VALUE OF HRXCT FOR SYSTEMATIC STUDIES OF PYRITIZED FOSSIL FRUITS

In recent years, both vertebrate and invertebrate paleontologists have embraced the use of computed tomography (CT-scan) technology as a nondestructive method for obtaining 3-D images, especially for rare and type specimens (e.g., Archaeopteryx). Paleobotanists have been slower to employ this technique. We have found High Resolution X-ray Computed Tomography (HRXCT) to be of value in our recent studies of pyritized fruits and seeds from the Early Eocene London Clay flora. This flora, the premier assemblage of Paleogene floras of Europe, has been interpreted as a paratropical rainforest. We studied fruits previously assigned to Toona sulcata (Meliaceae), today a tropical tree of Australasia. Working with the NSF-funded UTCT, pyritized specimens were scanned directly within their vials containing silicon fluid preservational medium. Fruits are ovoid, 5-loculate woody capsules 11-15 mm long x 10-14 mm wide, containing around 10 tightly imbricated seeds per locule. The wide central axis is prominently star-shaped, and placentation is axile. The fruits conform to the mahogany family Meliaceae, and are assignable to the traditional large subfamily Cedreloideae on the basis of winged seeds. The unusually thick fruit wall and characteristic seed morphology suggest that this fossil is most similar to the New World mahogany genus Swietenia, which bears around 12 seeds per locule. This Northern Hemisphere fossil occurrence has implications to the historical biogeography of the family. Based on present plant distribution, neontologists often evoke two very broad biogeographical patterns. A group either originated (1) in Gondwana and dispersed there, or (2) had a Northern Hemisphere origin and multiple dispersals between the Old and New Worlds via Beringia and the North Atlantic Land Bridge, eventually dispersing to the Southern Hemisphere. Today, particularly families like Meliaceae and Myrtaceae have patterns of distribution that have been described by neontologists as originating from Gondwana. However, fruit and seed records, found in such floras as the Late Paleocene Almont flora, and Eocene London Clay, Clarno Nut beds and Messel floras show these families were widespread in the Northern Hemisphere. Leaf records of comparable age (case: Myrtaceae) are in concordance with this pattern.