Petalophyllites speirsiae gen. et sp. nov. (Marchantiophyta: Fossombroniales), a fossil liverwort gametophyte from the Paleocene of Alberta, Canada

Although liverworts (Marchantiophyta) are postulated to be one of the oldest lineages of land plants, their documented fossil record is sparse. This study describes five new fossil liverwort gametophytes, one of which bears antheridia, from the Paleocene Paskapoo Formation of Alberta, Canada. The thalli are bilaterally symmetrical, with two undissected wings that bear oppositely arranged dorsal lamellae. One specimen is dichotomously branched, and one bears clusters of antheridia along the midrib area of its dorsal surface. These characters are consistent with those of extant Petalophyllum Nees & Gottsche ex Lehm. (Fossombroniales: Petalophyllaceae), and the genus Petalophyllites gen. nov. (Fossombroniales: Petalophyllaceae) is here erected for these fossils. They are the first representatives of Petalophyllaceae and the Fossombroniales from the fossil record. Most of the specimens are in growth position. Their geologic setting and associated fossils indicate that Petalophyllites lived at the margin of a lake or pond in a habitat that may have been intermittently dry, as is also common in extant species of Petalophyllum. The well-preserved thalli and antheridia of these fossils, and their well-documented middle Paleocene age, provide new evidence of liverwort diversification at the dawn of the Cenozoic Era.