Volumetric estimate for Giganotosaurus

During my sweeping tweaks to my carcharodontosaurid reconstructions I decided to do a volumetric estimate for my updated reconstruction of Giganotosaurus carolinii.

Giganotosaurus carolinii (MUCPv-ch1, holotype) multiview for GDI.

For the dorsal view and the anterior views of the limbs, I took the scanned Acrocanthosaurus skeleton from Bates et al. (2009), scaled and modified to fit the Giganotosaurus. I then took the dorsal view of the Carcharodontosaurus skull from Sereno (1996) and adjusted it to fit.

Using a Python script to perform a graphic double integration with one-pixel slices at a scale of 364 px = 1 metre, the estimated volumes for each section were as follows:

Section	Volume (L)
Head	497
Neck	593.5
Torso	5281.4
Tail	1621.2
Individual forelimb	7.2
Individual hindlimb	296.7
Total	8601

Volumes of sections for MUCPv-Ch1.

Sections can be downloaded here.

The estimated mass for MUCPv-ch1 using a density of 0.95 kg/L (Larramendi, Paul, & Hsu, 2020) would be around ~8.17 tonnes.

Readers might note that this is significantly larger than the 2013 mass estimate by Scott Hartman of 6.8 tonnes for the same specimen. This might seem egregious especially given how my own reconstruction bases several aspects on it due to the dearth of available data. Given how touchy people can get about megatheropod mass estimates, this might generate some less-than-friendly dissent thrown my way, but let me explain. There are a few reasons for this.

For one, the 2013 Hartman Giganotosaurus estimate has an average bodily density of about ~0.913 kg/L. With a density of 0.95 kg/L, it would raise the resulting mass to about ~7.1 tonnes even when the above factors were disregarded. Furthermore, there’s quite a precision level difference here that contributes to the disparity. The 2013 Hartman GDI used 46 equally spaced slices along the axial body, a few slices for the hindlimbs and 10 for the hindlimbs, rather than a pixel-precise one as done here.

Those differences by no means account for the large disparity in the final result – they aren’t close to being big enough on their own. By far, the biggest factor is the restoration of the shoulder girdle and its effect on the chest.

In Scott Hartman’s reconstruction the Giganotosaurus type scapulacoracoid (preserved length ~90 cm) was taken to be rather well-preserved, assuming that only a small portion of the distal end of the scapula was missing, and restores the total scapulacoracoid length at ~97 cm. This results in the diminutive pectoral girdle the animal had been traditionally reconstructed with.

Scapulacoracoids of (A) Giganotosaurus carolinii (MUCPv-Ch1) and (B) Tyrannotitan chubutensis (MPEF-PV 1156) in lateral view. From Canale et al. 2013, fig. 35.

However, the scapulacoracoid in actuality is very badly and incompletely preserved, with the acromial process and a major portion of the coracoid broken off, which lead to misinterpretation of its anatomy (Canale et al. 2013). Furthermore, comparisons with other carcharodontosaurids also indicate a significantly longer scapulacoracoid. I have estimated the complete length of the scapula alone at ~103.7 cm based on Acrocanthosaurus, assuming a 127.7 cm long femur for NCSM 14345 (Carpenter 2000) and the scapula:femur proportions being similar between the two taxa. This would be a minimalistic estimate, as the femur of NCSM 14345 is incompletely preserved and the aforementioned length being probably overinflated – comparisons with the complete femur of SMU 74646 yields an est. length of ~119.4-123.1 cm, which would consequently increase the estimated scapula length for Giganotosaurus. Nevertheless, the minimalistic estimate was used here. Adding in the coracoid gives a total length of ~125 cm.

The resulting larger pectoral girdle deepens the anterior chest significantly. With the estimated width here being identical (134 cm including soft tissues) the chest ends up considerably larger, and since it is the most voluminous portion of the animal’s body and comprises a major portion of the total, the overall volume and mass of the animal shoots up considerably. The hips of my reconstruction are a bit shallower, but due to the lower volume of the pelvic region it is nowhere near enough to counteract the aforementioned increase.

