarticulated skeleton and most of the skull bones. The authors reported, “Based on four phylogenetic analyses, the new dinosaur fits consistently on the sauropodomorph stem, but lacks several typical features of sauropodomorphs, showing dinosaur plesiomorphies together with some neotheropod traits.”


References
Cabreira SF, Schultz CL, Bittencourt JS, Soares MB, Fortier DC, Silva LR and Langer MC 2011. New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil. Naturwissenschaften (advance online publication) DOI: 10.1007/s00114-011-0858-0
Langer MC, McPhee BW, Marsola JCdA, Roberto-da-Silva L, Cabreira SF 2019. Anatomy of the dinosaur Pampadromaeus barberenai (Saurischia—Sauropodomorpha) from the Late Triassic Santa Maria Formation of southern Brazil. PLoS ONE 14(2): e0212543.
https://doi.org/10.1371/journal.pone.0212543
Martínez RN and Alcober OA 2009. A basal sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the early evolution of Sauropodomorpha (pdf). PLoS ONE 4 (2): 1–12. doi:10.1371/journal.pone.0004397. PMC 2635939. PMID 19209223. online article

wiki/Panphagia
wiki/Pampadromaeus

Dr. Baron tip-toes around the radiation of dinosaurs

Last year, Dr. Matthew Baron,
not even a year out from his PhD thesis, placed himself in the middle of controversy when Baron, Norman and Barrett 2017 resurrected the clade Ornithoscelida, wrongly uniting plant-eating Ornithischia with meat-eating Theropoda to the exclusion of plant-eating Sauropodomorpha, an invalid (due to taxon exclusion) hypothesis of relationships, we discussed earlier here.

Dr. Baron guessed,Maybe Ornithischia is actually so far removed from the base of the dinosaur tree that no studies, including my own, have been able to properly place them… Its an intriguing thought and one that needs examining properly.” By his own words, Dr. Baron is not yet an authority on the subject. That authority can only come from a wide gamut analysis that minimizes taxon exclusion, like the large reptile tree (LRT, 1236 taxa), which is something that anyone can produce. As noted last year (see citations below), Dr. Baron’s team excluded several relevant taxa.

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

Figure 1. Look familiar? Here are the pelves of Jeholosaurus and Chilesaurus compared. As discussed earlier, this is how the ornithischian pelvis evolved from that of Eoraptor and basal phytodinosauria.

Later Langer et al. 2017 argued against the Baron, Norman and Barrett interpretation. Baron, Norman and Barrett agued back, stating in Baron’s summary, “Langer et al.’s response showed that the alternative arrangement, that preserved the traditional model, was not statistically significantly different to our own hypothesis, and that was with much of our data having been altered, in ways that we perhaps disagree with strongly.”

Baron is correct is noting that Seeley’s original division, uniting sauropodomorphs with theropods based on pelvis orientation “just because a subgroup have gone on to lose a feature that was the ancestral condition for the wider group, it does not mean that we can then say that the other subgroups who have ‘hung on’ to the feature should be grouped together to the exclusion of the experimental group, at least based on that feature’s absence/presence, without other evidence.” Plus it would be one more example of pulling a Larry Martin.

Unfortunately
Dr. Baron pulls out a bad example as his example of the above. He states, “In fact, Cetacea is more closely related to Carnivora than either group are to the Primates.” In counterpoint, in the large reptile tree (LRT, 1236 taxa) there is no clade “Cetacea.” Odontoceti arise from tenrecs and elephant shrews. Mysticeti arise from hippos and desmostylians. Carnivora split apart in the first dichotomy in Eutheria. Thus all other eutherians, including primates, odontocetes and mysticetes have a last common ancestor that is not a member of the Carnivora.

Unfortunately
Dr. Baron bases the above quote on a phylogenetic error when he states, “Like I said before, you need to look at TOTAL EVIDENCE to come to this quite obvious conclusion, which means focusing on more anatomical evidence.” While this may sound reasonable and correct, a focus on anatomical evidence may lead to confusion due to convergence. Bottom line, it is more important to look at the phylogenetic placement of a taxon in order to determine what it is. This has to be done in the context of a wide gamut analysis that minimizes taxon exclusion using at least 150 (sometimes multi-state) characters (the LRT uses 238). Otherwise you’re cherry-picking taxa, something Baron, Norman and Barrett were guilty of by excluding bipedal crocs and several basal dinosaurs from their study (and we know this since the LRT includes them). Baron also cherry-picks traits in part 3 of his argument, pulling a Larry Martin several times in doing so. In a good phylogenetic analysis, like the LRT, you’ll see a gradual accumulation of traits. That means you’ll get a pubis with a transitional phase, a tiny predentary and other traits in gradual accumulation among the outgroups to Ornithischians.

