Crassigyrinus

Crassigyrinus Fossil range: Late Mississippian - Early Pennsylvanian (Visean-Namurian) 345.3–318.1 Ma PreЄ Є O S D C P T J K Pg N
Scientific classification
Superclass:	Tetrapoda
Class:	Amphibia
Genus:	Crassigyrinus
Species:	C. scoticus (type)
Synonyms:	Macromerium scoticum

Crassigyrinus (meaning "thick tadpole") is an extinct genus of carnivorous stem group tetrapod from the Early Carboniferous of Scotland and a possible specimen from 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 a couple of incomplete lower jaws. Crassigyrinus grew up to 1.5 meters in length, coupled with punitive limbs and unusually large jaws. Crassigyrinus is taxonomically enigmatic, having confused paleontologists for decades with its apparent fish-like and tetrapod features. Some paleontologists have even considered it as the most basal 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.

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.

Skull

Anatomy of the skull of Crassigyrinus. (click for better quality)

The holotype skull was discovered in Gilmerton near Edinburgh, Scotland. Crassigyrinus had large, diamond-shaped eye sockets that were placed high up on the skull and close together. The apparent large size of its eyes suggests that it was able to see well in dark or murky water. The cheek region of Crassigyrinus has canals within that are presumed to have housed a lateral line system, similar to the one seen in sharks. Furthermore, a distinctive ornament on the top of its skull suggests that it might have been a skin flap that appeared as seaweed, such as that seen in in wobbegong sharks.

The interpremaxillary fenestra connected the dorsal surface of the snout tip with the palate, but does not seem to serve any obvious function except as an exit point for an enlarged dentary fang pair; similar to other tetrapods where teeth from the lower jaw pierce the skull roof. Special notches on the lower jaws seem to have housed the enlarged palatal fangs.

The snout was also consolidated and buttressed, and with a kinetic inertial jaw mechanism, would have produced a bone-smashing snap-trap.

The third skull discovered, BMNH 30532, also from Gilmerton, has features of a unique fenestra between the premaxillae, which communicated with an anterior palatal fenestra. The function of this structure is unknown. Another unique feature of Crassigyrinus are the several large holes in the dorsal surface of the dentary which housed the massive palatal teeth when the jaws were closed. The lower jaws seem to be rather primitive in construction.

Postcranial skeleton and is Crassigyrinus a tetrapod?

The vertebrate of Crassigyrinus are simple and lack enlarged interlocking processes (zygapophyses), and the back of its skull does not have the distinct occipital condyle. However, this character is not thought to be primitive because a properly formed occipital condyle is present in many fishes. This feature is instead considered to be a paedomorphic one. The ribs are a well-developed feature, yet lack distinct facets for articulation with the vertebrae, and hence also appear primitive or degenerate compared to those of most other tetrapods. This combination of features has led some paleontologists to consider Crassigyrinus as the most basal tetrapod,^{p. 116} while others have not even considered it a tetrapod at all. Ahlberg & Milner (1994) even suggested that Crassigyrinus was "the last survivor of the primitive tetrapods that never left the water".^{(p. 512)}

However, recent phylogenetic studies have shown that Crassigyrinus is more likely to have been secondarily primitive, rather than a basal relict: it was fairly close to the base of Tetrapoda, but its alleged fish-like characters are either erroneous or secondarily reacquired. Previously, it had been argued that Crassigyrinus was close to the anthracosaurs and perhaps more closely related to amniotes than to stem-group tetrapods, but the evidence used to support this is generally regarded as erroneous now.

Because of its various unique features, new taxonomic groups have been created for Crassigyrinus, such as a new family, Crassigyrinidae, within its own order, Palaeostegalia.

Phylogenetic and anatomical dilemma

Skull of Crassigyrinus shown in dorsal view.

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.

Some scientists regard Crassigyrinus as a reptiliomorph. Panchen (1985) regarded Crassigyrinus as a relative to anthracosaurs. Panchen determined that on the basis of several characters, including dark dentine’ in the teeth and the form of the dermal bone ornament. However, other phylogenetic studies have lessened this likelihood, such as the one performed by (Clack 1998), which found out that Crassigyrinus is closely related to Whatcheeria and the embolomeres. More recent studies published support the conclusion that Crassigyrinus is the most basal tetrapod of the Carboniferous, with only the Devonian tetrapods that proceeded Crassigyrinus being more primitive in their features.

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

Life restoration of Crassygyrinus.

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. Crassigyrinus was probably a major predator of smaller vertebrates and may have been one of the dominant predators in its environment. Its proportionally tiny limbs indicate that it was unable to move on land.

Gallery

References

• Panchen, A. L., & Smithson, T. R. 1990. The pelvic girdle and hind limb of Crassigyrinus scoticus (Lydekker) from the Scottish Carboniferous and the origin of the tetrapod pelvic skeleton. Transactions of the Royal Society of Edinburgh, Earth Sciences 81, 31-44.
• Zimmer, C. 1998. At the Water's Edge: Macroevolution and the Transformation of Life. Free Press, New York.

External links

• Crassigyrinus