Crassigyrinus

Crassigyrinus (meaning "thick tadpole") was an early tetrapod from the Early Carboniferous period of Scotland and a possible specimen from Greer, West Virginia. Some scientists regard Crassigyrinus as a reptiliomorph.

Description
Crassigyrinus had a streamlined body up to 1.5 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 further suggest that it was a powerful predator with a strong bite. 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. It possessed large otic (spiracular) notches, probably accommodating a spiracle rather than a tympanic membrane.

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. 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). The tail is unknown but is assumed to have been long and laterally compressed.

Phylogenetic and anatomical dilemma
Due to the fish and tetrapod-like features in Crassigyrinus, phylogenetic affinities have proved controversial. However, it has recently been discovered that several of the apparent "fish-like" features have been erroneous, including a preopercular at the back of the skull and an anterior tectal and lateral rostral near the nostril. These bones have been misplaced and misinterpreted nasals and vomers.

While the bones of the nasal region have proved problematic, the nostril and its supposed associated openings are also problematic. Three Crassigyrinus skulls are now known, however, for several decades only the Hugh Miller specimen (see "History of discovery" section below), and this was always somewhat confusing as it seemed to have two nostril openings. Panchen (1967) argued that the lower of the two nostrils, the one closer to the edge of the mouth, was the true nostril, and that it was continuous with a groove that connected this opening with the jaw margin (which would then make this groove a nasolabial groove). However, he later regarded the lower opening as some sort of sensory structure.

However, Clack (1998) described how further examination of both the holotype and specimens discovered later demonstrated how the supposed sensory pit was merely an artifact of preservation; not a real anatomical feature. This new information now strongly suggests that there were no barbels, second nostril, and nasiolabial groove in Crassigyrinus.

At the back of the skull, several prominent notches just behind the eyes were once interpreted as being ear drums. However, recent analysis has suggested that these notches actually contained spiracles (small remnants of the gill system), similar to those seen in other stem-group tetrapod lineages.

History of discovery
The type specimen of Crassigyrinus was originally described as Macromerium scoticum and consisted of a a lower jaw from Fife, Scotland. Macromerium is regarded as the type specimen despite previous arguments that it was quite different and more likely to belong to a baphetid.

The holotype of Crassigyrinus is based upon a partial lower half of the skull collected by Scottish geologist Hugh Miller in the early 1850's. However, the specimen was not officially described until 1929 by David M. S. Watson who mistakenly thought that Crassigyrinus was shallow-skulled. Between 1970 and 1980, two more specimens were discovered, the better one of which was comprised of a nearly complete skeleton was discovered in Fife, Scotland by Stan Wood, a well known professional fossil collector. This new specimen revealed that Crassigyrinus had tiny limbs and primitive spinal column, as well as that the skull of Crassigyrinus, while long, was not shallow.

Paleoenvironment
During the Carboniferous, Scotland was near to the equator and covered with swamps and lakes that were inhabited by diverse fishes and tetrapods. These included the giant swimming anthracosaur Proterogyrinus, snake-like aïstopods, and small, terrestrial temnospondyls. Large scorpions, eurypterids and millipedes were also present.