Dibothrosuchus: a new ancestor for Gracilisuchus and Scleromochlus

According to Wikipediia,
Dibothrosuchus is a genus of sphenosuchian, a type of basal crocodylomorph, the clade that comprises the crocodilians and their closest kin. It is known from several partial skeletons and skulls. These fossils were found in Lower Jurassic rocks of YunnanChina.  Dibothrosuchus was a small terrestrial crocodylomorph.”

Here
in the updated crocodyomorph portion of the large reptile tree (LRT, 1658+ taxa; Fig. 6) Dibothrosuchus (Figs. 1, 2) nests among the most basal bipedal crocodylomorphs (phylogenetically far from Sphenosuchus (Fig. 7).

Figure 1. Dibothrosuchus skull fossil with colors added. Note the differences in this skull and the illustrated one in figure 2.

Figure 1. Dibothrosuchus skull fossil with colors added. Note the differences in this skull and the illustrated one in figure 2.

More specifically
an earlier sister to Late Jurassic Dibothrosuchus arose from a sister to Middle Jurrassic Junggarsuchus and Late Triassic Pseudhesperosuchus, (Fig. 3) and gave rise to smaller Middle Triassic Gracilisuchus (Fig. 4, 5), and Late Triassic Scleromochlus, Saltopus and Lagosuchus.

Figure 2. Images from Wu et al. 1993, colors and hind limbs added. Compare to skull in figure 1.

Figure 2. Images from Wu et al. 1993, colors and hind limbs added. Traditionally the postorbital is considered fused to the postfrontal. Compare to figure 1.

Dibothrosuchus elaphros (Early Jurassic, Simmons 1965; Wu and Chatterjee 1993) is known from an incomplete skeleton, lacking hind limbs and distal tail. In the LRT Dibothrosuchus nests at the base of bipeds AND was derived from bipeds, so phylogenetic bracketing indicates Dibothrosuchus was a biped, too.

Figue 1. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Figure 3. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

As in descendant taxa,
the rostrum of Dibothrosuchus was perforated between the premaxilla and maxilla (Figs. 1, 2). The lateral temporal fenestra was elaborated with a quadrate that had three dorsal heads for a strong articulation with the skull roof. The cervicals and their ribs were quite robust. As in few other tetrapods, the cervicals and their ribs were deeper than the skull. The proximal carpals were longer than the metacarpals.

Figure 4. Present reconstruction of Gracilisuchus with skull based on Romer 1971. See figure 4 for an updated on that skull.

Figure 4. Present reconstruction of Gracilisuchus with skull based on Romer 1971. See figure 4 for an updated on that skull. Inset at upper right shows the broken femur (blue on the digram) and likely proximal carpals (green on the diagram).

Gracilisuchus stipanicicorum (Romer 1972; Butler et al. 2014; Ladinian, Middle Triassic, ~230 mya, 30 cm long; holotype PULSR8) is a basal crocodilomorph. It was derived from a sister to Dibothrosuchus and preceded both Saltopus and Scleromochlus.

Figure 5. Gracilisuchus skull updated with new colors.

Figure 5. Gracilisuchus skull updated with new colors. Skull image from Butler et al. 2014. Note the tall fenestra separating the premaxilla from the maxilla, as in Dibothrosuchus (Fig. 2).

Gracilisuchus was originally considered
an ornithosuchid by Romer (1972). Others thought it nested between Parasuchus and Stagonolepis (Benton and Clark 1988), as the sister to Postosuchus (Juul 1994) or Postosuchus and Erpetosuchus (Benton and Walker 2002). Butler et al. (2014) nested Turfanosuchus, Gracilisuchus and Yonghesuchus together in a clade. Yonghesuchus is close (Fig. 5), but other omitted taxa are closer to the other two. Turfanosuchus is also close, but nests at the base of the Poposauria in the LRT (Fig. 6).

Figure 1. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

Figure 6. Subset of the LRT focusing on the Crocodylomorpha, dorsal scutes, elongate proximal carpals, bipedality and clades.

The Gracilisuchus hind limb paper by Leuona and Desojo (2011)
is about PVL 4597 (Fig. 7), a different genus, the last common ancestor of all archosaurs (crocs + dinos).

Figure 1. Taxa from the croc subset of the LRT to scale. Click to enlarge.

Figure 7. Taxa from the croc subset of the LRT to scale. Click to enlarge.

Lagosuchus talampayensis 
(Romer 1971) is a smaller specimen found on the same slab as the Gracilisuchus holotype. In the LRT Lagosuchus nests with Saltopus, both derived from a sister to Scleromochlus, which was derived from Gracilisuchus (Fig. 7)so all are members of the same tiny bipedal clade within Crocodylomorpha, all derived from a Middle to Early Triassic sister to Late Jurassic Dibothrosuchus.

This is an update
from blogpost #100 in October 2011 on basal bipedal crocs. Current blogpost # is something over 3000.


References
Benton MJ and Clark JM 1988. Archosaur phylogeny and the relationships of the Crocodilia in MJ Benton (ed.), The Phylogeny and Classification of the Tetrapods 1: 295-338. Oxford, The Systematics Association.
Butler RJ, Sullivan C, Ezcurra MD, Liu J, Lecuona A and Sookias RB 2014. New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and
the biogeography of the archosaur radiation. BMC Evolutionary Biology 14:1-16.
Juul L 1994. The phylogeny of basal archosaurs. Palaeontographica africana 1994: 1-38.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Lecuona A, Desojo JB and Pol D 2017. New information on the postcranial skeleton of Gracilisuchus stipanicicorum (Archosauria: Suchia) and reappraisal of its phylogenetic position. Zoological Journal of the Linnean Society XX:1–40.
Parrish JM 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287-308.
Romer AS 1971. The Chañares(Argentina) Triassic reptile fauna. Two new bu incompletely known long-limbed pseudosuchians. Breviora 378:1–10.
Romer AS 1972. The Chañares (Argentina) Triassic reptile fauna. An early ornithosuchid pseudosuchian, Gracilisuchus stipanicicorum, gen. et sp. nov. Breviora 389:1-24.
Simmons DJ 1965. The non-therapsid reptiles of the Lufeng Basin, Yunnan, China. Fieldiana Geology. 15: 1–93.
Wu X-C and Chatterjee S 1993. Dibothrosuchus elaphros, a crocodylomorph form the Lower Jurassic of China and the phylogeny of the Sphenosuchia. Journal of Vertebrate Paleontology 13:58-89.

wiki/Gracilisuchus
wiki/Dibothrosuchus

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