Tetragraptus approximatus

Description

The general outline of each Tetragraptus approximatus colony (rhabdosome) is highly distinctive. It resembles a long narrow letter H or X. The central process (the funicle) is about 2.5 mm long, each end bifurcating at right angles with each other. Each pair of branches (stipes) curve away sharply from the ends of the funicle then run more or less parallel with each other at a distance of 5 to apart. Each of the stipes can reach more than 45 mm in length, with approximately ten cup-like structures (thecae) for every 10 mm of the stipes. The thecae are tilted at a 45° angle to the axis.

Taxonomy

Tetragraptus approximatus is classified under the genus Tetragraptus of the family Dichograptidae. It was first described by the British paleontologist Henry Alleyne Nicholson in 1873 from specimens recovered from Lévis, Quebec, Canada.

Distribution

Tetragraptus approximatus is found worldwide. It has been identified in graptoliferous rocks from Australia, New Zealand, Canada (Newfoundland), the United States (Texas), Kazakhstan, Russia (Taimyr), China, South America, Norway, and Sweden. It is unknown, however, in areas which lack coeval graptoliferous rocks like the United Kingdom, Spitsbergen, and Africa.

Biostratigraphy

Tetragraptus approximatus is used in biostratigraphy as an index fossil. Its first appearance at the GSSP section of the Diabasbrottet Quarry in Västergötland, Sweden is defined as the beginning of the Floian Age ( million years ago) of the Ordovician.

References

References

1. (1963). "Ordovician Graptolites of the Basin Ranges in California, Nevada, Utah, and Idaho". Geological Survey Bulletin.
2. "''Tetragraptus (Tetragraptus) approximatus'' Nicholson, 1873". New Zealand Organisms Register (NZOR).
3. (1936). "On some Fossils from the Quebec Group of Point Lévis, Quebec". Transactions and Proceedings of the Royal Society of New Zealand.
4. Henry Alleyne Nicholson. (1873). "On some Fossils from the Quebec Group of Point Lévis, Quebec". Annals and Magazine of Natural History.
5. (2004). "The GSSP of the Second (Upper) Stage of the Lower Ordovician Series: Diabasbrottet at Hunneberg, Province of Västergötland, Southwestern Sweden". Episodes.