Reproductive anatomy of the conifer family Cheirolepidiaceae

STOCKEY, RUTH; ESCAPA, IGNACIO; ROTHWELL, GAR

Tokyo

Congreso; International Organization of Palaeobotany Conference; 2012

Chuo University

The Cheirolepidiaceae is a large family of extinct conifers of several genera that formed a dominant component of many Mesozoic floras worldwide. The unifying feature of these conifers is the unique pollen type, Classopollis (Upper Triassic-Upper Cretaceous). Until recently, the family has been represented primarily by compression/impression fossils with excellent cuticular remains. More recently, however both pollen and seed cones with excellent internal anatomy have been discovered. The pollen cone Classostrobus crossii from the Jurassic of the UK contains Classopollis pollen within clusters of pollen sacs that occur abaxially on the inner surface of the distal lamina on helically arranged sporophylls of simple cones. The permineralized seed cones described as Pararaucaria have recently been reexamined, and new specimens have allowed for their reinterpretation as cones of Cheirolepidiaceae showing for the first time anatomical characters of seed cones for this enigmatic family. The genus Pararaucaria was erected by for permineralized seed cones from the historic Jurassic Cerro Cuadrado Petrified Forest in the Santa Cruz Province of southern Argentina (Patagonia). New specimens of of Pararaucaria patagonica from the Cerro Cuadrado and cones of two new Pararaucaria species from the Late Jurassic Cañadón Calcáreo Formation (Chubut Province, Argentina) and the Middle Jurassic Trowbridge Formation (Oregon, USA), reveal previously unknown systematically informative characters. Whereas, the Argentine fossils are preserved in a silica matrix like the type species, the Oregon cone occurs in a marine volcanoclastic carbonate concretion. Different preservational attributes of the three species provide complementary information about previously unknown or incompletely characterized features (e.g., the presence of distal ovuliferous scale lobes, distal branching of ovuliferous scale trace, number of seeds per bract/scale complex and a reinterpretation of the putative seed wing as a seed enclosing tissue), and allow for an improved understanding of the genus. Together, the newly interpreted characters demonstrate that Pararaucaria represents the conifer family Cheirolepidiaceae, thus providing the first anatomical information for seed cones of this important Mesozoic conifer family. This new information also emphasizes the dependence of conifer taxonomy on homology hypotheses, and on structural interpretations of the different seed cone structures. All three species of Pararaucaria occur in association with seed cones assignable to the genus Araucaria, demonstrating the geographical and chronological extension of Cheirolepidaceae-Araucariaceae forest biomes from the Middle Jurassic to the Late Jurassic and from the Southern Hemisphere to the Northern Hemisphere.