Circinate vemation in Jurassic foliage: new data from Sardinia

Palaeobotany deals with the recovery and identification of plant remains from geological contexts. Plant fossils are usually represented by disarticulated parts of plants and because of that parataxonomy has been introduced for identifying these fossils. The organ-genus Spiropteris Schimper, 1869 is used to identifY coiled and immature leaves. This is the only genus erected for this kind of developmental stage and, according to some authors, it includes only ferns and seed ferns. This interpretation is challenged by the general diagnosis given by Schimper in its "Traite de Paleontologie Generale, Tome Premiere". Indeed, circinate vernation arrangement of immature leaves is known in the fossil record since the appearance of land plants and comprises gymnosperms. Due to a relocation and a new organization of the expositions, a lost portion of the Jurassic Lovisato plant collection of the University of Cagliari is now available and shows bennettitalean fragments arranged in coils. These remains are preserved in sandy lithology and for this reason the microstructures of cuticles are not preserved. The Bennettitales (Engler, 1892), a Mesozoic (Triassic to Cretaceous) group of gymnosperms, is extinct today. They have entire or simple pinnate leaves, morphologically similar to those of cycads (like the extant Cycas revoluta Thunb, 1782) and only distinguishable by epidermal features (syndetocheilic stomata). Recent studies show two different evolutionary lines for these two groups of plants. Even if they are distinct, they share ontogenetic similarities. Leaves of Pterophyllum Brongniart, 1825 with circinate vernation for the first time were discovered in the Carnian of Austria (Pott & Krings, 2007). The remains now assigned to Spiropteris in the Lovisato Collection might belong to the genus Ptilophyllum Morris in Grant, 1840, in view of their good preservation in this kind of Sardinian rock, which is generally not suitable for the preservation of fern remains.