Sahnioxylon rajmahalense

Sahnioxylon rajmahalense Temporal range:
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Gymnospermae
Division:	Cycadophyta
Genus:	†Sahnioxylon
Species:	†S. rajmahalense
Binomial name
†Sahnioxylon rajmahalense (Sahni) M.N. Bose, S.C.D. Sah
Synonyms
Homoxylon rajmahalense

Sahnioxylon rajmahalense was a piece of secondary wood found in the Rajmahal Hills of Bengal, northwest to Calcutta. It was described by Birbal Sahni in 1954, and later that year S. andrewsii was found by Bose & Sah in the same general area. Although its exact location of excavation is unknown, evidence states that it ranged from the lower Triassic Period, until the Upper Cretaceous. The fossil of the leaves resemble a gymnosperm in that it has no vessels, yet the tracheid walls bear a mixture of scalariform.

History

The form-genus Sahnioxylon was first coined by paleobotanist Birbal Sahni, and is used for fossilized woods, whose primitive features place them in between Cycadeoidea, Gnetales, and homoxylous angiosperms. These plants have been found in Siberia, India, Antarctica, Romania, Middle-Asia, and New-Caledonia. Thus it seems that the genus be of little paleobiogeographical interest. A review by Torres and Phillipe proposes to keep the species below within Sahnioxylon.

1. S. andrewsii: Bose & Sah, 1954 (Rajmahal Hills, India, Late Jurassic or Early Cretaceous);
2. S. antarcticum: Lemoigne & Torres, 1988 (South Shetland, Antarctica, Late Cretaceous);
3. S. rajmahalense: (Sahni) Bose & Sah, 1954 (Rajmahal Hills, India, Late Jurassic or Early Cretaceous);
4. S. seward: Kumarasamy & Jeyasingh, 1995 (Madras area, India, Early Cretaceous, maybe conspecific with Mesembrioxylon seward Sahni 1931).

Distribution

The article by Torres and Phillipe stated that “the distribution of Sahnioxylon, is astonishing and does not comply with Mesozoic vegetation maps as proposed, for example, by Vakhrameev [1991].” Further in their article they stated 3 hypotheses on how the gymnosperm has been found in deposits all over the world.

1. The first bias is that data is lacking. The range of the plant could be larger but is unidentified. It may, in places, not be preserved, found, or published. In rebuttal to the hypothesis, is that sediments similar to the ones found in Sahnioxylon localities are all over the world.
2. The next bias is that samples that actually belong to Sahnioxylon, have not been described as such.
3. The final possibility is that of parallel evolution.