NEW OBSERVATIONS ON SALVINIA MACROFOSSILS AND MICROFOSSILS FROM THE MIOCENE OF MISSISSIPPI, USA

Heterosporous water ferns (order Salviniales) include five extant and multiple extinct genera and have a fossil record extending back to at least the Early Cretaceous. Macrofossils and dispersed and in situ spores are important for understanding the evolution of water ferns and the ancient environments that they inhabited. Salvinia (~12 extant species) is a subtropical to tropical floating fern with a fossil record extending to the Late Cretaceous. Most North American fossil records of Salvinia are Paleogene-aged sterile sporophytes assigned to Salvinia preauriculata, whereas fertile material and dispersed spore records are rare. In 2019, McNair et al. (Palaeontologia Electronica 22.2.40A) described an unnamed Salvinia species based on sterile and fertile macrofossil specimens from the Miocene Hattiesburg Formation, Mississippi, USA. Further study of the Hattiesburg Formation Salvinia material has confirmed some of their prior observations and has also revealed new details. The Hattiesburg Salvinia sporophytes have three types of lateral appendages (leaves or leaf homologues), including: 1) pairs of elliptical floating leaves with reticulate venation and newly reported simple trichomes; 2) poorly preserved, possibly inflated structures (sometimes called “floats”), newly identified here; and 3) branched submerged organs. Bisporangiate sori containing multiple sporangia are borne on the submerged organs. Samples of fossil-bearing matrix were macerated for this study, allowing for characterization of the spore morphology. Each microsporangium contains a single massula that encloses multiple microspores within a common pseudovacuolate perine. Each megasporangium contains a single elliptical to ovate megaspore that has three broad triangular flaps at the proximal pole and a surface that varies in morphology from scabrate and perforate to finely wrinkled and/or tuberculate. The Hattiesburg Salvinia material is significant because: 1) It has features (“floats,” bisporangiate sori) not characteristic of modern Salvinia species but found in some other fossil species; 2) The vegetative and reproductive organs have been found in attachment; 3) It adds to the relatively sparse North American spore record of Salvinia.