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

Figure 2. Chilesaurus and kin, including Damonosaurus and basal phytodinosauria.

Baron promises
“I will eat my shoes!” if Seeley’s dichotomy is correct. That’s an easy promise to make knowing there is a third hypothesis out there: the Theropod/Phytodinosaur dichotomy presented by Bakker (1986) and confirmed by the LRT in 2011.

Pertinent to this discussion
sometimes what a paleontologist does not say about a particular subject can be more important that what a paleontologist does say. I lump taxon exclusion and citation exclusion in the category of ‘what is not said.’

References
Bakker RT 1986. The Dinosaur Heresies.New Theories Unlocking the Mystery of the Dinosuars and Their Extinction. Illustrated. 481 pages. William Morrow & Company.
Baron MG and Barrett PM 2017. A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biology Letters 13, 20170220.
Baron MG, Norman DB and Barrett PM 2017.
A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543: 501–506;  doi:10.1038/nature21700
Baron MG, Norman DB and Barrett PM 2017. Baron et al. reply. Nature 551: doi:10.1038/nature24012
Langer et al. (8 co-authors) 2017. Untangling the dinosaur family tree. Nature 551: doi:10.1038/nature24011
Novas FE et al. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature 522(7556), 331.

Relevant blogposts and theses from Dr. Baron:

https://www.academia.edu/36002282/THE_ORIGIN_AND_EARLY_EVOLUTION_OF_THE_DINOSAURIA

What I think about Ornithischia

Thoughts on Ornithoscelida … over one year on … (part 1)

Thoughts on Ornithoscelida … over one year on … (part 2)

Chilesaurus – what is it?

https://pterosaurheresies.wordpress.com/2017/03/23/new-radical-dinosaur-cladogram-baron-norman-and-barrett-2017

https://pterosaurheresies.wordpress.com/2017/03/24/baron-2017-21-unambiguous-theropodornithischian-synapomorphies-dont-pan-out/

https://pterosaurheresies.wordpress.com/2015/06/25/the-dinosaur-heresies-nytimes-book-review-from-1986/

https://pterosaurheresies.wordpress.com/2017/11/03/dinosaur-family-tree-langer-et-al-responds-to-baron-et-al-2017-in-nature/

https://pterosaurheresies.wordpress.com/2017/08/16/you-heard-it-here-first-chilesaurus-is-a-basal-ornithischian-confirmed/

The last common ancestor of all dinosaurs in the LRT: ?Buriolestes

Müller et al. 2018
describe a new dinosaur skeleton they attribute to Buriolestes shultzi (Cabreria et al. 2016, ULBRA-PVT280, Figs. 2, 3). In the large reptile tree (LRT, 2015 taxa; subset Fig. 1) the holotype now nests at the base of the Phytodinosauria. The referred specimen is different enough to nest between the herrerasaurs and all other dinosaurs. This, of course, removes herrerasaurs from the definition of the Dinosauria (Passer + Triceratops, their last common ancestor (= CAPPA/UFSM 0035) and all descendants).

Figure 1. Subset of the LRT including the new specimen of Buriolestes (CAPPA/UFSM 0035) nesting at the base of all dinosaurs.

Figure 1. Subset of the LRT including the new specimen of Buriolestes (CAPPA/UFSM 0035) nesting at the base of all dinosaurs.

 

Buriolestes schultzi (Cabreria et al. 2016; Late Triassic, Carnian; 230mya) was originally and later (Müller et al. 2018) considered a carnivorous sauropodomorph, but here two specimens nest as the basalmost dinosaur (CAPPA/UFSM 0035) and the basalmost phytodinosaur (ULBRA-PVT280).

Figure 2. The two skulls attributed to Buriolestes (holotype on the right). The one on the left nests as the basalmost dinosaur, basal to theropods and phytodinosaurs.

Figure 2. The two skulls attributed to Buriolestes (holotype on the right). The one on the left nests as the basalmost dinosaur, basal to theropods and phytodinosaurs. It should have a distinct name.

