Synchrotron computer tomographic (CT) scans complement traditional techniques in understanding the internal anatomy of permineralised Fontainocarpa (Crotonoideae, Euphorbiaceae) fruits from the Oligocene of eastern Australia

Abstract

The internal morphology and anatomy of silicified fruits of Fontainocarpa were studied using traditional thin sectioning techniques, SEM and synchrotron computed tomographic (CT) imaging and animations, to enable comparative analyses with extant, indehiscent-fruited genera in the Euphorbiaceae including Fontainea, Aleurites and Hylandia from Australia, and other non-Australian crotonoid genera. Thin sections and sectioning show that the fruits of Fontainocarpa are indehiscent, multicarpellate and usually 3- to 5-loculate, with axial placentation, a single ovule per carpel and the ovules are anatropous and have antitropous curvature. A ventral vascular trace that supplies each ovule is embedded in the bitegmic seed coat. The internal anatomy is therefore consistent with the Euphorbiaceae. Additional characters, including indehiscent fruits, distinctive vascular channels (foramina) that penetrate through the fruit wall into the locule, and thin membranous seed coats are restricted to very few genera in the Euphorbiaceae, but occur together in extant Fontainea. The seed coat in extant Fontainea and fossil Fontainocarpa seeds is membranous, and appears to lack the palisadal exotegmen of most genera in the Euphorbiaceae. Fontainocarpa fruits were compared with those of extant Fontainea and the fossil has a combination of features unlike those of extant taxa. It shares with Fontainea picrosperma in having endocarps with convex intersutural surfaces lacking ornamentation and a similar number of locules and with Fontainea venosa in having conspicuous foramina. This study therefore supports a close relationship between Fontainea and Fontainocarpa and is further evidence of the Crotonoideae in the fossil record in Australia, and is one of the few records of this subfamily worldwide. This study is one of the few, to date, using synchrotron CT imaging to reveal the internal morphology of silicified fruits and to utilize animations to examine the structure of these fruits