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.

 

 

Poposaurs – Topology Shift

Yes, I was wrong. And it’s time to man-up.
Earlier, based on available data, the large reptile tree nested poposaurids with phytodinosaurs and attributed the appearance and growth of the calcaneal tuber in certain poposaurs to convergence with the Crocodylomorpha. So the earlier data recovered poposaurs as dinosaurs with an odd ankle, not crocs with a “massive convergence” with dinosaurs, which is still the widespread hypothesis (see Nesbitt 2011 and others).

That seemed to make sense — except some poposaurs, like Lotosaurus and Asilisaurus (Fig. 1), appeared a little too early in the Triassic. They seemed to be anachronistic, and that can be a red flag.

So going back to the phylogenetic analysis,
I reexamined certain specimens, discovered a few items not originally presented (I trusted original tracings instead of making my own from in situ photos) and I found several bad scores. The newly recovered tree finds poposaurids derived from Turfanosuchus, a taxon that earlier stood alone at the base of the Archosauria (basal to crocs and dinos). Now things seem to make more sense, phylogenetically, chronologically (Fig.1) and morphologically. See if you agree…

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos.

Figure 1. Poposauridae revised for 2014. Here they are derived from Turfanosuchus at the base of the Archosauria, just before crocs split from dinos. Among these, only Silesaurus and Asilisaurus lost the calcaneal tuber.

Figure 2. The Euarchosauriformes featuring a new nesting for the Poposauridae.

Figure 2. The Euarchosauriformes featuring a new nesting for the Poposauridae.

Now the odd thing is: 
Poposaurs appear to provide a sort of preview to what would eventually evolve in the Dinosauria itself, likely filling similar niches in earlier strata.

Turfanosuchus and Poposaurus (Fig. 1) were convergent with theropods. The larger Asilisaurus was convergent with sauropodomorphs. The remainder were convergent with various ornithischians, even down to the toothless predentary they shared by convergence. Lotosaurus was a stegosaur mimic. Shuvosaurus was a Dryosaurus mimic. Silesaurus was a Camptosaurus mimic, down to losing the calcaneal tuber. Sacisaurus was a little Agilisaurus mimic. Effigia was still the oddball with those vestigial hands and back-sloped braincase.

So poposaurids are not dinosaurs. They are also not basal to rauisuchidae (contra Nesbitt 2011), but were derived from basal rauisuchia like Decuriasuchus and Vjushkovia. They are the most basal archosaurs. Basal poposaurs were the last common ancestors of crocs and birds. From their basalmost taxon, a sister to little Turfanosuchus, both tiny basal bipedal crocs and tiny bipedal basal dinos evolved.

Size
Poposaurs, in the form of Nyasasaurus, Asilisaurus and Lotosaurus (Fig. 1), were the first archosaurs to evolve substantial size in the Middle Triassic. Crocs and dinos remained small until the late Triassic (mid-Triassic for the basal Herrerasaurus) when they had their great radiation and poposaurs began to fade. This is an unrecognized faunal turnover.

Discovering and correcting errors is what scientists do. 
And I was happy that these new insights appeared.

What took so long?
Inattention to red flags. We should all look more closely at problems. They lead to new insights.

M.M. I hope this helps the cause. And yes, I have made and will make changes to earlier posts on this subject.

References
Brusatte SL, Benton MJ, Desojo JB and Langer MC 2010. The higher-level phylogeny of Archosauria (Tetrapoda: Diapsida), Journal of Systematic Palaeontology, 8:1, 3-47.
Irmis RB, Nesbitt SJ, Padian K, Smith ND, Turner AH, Woody D and Downs A 2007. A Late Triassic dinosauromorph assemblage from New Mexico and the rise of dinosaurs. Science 317 (5836): 358–361. doi:10.1126/science.1143325. PMID 17641198.
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.
Nesbitt SJ, Irmis RB, Parker WG, Smith ND, Turner AH and Rowe T 2009. Hindlimb osteology and distribution of basal dinosauromorphs from the Late Triassic of North America. Journal of Vertebrate Paleontology 29 (2): 498–516. doi:10.1671/039.029.0218

MCSNB 3496 reconstructed

And one more Triassic pterosaur first reported by Wild (1978) and reviewed by Dalla Vecchia (2003), MCSNB 3496 (Fig. 1). So few parts are known that it would be very difficult to accurately nest this one. The fused pubis/ischium is like Dimorphodon. So is the inturned femoral head. The wing phalanges appear to be very gracile, but perhaps they are only partly exposed or misidentified.