All cladograms agree that Buriolestes
is a very basal dinosaur. Taxon exclusion changes the tree topology of competing cladograms. The broad autapomorphic ‘eyebrow’ of the CAPPA specimen indicates it is a derived trait in this Late Triassic representative of an earlier genesis.

Figure 3. Herrerasaurus, Buriolestes and Tawa to scale.

Figure 3. Herrerasaurus, Buriolestes and Tawa to scale.

The Müller et al. cladogram
combined both specimens attributed to Buriolestes (never a good idea, but it happens all the time). The Müller et al. cladogram excluded a long list of basal bipedal crocodylomorpha, but did include Lewisuchus. It excluded the archosaur outgroups PVL 4597Turfanosuchus and Decuriasuchus. The Müller et al. cladogram nested Ornithischia basal to Saurischia (= Herrerasauridae + Agnophitys, Eodromaeus, Daemonosaurus + Theropoda + Sauropodomorpha) with Buriolestes nesting between Eoraptor and Panphagia. The CAPPA specimen of Buriolestes is also a sister to the basalmost theropod, Tawa (Fig. 3)… and not far from the other basal archosaur, Junggarsuchus (Fig. 4).

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

Figure 4. The CAPPA specimen of Buriolestes compared to Junggarsuchus, basal to the other branch of archosaurs, the crocs.

References
Cabreira SF et al. (13 co-authors) 2016. A unique Late Triassic dinosauromorph assemblage reveals dinosaur ancestral anatomy and diet. Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2016.09.040
Müller RT et al. (5 co-authors 2018. Early evolution of sauropodomorphs: anatomy and phylogenetic relationships of a remarkably well-preserved dinosaur from the Upper Triassic of southern Brazil. Zoological Journal of the Linnean Society, zly009 (advance online publication) doi: https://doi.org/10.1093/zoolinnean/zly009

Dinosaur family tree: Langer et al. responds to Baron et al. 2017 in Nature

Earlier
Baron et al. revised the dinosaur family tree by uniting Ornithischia with Theropoda to the exclusion of Herrerasaurus + Sauropodomorpha. Then Baron and Barrett 2017 moved Chilesaurus (Fig. 1) from Theropoda to Ornithischia, confirming the earlier hypothesis advanced here in 2015, but in the context of uniting Ornithischia with Sauropodomorpha (= Phytodinosauria) to the exclusion of Herrerasaurus + Theropoda.

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

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

Today
Langer et al. 2017 argue, “we evaluate and reanalyse the morphological dataset underpinning the proposal by Baron et al.5 and provide quantitative biogeographic analyses, which challenge the key results of their study by recovering a classical monophyletic Saurischia and a Gondwanan origin for dinosaurs. Our international consortium of early dinosaur evolution specialists has come together to critically assess the Baron et al.5 dataset.”

The Langer team recovered a traditional Saurischia/Ornithischia tree, but noted it would take only 2-3 additional steps to enforce a Sauorpodomorpha/ Ornithoscelida split, as recovered by the Baron team – this after changing 2,500 scorings (10% of the dataset). The Langer team also confirmed the origin of dinos in southern Pangaea and left with three conclusions (my comments follow):

  1. There is currently great uncertainty about early dinosaur relationships and the basic structure of the dinosaur family tree. Not in the large reptile tree (LRT, 1119 taxa)/
  2. Dataset construction is key. No, taxon inclusion is the key. Neither the Baron team nor the Langer team included the correct outgroup taxa nor a long list of basal dinosaur taxa (see below) that direct the tree topology toward the phytodinosaur clade.
  3. It is important to use appropriate computational analytical tools before making macro-evolutionary claims. No, taxon exclusion will lead to wrong results. Trait selection matters, but not as much. Scoring correctly matters, but not as much. Employing decades old software does not matter because the math and statistics are the same. Remember, only a poor workman blames his tools so don’t  blame the “computational analytical tools” for poor macro-evolutionary claims.

Bottom line:
The Langer team used the same incomplete taxon list as the Baron et al. team did. So they were looking for their ‘keys’ beneath the bright lamp, while the keys were lost in the dark alley they ignored.

This happens so often.

And
Baron et al. 2017 reply. “This  extensive re-scoring results in recovery of the ‘traditional’ topology, although with less resolution and very weak support; their result is statistically indistinguishable from the possibility that our topology provides a better explanation of the data. This weak support, despite these extensive changes, suggests that the ‘traditional’ tree struggles to account for many character distributions.”

