Format: ORAL

Stephen McLoughlin

Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden

Peltasperms are extinct seed plants with simple, forked, or variably dissected fern-like leaves having thick cuticle and sunken actinocytic stomata. Triangular ovules were borne on the underside of ovate or peltate appendages attached to a slender stalk. Clustered pollen sacs were arranged at the tips of loosely branched stalks. The attachment of reproductive and vegetative organs remains unresolved, but is critical to resolving their phylogenetic placement. The pollen is monosulcate or taeniate to non-taeniate bisaccate. Their wood and growth habit is unknown but they are commonly assumed to have been shrubs to trees of open habitats. Phylogenetic analyses have focused on only a few representatives of this palaeoecologically heterogeneous group. Past analyses have resolved a close relationship between Peltaspermales and Umkomasiales, but inferred affinities of these groups to other seed-plants vary widely. Peltaspermales were one of the few Paleozoic seed fern clades to persist into the early Mesozoic. This order originated in the Late Carboniferous and diversified primarily within northern low- to middle-palaeolatitudes in the Permian. Several distinct groups have been recognized, generally occupying discrete regions during the Permian: supaioids (southwestern United States), callipteroids (euramerican province), tatarinoids (Volga-Uralian region), and possibly comioids (palaeotropical belt). Peltaspermales sparsely penetrated Gondwana and Cathaysia in the mid-Permian. However, following the hyperthermal end-Permian extinction event, the family abruptly spread globally and Early Triassic Lepidopteris/Germaropteris species developed thick cuticles, strongly protected stomata and small leaflets adapted to hot and seasonally dry conditions. The family rediversified through the Triassic, encompassing forms with complex reticulate venation. Lepidopteris ottonis markedly increased in abundance near the end-Triassic extinction event and produced aberrant pollen suggesting major disruption to reproductive strategies. Peltaspermales disappeared during the end-Triassic Extinction Event apart from two relictual Jurassic occurrences.