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

 

Lepidosaur bipedality and pelvis morphology: Grinham and Norman 2019

Grinham and Norman 2019
brings us a new look at 34 lepidosaur pelves with an emphasis on trends associated with bipedal locomotion. The authors illustrated 11 pelves (Fig. 1, white and yellow areas).
Figure 1. On the left, lepidosaur pelves from Grinham and Norman 2019, reordered phylogenetically here. On the right several tritosaur pelves and prepubes, most of which strongly demonstrate bipedal traits (elongate anterior ilium, increased sacral number). Yellow boxes indicate facultatively bipedal extant lepidosaurs.

Figure 1. On the left, lepidosaur pelves from Grinham and Norman 2019, reordered phylogenetically here. On the right several tritosaur pelves and prepubes, most of which strongly demonstrate bipedal traits (elongate anterior ilium, increased sacral number). Yellow boxes indicate facultatively bipedal extant lepidosaurs.

From the Grinham and Norman abstract:
“Facultative bipedality is regarded as an enigmatic middle ground in the evolution of obligate bipedality and is associated with high mechanical demands in extant lepidosaurs. Traits linked with this phenomenon are largely associated with the caudal end of the animal: hindlimbs and tail. The articulation of the pelvis with both of these structures suggests a morphofunctional role in the use of a facultative locomotor mode. Using a three-dimensional geometric morphometric approach, we examine the pelvic osteology and associated functional implications for 34 species of extant lepidosaur. Anatomical trends associated with the use of a bipedal locomotor mode and substrate preferences are correlated and functionally interpreted based on musculoskeletal descriptions. Changes in pelvic osteology associated with a facultatively bipedal locomotor mode are similar to those observed in species preferring arboreal substrates, indicating shared functionality between these ecologies.”
Unfortunately, Grinham and Norman omitted
tritosaur lepidosaurs from their study. In the Triassic many of them became bipeds and among these, pterosaurs achieved bipedalism supported with four, five and more sacral vertebrae between horizontally elongate ilia, convergent with dinosaurs. The addition of the prepubis virtually extended the anchorage for the puboischial muscles. After achieving flight, beach-combing pterosaurs reverted to a quadrupedal configuration with finger 3 pointing posteriorly. Giant Korean bipedal pterosaur tracks are best matched to large dsungaripterid/tapejarid clade taxa.
Unfortunately, Grinham and Norman reported,
“A recently published molecular-based time-calibrated phylogeny for Squamata was pared down to match the species in our dataset.” Their genomic cladogram bears little to no resemblance to the large reptile tree (LRT, 1635+ taxa), which tests traits, not genes. Once again, genes produce false positives. 
The authors’ principal component analysis of the pelvis failed 
to isolate bipedal lepidosaurs from the rest. Grinham and Norman reported, “The shape of the pelvis in facultatively bipedal extant lepidosaurs falls within the overall morphospace of lepidosaurs generally.” This is also visible in their illustrated pelves (Fig. 1). They also reported, However, it is generally found in a very concentrated area of that morphospace.” And Conclusions can be drawn regarding pelvic morphology and substrate use, although not with the same clarity as for locomotor mode.”
Grinham and Norman 2019 conclude,
“we have used 3D landmark-based geometric morphometrics to demonstrate that the overall morphospace for the lepidosaur pelvis is broad and wide-ranging. Within this overall morphospace, a small region is occupied by facultative bipeds. The vast majority of this smaller morphospace overlaps that occupied by species that show a preference for arboreal habitats. Pelvic morphological adaptations relevant for living in an arboreal environment are similar to those necessary to facilitate facultative bipedality.”
That’s interesting with regard to
the arboreal abilities of volant basal bipedal pterosaurs and their ancestors. Maybe next time Grinham and Norman will expand their study to include tritosaur lepidosaurs.

References
Grinham LR and Norman DB 2019. 
The pelvis as an anatomical indicator for facultative bipedality and substrate use in lepidosaurs. Biological Journal of the Linnean Society, blz190 (advance online publication) doi: https://doi.org/10.1093/biolinnean/blz190
https://academic.oup.com/biolinnean/advance-article-abstract/doi/10.1093/biolinnean/blz190/5687877Â
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Snyder RC 1954. The anatomy and function of the pelvic girdle and hind limb in lizard locomotion. American Journal of Anatomy 95:1-46.

Pterosaur prebubis

 

Sacral Number and Bipedalism

Do more sacrals mean a bipedal stance?
Short answer: yes and no. Sacral vertebrae connect the backbone to the pelvis. Occasionally other vertebrae lie between the ilia without connecting to them. These are not real sacrals. Occasionally other vertebrae do not lie between the ilia, yet converge on them. These are real sacrals. Sometimes there is a relationship between ilial length and number of sacrals. Sometimes there isn’t.