And they disagree with many of the re-scorings. Their re-scoring of just Pisanosaurus reproduced the clade Ornithoscelida in their revised tree.

Both presented trees were poorly resolved.
The LRT is fully resolved. Baron et al. defended the possibility of a Northern origin for dinosaurs. That big ‘maybe’ does not follow the data in the LRT.

On a similar, but side note
Biology Letters was kind enough to publish my reply to the Baron and Barret 2017 paper on Chilesaurus, but much of it has bearing for today’s discussion. Here is that letter in its entirety:

Baron and Bennett [1] nest Chilesaurus [2] as the sister group to Ornithischia, rather than a tetaneuran theropod as previously proposed [2]. Unfortunately, the Baron and Bennett [1] taxon list, like the Novas et al. [2] taxon list before it, did not include many of the taxa essential to resolve this issue.

A larger study of over 1060 taxa [3] includes more taxa essential to resolve this issue. The matrix was created using MacClade [4]. Analyses were run in PAUP 4.0b10 [5] using a heuristic search and a Bootstrap/Jackknife search for 100 random addition replicates. Scores are indicated on the webpage.

On that cladogram Chilesaurus (Late Jurassic) nests as a basal ornithischian in a clade that also includes Daemonosaurus (Late Triassic) and Jeholosaurus (Early Cretaceous). The latter two taxa were not included in Baron and Bennett [1]. This clade of three taxa nested as a sister to the Sauropodomorpha with Leyesaurus at its base. The analysis recovered the clade Phytodinosauria as the sister taxa to the Theropoda. Herrerasaurus was the outgroup to these two clades as basalmost member of the Dinosauria. Basal phytodinosaurs not nesting within Sauropodomorpha + Ornithischia include Barberenasuchus, Eodromaeus, Eoraptor and Pampadromaeus.

On that cladogram Silesaurus nests within a clade Poposauridae outside the Archosauria. The clade Archosauria includes only the Crocodylomorpha + the Dinosauria. Lagerpeton nests with Tropidosuchus and other proterochampsids. The pterosaur, Dimorphodon, nests with lepidosaurs like Huehuecuetzpalli, Macrocnemus and Cosesaurus (the last of which had an antorbital fenestra by convergence [6, 7]). None of these are archosauriformes nor prolacertiformes [contra 7]. The following pertinent taxa were not included in Baron and Bennett [1]: Daemonosaurus, Jeholosaurus, Haya, Barberenasuchus, Buriolestes, Segisaurus, Procompsognathus, PVL 4597, Junggarsuchus, Pseudhesperosuchus, Carnufex, Trialestes, Gracilisuchus, Scleromochlus, Decuriasuchus, Turfanosuchus, Poposaurus, Lotosaurus, Shuvosaurus, Effigia and Tropidosuchus.

Chilesaurus was first nested as a basal ornithischian in April 2015 [8] in an earlier version of the above analysis, then with fewer taxa. With the addition of more pertinent taxa the position of Chilesaurus is indeed well resolved contra the previous e-letter [9].

1. Baron MG and Barrett PM 2017. A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biology Letters 13, 20170220.
2. Novas FE et al. 2015. An enigmatic plant-eating theropod from the Late Jurassic period of Chile. Nature 522(7556), 331.
3. http://www.ReptileEvolution.com/reptile-tree.htm .nex file link on that webpage
4. Maddison DR., & Maddison WP 2001 MacClade 4.08: Analysis of Phylogeny and Character Evolution. Version 4.03. Sinauer Associates.
5. Swofford D 2002 PAUP 4.0 b10: Phylogenetic analysis using parsimony. Sinauer Associates.
6. Ellenberger P. and de Villalta JF 1974. Sur la presence d’un ancêtre probable des oiseaux dans le Muschelkalk supérieure de Catalogne (Espagne). Note preliminaire. Acta Geologica Hispanica 9, 162-168.
7. Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
8.https://pterosaurheresies.wordpress.com/2015/04/28/chilesaurus-new-dinos…
9. King B 2017. Chilesaurus is not a basal ornithischian. http://rsbl.royalsocietypublishing.org/content/13/8/20170220.e-letters

References
Baron MG and Barrett PM 2017. A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs. Biology Letters 13, 20170220.
Baron MG, Norman DB and Barrett PM 2017.
A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543: 501–506;  doi:10.1038/nature21700
Baron MG, Norman DB and Barrett PM 2017. Baron et al. reply. Nature 551: doi:10.1038/nature24012
Langer et al. (8 co-authors) 2017. Untangling the dinosaur family tree. Nature 551: doi:10.1038/nature24011

Pisanosaurus: dinosaur or silesaurid?

