Fossil Mastixioid (Cornaceae) Alive in Eastern Asia

Abstract

The tree species Diplopanax stachyanthus, wrongly placed in the Araliaceae, belongs in fact to the mastixioid subfamily of Comaceae. Fruit features make it congeneric with Mastixicarpum, part of a woody-fruited complex known heretofore from Tertiary fossils. WHEN A GROUP of organisms known just from fossils turns out to have survivors, the finding attracts wide attention. Among animals the famed example is a fish, the coelacanth. Among higher plants the one example has been Metasequoia, found alive in the 1940s. We report the discovery of a living woody-fruited mastixioid, member of a plant group thought to have expired some 4 million years ago. No fossils are more prominent in lignite beds of Europe than mastixioid fruits. Collectors of the 19th century called them palm fruits, acorns, orAnnona seeds. By the early 1930s, however, British paleobotanists had trained their lenses on internal structure and had made correct comparison with fruits of Mastixia, a genus of trees now confined to Southeast Asia and islands of the western Pacific. The 19 living species of Mastixia have fruits that are much alike, but related fruits from fossil beds are diverse-so diverse that fossil species have been added to Mastixia and fossil genera established near Mastixia, to make taxonomic sense of the diversity. Species of Mastixia, extant and extinct, with species of related fossil genera, comprise subfamily Mastixioideae of the dogwood family, Cornaceae. Following the British lead, German paleobotanist Franz Kirchheimer searched the rich fossil fruit collections of the European mainland with mastixioids in mind and found kinds not represented in the British record (Kirch1 Received for publication 5 December 1989; revision accepted 25 January 1990. Research began while Xiang was a visiting fellow at the National Museum of Natural History. A. L. Takhtajan, leaming of our work, graciously gave up his research on Diplopanax and sent us his slides. We thank M. Leighton for information, D. Dean and S. Yankowski for technical help, and V. Krantz for photographs. 2 Current address: Department of Botany, Washington State University, Pullman, WA 99164. heimer, 1957, for review). Fifteen genera of mastixioid fruit remains came into print eventually, but current taxonomic views reduce the worldwide list to eight fossil genera, 33 fossil species. Mastixioids are relatively few in North America, and the record ends in the early Tertiary. The European record, however, reaches from uppermost Cretaceous to late Miocene, roughly from 65 million to 7 million years ago. Some of these fruits are superbly preserved, and in some places they abound; Poland's Turow mine alone has yielded 2,400 fruits in five mastixioid genera. Though Mastixia-like leaves and pollen grains occur as fossils, they lack the diversity that makes mastixioid fruits important for inferring past climatic changes. Mastixioid flora is the European term for the subtropical to paratropical broad-leaved evergreen vegetation that once covered much of the northern hemisphere. Mastixioid fruits are stone fruits; as the fruit grows, its inner cell layers turn into a woody housing for a chamber that contains a single seed. Some ofthe ancient mastixioids had multiple seed chambers in their stones, and the various kinds differed with regard to the hardness of the fruit's outer layers. Fossils indicate that the Mastixioideae evolved in two main lines: a single-genus line (Mastixia itself) having fruits with soft outer cell layers, and a several-genus line having fruits with hard outer layers. For convenience we call the latter group woody-fruited, even though the outer tissue was more leathery than woody in some species. Tooth marks and gnawed holes in fossil and modern stones show that mastixioid fruits can be moved about, and often destroyed, by rodents. The fleshy outer layers of Mastixia's fruits give it the advantage of nondestructive bird dispersal, and that may be the reason for Mastixia's survival into modern times. It was long supposed that Mastixia's woody-