Amphibians
Generally in basal tetrapods (pre-reptiles/amphibians), there is only one sacral. However, these basal tetrapod/pre-reptiles appear to have two sacrals: Batropetes, Microrater and Cacops. Let me know if this is wrong.

Reptiles – The New Lepidosauromorpha
Most reptiles have two sacrals or more, but two taxa in this study revert to one sacral: Milleretta and Eunotosaurus. Others in the Milleretta clade, like Acleistorhinus and the bolosaurids, may also have had one sacral, but they are known only from skulls.

Eunotosaurus

Figure 1. Eunotosaurus, a reptile with only one sacral vertebrae. Click to learn more.

Casea is the most basal taxon with three sacrals. Diadectes through Owenetta (sans Stephanospondylus + turtles) also have three or four sacrals. Noe bipeds here.

Saurosternon reverses this pattern back to two sacrals. It was much smaller than its predecessors and likely arboreal.

Jesairosaurus, Hypuronector and drepanosaurs have more than two sacrals. The fenestrasaurs continue this pattern. Cosesaurus has four. Sharovipteryx has nine. Longisquama has five. Pterosaurs have more than four. I have argued (Peters 2000) that the fenestrasaurs were increasingly bipedal with occasionally bipedal Rotodactylus tracks matching Cosesaurus feet. Beachcombing pterosaurs reverted to quadrupedalism, especially while feeding. There is no doubt that Sharovipteryx was a biped, although all that Google shows on this subject for the first few pages have been originated by me! No one else wants to concur? This is particularly strange and bears the mark of widespread bias or blindness. The drepanosaurs were likely arboreal lizards, some of which probably adopted a tripedal stance assisted by a prehensile tail. Jesairosaurus is a puzzle in this department.

Sharovipteryx mirabilis

Figure 2. Sharovipteryx mirabilis in various views. Not sure why this obvious biped has not been more widely acknowledged. Click to learn more.

Snakes from both branches revert to one sacral or none.

Reptiles – The New Archosauromorpha
Sphenacodont pelycosaurs have more than two sacrals as do therapsids. No bipeds here.  This has been attributed to increasing size and increasing leg length raising the body further off the substrate (if only slightly). The initiation of elongated dorsal spines may have contributed to this support between the pecs and pelvis.

The clade of Placodontia + Sauropterygia have more than two sacrals with convergence upon a very small ilium. This is the clade, mentioned above, with the converging sacrals. No bipeds here.

Pachypleurosaurus had more than two sacrals all converging on a tiny ilium.

Figure 3. Pachypleurosaurus had more than two sacrals all converging on a tiny ilium. Click to learn more.

Doswellia + Choristodera have more than two sacrals. So does the new Proterochampsa. No bipeds here. The biped Lagerpeton nests near here, but it had only two sacrals.

Ornithosuchids have more than two sacrals. So do a sprinkling of rauisuchians, including Vjushkovia, Smok and Postosuchus. I’m guessing on the number of sacrals in Vjushkovia based on a lateral view (which can be misleading) and would appreciate any better data. The other three look to be tentative bipeds.

Vjushkovia.

Figure 4. Vjushkovia. Not sure if this taxon had two or more sacrals, but the number of vertebrae between the pelvis indicate the possibility of the latter. Click to learn more.

Among archosaurs on the basal croc line, Pseudhesperosuchus has three or four sacrals. So do Scleromochlus + Saltopus. So do Terrestrisuchus + Saltoposuchus. All were likely bipeds. The number of sacrals decreased to two in derived quadrupedal crocs, but the quadrupedal Protosuchus may have retained three (covered by scutes).

Only two sacrals were reported for the theropod dinosaur Tawa, but three vertebrae were pressed between the ilia. Coelophysis had five or more sacrals highly pressed between the ilia. Herrerasaurus had only two sacrals, but they were greatly expanded as they attached to the broad ilia. Two other verts, one fore and one aft, had tiny sacrals between the ilia. These theropods were bipeds.

The phytodinosaurs (poposaurs + sauropodomorphs) all had three or four sacrals. Basal forms were bipeds. A few, like Lotosaurus and Shuvosaurus, reverted to quadrupedal locomotion. The other phytos – the Ornithischia had five or more and these likewise began as bipeds but most reverted to quadrupedalism, some of them quite early on.

Now, how do they stack up with regard to bipedalism?
As you can see from the above list, having more than two sacrals is a convergent trait among many clades, some of them completely aquatic. Some of this increase in sacral number was due to nothing more than increasing mass. Even so, an increased number of sacrals did develop among tentative and facultative bipeds at the fulcrum of the lever developing at the acetabulum, as a response to the stresses developing there when the forelimbs are raised. In later dinosaurs and pterosaurs the reversion to quadrupedal locomotion did not diminish the number of sacrals as members of both clades occasionally rose to hind limbs for feeding, fighting or takeoff and landing.

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.