A new paper by Agnolin and Rozadilla 2017
includes new photographs of the holotype that shed new light on Pisanosaurus (Casamiquela 1967, Bonaparte 1976; Late Triassic). This taxon was previously known in the literature chiefly (not exclusively) from drawings. The large reptile tree (LRT, 1043 taxa) nested Pisanosaurus with Haya as a basal ornithischian, confirming prior assessments. Now Agnolin and Rozadilla provide evidence for a Silesaurus affinity among the Poposauridae. Echoing others, they report, “the poor preservation of the specimen is the largest difficulty to overcome when interpreting its morphology. Its phylogenetic position within ornithischians is problematic.”

So, with the new evidence,
let’s test and nest Pisanosaurus 2017! (There are so few traits that can be scored for Pisanosaurus, that the rest of the discussion might seem like I’m pulling a Larry Martin. That happens sometimes, but I’m trying to report results from the LRT.

Before we start…
with present data, shifting Pisanosaurus to Silesaurus in the LRT adds 24 steps. Moreover, Agnolin and Rozadilla did not mention the proximal relatives of Pisanosaurus in the LRT:  Haya, Daemonosaurus, Chilesaurus, Scelidosaurus and Emausaurus. This may be the key to their novel results: taxon exclusion… once again. 

Some general notes to start with:

  1. Silesaurus and other poposaurs have a metatarsus no longer than the longest digit. The same hold true for many basal phytodinosaurs, but Pisanosaurus has a longer metatarsus, like its sister in the LRT, Haya.
  2. The photo of the pelvis does little to clarify any issues. It is a broken up mess (Fig. 2) with, what appear to be smaller pelvis bones (greens)  and several sacral bones (blues) stirred up in a conglomeration. Not much matches the published drawings. And my earlier imagination describing a rotated pubis based on simple published drawings did not pan out.
  3. The anterior dentary appears to be missing a predentary bone, a trait common to the clade Ornithischia, but something like it also appears in Silesaurus.
  4. Pisanoaurus comes from South America, home of most of the other basalmost Triassic phytodinosaurs. Popposaurids, all except Sacisaurus, come from somewhere else on the globe. Haya, the LRT sister to Pisanosaurus, comes from China, but it is Late Cretaceous in age.
  5. Agnolin and Rozadilla consider Silesaurus part of a clade “that is currently recognized as the sister group to Dinosauria.” The LRT recovers Crocodylomorpha closer to Dinosauria and Silesaurus nests within the next proximal outgroup, Poposauridae.
  6. Agnolin and Rozadilla report, “because Pisanosaurus is a unique and very valuable specimen, it is not currently possible to [CT] scan it.”
  7. Authors have not agreed whether the pelvis, represented by fragments of bones and bone impressions in rock. is preserved in medial or lateral view. Agnolin and Rozadilla report, “the sacrum is articulated and preserved in life position with respect to the pelvis.”
Figure 1. The Pisanosaurus pelvis here flipped right to left along with drawings and reconstructions by Agnolín and Rozadilla, plus DGS colors applied to what I can see here. Nothing is clear, but it seems like the pelvic elements are smaller that published and that several sacral vertebrate are sprinkled in this mass. Perhaps a CT scan would be helpful here. Blue = vertebrae. Green = pelvi elements.

Figure 1. The Pisanosaurus pelvis here flipped right to left along with drawings and reconstructions by Agnolín and Rozadilla, plus DGS colors applied to what I can see here. Other than the sacral vertebrate on top, not much is clear, but it seems like the pelvic elements are smaller that published and that several sacral vertebrate are sprinkled in this mass. Perhaps a CT scan would be helpful here. Blue = vertebrae. Green = pelvi elements.

Agnolin and Rozadilla provided an emended diagnosis.
Pisanosaurus is a basal dinosaurifordiagnosable by the following autapomorphies:

  1. “central teeth bilobate in occlusal view, showing well-developed mesial and distal grooves;
  2. distal end of the tibia anteroposteriorly longer than transversely wide;
  3. bilobate astragalus in distal view;
  4. ascending process of the astragalus being subquadrangular and robust in lateral view;
  5. intense transversal compression of the calcaneum.”
Figure 3. Skull of Haya and restored skull of Pisanosaurus compared. The resemblance of preserved elements is apparent here. In both cases the mandibular fenestra is filled in. The other holes in the Pisanosaurus mandible are artifacts of taphonomy. Pisanosaurus data from Irmis et al. 2007b.

