Nasal crests on theropods… insight from Sinosaurus

The latest addition 
to the large reptile tree is the crested theropod, Sinosaurus (Young 1948, Hu 1993; Figs. 1,2, Early Jurassic). So far (provisionally), It nests basal to the Allosaurus + Yutyrannus clade and is a sister to the Proceratosaurus+ Spinosaurus clade that also gave rise Deinocheirus (Fig. 1).

Figure 1. Here's where the crested theropod, Sinosaurusn nests in this subset of the large reptile tree.

Figure 1. Here’s where the crested theropod, Sinosaurusn nests in this subset of the large reptile tree. Note that other crested theropods, like Dilong and Gaunlong nest nearby. Proceratosaurus might have had a crest, larger than the tiny remnant that was preserved.

 

Figure 2. Sinosaurus cast with premaxilla in yellow.

Figure 2. Sinosaurus cast with premaxilla in yellow.

Two nasal crests
top the rostrum in Sinosaurus, but they are very close together. Because Sinosaurus does not nest with Dilophosaurus, and crestless taxa separate them, those two taxa developed similar crests by convergence.

Not so oddly
those two crests on Sinosaurus are barely a crest width apart from one another. Moreover, those those two crests include a healthy portion of premaxillae — and that’s unexpected because in most dinosaurs the ascending process of the premaxilla generally does not extend beyond the naris. But it does in this clade, as we saw earlier with Deinocheirus.

Figure 3. Sinosaurus skull model along with tracing and line art from Hu 1993 then stretched to fit the model.

Figure 3. Sinosaurus skull model along with tracing and line art from Hu 1993 then stretched to fit the model. Note the extent of the premaxilla. Hu considered this another Dilophosaurus, but other workers corrected that hypothesis.

 

That sent me back looking at other clade members. 
And I found premaxillae extending beyond the naris in basal clade members. The derived forms have their own story to tell with the fused nasal boss overlying whatever premaxillary ascending process might have remained below it.

Figure 3. Yutyrannus with revised premaxilla extending beyond the naris.

Figure 3. Yutyrannus with revised premaxilla extending beyond the naris. Sinovenator and Majungasaurus both overlap the ascending process of the premaxilla with the nasal boss, so it appears much shorter. 

Is the crest of Ceratosaurus 
an emerging ascending process of the premaxilla? We’d have to look at the fossil itself, but this colorized version of an old illustration is a tantalizing clue to that possibility. In Allosaurus (Fig. 4) and Acrocanthosaurus the ascending process of the premaxilla is reduced to a slender rod that looks like a medial ridge of the nasal. I think this has been overlooked by prior workers.

Figure 4. Ceratosaurus with extended premaxilla. It is the horn sticking up between the nasals.

Figure 4. Ceratosaurus with extended premaxilla. It is the horn sticking up between the nasals.

What we’re seeing here is 
the counterintutiive phenomenon that crests came first in this clade, with convergent reduction, or coverup. in derived larger taxa.

Figure 5. Allosaurus skull with bones colorized. Note the extent of the premaxillary ascending processes over the nasals.

Figure 5. Allosaurus skull with bones colorized. Note the extent of the premaxillary ascending processes over the nasals.

With present taxa as data
the twin, barely separated crests of Sinosaurus apparently evolved from the similar morphology in its sister, Proceratosaurus, (Figs. 6, 7).

Figure 6. Proceratorsaurus skull with premaxilla (yellow) and nasal (pink) demonstrating how twin nasal crests could have originated with the loss of the premaxillary ascending process.

Figure 6. Proceratorsaurus skull with premaxilla (yellow) and nasal (pink) demonstrating how twin nasal crests could have originated with the reduction of the premaxillary ascending process.

A closeup in anterior view
of Proceratosaurus (Fig. 7) shows the early stages of splitting of the premaxilla and nasals to form two crests.

FIgure 7. Proceratorsaurus premaxilla. Note the tentative split at the top.

FIgure 7. Proceratorsaurus premaxilla. Note the tentative split at the top of the premaxilla. 

References
Hu S-J 1993. Short Report on the Occurrence of Dilophosaurus from Jinning County, Yunnan Province. Vertebrata PalAsiatica 31(1):65-79. Translated by Will Downs 1998.
Young CC 1948. On two new saurischians from Lufeng, Yunnan. Bulletin of the Geological Society of China (Acta Geologica Sinca) 28 (1–2): 75–90. doi:10.1111/j.1755-6724.1948.mp281-2007.x.

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