Tiny ecosystems: bryophytes and other biotic interactions around an osmundaceous fern from the Eocene of Patagonia.

BIPPUS, ALEXANDER C.; ESCAPA, IGNACIO; TOMESCU, ALEXANDRU M. F.

Savannah

Congreso; Botany 2016; 2016

Botanical Society of America

The Eocene La Huitrera Formation is well-known for its remarkably diverse angiosperm-dominated compression flora, including the classic locality Laguna del Hunco. In addition to this, recent collections from this rock unit in the vicinity of Laguna del Hunco have uncovered anatomically-preserved plant fossils, including numerous wood and vascular plant fragments. One of these fossils is a well-preserved Todea (Osmundaceae) rhizome. Thin sectioning of this rhizome reveals a mat of densely arranged small leafy plants around its periphery. The small size of these plants, together with their organization and the lack of lignified tissue, indicate that they are bryophyte gametophytes. Additionally, the intimate association between these bryophytes and the Todea rhizome suggests that they are in situ epiphytes. Large patches of bryophytes are found between and around the outer surface of the leaf bases. Aggregates of fungal hyphae and trails of small coprolites strongly resembling those of oribatid mites are also interspersed throughout both the bryophyte mat and the Todea leaf bases. Our samples contain a minimum of 250 bryophyte shoots associated with this rhizome. The shoots are 0.3 mm in diameter and at least 2 mm long. Stems, 80 µm in diameter, consist of small rectangular cells (12 x 65 µm) and bear very thin (12 µm) leaves with opposite insertion. Many of the shoots are branched, several exhibiting extensive pinnate branching. Leaves are up to 500 µm long, with incompletely preserved apices. The opposite phyllotaxis and highly-branched architecture, along with the extremely thin leaves (probably unistratose and ecostate), suggest that these epiphytic bryophytes are leafy liverworts. However, incomplete leaf preservation precludes narrower taxonomic circumscription. Despite the lack of taxonomic precision, these fossils substantially augment the bryophyte fossil record. They are among the very few fossil bryophytes described from South America, and the first described from La Huitrera Formation. This discovery also expands the fossil record of epiphytic bryophytes, represented currently by only three reports of epiphyllous moss protonemata. Thus, this bryophyte mat is the first account of mature bryophyte gametophytes as epiphytes and the first report of epiphytic liverworts in the fossil record. More generally, this fossil association records biotic interactions between a fern, bryophytes, fungi, and arthropods. This information may help to better understand the immediate growth environment of these organisms and contributes to the growing body of evidence documenting the evolution of complex interactions between epiphyte communities and host plants.