Figure 2. Skull of Haya and restored skull of Pisanosaurus compared. The resemblance of preserved elements is apparent here. In both cases the mandibular fenestra is filled in. The other holes in the Pisanosaurus mandible are artifacts of taphonomy. Pisanosaurus data from Irmis et al. 2007b.

Other factors of interest:

  1. The number of tooth positions (15) in Pisanosaurus matches both silesaurids and pertinent ornithischians.
  2. “Central teeth are bilobate in occlusal view, and show well-developed mesial and distal grooves, a condition unknown in other herbivorous taxa and a trait that may be an autapomorphy of Pisanosaurus.” Not sure if the teeth in Haya are the same, but they look similar in lateral view (Fig. 2). Neither have denticles. Silesaurid teeth are leaf-shaped.
  3. “the teeth do not form a palisade or continuous masticatory surface as advocated by some authors.” As in Haya.
  4. “Pisanosaurus is similar to saurischians and basal dinosauriforms in having overlapping proximal metatarsals, differing from the non-overlapping condition in ornithischians.” Except Haya.
Figure 1. Haya in lateral view.

Figure 3. Haya in lateral view. Note the dorsal laminae, similar to those in Pisanosaurus.

Agnolin and Rozadilla describe the dorsal vertebrae
as having a strong and complex system of laminae. Haya (Fig. 3).has similar laminae. Poposauridae do not.

Silesaurus

Figure 4 Silesaurus as a biped and occasional quadruped. Note the squareish cervicals, unlike the parallelograms in figure 5.

Agnolin and Rozadilla considered the vertebrae
(Fig. 5) very different from the cervical vertebrae described for basal dinosauriforms and ornithischians. But they did not look at Haya, which has similar cervicals 1 and 2 (Fig. 5). They considered the cervicals ‘indistinguishable from Sacisaurus cervicals, but Langer and Ferigolo 2013, did not refer the cervical to Sacisaurus due to its relatively large size. Concluding Agnolin and Rozadilla considered these verts to be on uncertain position.

Figure 4. Pisanosaurus cervical vertebrae in left lateral view (not right as published) matches cervical vertebrae 1 and 2 in Haya.

Figure 5. Pisanosaurus cervical vertebrae in left lateral view (not right as published) matches cervical vertebrae 1 and 2 in Haya – and does not match the simpler vertebrae in Silesaurus (Fig. 4).

Sacrals are preserved as moulds in Pisanosaurus. 
Various authors have interpreted five, to two sacrals. Agnolin and Rozadilla concurred with Irmis et al. 2007, who found no trace of sacral elements, reporting, “some features previously considered to be impressions of sacral ribs are actually cracks in the matrix, and there is insufficient fidelity to determine whether any of the centra are fused to each other.” 

Figure 6. Pisanosaurus right pes with digit 2 ghosted in and digit 4 rotated into in vivo position. PILS added. Nnte the brevity of the toes compared to the metatarsus, a trait shared with Haya.

Figure 6. Pisanosaurus right pes with digit 2 ghosted in and digit 4 rotated into in vivo position. PILS added. Nnte the brevity of the toes compared to the metatarsus, a trait shared with Haya.

Is the acetabulum open or closed?
Agnolin and Rozadilla ‘suggest’ it is closed, as in poposaurs. If so the closed portion is buried. With available evidence and phylogenetic bracketing, it was probably open. Haya has an acetabulum with a keyhole shape (Fig. 3).

The tibia, tarsus and metatarsus
in Pisanosaurus the cnemial crest does not peak at the knee, but somewhat lower. Haya is similar. The fibula diameter is 70% that of the tibia, as in Scelidosaurus. The fibula for Haya is unknown. Anolín and Rozadilla identified a calcaneal tuber. That is odd because it is so small that it does not extend as far as the fibula does. in Haya the calcaneum extends slightly beyond the astragalus. The astragalus of Pisanosaurus is longer than wide (when the medial condyle is included), which is distinctly different from Haya and other sister taxa and different from Silesaurus.

Figure 8. Calcaneum of Pisanosaurus. You can see why some authors saw a tuber while others did not.

