Fruits of an “Old World” tribe (Phytocreneae; Icacinaceae) from the Paleogene of North and South America

The Phytocreneae (Icacinaceae) are a tribe of scrambling shrubs and lianas presently distributed in tropical Africa, Madagascar, and Indo-Malesia. We describe the oldest known fossils of this tribe and provide the first recognition of this group in the Neotropical fossil record based on distinctive fruit remains. Palaeophytocrene piggae sp. nov., from the late Paleocene of western North America, and Palaeophytocrene hammenii sp. nov. and cf. Phytocrene sp., from the middle-late Paleocene of Colombia, constitute the oldest confirmed records of this tribe. Pyrenacantha austroamericana sp. nov., from the Oligocene of Peru, represents an extant Old World genus known also from the Eocene fossil record of North America and Europe. Collectively, these fossils indicate that the Phytocreneae were previously established in the Neotropics, despite their current absence from the region, and may provide evidence for Paleogene floristic exchange between North and South America. Keywords—Biogeography, fossil endocarps, Icacinaceae, Neotropics, Paleogene. The Icacinaceae are a pantropical family within the Lamiidae (Soltis et al. 2011) consisting of trees, shrubs, and woody climbers. As currently circumscribed (Kårehed 2001; Lens et al. 2008), the family contains 35 genera and 150 species divided among four informal groups: the Icacina, Apodytes, Cassinopsis, and Emmotum groups. Although these groups probably do not together form a monophyletic assemblage, the Icacina group itself is strongly supported in molecular and morphological phylogenetic analyses as monophyletic (Kårehed 2001; Lens et al. 2008). This clade, which is predominantly Paleotropical today, includes all of the genera of the traditional tribes Iodeae, Phytocreneae, and Sarcostigmateae, and some of the genera of the traditional tribe Icacineae (tribes sensu Engler 1893; Sleumer 1942). Relationships within the Icacina group remain poorly understood, but it appears that, of the tribes within this clade, perhaps only the Phytocreneae are monophyletic (Kårehed 2001; Lens et al. 2008). The Phytocreneae (Engler 1893; Sleumer 1942), which consist of the lianescent genera Chlamydocarya Baill. ( five species), Miquelia Meisn. ( eight species), Phytocrene Wall. ( 12 species), Pyrenacantha Wight ( 30 species), and Polycephalium Engl. ( two species), occur today in lowland tropical forests of Africa, Madagascar, and Indo-Malesia (Sleumer 1971; Fig. 1). Fruits of this group are easily recognized and well documented in the fossil record (Fig. 1) due to the distinctive features of their endocarps, including deeply pitted outer surfaces formed by tuberculate indentations, which, in all genera except Phytocrene, penetrate into the locule, and transversely oriented, interlocking undulate to digitate cells making up the endocarp wall (Reid and Chandler 1933; Villiers 1973; Manchester 1994; Potgeiter and van Wyk 1994). Fossils of the Phytocreneae are well represented in the Paleogene of Europe (Reid and Chandler 1933) and North America (Crane et al. 1990; Manchester 1994; Rankin et al. 2008; Stull et al. 2011), suggesting that they were an important element of mid-latitude forests during the warm interval of the early Paleogene. Younger occurrences of Phytocreneae (Pyrenacantha and Chlamydocarya) are also known from the Oligocene Fayum flora of Egypt (Manchester and Tiffney 1993), indicating that this group has been present in tropical forests of Africa for at least 30 Ma. To date, the tribe has not been recognized in the fossil record of the Neotropics. In this paper, we describe the oldest known fossils of this tribe and provide the first recognition of this group in the Neotropical fossil record. Palaeophytocrene piggae sp. nov., based on endocarps from the late Paleocene ( 58 Ma) of western North America, and Palaeophytocrene hammenii sp. nov. and cf. Phytocrene, from the middle-late Paleocene of Colombia ( 60–58 Ma), represent the oldest known fossil records of this tribe (Fig. 2). Pyrenacantha austroamericana sp. nov., a carbonatepermineralized endocarp from the late early Oligocene of Peru ( 30–28.5 Ma), represents an extant genus also known from the Eocene fossil record of North America and Europe (Fig. 3). These fossils provide important information on the geographic history of this group, indicating that it was previously established in the Neotropics, despite its current absence from the region. The presence of Palaeophytocrene and Pyrenacantha in both North and South America may also reflect a broader pattern of floristic exchange between these regions during the Paleogene (Jaramillo and Dilcher 2001; Pennington and Dick 2004; Herrera et al. 2011). Materials and Methods Geological Settings—Palaeophytocrene piggae is based on endocarps from several localities in the Paleocene of the Great Plains of western North America. Two permineralized specimens were collected from the Almont locality, Morton County, North Dakota (Sentinel Butte Formation), which is considered late Paleocene ( 58 Ma) in age (Crane et al. 1990; UF loc. 15722, 46 55012.8600N, 101 30017.4000W). Another permineralized specimen was recovered from Beicegel Creek, McKenzie County, North Dakota (UF loc.18907, 47 21.9090 N, 103 25.4250 W). The other specimens, which consist of endocarp impressions, casts, and molds, were collected from the Fort Union Formation in Montana and Wyoming. Brown (1962, pl. 67, Fig. 26) figured a specimen as “impression of a seed showing pits arranged in longitudinal rows” from Lebo Creek, Montana (USGS loc. 4618), and listed the same taxon from Sand Creek, 7 miles N. of Glenrock, Wyoming (USGS loc. 8551). Additional Fort Union Formation occurrences are Sand Draw (USGS loc. 9532, 42 48.0490N, 108 10.9680W), Hells Half Acre (UF loc. 15740D, 43 01.020N, 107 04.660W), Leffingwell Bluff (UF loc. 15776, 43 18.420N, 105 02.090W), and Linch (UF loc. 18255, 43 38.020N, 106 12.320W) in Wyoming, and Traub Ranch (USGS loc. 8910, 45 9.3440N, 105 41.5930W) in southeastern Montana. These fossils are stored at the Florida Museum of Natural History (UF), the Field Museum (PP), and the Smithsonian Institution (USNM). Although Crane et al. (1990) previously reported several of these fossils, they did not formally name or describe the species; here we provide a full treatment of these stratigraphically significant records.