Lewisuchus and Pseudolagosuchus

One and the Same?
Recently Nesbitt et al. (2010) attempted to synonymize Lewisuchus admixtus (Romer 1972, Middle Triassic, 235 mya ~70 cm, Fig. 1) and Pseudolagosuchus major (Arcucci 1987, Middle Triassic, 235 mya ~70 cm, Fig. 2). Nesbitt et al. (2010) reported in their supplementary material, “Given that both Lewisuchus and Pseudolagosuchus share apomorphies with other silesaurids, are approximately the same size, and are from the same stratum and locality (Romer, 1972; Arcucci, 1987; Bonaparte, 1997), it is reasonable to suspect that these specimens belong to the same taxon.” They found, “When Lewisuchus and Pseudolagosuchus are placed in a phylogeny as separate taxa, all most parsimonious trees recover both taxa at the base of Silesauridae.” Then reported, “Unfortunately, this lack of overlapping material also means that the two taxa cannot be formally synonymized.”

Figure 1. Lewisuchus, a tiny predecessor to crocs and dinos (including birds).

Figure 1. Lewisuchus, a tiny predecessor to crocs and dinos (including birds).

Romer and Arcucci
Romer (1972) considered Lewisuchus a “thecodont,” but did not delve further.  Arcucci (1987) considered Pseudolagosuchus a basal dinosauromorph.

 Pseudolagosuchus.

Figure 2. Pseudolagosuchus. Gray areas are restored according to Silesaurus. The elongated and gracile posterior ribs are atypical of sister taxa. The ribs were bunched together in situ suggesting they were connected to vertebrae with shorter centra, as shown here, or they could have just drifted closer together during taphonomy.

Non-Overlapping Data
Both Lewisuchus and Pseudolagosuchus shared traits with Silesaurus and and the smaller silesaurid, Asilisaurus, according to Nesbitt et al. (2007) and their analysis (Fig. 3). They were nested as one taxon, which forced them together rather than testing them against other taxa.

Nothing in Common?
Just because Lewisuchus and Pseudolagosuchus share no preserved parts they can still be nested with other, more complete taxa. Here, in the large reptile tree, they don’t nest together.

The Large Study
In the large reptile study Lewisuchus nested with Decuriasuchus and  Pseudhesperosuchus,  two protoarchosaurs outside the Archosauria. Lewisuchus was much smaller than its sisters. This clade was basal to Turfanosuchus and the Archosauria with Trialestes at its base. This is several nodes away from Pseudolagosuchus which nested with Silesaurus nesting within the Paraornithischia within the Dinosauria.

Comparing the Nesbitt (2010) analysis

Figure 3. Comparing the Nesbitt (2010) analysis to a small subset of the large reptile tree, which pretty much follows the pattern of the large tree. See text for details and problems.

Matching the Nesbitt et al. (2012) Taxon List
Decuriasuchus and Pseudhesperosuchus were not included in Nesbitt et al. (2012). I attempted to match the Nesbitt et al. (2012) taxon list (Fig. 3 left) by culling the large reptile taxon list (Fig. 3 right). Five most parsimonious trees were recovered (probably because Pisanosaurus and Pseudhesperosuchus are so incomplete). Most recovered nestings matched the Nesbitt et al. (2012) tree, including nesting the pterosaur with Aetosaurus (ridiculous, isn’t it?). Lewisuchus nested closer to the crocodylomorph Dromicosuchus rather than the silesaurid Pseudolagosuchus, in the absence of Decuriasuchus and Pseudhesperosuchus.

Taxon Exclusion
Please compare the tree on the right in figure 3 to the large reptile tree to gauge the effects of taxon exclusion. Once again, the differences in tree recovery seem to hinge on taxon exclusion, which was minimized in the large reptile tree because it is many times larger than the Nesbitt et al. (2010) tree.

So what happened?
I took data from Romer (1972) while Nesbitt et al. (2012) had access to the specimens. On the other hand, Nesbitt et al. (2012) clearly did not have a large enough taxon list for the gamut of taxa they were testing. I’ll have to see the specimen and Nesbitt et al. (2012) will need to add more taxa before we can come a better understanding here.

Was Asilisaurus the Oldest Known Dinosaur Relative?
Could be. But Lotosaurus may be just as old.

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
Arcucci AB 1987. Un nuevo Lagosuchidae (Thecodontia-Pseudosuchia) de la fauna de Los Chañares (Edad Reptil Chañarense, Triasico Medio), La Rioja, Argentina. .Ameghiniana 24(1-2):89-94.
Nesbitt SJ, Sidor CA, Irmis RB, Angielczyk KD, Smith RMH and Tsuji LMA 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464 (7285):95-98. doi:10.1038/nature08718PMID 20203608.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna; XIV, Lewisuchus admixtus, gen. et sp. nov., a further thecodont from the Chañares beds. Breviora 390:1-13.

wiki/Lewisuchus
wiki/Pseudolagosuchus

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