The Swamp Monster Revisited: A New Face for an Ancient Tetrapod

Over 330 million years ago, a strange creature patrolled the ancient swamps of present-day Scotland. Known as Crassigyrinus scoticus (Crass-ig-ah-rye-nus), this stem tetrapod – an early cousin of the lineage that would lead to higher vertebrates (amphibians, reptiles, birds, and mammals) – functioned as a vicious aquatic predator. With two rows of teeth (including numerous fangs that Dracula would envy) paleontologists are confident that Crassigyrinus would have been an adept killer.

Yet one crucial aspect of Scotland’s “killer tadpole” was previously misinterpreted: its face.

That’s right! Although most fossil material consists of skulls, with some specimens being known to paleontologists for nearly a century, it turns out paleontologists have got its facial appearance wrong. In years past, depictions of Crassigyrinus have featured a narrow and elevated head comparable to an eel[i]. The notion of a “thick” skull, paired with downright miniature limbs and a robust tail that powered Crassigyrinus’ movement, makes the comparison between the swamp monster and a supersized tadpole a bit clearer.

However, new research has called this outlook into question. On May 2^nd, a study published by Dr. Laura Porro of the UCL in Britain used CT-scanning and 3-D imaging to reconstruct the skull of Crassigyrinus[ii]. Utilizing fragments of four fossil specimens, Dr. Porro and her team crafted a new model vastly different from previous interpretations. Instead of a tall and narrow face, the new visage of Crassigyrinus is much flatter and broader, with a shape much more akin to that of a crocodile:

Not only does the new model align much better with the cranial anatomy of Crassigyrinus, but with contemporary stem tetrapods as well. While there are exceptions, numerous stem tetrapods – including Tiktaalik, Ventastega, and Ichthyostega – had relatively flat skulls. While the evolutionary relationships between Crassigyrinus and these taxa are poorly understood, it’s more logical to assume that Crassigyrinus did not deviate too heavily from close relatives.

If this is true, why didn’t paleontologists identify it as such? The answer lies in the preservation of Crassigyrinus fossils. Despite there being quite a few Crassigyrinus specimens, almost all of them have become distorted. Some fossil bones have become crushed, while others (such as the holotype, the first known specimen) have been flattened or compressed. These processes have altered the appearance of the fossils, which can be hard to discern with the naked eye.

The physical alterations to these fossils wouldn’t be a massive issue under most conditions. However, some specimens (including the holotype) lie within a type of rock that makes the identifications of irregularities difficult: ironstone. Ironstone is a hard sedimentary rock, which makes it difficult to break without damaging the fossil within. To examine the inner contents of the rock, advanced imaging technologies are required, which have only been made widely available to paleontologists in recent years. Thus, the previous inaccuracies regarding Crassigyrinus were no fault of the paleontologists describing it in the 20^th century but rather the result of the technological limitations of their time.

The advent of modern imaging technology has helped correct these errors, providing paleontologists a better view of an enigmatic predator. A wider jaw would have enabled Crassigyrinus to have a much broader diet, allowing it to gobble up more sources of potential prey. Crassigyrinus would have packed quite the punch in the Carboniferous swamps of prehistoric Scotland…

Crassigyrinus scoticus will always be known as the swamp monster to me because of its appearance in Nigel Marven’s classic series Prehistoric Park. While it may not have been a prominently featured animal, it still made an impact when it took a bite out of Nigel’s leg. Luckily for Nigel, he was bitten by the box-head Crassigyrinus – it isn’t hard to imagine that the flat-head would inflict just a little more damage.

Thank you for reading today’s article! If you would like to know more about the Carboniferous Period and some of its creepier fauna, I suggest you read about how the Carboniferous was bugging out, here at Max’s Blogosaurus!

Works Cited

I do not take credit for any image found in this article. All works belong to credited illustrators and creators.

Header image courtesy of Mario Lanzas, whose work can be found at his DeviantArt here.

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[i] Ashworth, James. “Crushed Scottish Fossils Reconstructed to Reveal Ancient Predator’s Skull.” Natural History Museum, 2 May 2023, https://www.nhm.ac.uk/discover/news/2023/may/crushed-scottish-fossils-reconstructed-reveal-ancient-predators-skull.html.

[ii] Porro, Laura B, et al. “Computed Tomography and Three-Dimensional Reconstruction of the Skull of the Stem Tetrapod Crassigyrinus Scoticus Watson, 1929.” Journal of Vertebrate Paleontology, 2 May 2023, https://doi.org/10.1080/02724634.2023.2183134.