The Origin of Dinosaurs x2 (2010) revisited

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

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

What has stood the test of time:

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

What has not stood the test of time:

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

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

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

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

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

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

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

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

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

Traits found in dinosaurs exclusive of Herrerasaurus:

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

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

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

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News on the Origin of Dinosaurs: Megapnosaurus and Zupaysaurus

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

Figure 6. Proto-dinosaurs including Zupaysaurus and Megapnosaurus.

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

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

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

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

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

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

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

Megapnosaurus_skull588

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

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

This is what happens with taxon exclusion.

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

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

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

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 5. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

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

Figure 4. Coelophysis skull for comparison.

Figure 6. Coelophysis skull for comparison.

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

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

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

wiki/Zupaysaurus
wiki/Coelophysis_rhodesiensis

The Chronological Origin of Dinosaurs – Nesbitt vs. Peters

Dr. Sterling Nesbitt (2011) recently published his chronological family tree of archosaurs and their ancestors (Fig. 1). It’s too small to read, so please click on it to enlarge it.

The family tree of Archosaurs and their ancestors by Sterling Nesbitt.

Figure 1. Click to enlarge. The family tree of Archosaurs and their ancestors by Sterling Nesbitt, who includes pterosaurs, phytosaurs, and proterochampsids (including lagerpetids) that do not belong here according to the large reptile tree of Peters.

Taking the same chronological data, I rearranged and edited the taxa according to the results of the large reptile tree (Fig. 2).

Figure 2. Click to enlarge. The family tree of Archosaurs and their ancestors according to the large reptile tree by Peters. Note the absence here of phytosauria, proterochampsia (including lagerpetidae) and pterosauromorpha all of which nest far afield in the large reptile tree. A few taxa are added here. As in Nesbitt's tree, the major radiation occurred during the early Triassic and rather quickly. Here most dinosaur groups had a long ghost lineage into the Early Triassic, except, possibly, the Ornithischia evolving from Daemonosaurus

Figure 2. Click to enlarge. The family tree of Archosaurs and their ancestors according to the large reptile tree by Peters. Note the absence here of phytosauria, proterochampsia (including lagerpetidae) and pterosauromorpha all of which nest far afield in the large reptile tree. A few taxa are added here. As in Nesbitt’s tree, the major radiation occurred during the early Triassic and rather quickly. Here most dinosaur groups had a long ghost lineage into the Early Triassic, except, possibly, the Ornithischia evolving from Daemonosaurus

The evolution of dinosaurs and their ancestors is an intriguing question made more puzzling by the early appearance of Lotosaurus and Asilisaurus, two highly derived forms appearing in the early Middle Triassic, much sooner than the rest of the known fossils.

The question of poposaur nesting is the big difference here. Nesbitt finds that Ticinosuchus is basal to poposaurids. The large reptile tree found that Ticinosuchus was basal to aetosaurs and Sacisaurus was basal to poposaurids, despite its late appearance in the fossil record. The addition of Daemonosaurus (Fig. 2) provides a short path toward the Ornithischia. Nesbitt (Fig. 1) has a much longer path to the Ornithischia, close to Pisanosaurus, but running to the Early Triassic.

Poposaurids have been described as extremely convergent with dinosaurs, but their croc-like ankle gives them away. The large reptile tree found the development of a croc-like ankle was convergent in crocs and poposaurids, both developing from ankles without a calcaneal tuber or much of one.

The clade of Eoraptor, Panphagia and Pampadromaeus turns out to be the clade from which all phytodinosaurs (sauropodomorphs, ornithischians and poposaurids) evolved.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.