Lectotypification of the name Regnellidium diphyllum (Marsileaceae)

Regnellidium Lindman (1904: 2) is a genus that includes only the extant species Regnellidium diphyllum Lindman (1904: 14) in the family Marsileaceae Mirbel (1802: 126), which encompasses a total of three extant genera of aquatic, heterosporous ferns. While the other two genera within the family, Marsilea L. (1753: 1099) with 50–75 spp., and Pilularia L. (1753: 1100) with 3–6 spp., are nearly cosmopolitan in distribution, the extant species of Regnellidium is restricted to a small region of South America, found only in northeastern Argentina, southern Brazil, and Uruguay (Schultz, 1949; de la Sota & Mitchel 1970; Tryon & Tryon 1982; Kramer, 1990; Alonso Paz & Bassagoda, 2002; Cúneo et al., 2013). It was only in 1892, on the first Regnell expedition, that Lindman discovered the plants, which he described as Regnellidium (Lindman, 1904), on the banks of the Rio Grande do Sul, in southern Brazil. The species was not found again until November 1935, when it was located at Santa Maria by Dr. W. Rau and at São Leopoldo by Dr. J. Dutra (Johnson & Chrysler, 1938). Today it is widely cultivated in Botanic Gardens.

Mid-Cretaceous megaspore floras from Maryland, USA

Bulk maceration of unconsolidated Potomac Group sediments collected from 12 samples of middle–late Albian and early Cenomanian age on the Elk Neck Peninsula, Maryland, yielded 14 genera and 18 species of megaspores. The identified megaspores have affinities to both heterosporous ferns, e.g., Arcellites and Molaspora, and to heterosporous lycopsids, e.g., Erlansonisporites, Minerisporites, and Paxillitriletes. Morphological variation within species is high, and most morphotypes differ from established taxa in one or more characters. Two new species—Dijkstraisporites praetextus and Verrutriletes diversus—are proposed. The composition of these middle–late Albian and early Cenomanian floras is a mixture of taxa previously recorded from Aptian and Santonian floras in North America. In addition, a fragmentary strobilus of selaginellalean affinity containing a megaspore and microspores is described.

Late Santonian megaspore floras from the Gulf Coastal Plain (Georgia, USA)

Fossil megaspore floras from the Late Cretaceous of North America have been studied extensively, but primarily from the Western Interior Basin. Two new megaspore floras are described from eastern North America along the Gulf Coastal Plain. Cumulatively, 10 genera and 16 species of megaspores are recognized from Allon, Georgia and along Upatoi Creek, Georgia (both late Santonian in age, ~84 Ma). Megaspores identified have affinities to both heterosporous lycopsids, e.g., Erlansonisporites, Minerisporites, and Paxillitriletes, and to heterosporous ferns, e.g., Ariadnaesporites, and Molaspora. Lycopsid megaspores are more diverse than fern megaspores in the Allon and the Upatoi Creek floras. Two new species—Erlansonisporites confundus n. sp. and Erlansonisporites potens n. sp.—are proposed.

Arcellites stellatus new species, a new megaspore from the Lower Cretaceous of Maryland, USA

Dispersed megaspores with affinities to aquatic heterosporous ferns are relatively common in mesofossil assemblages from the Early Cretaceous to the Recent. Extant heterosporous ferns are free floating or shallow rooted freshwater plants, with a dominantly tropical to warm-temperate distribution (Tryon and Lugardon, 1991). Their heterosporous life cycle (including both megaspores and microspores) is likely to be an adaptation to their aquatic habit (Collinson, 1991; Hemsley et al., 1999; Kar and Dilcher, 2002). Thus the abundance of heterosporous fern megaspores, or the presence of heterosporous fern macrofossils, within a stratigraphic interval may be indicative of a shallow, calm, freshwater depositional environment (Hall, 1963; Batten et al., 1996; Rich et al., 2001).

Diversity changes in lycopsid and aquatic fern megaspores through geologic time

Quantitative data on lycopsid and aquatic fern megaspore taxa recovered from Carboniferous, Mesozoic, and Tertiary strata have been compiled in order to analyze the changes in diversity of the two groups of fossil plants that produced them. Numbers of species of lycopsid megaspores are similar in the Carboniferous and Mesozoic, whereas the diversity of megafossils is much lower in post-Paleozoic deposits. Our data suggest that lycopsids were more diverse in the Mesozoic than previously thought and that there is a preservational bias against the megafossils, because the plants were probably mainly herbaceous. Heterosporous aquatic ferns first appeared in the Neocomian and gradually diversified until the early Late Cretaceous, after which their numbers remained relatively stable, whereas the variety of lycopsids declined dramatically during the Late Cretaceous. These changes occurred at a time of rapid angiosperm diversification. The reduced diversity of the lycopsids may have been caused by the invasion of their aquatic and damp forest-floor habitats by heterosporous ferns and by aquatic and herbaceous angiosperms. These diversity changes do not seem to be directly related to the global events at the Cretaceous-Tertiary boundary, but the relatively few samples available and the resulting range truncation would make detection of such correlations difficult.