Figure 8. Calcaneum of Pisanosaurus. You can imagine why some authors saw a tuber while others did not.

A flawed phylogenetic analysis
Other than excluding several taxa that nest close to Pisanosaurus in the LRT, Agnolin and Rozadilla employed the invalid Nesbitt (2011) database, also suffering greatly from taxon exclusion. It does not nest sauropodomorphs with ornithischians as phytodinosaurs, but nests sauropodomorphs, like Pampadromaeus, with Tawa and other theropods. In their first analysis, 20 trees resulted with Pisanosaurus nested as an unresolved polytomy of several dinos and non-dinos. After excluding wild card taxa, 82 trees resulted with Pisanosaurus within the Silesauridae. Bremer support is low in their analysis, but Bootstrap support is high in the LRT.

Discussion
Agnolín and Rozadilla discuss several traits of Pisanosaurus (typically related to herbivory) and their appearances elsewhere within the Archosauria. They find no epipophyses in the cervicals, but Haya lacks these, as well on the pertinent first two verts. Agnolín and Rozadilla note “The vertebral centra are very elongate and transversely compressed, differing from the short and stout dorsal vertebrae of known ornithischians, including heterodontosaurids.” They do not realize the close relationship of Pisanosaurus to sauropodomorphs like Saturnalia and the basalmost ornithischian, Chilesaurus, both with elongate dorsals. Agnolín and Rozadilla made a “tentative reconstruction” of the pelvis (Fig. 1), but it bear little to no resemblance to the in situ fossil. In every comparison made, Agnolín and Rozadilla delete or ignore Haya and related taxa and thus recover semi-blind results.

Today and in the future
we can’t keep going back to the same short lists of taxa for our inclusion sets. We know of so many more now that need to be included in phylogenetic analyses. The LRT can be your guide.

References
Agnolín FL and Rozadilla S 2017. Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina. Journal of Systematic Palaeontology. http://dx.doi.org/10.1080/14772019.2017.1352623
Ferigolo and Langer 2006. A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology, 2006; 1–11, iFirst article
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.

wiki/Sacisaurus
wiki/Pisanosaurus
wiki/Haya

 

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.

New radical dinosaur cladogram: Baron, Norman and Barrett 2017

Baron, Norman and Barrett 2017
have just allied Ornithischia with Theropoda to the exclusion of Sauropodomorpha. That radical hypothesis was not recovered by the large reptile tree (LRT, 980 taxa) nor any other study in the long history of dinosaurs. Despite the large size of their study, it was not large enough. And so taxon exclusion bites another group of well-meaning paleontologists who used traditional small inclusion sets.

From the Baron et al. abstract:
“For 130 years, dinosaurs have been divided into two distinct clades—Ornithischia and Saurischia. Here we present a hypothesis for the phylogenetic relationships of the major dinosaurian groups that challenges the current consensus concerning early dinosaur evolution and highlights problematic aspects of current cladistic definitions. Our study has found a sister-group relationship between Ornithischia and Theropoda (united in the new clade Ornithoscelida), with Sauropodomorpha and Herrerasauridae (as the redefined Saurischia) forming its monophyletic outgroup. This new tree topology requires redefinition and rediagnosis of Dinosauria and the subsidiary dinosaurian clades. In addition, it forces re-evaluations of early dinosaur cladogenesis and character evolution, suggests that hypercarnivory was acquired independently in herrerasaurids and theropods, and offers an explanation for many of the anatomical features previously regarded as notable convergences between theropods and early ornithischians.”

As a reminder, the fully resolved cladogram
at ReptileEvolution.com/reptile-tree.htm finds Herrerasaurus as a basal dinosaur arising from the Pseudhesperosuchus clade. Tawa (Fig. 1) and Buriolestes lead the way toward Theropoda. Barberenasuchus and Eodromaeus are basal to Phytodinosauria, which includes Sauropodomorpha + Ornithischia. So the Nature piece is totally different due to taxon exclusion and improper taxon inclusion.

Earlier heretical dinosaur origins were presented here with images and complete resolution with high Bootstrap scores at every or virtually every node.

