How Heavy Was Tiktaalik roseae? A Body-Mass Estimate Tested on Living Fishes, with Explicit Uncertainty, and Why Specimen Length Is the Open Question

I know of only one published body mass value for Tiktaalik roseae, given without uncertainty: 6.94 kg from a volumetric model (Hohn-Schulte et al., 2013). Here I make a new estimate using a model tested on living fishes, and propagate the primary measurement and calibration uncertainties through to the estimate. The equation is M = C·L·w·d, where L, w and d are the length, maximum width and maximum depth of the body. C is a coefficient measured on 157 extant fishes of known mass. In leave-one-species-out tests across seven species, the model predicts mass with median errors of 4–31% (4–14% for the five species retained for calibration; the two deep-bodied breams have the largest errors). None of these fishes is flattened like Tiktaalik, so I am extrapolating the model to shapes other than those tested. Hohn-Schulte et al. appear to use the body proportions of the published skeletal reconstruction of the holotype, NUFV 108, the smallest of the three articulated specimens. This reconstruction assumed about 45 presacral vertebrae; the count has since been revised to about 31. With 45 and 31 vertebrae respectively, the model estimates 5.5 kg (95% interval 3.0–10.4 kg) and 4.3 kg (2.1–8.9 kg) for the holotype. The holotype is dorsoventrally crushed. If crushing conserved cross-section area, these values would stay the same. If true depth was 25% greater, they would rise by about a quarter. Isometric scaling using published fin and humerus size ratios suggests 14–18 kg for NUFV 110 and 34–44 kg for the largest articulated specimen, NUFV 109 (95% interval 15–104 kg). Across fish species, C increases as bodies become flatter. Projecting that pattern out to Tiktaalik’s shape, well outside the data, would multiply all these masses by about 1.6–2.0. The intervals quoted here do not include this model-form uncertainty. A rough comparison suggests that under the published proportions, Tiktaalik is about as heavy as a generic fish of the same total length: the wide body compensates for the flattening. If the factors in the model are independent, a decomposition of the log mass variance shows that body length and relative width are the main sources of measurement uncertainty. If the commonly quoted 1.25–2.75 m is total length, it is 1.3–1.8 times the lengths suggested for the articulated specimens. A 2.75 m individual would weigh about 140 kg (50–400 kg) if that is total length, or about 220 kg (80–630 kg) if the caudal fin is not included. Testing C on flat-bodied animals and clarifying the basis of the length range would be the most useful next steps. All data and code are available.