STUDIES ON FOSSIL AZOLLA: AZOLLA MONTANA, A CRETACEOUS MEGASPORE WITH MANY SMALL FLOATS

Azolla montana is a new species from the late Cretaceous; it has a megaspore apparatus with a single large and conspicuous columella which resembles the floats of the megaspore apparatus of many extant species. The columella differs from true floats in being pilose, like the perispore of the megaspore body. The 10-20 small true floats are appressed to the columella, and are difficult to distinguish from it. It is suggested that the floats were derived, phyletically, by segmentation of the columella; the columella represents the primitive "float" of the megaspore apparatus. THE FOSSIL RECORD of the aquatic fern Azolla is based largely on its highly distinctive massulae (which bear microspores) and megaspore apparatuses, of which there are records dating back to the late Cretaceous in North America (Hills and Gopal, 1967; Hall, 1968). There is a total of 31 species, 6 of which are extant. Two of the extant species have a fossil record in the Pleistocene; these two species occur in interglacial deposits in Europe, but they became extinct there during the latest glaciation and were reintroduced as now successful adventives. Table 1 gives the known characteristics and stratigraphic ranges of the named species. Taxonomically, the species of Azolla are placed in four sections of the genus, but only two of these (Rhizosperma and Azolla) are well known. Sterile vegetative fronds are rather uniform throughout the genus; the four sections are based on characters of massulae and megaspore apparatuses, as follows: Section Azolla: Massulae with hooked or anchorshaped glochidia; megaspore apparatus with three "floats" in the "swimming apparatus."2 Section Rhizosperma: Massulae with few, simple, straight or branched glochidia, or no glochidia. Megaspore apparatus with nine floats. Section Antiqua: Megaspore apparatus with about nine floats, all of equal size. (The validity of this section, known from one fossil species, may be questioned.) Section Filifera: Massulae with hair-like, coiled or knobbed glochidia. There is only one fossil species in this section (Hall, 1968). Massulae of section Antiqua (Dorofeev, 1959) 1 Received for publication 11 March 1968. Supported by NSF Grant GB4090 and by funds from the Graduate School, University of Minnesota. Appreciation is expressed to Dr. R. K. Jain for his contributions toward the reconstruction of A. montana. 2 As long ago as 1893 Campbell showed that the megaspore apparatus sinks when shed; the term swimming apparatus is a misnomer, but it is entrenched in the literature. and megaspores of section Filifera (Hall, 1968) have not yet been described. Prior to the discovery of the Paleocene species with megaspore apparatuses having many floats and the Cretaceous species which have hooked glochidia, the phylogeny of the genus seemed reasonably clear: section Azolla occurred before species of section Rhizosperma, and the theoretical bases for deriving the latter from the former were substantiated by their fossil record. However, Table 1 includes a number of fossil species that cannot be readily assigned to either of the sections defined on the basis of the living species. Several examples stand out. The megaspore apparatuses of the Paleocene species A. teschiana Florschuitz and A. schopfi Dijkstra have more than nine floats-not typical of members of section Rhizosperma, and certainly not of section Azollabut massulae with hooked glochidia are attached to the megaspore apparatuses of A. teschiana and are associated with those of A. schopfi (Dijkstra, 1961). Then, too, the megaspore apparatuses of A. primaeva (Penhallow) Arnold have never been shown to have floats (Arnold, 1955) but Azolla-type glochidia occur on the massulae. A. prisca Reid and Chandler is generally assigned to section Rhizosperma on the basis of the nine-float megaspore apparatus, but Azollatype massulae are associated but not attached. In this instance Reid and Chandler (1926) suggest that A. prisca is a transitional species. As more species are found, particularly from the late Cretaceous and Paleocene, additional taxonomic anomalies are sure to occur. These examples are of species where both types of reproductive structures are known; where only one is known, as in A. cretacea Stanley which has hooked glochidia, it would be prudent not to imply the occurrence of the corresponding type of megaspore apparatus. A. geneseana Hills and Weiner, referred to briefly later, does seem to have both massulae and megaspore apparatuses of the section Azolla; if this is reallv so, this