Fossil ferns of the Asian tropics: Salvinia and Acrostichum macrofossils and diverse fern spores from the early Eocene Ghazij Formation (Balochistan, Pakistan)

Background: Ferns are most taxonomically and ecologically diverse in the tropics, but Cenozoic tropical fern fossils are rare, limiting understanding of fern evolution and biodiversity through time. Both Salvinia (Salviniaceae) and Acrostichum (Pteridaceae) are widely distributed ferns that are most diverse and abundant near the equator. Salvinia and Acrostichum macrofossils and spores have a rich Cenozoic record from subtropical and temperate regions, but only a handful of fossils have been discovered in the tropics, and few are from Asia. Here we report new fossil Salvinia and Acrostichum macrofossils and a diverse fern-spore assemblage from the early Eocene Ghazij Formation (Balochistan, Pakistan), palaeolatitude 0–5°N. Methods: As part of a larger study of the Ghazij flora, we collected 29 fern macrofossils and 39 palynology samples from multiple localities. The fern fossils were compared with similar extinct and extant vascular plants and systematically described. Key Results: Salvinia dashtensis sp. nov. is described from four localities in the upper Ghazij Formation based on vegetative leaves, submerged rhizomorphic leaves, and inflated submerged structures (floats), along with Salvinia microspore massulae. These Salvinia occurrences serve as important freshwater palaeoenvironmental indicators. Acrostichum pinnae are identified at one locality in the upper Ghazij Formation from their characteristic reticulate secondary venation, forming rectangular to hexagonal polygons without freely ending veinlets. Acrostichumsporites spores are also identified. Spores of a total of 21 palynotaxa representing ten pteridophyte families (Gleicheniaceae, Lindsaeaceae, Lygodiaceae, Matoniaceae, Polypodiaceae, Pteridaceae, Osmundaceae, Schizaeaceae, Salviniaceae, and Thelypteridaceae) record a diverse fern assemblage from various ecological niches and the first occurrences of Pteris, Baculatisporites, and Verrucosisporites for Palaeogene Indo-Pakistan. Conclusions: The Salvinia and Acrostichum occurrences from the Ghazij Formation provide rare fossil examples of extant fern genera in the ancient tropics and represent some of the only fern fossils for all of Palaeogene tropical Asia.