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

Lewisuchus – new reconstruction

Be wary of the scale bars, but the new data on Lewisuchus by Bittencourt et al. (2014) brings a fresh look to old Romer (1972) data. Here (Fig. 1) the scale of the posterior skull and tibia are pure guesswork until I hear back from the authors on my query.

Figure 1. Lewisuchus from Romer 1972. Tibia and skull do no match published scale bars. Pelvis and femur are hypothetical. Skull above left per Romer. Below left per Bittencourt et al.

Figure 1. Lewisuchus from Romer 1972. Tibia and skull do no match published scale bars. Pelvis and femur are hypothetical. Skull above left per Romer. Below left per Bittencourt et al.

Unfortunately the authors did not include basal archosaurs (Decuriasuchus and Pseudhesperosuchus) in their analysis, but restricted their taxon list to dinos and pre-dinos. Not surprisingly, Lewisuchus nested with Marasuchus. The large reptile tree nests Lewisuchus with Pseudhesperosuchus.

Lewisuchus is the reason why basal crocs and dinos were bipeds. It nests at the base of all archosaurs. The palate is a close match to that of the basal bipedal croc Scleromochlus.

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
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

wiki/Lewisuchus

The Nesbitt 2011 Tree – Mixes and Matches

A few years ago Sterling Nesbitt (2011) recovered a tree of the Archosauria (Fig. 1) using several dozen taxa and several hundred traits. This is laudable. Unfortunately, Nesbitt’s inclusion set did not follow a larger gamut analysis (like the large reptile tree) to determine which taxa to include and which to exclude. So Nesbitt’s tree included several taxa that were not at all related to Archosauria and their ancestors, according to the larger gamut results. These outliers included Mesosuchus and pterosaurs (both nesting on the new Lepidosauromorpha branch, far from the archosaurs) and Vancleavea (a thalattosaur).

Figure 1. Click to enlarge. On the left is the Nesbitt (2011) tree. On the right is a subset of taxa from the large reptile tree. Pink double arrows, and there are lots of them, are pretty good matches. Green double arrows are mismatches.

Figure 1. Click to enlarge. On the left is the Nesbitt (2011) tree. On the right is a subset of taxa from the large reptile tree. Pink double arrows, and there are lots of them, are pretty good matches. Green double arrows are mismatches. The lack of resolution in the Peters Partial Tree reflects the lack of real relationships among those many “by default” nested sisters. Eoraptor was inadvertently  pruned from the Peters tree, but nested with Saturnalia.

In an attempt at testing Nesbitt’s tree, I included as many of the same taxa, massively pruning the large reptile tree to do so. There are also several taxa in the Nesbitt tree that are not found in the large reptile tree, chiefly from the higher crocs, the higher dinos, the partial rauisuchians and a few odds and ends known from very incomplete material. The results (Fig. 1) largely echoed the Nesbitt tree, but several mismatches also arose.

Mismatch: Pterosaurs
Among the tree mismatches are pterosaurs, which nest close to dinosaurs in the Nesbitt tree but nest with Vancleavea in the large reptile tree after pruning the real sisters of pterosaurs among the lepidosaur / tritosaur / fenestrasaurs.

Lagerpeton
Lagerpeton is at the base of the dinosaurs in the Nesbitt tree, but nests with Tropidosuchus in the Peters tree. Lagerpeton has many ankle traits in common with dinos, which were treated in the Peters tree, but emphasized in the Nesbitt tree.

Lewisuchus
Lewisuchus nests at the base of the Silesauridae, the outgroup to the Dinosauria, in the Nesbitt tree, but at the base of the Archosauria in the Peters tree. Nesbitt excluded the Lewisuchus sisters Pseudhesperosuchus and Decuriasuchus found by the Peters tree.

Turfanosuchus and Gracilisuchus
These two nest close to the Ornithosuchia (basal Pseudosuchia) in the Nesbitt tree, but at the base of the Archosauria/Crocodylomorpha in the Peters tree. Not sure why these two nest so far from the basal crocs in the Nesbitt tree, but the lack of the basal archosaurs and Gracilisuchus sister taxa, Scleromochlus SaltopusPseudhesperosuchus and Decuriasuchus may have something to do with it.

Poposaurs
Nesbitt nests poposaurs (those “dinosaur-like pseudosuchians”) with rauisuchians. Peters nests them with dinosaurs (despite the calcaneum tuber) and includes in their number Sacisaurus and Silesaurus. Nesbitt, like Mortimer, nests beaked Sacisaurus and beaked ornithischian dinosaurs like Lesothosaurus, and other plant eaters, like Saturnalia, basal to non-beaked theropod dinosaurs, which appears to be due to the creation of an upside-down topology (correct but inverted) and far from Marasuchus, another theropod, according to the large reptile tree.

The differences between the two trees arise from the choice of taxa and characters.
The Peters tree uses many more taxa. This allows pterosaurs, Mesosuchus and Vancleavea to nest more parsimoniously elsewhere and to separate parasuchians, chanaresuchians (including Lagerpeton) to separate from the rest. Nesbitt did not employ any younginids or choristoderes, which nest at the base of the Archosauriformes.

