Finding two more sacral impressions in Lagosuchus

With fragile, damaged fossils like
Lagosuchus talampayensis (Romer 1971; PULR 09; Late Triassic; Fig. 1), sometimes the fossilized bones alone tell only part of the story.

Figure 1. Lagosuchus in situ preserves impressions of two additional sacrals. Phylogenetic bracketing between taxa with four sacrals suggesting looking for these impressions.

Figure 1. Lagosuchus in situ from Agnolin and Ezcurra 2019 preserves impressions of two additional sacrals. The length of the two ilia alone strongly suggest the presence of four sacrals or two very reinforced sacrals, a la Herrerasaurus, which was not the case here.

When bones disappear through taphonomy,
sometimes they leave impressions of their past presence providing workers subtle data that can be used for scoring in phylogenetic analysis, unless overlooked.

Figure 1. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared. The radius and ulna are longer in marasuchus. The hind limbs are more robust in Lagosuchus. The length of the torso in Lagosuchus is based on the insitu placement of the pectoral girdle and forelimb, which may have drifted during taphonomy.

Figure 2. Marasuchus lilloensis (above) and Lagosuchus talampayensis (below) compared. The radius and ulna are longer in Marasuchus. The hind limbs are more robust in Lagosuchus. The length of the torso in Lagosuchus is based on the insitu placement of the pectoral girdle and forelimb, which may have drifted during taphonomy.

These two bipedal archosaur taxa
(Fig. 2) are similar, but the large reptile tree recovered them in distinct clades. The LRT nests Marasuchus with basal theropod Dinosaurs. Lagosuchus nests with Saltopus and other basal bipedal crocodylomorphs. These novel nestings were recovered due to including taxa overlooked by traditional studies.

Lagosuchus sister taxa in the LRT,
also have four sacrals. That fact provided the impetus for the present study searching for additional sacral impressions in high resolution photos from Agnolin and Ezcurra 2019.

As mentioned earlier,
a review of taxa tested within the Crocodylomorpha, like this one (Fig. 1), has resulted in several revisions to the croc subset of the LRT. The work is exciting, necessary and in progress. Results will be announced when they are known.


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. Gaia15: 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

 

6 thoughts on “Finding two more sacral impressions in Lagosuchus

  1. It’s pretty obvious that the bulk of the “left ilium” you identify is just a shadow if you look closely at the higher-resolution image in the source. And it makes much more sense for the “right” ilium to be a right pubis considering its length and how its posterior portion maintains a proximity to the femoral head. So the claim that iliac length justifies additional sacrals is unfounded.

    Furthermore, the sacral rib impressions you claim to identify clearly do not exist once you look more closely. Figure 2B-C in Agnolin & Ezcurra (2019) shows the two posterior dorsals (which you interpret as anterior sacrals) in detail on the left. The matrix is completely irregular, having been cleared away by preparators and preserved in glue. Prepared Chanares fossils do not have impressions, as is clear to anyone with experience in fossil preparation.

    Finally, I noticed this statement: “That fact provided the impetus for the present study searching for additional sacral impressions”. Expecting four sacrals in Lagosuchus made you look for any potential textural change that justified your expectations, without considering the specimen’s preservational context or preparational history. Pareidolia strikes again.

    • You wrote: “Prepared Chanares fossils do not have impressions, as is clear to anyone with experience in fossil preparation.” That seems odd and unique. The number of preparators dealing with Chanares fossils is a pretty small number. I did not realize you were in that clade.

      Here today…gone tomorrow. vs. Never were here…Don’t even bother. That doesn’t make sense.

      In science it’s always better to show, rather than tell. Repeat the experiment, including the phylogenetic analysis that recovered a two-sacral taxon in the middle of four-sacral taxa. Or perhaps your Lagosuchus will nest with other two sacral taxa? Which ones?

      None of your arguments make sense at present, but I’m still eager to learn. The ball is back in your court.

      • By ‘sacral vertebrae’ you mean vertebrae that lies between the ilia or vertebrae that contact the ilia?

        For instance, in Saturnalia, the holotype does not have a dorsosacral vertebra, it has a caudosacral vertebra instead (Langer, 2003). On the other hand, one of the paratypes preserved a dorsosacral vertebra, but doest not present a caudosacral (Marsola et al. 2019).. Although both specimens have at least four vertebrae between the space of the ilia and both have the same number of sacrals (3, two primordials + one additional), these are not the same vertebrae composing the sacrum.

        Some other taxa also presenta degree of variation, so the two sacrals preserved in PULR 09 may not be the only ones it would have in life, though it certainly would not have more than three or four vertebrae between its ilia. Furthermore, even when I was looking at other photographs of PULR 09 I failed in recognize the existence of the impressions you colored in Fig. 1 (I agree with NP in this). Anyway, I’m not saying it is impossible for Lagosuchus to have more than two ‘true’ sacrals, but at least in PULR 09 (and PVL 3871), this is not the case. Since there are few specimens, we have no idea of how much this character varies (whether by ontogeny, individual variation, sexual dimorphism, etc.). There is even disagreement if the dorsosacral/caudosacral I mentioned before are considered ‘true’ sacrals, since usually they barely touch the ilia.

        Indeed, if there were impressions of the bones, they would have been long ago destroyed during preparation, so what we see are just irregularities on the matrix and nothing more.

      • Maurício, this is what I go by: Almost everywhere within Tetrapoda the sacral ilia not only lie between the ilia, they also contact the ilia. I am aware of some exceptions. I score simply: 1) 0 or 1 sacral; 2) 2 sacrals; 3) 3-4 sacrals; 4) 5 or more sacrals. Surprisingly, given their geographic distance from each other, Lagosuchus and Saltopus are similar in size, similar in morphology and similar in taphonomy. I observe 4 sacrals in both. I see Rauhut and Hungerbühler in 2000 also found this hypotheses of relationships, connecting Lagosuchus and Saltopus.

  2. I think the close relationship between Saltopus and Lagosuchus is plausible, but I do not agree with the presence of 4 sacrals in Lagosuchus based on the specimen PULR 09. As I mentioned before, when examining the specimen through photos from different angles reveals that much of what you colored was really matrix. In Saltopus, the specimen is so poorly preserved that, in my view, it is difficult to be able to state the exact amount of ‘true’ sacrals (but I saw what you said about it using DGS).

    New and well-preserved specimens are welcome to elucidate this issue, but for the moment regarding Saltopus there seems to be a lot of speculation about dubious materials or simply wrong observations in the case of Lagosuchus.

    • A good observation! What I colored was indeed matrix. As mentioned in the text, I observed plausible impressions in the matrix. Nothing more substantial than that. If I am mistaken about Lagosuchus and Saltopus on sacral number, that one score change will not affect the nesting of both taxa with Scleromochlus, a similar taxon with four sacrals.

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