Crassigyrinus (from Latin: crassus, 'thick' and Greek: γυρίνος Greek: gyrínos, 'tadpole') is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous Limestone Coal Group of Scotland and possibly Greer, West Virginia.^[1]

Quick Facts Scientific classification, Type species ...

Crassigyrinus Temporal range: Viséan ~345–329 Ma PreꞒ Ꞓ O S D C P T J K Pg N
3D skull reconstruction
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Sarcopterygii
Clade:	Tetrapodomorpha
Clade:	Stegocephali
Family:	†Crassigyrinidae von Huene, 1948
Genus:	†Crassigyrinus Watson, 1929
Type species
†Crassigyrinus scoticus Watson, 1929

Close

The type specimen was originally described as Macromerium scoticum and lacked a complete skull. With subsequent discoveries, Crassigyrinus is now known from three skulls, one of which is in articulation with a fairly complete skeleton, and two incomplete lower jaws. Crassigyrinus grew up to 2 meters in length, coupled with tiny limbs and unusually large jaws. Crassigyrinus is taxonomically enigmatic, having confused paleontologists for decades with its apparent fish-like and tetrapod features.^[2] It was traditionally placed within the group Labyrinthodontia along with many other early tetrapods. Some paleontologists have even considered it as the most basal crown group tetrapod, while others hesitate to even place it within the Tetrapoda superclass.^{[citation needed]}

Crassigyrinus had a streamlined body up to 2 meters in length. Its limbs were tiny and virtually useless, implying that the animal was almost completely aquatic. Crassigyrinus had unusually large jaws, equipped with two rows of sharp teeth, the second row having a pair of palatal fangs. Studies have shown that Crassigyrinus may have been able to open its mouth as wide as 60 degrees, which suggests that it was a powerful predator with a strong bite.^[2] This strongly suggests that it was ideally suited for catching fish, and the animal was probably a fast-moving predator.

Several thickened bony ridges ran along the dorsal midline of the snout and between the eyes, and several paleontologists have suggested that they helped the skull to withstand stress when the animal bit prey. Crassigyrinus had large eyes, suggesting that it was either nocturnal, or lived in very murky water.^[3] It possessed large otic (spiracular) notches, probably accommodating a spiracle rather than a tympanic membrane.^[4]

Its peculiar stunted forelimbs were tiny and the humerus was only 35 mm long (the whole animal was about 1.5 m long). Various foramina on the humeral surfaces are very similar to those seen in Ichthyostega, Acanthostega, and lobe-finned fishes like Eusthenopteron.^[5][6] The hindlimbs were much larger than the forelimbs, and in the pelvis the ilium lacked a bony connection to the vertebral column (a classic feature of aquatic tetrapods). Although there is evidence that the Crassigyrinus eventually lost its limbs,^[7] there is counterevidence in that it used its limbs for movement,^[8] as proven by healing in the bones in case of injury.^[9] The fact that there is a need to heal the limbs must mean there was an importance of the limbs at some point, and was then lost. The tail, only known from a few vertebrae fragments, is assumed to have been long and laterally compressed.^[2]

A crushed skull from the Dora bonebed near Cowdenbeath was described by Panchen (1985).^[5] The skull was redescribed by Porro et al. (2023), based on a digital reconstruction derived from CT scans. The 1985 reconstruction of the skull was much taller and narrower than in most early tetrapods, though the 2023 reconstruction is relatively low and broad.^[10]

Crassygyrinus was an aquatic predator. It was capable of opening its jaws widely at a 60 degree angle, with a powerful bite with rapid jaw closure, allowing it to grasp and consume relatively large prey items.^[10]