Problems with the Baron et al. report

  1. Lack of resolution: Over dozens of nodes, only 5 bootstrap scores were over 50 (the minimum score that PAUP shows as fully resolved).
  2. Lack of correct proximal outgroup taxa (taxon exclusion) and they chose several wrong outgroup taxa (see below) because they had no large gamut analysis that established the correct outgroup taxon out of a larger gamut of choices
  3. Lack of several basal dinosaur taxa. (again, taxon exclusion, see below)
  4. Improper taxon inclusion: poposaurs, pterosaurs and lagerpetons are not related to dinos or their closest kin
  5. Lacking reconstructions for all pertinent basal/transitinal taxa so we can see their data at a glance, see if a gradual accumulation of traits can be observed and not have to slog through all the scores
Figure 1. Unrelated archosaurs. Silesaurus is a poposaur. Eoraptor is a phytodinosaur (note the big belly). And Tawa is a lean theropod.

Figure 1. Unrelated archosaurs mentioned in this blog. Silesaurus is a poposaur. Eoraptor is a phytodinosaur (note the big belly). And Tawa is a lean theropod.

LRT differences with the Baron et. al results

  1. Carnivorous Staurikosaurus, Herrerasaurus, Chindesaurus and Sanjuansaurus nest at the base of the herbivorous Sauropodomorpha.
  2. Herbivorous Eoraptor nests at the base of the Theropod with Tawa.
  3. Poorly known Saltopus sometimes nests as the last common ancestor of Dinosauria.
  4. Six taxa nest basal to dinosaurs in SupFig1 including the poposaur Silesaurus and kin. Silesaurus has ornithischian and theropod traits and so appears to make an ideal outgroup taxon,  but nests with neither clade when more taxa are included. This is the key problem with the study: pertinent taxon exclusion. 
  5. The lack of Gracilisuchus and other bipedal basal crocs that nest basal to dinos in the LRT certainly skewed results.

In an effort to understand Baron et al. I duplicated their outgroup taxon list
but retained all the LRT dinosaurs to see what would happen. The SupFigs are available free online at Nature.com

  1. SupFig 1: When Euparkeria is the outgroup and Postosuchus is included: 3 trees result and (theropods Herrerasaurus + Tawa + Buriolestes) + (poposaurs Sacisaurus + Silesaurus) nest as the base of the Phytodinosauria, while bipedal croc Saltopus nests at the base of the Theropoda.
  2. SupFig 2: When the lepidosaur pterosaur Dimorphodon is the outgroup and Euparkeria + Postosuchus are excluded: 12 trees and basal scansoriopterygid birds (come to think of it, they DO look like Dimorphodon!) nest as basal dinosaurs, then the bird cladogram gets reversed such that basal becomes derived, but Phytodinosauria is retained.
  3. SupFig. 3: when Silesaurus is the outgroup: 12 trees and Phytodinosauria is retained in the LRT
  4. SupFig. 4: when no characters were treated as ordered. Neither does the  LRT order any characters, so this test was moot.

Dr. Kevin Padian said, 
“‘original and provocative reassessment of dinosaur origins and relationships”. And because Baron and his colleagues used well-accepted methods, he notes, the results can’t simply be dismissed as a different opinion or as mere speculation. “This will send people back to the drawing board,” he added in an interview.”

“There have been a lot of studies on the phylogenetic relationships, the family tree of the dinosaurs, but they’ve mostly been on individual dinosaurian groups. They haven’t really examined the entire dinosaur tree in such depth. And so this analysis had the advantage of using a different and larger set of critters than most previous trees. They’ve analyzed the characters used by others before and then also adding their own characteristics and getting their selves quite different configurations, radically different in fact.

The LRT has had, for several years, an even larger set of taxa, so large that any bias in selecting an outgroup taxon list has been minimized. Unfortunately, Baron et al. were biased and used traditional outgroup taxa that skewed their results.

Dr. Hans-DieterSues reported,
“For one thing, palaeontologists’ analyses of relations among species are keenly sensitive to which species are considered, as well as which and how many anatomical features are included, he says.”

True.
Many more outgroup taxa would have minimized the inherent bias clearly present in Baron et al. When Silesaurus is your outgroup, herbivores will nest with carnivores. When you start your study with a goal in mind (read and listen to Baron’s comments) that’s never good. When you exclude taxa that have been shown to be pertinent to your study, that’s never good.

That’s what ReptileEvolution.com is here for (on the worldwide web). Free. Testable. And with a demonstrable gradual accumulation of traits along with minimal bias due to its large gamut.

I was surprised to see Nature print this
because they have not published relationship hypotheses in favor of  new specimens of note. Co-author Dr. David Norman has published for several decades and has a great reputation.

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