Fossil ceratophyllum (Ceratophyllaceae) from the tertiary of North America

Fossil fruits and a vegetative axis assignable to the extant genus Ceratophyllum are described from four North American Tertiary localities. Fossil fruits assignable to the extant species C. muricatum and C. echinatum are reported from the Eocene Green River and Claiborne formations, and the Miocene Esmerelda Formation, respectively. An extinct species, C. furcatispinum, is described from the Paleocene Fort Union Formation and represents the oldest published report of Ceratophyllum in the fossil record. The existence of extant angiosperm species in the Eocene is very unusual and may be attributable in this case to slow evolutionary rates and unusual evolutionary properties associated with hydrophily in the genus Ceratophyllum. CERATOPHYLLUM is a submersed monoecious freshwater aquatic angiosperm with underwater pollination (hydrophily) and distinctive, often spiny fruits. One might expect such taxa to have an abundant fossil record, and in this paper we describe and illustrate fossil vegetative and reproductive Ceratophyllum specimens from four localities ranging in age from Paleocene to Upper Miocene. These specimens expand the known geological history of the group (Les, 1 988a) and provide an opportunity to evaluate the suggestion that slow rates of evolution and low diversity are consequences of hydrophilous pollination in perennial aquatic genera (Les, 1988c). Inefficient pollen transfer, reduced sexuality, widespread clonal growth and diminished seed production have been suggested as causal factors to explain the low species diversity in many hydrophilous genera (Les, 1988c). Hydrophily occurs in a phylogenetically heterogeneous group consisting of seven families and 18 genera, 16 of which consist of fewer than ten species (Les, 1 988c). The Ceratophyllaceae have been suggested to be a living representative of some of the oldest flowering plants (Les, 1988a). Therefore, the 1 Received for publication 16 December 1988; revision accepted 13 June 1989. We wish to thank Steven Manchester for photographing fossil specimens at the U.S. National Museum and for providing the Green River fossil fruits. We thank Lance Grande for providing access to the Green River fossil fish material. Figure 21 was prepared by the Cartographic Services Laboratory of the University of Wisconsin-Milwaukee. We thank Peter Crane for helpful comments and suggestions on an earlier draft of this paper. This work was supported in part by a National Science Foundation Grant BSR 8516657 to DLD and Indiana Academy of Science Grant to PSH. fossil history of Ceratophyllum is of interest particularly because it should allow for an evaluation of some of the hypotheses concerning diversity and rates of evolution in hydrophilous taxa (Les, 1988c). The Ceratophyllaceae have been reported previously in the paleobotanical literature. The extinct genus Ceratostratiotes Gregor from the Miocene of Europe has been suggested to have taxonomic affinities to Ceratophyllum. It is treated as a separate extinct genus in the Ceratophyllaceae because of differences from Ceratophyllurn in placentation, fruit wall ornamentation and symmetry (Buiz:ek, 1982; Les, 1988a). In addition to the reports of fossil Ceratophyllum speciescitedin Les (1 988a), several other reports are in the literature. Ceratophyllum miodemersum Hu and Chaney was reported from the Miocene of Shantung Province, China (Hu and Chaney, 1940). The fossil vegetative axes have whorled leaves which appear to be at least twice dichotomously divided. Reports of fossil material of twelve Ceratophyllum species, several of which were originally described in a number of different papers, are summarized by Dorofeev (1974). However, the report of C. praedemersum Ashlee (Ashlee, 1932; Brown, 1937) is erroneous. The axis illustrated by these authors is perhaps an immature platanoid inflorescence, not a vegetative shoot of Ceratophyllum. The Ceratophyllaceae appear to have been relatively diverse in the past. Thus the paleobotanical record provides an understanding of the extent of past morphological diversity as compared to that observed in extant Ceratophyllum species. Historically, taxonomic studies of Cerato-