The Nesbitt tree uses more and different characters, many too small or hidden to be observed in a typical reconstruction (though they may and are still important!), which is the prime source for the Peters characters.

In the Peters tree all taxa share a large number of traits. In other words, very few traits separate sisters. They could have evolved from one another.

The Nesbitt tree ignores the strange bedfellows it produces in which sister taxa share very few traits, but nest by default, as some do in the pruned Peters tree.

Most sisters in the Nesbitt tree are also sisters in the Peters tree, providing confirmation for those relationships in both studies.

The Peters tree finds that enough taxa are known to produce a gradual accumulation of traits in derived taxa and a gradual spectrum of morphologies.

The Nesbit tree assumes (or hopes) that there are large numbers of taxa that will someday fill out and blend in the disparate morphologies among putative sisters. In many cases, these taxa are indeed known, just not used by Nesbitt.

There are also some irreversible reversals in the Nesbitt tree that are absent in the Peters tree when you get down to details. More plesiomorphic taxa can be found in basal positions in the complete Peters tree (the large reptile tree), like the many younginids missing from the base of the Nesbitt tree.

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.

Late News Item:
Nesbitt (2011) nested Eoraptor with theropods. The large reptile tree nested Eoraptor with basal phytodinosaurs like Pampadromaeus and Panphagia. Today’s paper by Sereno et al. (2013) confirms the large reptile tree nesting.

What is Faxinalipterus? (hint: not a pterosaur…)

Kellner et al. 2022 updates this specimen,
splitting it into two genera. Click here for that update.

Faxinalipterus minima (Bonaparte 2010) has been described from bits and pieces of a sparrow-sized archosaur. The holotype consists of short robust arm bones and much longer leg bones. A displaced maxilla with a large antorbital fenestra and narrow fossa is also referred to the specimen.

Wikipedia reports, “The describers have assigned Faxinalipterus to the Pterosauria, based on its long hollow limbs and saddle-shaped upper joint of the relatively short and robust humerus, suitable to perform a wing stroke. They see it as perhaps the oldest pterosaur known, as it possibly predates European finds from the Norian. That the possible age difference cannot be large, they see as an indication of rapid evolution in early pterosaurs. Because the Caturrita Formation consists of terrestrial sandstones, that evolution would have had its origins in a terrestrial, not coastal, habitat. They also concluded Faxinalipterus is the most basal known pterosaur, basal features including a lack of fusion between tibia and fibula, a thin radius and a coracoid that has not fused to the scapula. However, Alexander Kellner has suggested Faxinalipterus might be not be a pterosaur but a basal member of the Pterosauromorpha instead or, if the lack of fusion between tibia and fibula is plesiomorphic, even a sister taxon of the Ornithodira.”

Faxinalipterus matched to Scleromochlus. The former is more primitive, like Gracilisuchus, in having shorter hind limbs and more robust fore limbs. The maxilla with fenestra and fossa, plus the teeth, are a good match.

Figure 1. Click to enlarge. Faxinalipterus matched to Scleromochlus. The former is more primitive, like Gracilisuchus, in having shorter hind limbs and more robust fore limbs. The maxilla with fenestra and fossa, plus the teeth, are a good match. Faxinalipterus was not phylogenetically analyzed, but I’m not sure what other Triassic taxon could be closer.

This is going to get some people excited, others not
The maxilla assigned to Faxinalipterus (and I don’t doubt the assignment) has a large squarish antorbital fenestra surrounded by a narrow fossa. No pterosaur has a fossa. Basal pterosaurs always have an angled maxillary ascending process. Basal pterosaurs also have a much more slender fibula. And there are several other mismatches despite the few bones representing the animal. The putative coracoid is more likely a pubis or ischium.

The best match I found (not via phylogenetic analysis) is with Scleromochlus (Fig. 1) a basal bipedal crocodylomorph. Virtually every aspect of Faxinalipterus seems to be a good match, including chronological age and overall size, other than relative limb length. Faxinalipterus is just more primitive in having shorter hind limbs and more robust front limbs. Check out the distal tibia and fibula. A close match to bipedal crocs. Nothing like pterosaurs.

So, for those who like to match Scleromochlus with pterosaurs in the Pterosauromorpha and the Ornithodira, you now have another taxon that doesn’t look like a pterosaur!

Since every discovery can be discovered only once
it’s only human nature that a paleontologist finding a partial skeleton would jump on the most exciting possibility, like “the most primitive known pterosaur.” Unfortunately you also have to play by the rules and compare the new specimen to every other taxon sharing a majority of its traits (even if incomplete) and you have to go with the recovered results.

On the other hand…
Faxinalipterus
 does offer insight into the origin of Scleromochlus and basal crocs, and by extension, basal archosaurs. I’d like to see thefolks toying with Lagerpeton (a convergent biped close to Tropidosuchus) drop it in favor of these two croc bipeds at the base of the archosaur family trees.

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
Bonaparte JF, Schultz CL and Soares MB 2010. Pterosauria from the Late Triassic of southern Brazil. In S. Bandyopadhyay (ed.), New Aspects of Mesozoic Biodiversity, Lecture Notes in Earth Sciences 132:63-71.

Wiki/Faxinalipterus