| Named By: | Watson in 1929 |
| Time Period: | Visean 345.3-328.3 Ma |
| Location: | Scotland |
| Size: | 2 meters long |
| Diet: | unavailable |
| Fossil(s): | Several incomplete specimens, three skulls |
| Classification: | | Chordata | Amphibia | Labyrinthodontia | Ichthyostegalia | Crassigyrinidae | |
Crassigyrinus (meaning "thick tadpole") is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous of Scotland and possibly Greer, West Virginia. 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. 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. Crassigyrinus had unusually large jaws, enabling it to eat other animals it could catch and swallow. It had two rows of sharp teeth in its jaws, the second row having a pair of fangs. Crassigyrinus had large eyes, suggesting that it was either nocturnal, or lived in very murky water.