Pisanosaurus: dinosaur or silesaurid?

A new paper by Agnolin and Rozadilla 2017
includes new photographs of the holotype that shed new light on Pisanosaurus (Casamiquela 1967, Bonaparte 1976; Late Triassic). This taxon was previously known in the literature chiefly (not exclusively) from drawings. The large reptile tree (LRT, 1043 taxa) nested Pisanosaurus with Haya as a basal ornithischian, confirming prior assessments. Now Agnolin and Rozadilla provide evidence for a Silesaurus affinity among the Poposauridae. Echoing others, they report, “the poor preservation of the specimen is the largest difficulty to overcome when interpreting its morphology. Its phylogenetic position within ornithischians is problematic.”

So, with the new evidence,
let’s test and nest Pisanosaurus 2017! (There are so few traits that can be scored for Pisanosaurus, that the rest of the discussion might seem like I’m pulling a Larry Martin. That happens sometimes, but I’m trying to report results from the LRT.

Before we start…
with present data, shifting Pisanosaurus to Silesaurus in the LRT adds 24 steps. Moreover, Agnolin and Rozadilla did not mention the proximal relatives of Pisanosaurus in the LRT:  Haya, Daemonosaurus, Chilesaurus, Scelidosaurus and Emausaurus. This may be the key to their novel results: taxon exclusion… once again. 

Some general notes to start with:

  1. Silesaurus and other poposaurs have a metatarsus no longer than the longest digit. The same hold true for many basal phytodinosaurs, but Pisanosaurus has a longer metatarsus, like its sister in the LRT, Haya.
  2. The photo of the pelvis does little to clarify any issues. It is a broken up mess (Fig. 2) with, what appear to be smaller pelvis bones (greens)  and several sacral bones (blues) stirred up in a conglomeration. Not much matches the published drawings. And my earlier imagination describing a rotated pubis based on simple published drawings did not pan out.
  3. The anterior dentary appears to be missing a predentary bone, a trait common to the clade Ornithischia, but something like it also appears in Silesaurus.
  4. Pisanoaurus comes from South America, home of most of the other basalmost Triassic phytodinosaurs. Popposaurids, all except Sacisaurus, come from somewhere else on the globe. Haya, the LRT sister to Pisanosaurus, comes from China, but it is Late Cretaceous in age.
  5. Agnolin and Rozadilla consider Silesaurus part of a clade “that is currently recognized as the sister group to Dinosauria.” The LRT recovers Crocodylomorpha closer to Dinosauria and Silesaurus nests within the next proximal outgroup, Poposauridae.
  6. Agnolin and Rozadilla report, “because Pisanosaurus is a unique and very valuable specimen, it is not currently possible to [CT] scan it.”
  7. Authors have not agreed whether the pelvis, represented by fragments of bones and bone impressions in rock. is preserved in medial or lateral view. Agnolin and Rozadilla report, “the sacrum is articulated and preserved in life position with respect to the pelvis.”
Figure 1. The Pisanosaurus pelvis here flipped right to left along with drawings and reconstructions by Agnolín and Rozadilla, plus DGS colors applied to what I can see here. Nothing is clear, but it seems like the pelvic elements are smaller that published and that several sacral vertebrate are sprinkled in this mass. Perhaps a CT scan would be helpful here. Blue = vertebrae. Green = pelvi elements.

Figure 1. The Pisanosaurus pelvis here flipped right to left along with drawings and reconstructions by Agnolín and Rozadilla, plus DGS colors applied to what I can see here. Other than the sacral vertebrate on top, not much is clear, but it seems like the pelvic elements are smaller that published and that several sacral vertebrate are sprinkled in this mass. Perhaps a CT scan would be helpful here. Blue = vertebrae. Green = pelvi elements.

Agnolin and Rozadilla provided an emended diagnosis.
Pisanosaurus is a basal dinosaurifordiagnosable by the following autapomorphies:

  1. “central teeth bilobate in occlusal view, showing well-developed mesial and distal grooves;
  2. distal end of the tibia anteroposteriorly longer than transversely wide;
  3. bilobate astragalus in distal view;
  4. ascending process of the astragalus being subquadrangular and robust in lateral view;
  5. intense transversal compression of the calcaneum.”
Figure 3. Skull of Haya and restored skull of Pisanosaurus compared. The resemblance of preserved elements is apparent here. In both cases the mandibular fenestra is filled in. The other holes in the Pisanosaurus mandible are artifacts of taphonomy. Pisanosaurus data from Irmis et al. 2007b.

Figure 2. Skull of Haya and restored skull of Pisanosaurus compared. The resemblance of preserved elements is apparent here. In both cases the mandibular fenestra is filled in. The other holes in the Pisanosaurus mandible are artifacts of taphonomy. Pisanosaurus data from Irmis et al. 2007b.

Other factors of interest:

  1. The number of tooth positions (15) in Pisanosaurus matches both silesaurids and pertinent ornithischians.
  2. “Central teeth are bilobate in occlusal view, and show well-developed mesial and distal grooves, a condition unknown in other herbivorous taxa and a trait that may be an autapomorphy of Pisanosaurus.” Not sure if the teeth in Haya are the same, but they look similar in lateral view (Fig. 2). Neither have denticles. Silesaurid teeth are leaf-shaped.
  3. “the teeth do not form a palisade or continuous masticatory surface as advocated by some authors.” As in Haya.
  4. “Pisanosaurus is similar to saurischians and basal dinosauriforms in having overlapping proximal metatarsals, differing from the non-overlapping condition in ornithischians.” Except Haya.
Figure 1. Haya in lateral view.

