A unique Late Triassic dinosaur assemblage

Cabreria et al. 2016
bring us two new taxa, Ixalerpeton and Buriolestes from the Late Triassic. Originally considered a lagerpetid and a carnivorous sauropodomorph. “This is the first time nearly complete dinosaur and non-dinosaur dinosauromorph remains are found together in the same excavation, clearly showing that these animals were contemporaries since the first stages of dinosaur evolution.”

Figure 1. Ixalerpeton bones (above) as originally reconstructed (below) and a new reconstruction (middle).

Figure 1. Ixalerpeton bones (above) as originally reconstructed (below) and a new reconstruction (middle) as a protorosaur with a small scapula, as in Malerisaurus, and short cervicals, as in Boreopricea.

Ixalerpeton polesinensis
(ULBRA-PVT059) was originally considered a laterpetid (close to Lagerpeton, which they considered a dinosauromorph), but distinct from Lagerpeton in several ways. The large reptile tree (LRT) nests Ixalerpeton among the basal protorosaurs. Protorosaurs were not part of the original inclusion set presented by Cabreira et al., so taxon exclusion may be an issue here. As in the protorosaur, Malerisaurus, the scapula was not large in Ixalerpeton. As in another protorosaur, Boreopricea, the cervicals were not elongated.

Cabreira et al. report
“The parietal and frontal bones of Ixalerpeton polesinensis form a skull roof broader than that of most early dinosaurs. A large postfrontal fits laterally to the frontal, as more common to non-archosaur archosauromorphs.” In other words, these are clues that the inclusion set needs to be expanded. Taking a look at the pelves of candidate sisters provides some clues to the affinities of Ixalerpeton, but, of course, all the traits count.

Figure 2. Ixalerpeton pelvis compared to Lagerpeton, Tropidosuchus, Chanaresuchus, Prolacerta and the SAM K 7710 specimen of Youngina.

Figure 2. Ixalerpeton pelvis compared to Lagerpeton, Tropidosuchus, Chanaresuchus, Prolacerta and the SAM K 7710 specimen of Youngina. The latter two are taxa that frame Ixalerpeton in the LRT. No perfect matches here, but the vertical pubis on two of them match them.

Buriolestes schultzi
(ULBRA-PVT280) was originally considered a carnivorous basal sauropodomorph. Several cladograms were presented based on the addition of these taxa to prior analyses. The LRT nests Buriolestes with the basal theropod, Tawa, which was part of their analyses, but none of their topologies match the topology of the LRT, in many regards based on taxon exclusion.

Figure 2. Buriolestes reconstructed along with skeletal elements, some of which have been colorized for segregation.

Figure 2. Buriolestes reconstructed along with skeletal elements, some of which have been colorized for segregation.

Cabreira et al. report
Buriolestes schultzi corresponds to a sauropodomorph dinosaur, as indicated by

  1. a mandible tip with a ventrally inclined dorsal surface
  2. a deltopectoral crest that extends for more than 40% of the humeral length.”

Drawings of Tawa (Fig. 4) indicate a straight or elevated mandible tip, but the fossil has a ventrally inclined dorsal surface.

Indeed Tawa does not have a 40% deltopectoral crest, but it also does not have a reduced antebrachium. Similarly in T. rex, Segisaurus and other reduced forelimb theropods the deltopectoral crest extends relatively further down the short humerus.

Figure 4. Skull of Tawa. Note the descending mandible tip.

Figure 4. Skull of Tawa. Note the descending mandible tip not reflected in the drawing.

The Cabreira et al. study finds:

  1. Dinosauromorpha is composed of Lagerpetidae and Dinosauriformes. (but see this recent post in which Novas and Agnolin 2016 nest Lagerpeton with Tropidosuchus in the Chanaresuchidae)
  2. Lagerpetidae is composed of Lagerpeton, Ixalerpeton, and Dromomeron. (but see above)
  3.  Ixalerpeton and Dromomeron are sister taxa. (not when tested with protorosaurs)
  4. Dinosauriformes includes Marasuchus and a clade with all other members of the group. (Marasuchus nests with a clade of theropods that are not often included in analyses in the LRT.
  5. Saltopus, Lewisuchus, and Pseudolagosuchus form a polytomy with Dinosauria. (additional taxa in the LRT disrupt this polygamy and other taxa nest as outgroups to Dinosauria).
  6. Dinosauria of composed of the Saurischia and Orithischia lineages. (in the LRT the division is between Theropoda and Phytodinosauria).
  7. Asilisaurus is the sister group of all the other ornithischians, including Silesauridae. (in the LRT Asilisaurus and Silesaurus are poposaurs, not related to ornithischians.)
  8. Silesauridae is composed of Eucoelophysis, Silesaurus, Sacisaurus, and Diodorus. (only Silesaurus and Sacisaurus are tested in the LRT)
  9. Silesauridae is the sister-clade of the group composed of broadly accepted ornithischians. (not in the LRT)
  10.  Herrerasauria is the sister group to all other saurischian dinosaurs. (in the LRT, all other dinosaurs)
  11.  Herrerasauridae is composed of Staurikosaurus, Herrerasaurus, and Sanjuansaurus. (not in the LRT where Staurikosaurus is close, but at the base of the Marasuchus clade at the base of the Theropoda)
  12. Herrerasaurus and Sanjuansaurus are sister taxa (not tested in the LRT)
  13. Tawa and Chindesaurus are sister taxa. (not tested in the LRT)
  14.  Guaibasaurus, Eodromaeus, Tawa + Chindesaurus, and Daemonosaurus are saurischians belonging neither to Theropoda nor to Sauropodomorpha (i.e. non-Eusaurischia). (in the LRT Daemonosaurus nests at the base of the Ornithischia.)
  15. Eusaurischia is composed of the theropod and sauromopodorph branches. (paraphyletic in the LRT)
  16. Buriolestes is the sister group of all other sauropodomorphs. (nests with the basal theropod Tawa in the LRT)
  17. Eoraptor is the sister group of all other sauropodomorphs with the exception of Buriolestes. (Eoraptor is close to the base of the Phytodinosauria in the LRT).
  18. Pampadromaeus is the sister group of Panphagia, Saturnalia + Chromogisaurus, and all other sauropodomorphs. (confirmed in the LRT, but Pampadromaeus is also the sister group of the Ornithischia in the LRT)

References
Cabreira SF et al. (13 authors) 2016. A unique Late Triassic dinosauromorph assemblage reveals dinosaur ancestral anatomy and diet. Current Biology (2016), http://dx.doi.org/10.1016/j.cub.2016.09.040

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