Steve O

Due to the incomplete remains, all megalodon length estimates are built on a chain of assumptions. Changing any one of the variables can have a large effect on the estimated length. I am certainly not saying these estimates are perfect either. Regardless, megalodon is a stupidly large shark.

In fairness to Shimada et al., they do discuss the nuance in more detail in the paper, and I have not personally measured IRSNB P 9893 so the scaling ratios used above can be questioned. But I think there is reason to not treat the 24.3 m estimate as gospel.

Whilst these estimates still suggest a huge shark, it effectively chops off nearly 4 or 5 m off the estimate favoured by Shimada et al.

Because of the incomplete nature of the column, it's important to consider other interpretations. Applying these ratios to the Cooper et al. reconstruction at 15.9 m, suggests somewhere around 18.43 - 19.86 m for the Bendix-Almgreen vertebra.

Scaling up from IRSNB P 9893, using the range of 6.4 to 6.9 cm, assuming a TL of 16.4 m, and using a length of 8 cm for the Danish verts, suggests somewhere between ~19 and 20.5 m for the Bendix-Almgreen vertebra. (8 / 6.4 = 1.25, 8 / 6.9 = 1.159)

However, for the sake of discussion, let's assume the 23 dimeter vert was in fact 8 cm long and consider the implications to the Shimada length estimates.

Whilst Bendix-Almgreen (1983) states that the larger verts have suffered less distortion, it's certainly possible that these length estimates are not correct and, therefore, that the diameter is a better metric to use in this case.

It's not clear if the largest estimated length of 8 cm is for the the 23 cm diameter vertebra, but if we take this at face value, it suggests very tall/wide, but not very long vertebrae.

Bendix-Almgreen (1983) states; ''The 20 or so imperfectly preserved specimens range in diameter from about 10 cm to 23 cm. The few larger centra that are less affected from compaction in the deposits, permit an estimate of their original front to back dimensions ranging from about 5 cm to 8 cm.''

Frustratingly, I'm not aware of vertebral length measurements for IRSNB P 9893. However, using the 3d file provided by Cooper et al., I was getting around 6.4-6.9 cm depending on where I measured, biasing towards the larger anterior vertebra.

However, more important than comparing diameter is the comparing length of the vertebra.

The largest diameter vertebra of IRSNB P 9893 is 15.5 cm. Bendix-Almgreen (1983) described a vertebrae at ~23 cm in diameter, implying a scaling a ratio of 1.484. Shimada et al. then scale up their 16.4 m estimate x1.484 to produce 24.3 m.

Both Cooper et al. and Shiamda et al, are then adding a head and tail to get their respective estimates of 15.9 and 16.4 m, respectively.

It's difficult to be 100% certain where the precaudal region ends and the caudal begins in sharks, especially if the caudal lobe is angled shallow. Any vertebral chamfering will be subtle. Somewhat arbitrarily, Shimada et al. chop off the last 0.1 m.

Cooper et al. had previously restored the column, as preserved, coming to ~11.1 m, which they assumed represented the precaudal region (sans head). However, the last several vertebrae are tiny. Sterns et al. (2024) and Shimada et al. have suggested that some of these might be caudal verts.

To produce this estimate, they first estimate the incomplete column IRSNB P 9893 belonging to an individual 16.4 m. Cooper at al. (2022) had estimated this individual smaller at ~15.9 m. The differences are primarily due to differing head and tail interpretations.

Shimada et al. (2025) published a reassessment of megalodon and produced an estimate of 24.3 m by comparing a large 23 cm diameter vertebrae from Denmark to the fairly complete vertebral column of IRSNB P 9893.

How reliable is that estimate?

May I offer some dinosaur Stop Motion animation for your timeline

I didn't follow any specific reference, it was an exercise of "let's see where this takes me". I saw some ducks scratching their heads so I had to animate it here

Armature by Animation Toolkit. Head sculpt by Marco Cavassa