New record of the aquatic fern marsilea, from the olmos formation (Upper campanian), Coahuila, Mexico

Premise of research. In the past two decades, numerous aquatic ferns related to Marsileaceae from the Upper Cretaceous have been described. Most fossils resembling Marsilea L. but lacking reproductive structures have been included in the fossil genus Marsileaceaephyllum. Here we describe a new species of the aquatic fern genus Marsilea from the Olmos Formation (upper Campanian), northern Mexico, that is represented by leaves, rhizomes, and a sporocarp. This new fossil represents the first unequivocal record of the extant genus Marsilea worldwide from the Late Cretaceous. Methodology. The fossils we describe in this article were collected from sediments of the Olmos Formation, Coahuila, Mexico. The samples were identified by direct comparison with fossils from other localities and extant species. Pivotal results. The aquatic fern described here consists of leaves that are composed of four opposite leaflets in a cruciform arrangement. The fossil leaflets can be sessile or attached to a short petiolule, and their base is straight to cuneate. The leaflet apex can be obtuse and/or round, and the margin is entire but is sometimes slightly wavy toward the apex of the leaflet. The leaflets have straight venation, dichotomous with anastomoses (reticu-late) throughout the leaflet. The areoles are polygonals and fusiform, and they fuse to form a marginal vein. Small sporocarp without teeth, oblong in lateral view. The combination of the features discussed above supports the creation of a new species of Marsilea from the upper Campanian of northern Mexico. Conclusions. The new species of Marsilea reported here represents the first record of the genus from the Cretaceous and provides additional evidence that aquatic plants such as Marsilea—as well as Salvinia, Dorfiella, Pistia, Nymphaceae, and Nelumbonaceae—were an important element of the flora preserved in the Olmos Formation.