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Comments on Therrien and Henderson's new paper

While reading through the new paper in JVP on size estimates in theropods, I came across several points which I think deserve comment-

The authors find a relationship between skull and body length in theropods (excluding therizinosaurs, oviraptorosaurs and ornithomimosaurs), then test it using Sinraptor dongi, Velociraptor, Carnotaurus, NGMC 91 and Mei (none of which were used to model the relationship originally). They find-

"The body length estimates for Sinraptor and Velociraptor are extremely close to the published length for the specimens considered (<3.3%), which reinforce the validity of SK-BL."

And yet the published length of Sinraptor dongi is just another estimate, since only a few proximal caudals were preserved. Same for Velociraptor, as the specimen used (AMNH 6515) is just a skull and phalanges.

"Interestingly, the body length of Mei long and of the small feathered NGMC 91 is overestimated (36% and 85%, respectively) by the SK-BL regression whereas that of Carnotaurus is underestimated (28%). These latter results reflect the facts that juvenile theropods have larger heads relative to their body size than adults (Ji et al., 2001; Xu and Norell, 2004) and that abelisaurids have shorter heads relative to their body size than other theropods (Bonaparte et al. 1990). Consequently, the SK-BL regression should not be used for abelisaurids or juvenile individuals."

Er, maybe it's because Mei and NGMC 91 both have much shorter tails (3.17 and 3.6 times femoral length respectively) than any other theropods used, especially small ones.

Indeed, their figure 2 shows sillouettes of Compsognathus, Coelophysis, Allosaurus and Tyrannosaurus scaled to a standard total length to illustrate how relative skull length supposedly increases with increased total length. What should be obvious from the picture is that this is due to the increasingly shorter tails of the taxa! Compsognathus' tail is >6.4 times its femoral length, Allosaurus' is ~5.1, and Tyrannosaurus' is ~4.6.

The dataset shows a bias for short-tailed larger theropods (7 tyrannosaurids vs. 3 carnosaurs, Dilophosaurus and Ceratosaurus) and long-tailed smaller theropods (2 coelophysids, 3 compsognathids and Deinonychus vs. 1 troodontid). Note also that tyrannosaurs (especially the larger tyrannosaurines, especially the largest Tyrannosaurus) are known to be comparatively more heavily built than other theropods. Examining the dataset, their Allosaurus fragilis specimen is not YPM 1930 (the fragmentary holotype), but is USNM 4734 (the topotype). I really don't see why Carnotaurus, Huaxiagnathus, Juravenator, Mei, NGMC 91 and Sinraptor hepingensis are included in the "incomplete" dataset. They're more complete than Acrocanthosaurus, Deinonychus, and the Compsognathus holotype, which are all in the "complete" dataset. Indeed, the Deinonychus specimen is just a partial skull.

The authors go on to estimate the length and mass of Spinosaurus, based on the new large snout (MSNM V4047) described by Del Sasso et al. (2005). They say-

"A note of caution concerns the dimension of the reconstructed Spinosaurus skull. First, the proposed skull restoration (Dal Sasso et al. 2005:fig. 5B) is a composite reconstruction with the front and back halves being from different genera. Because skull shape varies among spinosaurids (i.e., the shape of the rostrum, the relative dimensions of the maxillae and premaxillae, and the shape of posterior region of the skull differ among spinosaurids; Fig. 3), there is potential for overestimating the length of a composite spinosaur skull."

Figure 3 shows the skulls of Baryonyx, Suchomimus and Irritator, with known parts outlined darkly. Yet not only is the old reconstruction of Baryonyx used (from Charig and Milner, 1997), but the authors failed to realize the reconstruction of Suchomimus (from Sereno et al., 1998) is mostly based on Baryonyx (except the premaxilla and maxilla)! Furthermore, reconstructing Spinosaurus based on the elongate baryonychine morphology instead of Irritator would result in a longer skull length if anything.

First, the authors test their skull vs. length and skull vs. mass relationships with more complete spinosaurids. They get results close to previous estimates, claiming this proves the method works for spinosaurids. Yet Suchomimus' skull is only known from the snout, while the proximal snout of Baryonyx is missing. Even worse, the incomplete skeleton of Suchomimus is headless! The snout and other cranial elements (quadrates and dentaries) are referred specimens with unknown scaling relative to the postcrania! So we don't know the skull length of either taxon or the skeletal length relative to skull length in Suchomimus, making it impossible to confirm their method works on spinosaurids.

Then they estimate Spinosaurus' length and mass based on their equations and Dal Sasso et al.'s 1.75 meter skull length estimate. They get a much shorter (14.34 m) and heavier (20.9 tons) animal than Dal Sasso et al. did. Why? I'd say because of what I wrote above regarding their biased sample of large theropods. The large theropods are mostly short-tailed and massive tyrannosaurids, and the largest are especially massive tyrannosaurines. Spinosaurus probably had a much longer tail than tyrannosaurids and was more lightly built, yet because it's skull is longer than any theropod used in the model, it will be forced into the trendline formed by the largest tyrannosaurids.

Yet it gets worse, as the authors carpriciously revise Spinosaurus' skull length to 1.5 meters, resulting in a total length of 12.57 meters, "a value very close to the body length of the largest theropods known (e.g., Tyrannosaurus, Carcharodontosaurus, Giganotosaurus)." "Consequently, the body length and body mass estimates for this individual are subject to change and could be considerably lower?possibly 12.57 m and 12 tons?within the range of the largest theropods known." The Spinosaurus skull reconstruction is rather crammed posteriorly as it is, with the laterotemporal fenestra extending below the orbit and such. I don't see how you could make a realistic reconstruction 1.5 meters long. Look at figure 4 in Dal Sasso et al. (2005) and see if you think MSNM V4047 was only 22% larger than Suchomimus.

"It is generally accepted that large hadrosaurs, including those smaller than the 17-m-long Shantungosaurus, were primarily quadrupedal (Brett-Surman, 1997; Dilkes, 2001), which suggests
that bipedalism at extremely large body size is impractical. In light of the results presented above, we conclude that it is doubtful that a bipedal theropod with a mass exceeding 20 tons could have existed. Although it was a large theropod, Spinosaurus aegyptiacus was probably no larger than the currently known largest tyrannosaurids and carcharodontosaurids, a size close to the biomechanical limit for strictly bipedal animals (Henderson, 2005)."

Or maybe it suggests that spinosaurids weren't hadrosaurs. Hadrosaurs of any size were primarily quadrupedal. It's just as meaningless as showing large sauropods were quadrupedal.

In conclusion, while it would be nice to have a formula to estimate theropod length and weight with based on skull size, I believe it's wishful thinking. Therrien and Handerson show you can get decent results if you limit taxon selection, but it's much better to use related taxa and scale to known proportions. Even better, find allometric relationships within known taxa (e.g. tyrannosaurids).

Mickey Mortimer