The Changeling – The Skull

I’ve now covered the interesting things to do with the hand and the foot of Wulatelong gobiensis. There’s not much there to the rest of the body (there’s the pelvis, which is nearly complete, but that’s the next post), but now I move on to the skull, and a quandary.

[Skeletal diagram is CC-BY: You are free to distribute, modify, and otherwise mess with this illustration, so long as any presentation of this cites me as the original source.]

“Normal headed” reconstruction of Wulatelong gobiensis Xu et al., 2013. Grey areas of the skull describe the minimal extent of the skull as preserved in the nodule of sandstone, but not its possible preserved extent. [Skeletal diagram is CC-BY: You are free to distribute, modify, and otherwise mess with this illustration, so long as any presentation of this cites me as the original source.]

If the Skull Fits

When I first began to produce a skeletal for this taxon, thinking that it was reasonably complete this shouldn’t be a problem (I mean, nearly complete legs, some bits of tail and spine, arms, a hand, even most of the skull, it was good!) but I quickly noticed a problem. The original paper gave out several measurements for the hands, feet, and pelvis, but omitted several others, especially any measurements for the skull; and no scale bars were added to figures, so even tentative estimates couldn’t be made.

Wulatelong gobiensis holotyp skullThe skull was associated with the skeleton, but not described in what manner, and is shown in its own separate block. I emailed Xu Xing, and got a prompt reply: the skull is from tip to caudal end of the distal quadrates 115mm in length. Now, this is fairly small, and barely larger than the holotype skull of Conchoraptor gracilis (~100mm). It is also fantastically undersized for the body it is attached to, an estimate made by comparison to other oviraptorids:

The ratio of skull length to iliac and femur length (two measures that may be constrained in oviraptorids and other derived oviraptorosaurs) were drawn from Fanti et al. (2012), who presented a wonderful table for oviraptorid skeletal measurements (previously noted here). For oviraptorids, the skull is always shorter than the ilium or femur (and the femur longer than the ilium — except in Heyuannia huangi, in which they are quite close); the ilium ranges between 145-175% the skull length, while the femur is 160-180%. Taking the skull and estimating the ilium and femur length provides measures of 167 and 203mm when using basal “oviraptorine” oviraptorids like Rinchenia mongoliensis, but 201 and 207mm when using “conchoraptorines” like Khaan mckennai. Both of these numbers are significantly smaller than the actual iliac and femoral lengths of 225 and 255mm, indicating that either the taxon presents a vastly undersized skull for its size, or a large hip for its size (possible), or the skull is from a different, smaller specimen. These measures also predict proportions among the ilium and femur which differ from the actual specimen, but nonetheless fall within the range (78%-102%) for ilium:femur lengths among oviraptorids. The estimate of i:f range is similar between Wulatelong gobiensis and Nomingia gobiensis (88.2% vs. 86.4%).

This data suggests to me that the skull probably doesn’t belong to the same specimen, especially when considering that the relative length of skull to body size increases when moving basally: the longer jaws relate to larger skulls in caenagnathids, and appears to be the case in Oviraptor philoceratops (note, for example, the large size of the skull).

"Shrunken head" version of Wulatelong gobiensis, with skull shown at preserved size.

“Shrunken head” version of Wulatelong gobiensis Xu et al., 2013, with skull shown at preserved size. [Skeletal diagram is CC-BY: You are free to distribute, modify, and otherwise mess with this illustration, so long as any presentation of this cites me as the original source.]

I wouldn’t spend so much time on this taxon if it weren’t so interesting, its implications fascinating. Of these include its import on the phylogeny of oviraptorids, which as a potential most-basal taxon can be dramatic. For example, it allows us to make assumptions about the basal condition of caenagnathids, and thus how to evaluate the caenagnathid characters of various phylogenies.

When the skull is taken into account, there is much to say.

Skull of IVPP V18409, holotype of Wulatelong gobiensis Xu et al., 2013.

Skull of IVPP V18409, holotype of Wulatelong gobiensis Xu et al., 2013.

Xu et al. diagnosed Wulatelong gobiensis in part on the basis of several characters that occur in basal oviraptorids, but not other oviraptorosaurs:

1) external narial fenestra (ENF) less dorsally-positioned; 2) orbit rostrocaudally longer than infratemporal fenestra (ITF); 3) dorsal border of ITF narrower than ventral border;
4) anterior and posterior processes of lacrimal “intermediate” between basal oviraptorosaurs and oviraptorids; 5) ectopterygoid positioned rostrally; 6) external mandibular fenestra (EMF) is caudally positioned; 7) scapula shorter and more slender; 8) pubic peduncle of ilium wider than ischiadic peduncle; 9) ischium relatively short; 10) metatarsal III “high reduced,” by which it is meant MTIII is narrower in its dorsal extent, “pinched” between MTs II and IV.

Xu et al. further diagnose Wulatelong gobiensis on the basis of three “autapomorphies,” numbered below in continuation of the characters above: 11) ENF ventral margin located below mid-height of premaxilla; 12) strap-like caudal ramus (jugal process) of maxilla extends further caudally than preorbital bar (lacrimal), and overlaps the jugal; 13) rostrodorsal process of surangular constricted dorsoventrally in lateral view.

