The lower cretaceous flora of the gates formation from western Canada

Abstract

The Lower Cretaceous Gates Formation (late Early Albian) of western Canada is a sequence of paralic coal-bearing strata composed of siltstone, sandstone and coal. Macrofossil plants are abundant in the Gates Formation; most fossils are impressions; others include casts and molds of tree trunks. No permineralized fossils are found. The Gates flora consists of bryophytes (Marchantiolites and Thallites), Equisetites, ferns (Gleichenites, Acanthopteris and Coniopteris of Dicksoniaceae, Cladophlebis, Sphenopteris and a new genus), seed-ferns (Sagenopteris and a new genus), conifers (Pityocladus and Pityophyllum of Pinaceae, Athrotaxites and Elatides of Taxodiaceae, Elatocladus), cycads (Chilinia, Ctenis, Pseudocycas and Pterophyllum and two new genera), Gingko and Ginkgoites; leptostrobans (a new genus), Taeniopteris and unidentified angiosperms. In total, 52 species from 28 genera are described, including 5 new genera, 15 new species and 3 new combinations. Most plants of the Gates flora appears to have been deciduous. Only Elatides curvifolia and Elatocladus manchurica are convincingly evergreen. The interpreted paleoclimate based on the Gates flora is strongly seasonal with winter minimum temperature possibly below $-$15°C. Rainfall appears to have been abundant since coal deposits are common in the Gates Formation. Although low winter minimum temperature appears to be the main factor causing deciduousness of the Gates flora, low winter light levels may have also contributed to the deciduousness of the Gates flora, as the paleolatitude of the study area was situated at 50°-60° N. Three Early Cretaceous floral provinces are recognized: the Arctic Province, which lacks Cheirolepidiaceae; the Equatorial Province, which has Cheirolepidiaceae; and the Antarctic Province, which also lacks Cheirolepidiaceae. Floras similar to the Gates flora have been reported from throughout the Arctic Province, including Montana, the western Interior of Canada, the Bowser Basin of northwestern Canada, Alaska, western Greenland, Spitzbergen, Siberia, northern Mongolia, northeastern China and the Inner Zone of Japan. The Pacific-rim areas are excluded from the Arctic Province. Plant deciduous habit appears to have prevailed within the Arctic Province during the Early Cretaceous.