Zuolong: a basal theropod

Wikipedia reports,
Zuolong sallleei (Choinere et al. 2010; IVPP V15912; Fig. 1; 3m in length) is a coelurosaur (related to Ornitholestes) dinosaur from the lower Oxfordian of the Late Jurassic.

Figure 1. Zuolong skull, very basic, very basal for Theropoda.

Figure 1. Zuolong skull, very basic, very basal for Theropoda. Using color can predict certain bones, like the nasals, dentary and jugal in this case.

Figure 2. Zuolong skull revised with a backward tilting lacrimal and other minor modifications.

Figure 1 revised. Zuolong skull revised with a backward tilting lacrimal and other minor modifications. No scores changed on the matrix.

By contrast
the large reptile tree (LRT, 1209 taxa) nests Zuolong at the base of the rarely included Segisaurus/Marasuchus clade, near the origin of the Theropoda between Tawa and Sinocalliopteryx. Apparently taxon exclusion was a problem with the original Zuolong results.

Figure 2. Zuolong skeleton from Choiniere et al.

Figure 2. Zuolong skeleton from Choiniere et al.

Choiniere et al. counted 5 sacrals (Fig. 3).
The LRT finds only 4 sacrals (Fig. 3) when compared to the ilium, which has a truncated anterior, like that of fellow clade members. Most traits in Zuolong are common and plesiomorphic, as one might expect of a basal theropod.

Figure 3. Zuolong pelvis and sacrum

Figure 3. Zuolong pelvis and sacrum. Originally the dorsal vertebra was considered sacral #1.

At last! Skull material!
Zuolong is the first member of this near basal theropod clade (Segisaurus, et al.) known from substantial skull material… and it’s suitably plesiomorphic. Also note the truncated anterior ilium, a trait of this clade. Four sacrals down to two in smaller taxa separate clade members from most 5-sacral theropods.

Figure 5. Basal theropods with the addition of Zuolong and Megaraptor.

Figure 5. Basal theropods with the addition of Zuolong and Megaraptor. We’ll look at Megaraptor tomorrow.

Choiniere JN, Clark JM, Forster CA and Xu X 2010. A basal coelurosaur (Dinosauria: Theropoda) from the Late Jurassic (Oxfordian) of the Shishugou Formation in Wucaiwan, People’s Republic of China. Journal of Vertebrate Paleontology. 30 (6): 1773–1796.


3 thoughts on “Zuolong: a basal theropod

  1. The posterodorsal lacrimal process isn’t a horn (“The lacrimal lacks prominent lateral and dorsal rugosities”), it’s continuous with the skull roof, so that it should be rotated clockwise and makes the snout longer. More importantly, you left the quadrate head separate from the socket in the squamosal that holds it, so the squamosal needs to be shifted downward and rotated clockwise a bit too. Also, the preacetabular process isn’t short, it’s broken. Which would leave the fused anterior vertebra available to be a sacral, which makes sense as all Jurassic theropods have at least five sacrals and long preacetabular processes. Finally, that grey bit in the middle of the pubes really is part of the pubic apron. The hole left below it is the interpubic foramen, very common in tetanurines (unlike Segisaurus and Marasuchus, for instance).

    • Thanks, Mickey. Changes to the lacrimal and quadrate are entered. I’m not seeing the strong transverse processes on the last dorsal and the broken anterior ilium does not seem so broken as you indicate. We may have to wait for more data on that. I’m going to need to see at least one similar example of the interpubic foramen. The ones I’ve seen do not resemble what is shown on Zuolong, which may be bone, but looks like a patch at present. The proximal pubis strikes me as odd, but failed to make your list.

      • The new skull looks better, and thank you for leaving your old reconstruction up too. One thing I didn’t catch before is that the anteroventral part of the maxilla is missing (“Small portions of the dorsoventrally low, triangular left maxilla are missing at the anterior contact with the premaxilla…”), it didn’t have a subnarial gap like a coelophysoid (note the premaxillary palatal process is just projecting into nothing in your pic, whereas it would have met the broken maxillary palatal process if the latter bone were complete).

        As for the sacrum, the first sacral is so eroded the authors state “The neural arch of S1 is too poorly preserved to describe”, but note that some theropods do have smaller sacral 1 transverse processes (e.g. Megalosaurus: Benson, 2009 fig. 7F). For the ilium, you could always write Jonah, but at the very least should rescore it as unknown, so that you’re not just confirming your own biases. Regarding the proximal pubis, you should scale down the pubis to 87% of what you have it based on Choiniere et al.’s figure 1, whereas I assume you used the scale bars in the pelvic figures. That will make the proximal end match the ilium better, and note too that the ischial peduncle and associated obturator plate is missing (“Due to posterior breakage of this plate, the morphology of the
        obturator region cannot be determined”) so that the ischia wouldn’t just abut the broken posterior edge as in your figure.

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