When dealing only with the skull, only characters 1-6, 11 and 13 are relevant; of them, several of these characters are definite oviraptorid characters, and it is left undenied the issue of whether this specimen (either the skull alone, or together with the postcrania) is an oviraptorid. But their quantification will be left to the next (and last) post.

Fanti, F., Currie, P. J. & Badamgarav D. 2012. New specimens of Nemegtomaia from the Baruungoyot and Nemegt Formations (Late Cretaceous) of Mongolia. PLoS ONE 7(2):e31330.
Xu X., Tan Q.-w., Wang S., Sullivan, C., Hone, D. W. E., Han F.-l., Ma Q.-y., Tan L. & Xiao D. 2013. A new oviraptorid from the Upper Cretaceous of Nei Mongol, China, and its stratigraphic implications. Vertebrata PalAsiatica 51 (2): 85–101. [PDF]

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7 Responses to The Changeling – The Skull

  1. Andrea Cau says:

    If the skull does not belong to the same individual as the postcranium (an hypothesis further supported by the gap in preservation between skull and rest of the remains), scoring all material into a single taxon may create a chimaera: I would to re-score Wulatelong as two distinct individuals…

  2. Mickey Mortimer says:

    The skull is 115 mm and the femur 255 mm. This results in a ratio of 45%. The Citipati holotype has a ratio of 46% (160/305 mm), so is comparable. Gigantoraptor’s mandible is 40% of its femur length, which would result in a skull ~42% of femur length. Both of these taxa are near the base of Oviraptoridae. Thus I don’t see any good reason to doubt the skull belonging to the same individual as the postcrania.

    • Interesting points. I didn’t include Gigantoraptor erliansis because of the difficulty in assessing mandible:skull ratios, which may vary: The mandible is much longer in caenagnathids than in oviraptorids relative to skull lengths, a reflection of the rotation of the dentary ventrally corresponding to snout rotation — and perhaps some other factors. I also hesitate to consider Gigantoraptor erliansis due to the immense size of the animal: limb bone proportions change at increasing size, as indicated by tyrannosauroid/allosauroids compared to smaller/more basal taxa, and I would prefer a more secure phylogenetic placement for Gigantoraptor erliansis first. I further did not include all of the Citipati specimens because explicit measurement data on the holotype of Citipati osmolskae is not available; the referred specimen lacks it, and the holotype postcrania has not been described. I am not sure from where you are deriving your measurements. Edit: It occurs to me you are conflating MPC-D 100/42, with a 305mm femur, and MPC 100/979, with a 160mm skull; however, the skull of MPC-D 100/42 is 180mm, which results in a comparable length of ~270mm estimate, and thus a skull:femur ratio of 59%. But this is higher than the actual ratio of 45% in Wulatelong gobiensis.

      As further noted in the post, caenagnathids and Oviraptor philoceratops have relatively large skulls, although as I noted there is uncertainty in measuring things: the ilium is incomplete in the latter, and it lacks a femur.

      • Mickey Mortimer says:

        Your argument caenagnathids have shorter skulls compared to their mandibles would make Gigantoraptor’s skull even shorter than the estimate I gave, so would support my point even more strongly. Regarding allometry, while we all know larger theropods have different limb proportions on average, do they have longer femora compared to their size? My guess would be that microraptorians are the taxa with the longest femora for their size, and they’re tiny.

        The measurements of Citipati’s holotype were published in the supplementary info of the Yulong paper. So no, I was not conflating it with IGM 100/42.

        This all indicates the “basal caenagnathoids have bigger heads” hypothesis isn’t borne out by the data, which instead suggests a complex assortment of sizes with no simple trend.

        • You wrote: “The measurements of Citipati’s holotype were published in the supplementary info of the Yulong paper. So no, I was not conflating it with IGM 100/42.”

          Then you misread: Lü et al. give the femoral length for MPC-D 100/978 at 345mm, not 305mm. It seems you reported the correct proportion (fem:skull = 46% half femoral length). Nonetheless, this does show significant variance, amongst all taxa with a skull and ilium/femur by which to compare sizes, and that as you say Citipati osmolskae has a ridiculously small head. Relative skull:femoral length increases towards the smaller, crestless taxa. Given the not-included caudipterids, Avimimus portentosus, I can also say that purported non-caenagnathoids will have tiny heads relative to their femora, and that this would be true as the caenagnathoid symplesiomorphic condition. Splitting from that we have a basal oviraptorid with a much larger head, caenagnathids with larger heads, apparent caenagnathids with tinier heads but at ridiculously larger size, and a trend into the oviraptorid lineage for small heads or for large heads, or both.

          I may have illogically construed a mean from which to draw data. I’m not certain at this point that the diversity of other basal oviraptorids (“citipatiines” and “oviraptorines” in my hand diagram here) holds for any others; and may have overemphasized the value of Oviraptor philoceratops as the otherwise most-basal oviraptorid I could draw from — what with its huge head and all. Skull:femoral ratios in Rinchenia mongoliensis, Citipati osmolskae and 100/42 are 54%, 46% and 59%, respectively, suggest that of these “citipatiines,” a stable number there isn’t.

  3. Pingback: The Changeling – The Analysis | The Bite Stuff

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