MCSNB 3496 in situ and reconstructed from the very few parts here.

Figure 1. MCSNB 3496 in situ and reconstructed from the very few parts here.

Earlier we looked at other Triassic bits and pieces herehere and here.

References
Dalla Vecchia FM 2003. A review of the Triassic pterosaur record. Riv. Mus. civ. Sc. Nat. “E. Caffi” Bergamo 22:13-29.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

MFSN 19864 compared

MFSN 19864 is a Triassic pterosaur tail. Well… I imagine it’s a pterosaur tail. We don’t know the rest of this specimen. Compared to another basal pterosaur, MPUM 6009, MFSN 19864 is longer and more robust with what appears to be a tiny vane at the tail tip (Fig.1).

Figure 1. MFSN 19864 compared to MPUM 6009 another pterosaur with an attenuated tail without ossified reinforcements.

Figure 1. Click to enlarge. MFSN 19864 compared to MPUM 6009 another pterosaur with an attenuated tail without ossified reinforcements.

Dalla Vecchia (2003) noted that this tail had no ossified reinforcing “bundles” of pre- and post-zygopophyses, although such “bundles” were probably present and unossified. This appears to have been a larger pterosaur based on the size of the proximal caudals.

Earlier we looked at other Triassic bits and pieces herehere and here.

References
Dalla Vecchia FM 2003. A review of the Triassic pterosaur record. Riv. Mus. civ. Sc. Nat. “E. Caffi” Bergamo 22:13-29.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

MCSNB 2887 reconstructed

Another of Wild’s (1978) Triassic pterosaurs featured by Dalla Vecchia (2003) is MCSNB 2887. Here it is in situ and reconstructed (Fig.1) as a Triassic basal dimorphodontid. It nests with Preondactylus.

Figure 1. MCSNB 2887 in situ and reconstructed as a basal dimorphodontid/basal anurognathid.

Figure 1. MCSNB 2887 in situ and reconstructed as a basal dimorphodontid/basal anurognathid. It nests with Preondactylus.

Earlier we looked at other Triassic bits and pieces herehere and here.

References
Dalla Vecchia FM 2003. A review of the Triassic pterosaur record. Riv. Mus. civ. Sc. Nat. “E. Caffi” Bergamo 22:13-29.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

MCSNB 2886 reconstructed

MCSNB 2886 is another roadkill partial pterosaur from the Triassic (Wild 1978, Dalla Vecchia 2003). Here it is (Fig. 1) and here it is reconstructed (Fig. 2) as a basal dimorphodontid, probably close to Peteinosaurus.

Figure 1. MCSNB 2886 a Triassic pterosaur in situ

Figure 1. MCSNB 2886 a Triassic pterosaur in situ

Figure 2. MCSNB 2886 reconstructed as a basal dimorphodontid.

Figure 2. MCSNB 2886 reconstructed as a basal dimorphodontid close to Peteinosaurus.

Earlier we looked at other Triassic bits and pieces here and here.

References
Dalla Vecchia FM 2003. A review of the Triassic pterosaur record. Riv. Mus. civ. Sc. Nat. “E. Caffi” Bergamo 22:13-29.
Wild R 1978. Die Flugsaurier (Reptilia, Pterosauria) aus der Oberen Trias von Cene bei Bergamo, Italien. Bolletino della Societa Paleontologica Italiana 17(2): 176–256.

The manus of Poposaurus revised — again

 

Revised April 23, 2014 based on further study. 

 Figure 1. Poposaurus manus as originally restored and with digit 1 switched to 5.


Figure 1. Poposaurus manus as originally restored and with digit 1 switched to 5. Note the resemblance to dinosaur and basal croc hands in this basal archosaur.

The manus of basal archosaurs is very rare.
What few clues we have indicate that metatarsals 1-3 aligned distally and digit 5 is a vestige. Revising the manus of Poposaurus to that pattern is demonstrated here (Fig. 1).

References
Gauthier JA, Nesbitt SJ, Schachner ER, Bever GS and Joyce WG 2011. The bipedal stem crocodilian Poposaurus gracilis: inferring function in fossils and innovation in archosaur locomotion. Bulletin of the Peabody Museum of Natural History 52:107-126.
Mehl MG 1915. Poposaurus gracilis, a new reptile from the Triassic of Wyoming. Journal of Geology 23:516–522.

wiki/Poposaurus