I have no guarantees that everyone will be satisfied by this explanation, especially those among the many palaeoenthusiast communities all over the internet that take theropod mass estimates way too seriously, but it should serve to explain why my estimate is so much higher. But I digress.

If this estimate holds, then Giganotosaurus would truly deserve its name. For comparison, average African elephants mass around ~4 and ~6 tonnes for adult females and males respectively (Larramendi 2016). The chances of the single good specimen of Giganotosaurus we have being the largest that ever lived is also vanishingly small, and statistically speaking it would most likely have been among the average-sized ones as there are much more of those in any animal population than there are especially large or small individuals. There might have been some real monster giganotosaurs out there for all we know!

This also has implications on a rather…fringe hypothesis made recently: that allosauroids were supposedly obligate scavengers akin to vultures as proposed by Pahl & Ruedas (2021). While they used a 2-tonne Allosaurus as their model theropod, Giganotosaurus as an allosauroid would have also fallen under this umbrella. The hypothesis is hugely flawed in several aspects, but the relevant one to this post here pertains to the relationship between body size and the energy efficiency of a scavenger lifestyle.

A study by Kane et al. (2016) models theropods of various sizes foraging and competing for carrion to estimate the energy expenditures and gains from scavenging. According to their model the most efficient size range for scavenging in theropods was estimated at 27-1044 kg.

Fig. 3 from Kane et al. (2016), showing the estimated proportion of energy expenditure offset by the gains from scavenging across a range of body masses. Triangles represent species estimates and are, from left to right, Archaeopteryx, Marasuchus, Microraptor, Velociraptor, Coelophysis, Gorgosaurus, Dilophosaurus, Allosaurus, and Tyrannosaurus. Circles represent inferred points based on a regression analysis.

At body masses over a tonne, energy intake from scavenging plateaued due to a limited accessible supply of carcasses after competition. While gigantic megatheropods had much larger gut capacities and capability to take over carcasses from smaller animals, the costs of foraging outpaces the energy gain from scavenging at massive sizes. The 2-tonne Allosaurus used by Pahl & Ruedas for their model is already about twice as massive as the upper reaches of the optimal scavenging range, putting severe aspersions to their hypothesis. At over 8 tonnes, the Giganotosaurus estimated here is much too massive for scavenging to be a remotely sustainable lifestyle beyond opportunistic occasions. Forced into an obligate scavenger lifestyle, it would fail to meet its energy demands by far and starve to death. Theropods of this size are thus very unlikely to have been driven by scavenging and most likely were specialised hyperpredators.

A Giganotosaurus attacking an iguanodont, from BBC’s Walking With Dinosaurs special, Chased by Dinosaurs. The reconstructions are by now dated, but the apex predatory nature of the megatheropod holds. They grew to sizes far too large for a vulture-like specialised scavenger lifestyle to be sustainable. Iguanodonts, abundant during the early and middle Cretaceous, would have been a staple source of prey for the gigantic carcharodontosaurids.

Acknowledgements

Credit to Franoys for the Python GDI script.

References

• Sereno et al., 1996, “Predatory dinosaurs from the Sahara and Late Cretaceous faunal differentiation”
• Carpenter, 2000, “A new specimen of Acrocanthosaurus atokensis (Theropoda, Dinosauria) from the Lower Cretaceous Antlers Formation (Lower Cretaceous, Aptian) of Oklahoma, USA”
• Bates et al., 2009, “Estimating mass properties of dinosaurs using laser imaging and 3D computer modelling”
• Canale et al, 2013, “Osteology and phylogenetic relationships of Tyrannotitan chubutensis Novas, de Valais, Vickers-Rich and Rich, 2005 (Theropoda: Carcharodontosauridae) from the Lower
• Kane et al. (2016), “Body size as a driver of scavenging in theropod dinosaurs”
• Larramendi, 2016, “Shoulder height, body mass, and shape of proboscideans”
• Larramendi, Paul, & Hsu, 2020, “A review and reappraisal of the specific gravities of present and past multicellular organisms, with an emphasis on tetrapods”
• Pahl & Ruedas, 2021, “Carnosaurs as apex scavengers: Agent-based simulations reveal possible vulture analogues in Late Jurassic dinosaurs”

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