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

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.

Wiki writes, “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

2 thoughts on “What is Faxinalipterus? (hint: not a pterosaur…)

  1. Rhaeticodactylus — and possibly even Dimorphodon — has an antorbital fossa. You might disagree with this, you might quibble about the exact aspect of these from photographs, but the structures are there, even if slight. You may then TEST the hypothesis by examining the structure, to determine the feature is or isn’t a depressed region around the antorbital fenestra, but note: The mere presence of an antorbital fenestra refers to a paranasal sinus system, and thus to the diverticular branches, including the one that expands onto the lateral surface of the maxilla, jugal, and sometimes the nasal. If the structure appears on the maxilla, you can look for confirmation on other bones of the skull to find expression of diverticulae on the outside of the skull, which can include sharp rims, a smooth, unornamented surface, and so forth.

    However, regardless, I find the allocation to a Scleromochlus taylori-like animal amazing, given that it is generally found as a near-pterosaur, or basal dinosauromoprh near the split with pterosaurs (in the “restricted” Ornithodira). But certain features exist that firmly place this taxon with pterosaurs, including the elevated femoral caput with elongated neck, spherical caput, and the high, offset deltopectoral crest. Indeed, you don’t mention these contradictory features at all, leading a reader to assume your analysis might be astute if there weren’t familiar with pterosaurs already.

  2. Re; Raeticodactylus and Dimorphodon, don’t buy into the BS of Nesbitt and Hone (2010). The purported AOF on Dimorphodon bears no resemblance to the purported one on Rhaeticodactylus and no resemblance to any archosaur, like Faxinalipeterus. They were reaching by misidentifying a structural triangle only at the base of the ascending process. In Raeticodactylus you’re looking at the transverse width of the structure, typically buried in matrix. I’ll do a blog on this, thanks for the idea.

    With regard to Scleromochlus, don’t the tiny fingers, flat head, lack of a fifth toe, descending chevrons and a thousand other croc traits sway you? I’ve been living with four fenestrasaurs for 12 years. They’re better pterosaur sisters than anything you can propose from the archosaurs, hands down. If you like the deltopectoral crest on Faxinalipterus and Scleromochlus, you’ll love the one on Decuriasuchus. Also check out its femur.

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