Lagosuchus enters the LRT with Saltopus

A small partial skeleton named
Lagosuchus talampayensis 
(Romer 1971; PULR 09; Late Triassic; Fig. 1) was found on the same slab as the more completely known Gracilisuchus (Romer 1971, Fig. 1). Oddly and coincidentally, if not for the few duplicated posterior dorsals and sacrals, the two incomplete specimens could have created one more or less complete, specimen.

Figure 1. Gracilisuchus and its slab mate, Lagosuchus. Note the very few bones both specimens preserve in common are at the base of the dorsal column.

Figure 1. Gracilisuchus and its slab mate, Lagosuchus. Note the very few bones both specimens preserve in common are at the base of the dorsal column.

Back in Romer’s time in the 1970s,
Lagosuchus was considered congeneric with a second, more complete specimen before the two were separated by Sereno and Arcucci 1994. The later, more complete one was renamed Marasuchus lilloensis (Fig. 2). The two authors considered the less complete holotype of Lagosuchus to be undiagnosable.

Figure 2. Lagosuchus to scale with Marasuchus. Several proportions are different between the two.

Figure 2. Lagosuchus to scale with Marasuchus. Several proportions are different between the two.

According to Wikipedia
“Lagosuchus talampayensis is a genus of small avemetatarsalian archosaur from the Middle to Late Triassic period. It is generally thought to be closely related to dinosaurs, as a member of the Dinosauromorpha. Its fossils were found in the Chañares Formation of Argentina, the dating of which is uncertain; some sources date it to the Middle Triassic whilst others date it to the earliest Carnian.”

Just this year,
Agnolin and Ezcurra 2019 determined that Lagosuchus was indistinguishable from Marasuchus, returning to Romer’s original hypothesis of relationships.

Added to
the large reptile tree (LRT, 1623+ taxa) the nesting of Lagosuchus took place in three stages.

Stage one:
Testing all Archosauriformes (sans birds). Result: Due to the large number of skull-only and post-crania-only taxa, there was loss of resolution in the clade that recovered Lagosuchus: basal bipedal Crocodylomorpha. The rest of the tree topology was unaffected. Marasuchus, for instance, remained within the clade of least popular (most often overlooked) basal theropods.

Stage two: 
Further deletions, saving only Crocodylomorpha, Poposauria and basal Theropoda (including Marasuchus). Result: resolution was not improved in basal Crocodylomorpha due to the continued mixing of skull-only and skull-less taxa  In hindsight, this stage could have been skipped, but since every included taxon affects every other included taxon, this stage was deemed necessary.

Figure 4. Lagosuchus compared to Saltopus.

Figure 4. Lagosuchus compared to Saltopus. The lack of dorsal armor here links these two together. The larger number of laterally wider sacrals in Saltopus sets the apart.

Stage three:
Deletion of two skull-only basal crocodylomorph taxa: Yonghesuchus and the MCZ4116 specimen attributed to Gracilisuchus. Result: skull-less Lagosuchus nested with another skull-less crocodylomorph, Saltopus (Fig. 3), a slightly later taxon from Europe apparently overlooked in all prior Lagosuchus studies.

By the way,
the nesting of skull-only Yonghesuchus near the two skull-less taxa provides clues to the likely morphology of the missing skulls in Lagosuchus and Saltopus… and indicates a world-wide distribution for members of this clade.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Distinct from Saltopus,
Lagosuchus
 has only two laterally unexpanded sacrals and a shorter antebrachium.

Distinct from other related taxa,
the Lagosuchus and Saltopus share a lack of gastralia, a lack of dorsal scutes, a fibula greater than half the diameter of the tibia, a mesotarsal ankle joint without a calcanaeal heel and long feet (greater than half the tibia length) without compressed metatarsals. These relatively common traits are easy to overlook because they are found by convergence elsewhere, but are relatively rare at the base of the Crocodylomorpha. Everything comes back to a phylogenetic context. The LRT still works despite rampant convergence in the Tetrapoda.

After googling ‘Saltopusuchus, Lagosuchus
I would have been pleased to cite Rauhut and Hungerbühler 2000 who thought these two were closely related 20 years ago, according to the Saltopus article in Wikipedia. However, shortly after writing this line of text, I downloaded and read their paper. Rauhut and Hungerbühler report, “Characters like elongated hindlimbs, bipedality, mainly three digits used in locomotion, and cursorial habits are also present in primitive dinosauriformes like Marasuchus SERENO & ARCUCCI, 1994. Therefore, S. elginensis can only be treated as a probable dinosauriform nomen dubium.” So, they provided no analysis and they cited the wrong specimen. The holotype of Lagosuchus was not mentioned in the paper. I wonder why the Wiki writer got it wrong?

Should we give credit
to the mistaken writer of the wiki/Saltopus article? Do happy accidents count? If you know of a valid previous citation that links the holotype of Lagosuchus with Saltopus, please let me know so I can advertise that citation.

Otherwise,
the linking of Lagosuchus with Saltopus appears to be a novel hypothesis of interrelationships. This could be due to the fact that the basal bipedal crocs have been largely ignored as a clade or that both specimens have been traditionally held in low regard. Then again, the relationships of basal bipedal crocs to the clade Dinosauria has been overlooked and ignored (due to taxon exclusion) by other workers. Finally, too often, basal bipedal crocs in the LRT, like Gracilisuchus and Scleromochlus, have not been recognized as basal bipedal crocs by other workers. So, this turned out to be another case of a perfect storm meeting low hanging fruit.


References
Agnolin FL and  Ezcurra MD 2019.The validity of Lagosuchus talampayensis Romer, 1971 (Archosauria, Dinosauriformes), from the Late Triassic of Argentina. Breviora. 565 (1): 1–21.
Rauhut OMW and Hungerbühler A 2000. A review of European Triassic theropods. Gaia, 15: 75-88.
Romer AS 1971.
The Chañares (Argentina) Triassic reptile fauna. X. Two new but incompletely known long-limbed pseudosuchians. Breviora. 378: 1–10.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. XV. Further remains of the thecodonts Lagerpeton and Lagosuchus. Breviora. 394: 1–7.
Sereno PC and Arcucci AB 1994. Dinosaurian precursors from the Middle Triassic of Argentina: Marasuchus lilloensis, gen. nov. Journal of Vertebrate Paleontology. 14 (1): 53–73.

wiki/Saltopus
wiki/Lagosuchus

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