Figure 3. Haya in lateral view. Note the dorsal laminae, similar to those in Pisanosaurus.

Agnolin and Rozadilla describe the dorsal vertebrae
as having a strong and complex system of laminae. Haya (Fig. 3).has similar laminae. Poposauridae do not.

Silesaurus

Figure 4 Silesaurus as a biped and occasional quadruped. Note the squareish cervicals, unlike the parallelograms in figure 5.

Agnolin and Rozadilla considered the vertebrae
(Fig. 5) very different from the cervical vertebrae described for basal dinosauriforms and ornithischians. But they did not look at Haya, which has similar cervicals 1 and 2 (Fig. 5). They considered the cervicals ‘indistinguishable from Sacisaurus cervicals, but Langer and Ferigolo 2013, did not refer the cervical to Sacisaurus due to its relatively large size. Concluding Agnolin and Rozadilla considered these verts to be on uncertain position.

Figure 4. Pisanosaurus cervical vertebrae in left lateral view (not right as published) matches cervical vertebrae 1 and 2 in Haya.

Figure 5. Pisanosaurus cervical vertebrae in left lateral view (not right as published) matches cervical vertebrae 1 and 2 in Haya – and does not match the simpler vertebrae in Silesaurus (Fig. 4).

Sacrals are preserved as moulds in Pisanosaurus. 
Various authors have interpreted five, to two sacrals. Agnolin and Rozadilla concurred with Irmis et al. 2007, who found no trace of sacral elements, reporting, “some features previously considered to be impressions of sacral ribs are actually cracks in the matrix, and there is insufficient fidelity to determine whether any of the centra are fused to each other.” 

Figure 6. Pisanosaurus right pes with digit 2 ghosted in and digit 4 rotated into in vivo position. PILS added. Nnte the brevity of the toes compared to the metatarsus, a trait shared with Haya.

Figure 6. Pisanosaurus right pes with digit 2 ghosted in and digit 4 rotated into in vivo position. PILS added. Nnte the brevity of the toes compared to the metatarsus, a trait shared with Haya.

Is the acetabulum open or closed?
Agnolin and Rozadilla ‘suggest’ it is closed, as in poposaurs. If so the closed portion is buried. With available evidence and phylogenetic bracketing, it was probably open. Haya has an acetabulum with a keyhole shape (Fig. 3).

The tibia, tarsus and metatarsus
in Pisanosaurus the cnemial crest does not peak at the knee, but somewhat lower. Haya is similar. The fibula diameter is 70% that of the tibia, as in Scelidosaurus. The fibula for Haya is unknown. Anolín and Rozadilla identified a calcaneal tuber. That is odd because it is so small that it does not extend as far as the fibula does. in Haya the calcaneum extends slightly beyond the astragalus. The astragalus of Pisanosaurus is longer than wide (when the medial condyle is included), which is distinctly different from Haya and other sister taxa and different from Silesaurus.

Figure 8. Calcaneum of Pisanosaurus. You can see why some authors saw a tuber while others did not.

Figure 8. Calcaneum of Pisanosaurus. You can imagine why some authors saw a tuber while others did not.

A flawed phylogenetic analysis
Other than excluding several taxa that nest close to Pisanosaurus in the LRT, Agnolin and Rozadilla employed the invalid Nesbitt (2011) database, also suffering greatly from taxon exclusion. It does not nest sauropodomorphs with ornithischians as phytodinosaurs, but nests sauropodomorphs, like Pampadromaeus, with Tawa and other theropods. In their first analysis, 20 trees resulted with Pisanosaurus nested as an unresolved polytomy of several dinos and non-dinos. After excluding wild card taxa, 82 trees resulted with Pisanosaurus within the Silesauridae. Bremer support is low in their analysis, but Bootstrap support is high in the LRT.

Discussion
Agnolín and Rozadilla discuss several traits of Pisanosaurus (typically related to herbivory) and their appearances elsewhere within the Archosauria. They find no epipophyses in the cervicals, but Haya lacks these, as well on the pertinent first two verts. Agnolín and Rozadilla note “The vertebral centra are very elongate and transversely compressed, differing from the short and stout dorsal vertebrae of known ornithischians, including heterodontosaurids.” They do not realize the close relationship of Pisanosaurus to sauropodomorphs like Saturnalia and the basalmost ornithischian, Chilesaurus, both with elongate dorsals. Agnolín and Rozadilla made a “tentative reconstruction” of the pelvis (Fig. 1), but it bear little to no resemblance to the in situ fossil. In every comparison made, Agnolín and Rozadilla delete or ignore Haya and related taxa and thus recover semi-blind results.

Today and in the future
we can’t keep going back to the same short lists of taxa for our inclusion sets. We know of so many more now that need to be included in phylogenetic analyses. The LRT can be your guide.

References
Agnolín FL and Rozadilla S 2017. Phylogenetic reassessment of Pisanosaurus mertii Casamiquela, 1967, a basal dinosauriform from the Late Triassic of Argentina. Journal of Systematic Palaeontology. http://dx.doi.org/10.1080/14772019.2017.1352623
Ferigolo and Langer 2006. A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone. Historical Biology, 2006; 1–11, iFirst article
Nesbitt SJ 2011. The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History, 352, 1–292.

wiki/Sacisaurus
wiki/Pisanosaurus
wiki/Haya

 

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