On the earliest occurrence of Tolypella section Tolypella in the fossil record and the age of major clades in extant Characeae

摘要

The genus Tolypella is considered the basal-most characean genus according to modern molecular phylogenies. Its fossil record, however, provides contradictory evidence since fossil Tolypella section Tolypella has its first fossil occurrence in the Late Cretaceous, about the same time as the first occurrences of other genera of living characeans, i.e. Chara, Lamprothamnium, Nitellopsis and Lychnothamnus which are considered more derived. In this study, the first occurrence of Tolypella sp. aff. T. grambastii subsp. arctica (Tolypella section Tolypella) is now documented from a lacustrine bed, in the Lower Cretaceous of the Garraf Massif (Catalonia, Spain), ca. 125.0 Ma old. This indicates that Tolypella s.s. is indeed a very old genus and that its first fossil record should be extended back at least 55 million years. The first appearance of living Characeae in the fossil record correlates well with the topology of molecular phylogenies. The basal genera Tolypella, Nitella and the ancestors of the extant Chareae represent the first radiation of the characeans during Late Jurassic-Early Cretaceous. The oldest representative of the clade of Tolypella in the fossil record, belonging to Tolypella section Rothia, suggests that the divergence of Tolypella is at least of Kimmeridgian age (157.3-152.1 Ma). The splitting of Nitella and the Chareae is dated as Oxfordian in age (163.5-157.3 Ma). The extant representatives of the crown group (Chara, Lamprothamnium, Nitellopsis and Lychnothamnus) thus represent the remnants of the second radiation of the Characeae during the Upper Cretaceous, at least 83.6-72.1 Ma ago.