Arborescent lignophytes in the Tournaisian vegetation of Queensland (Australia): Palaeoecological and palaeogeographical significance

Abstract

A middle Tournaisian flora comprising several taxa of arborescent lignophytes is documented for the first time in Australia. It challenges earlier hypotheses considering that the early Mississippian vegetation of Australia was impoverished and dominated by small to medium sized arborescent lycopsids until the Serpukhovian when climatic conditions became favorable to the thriving of the famous lignophyte-dominated “ Rhacopteris flora” recorded in Australia but also South America. The present account is based on anatomically preserved fragments of trunks and branches from two localities of Queensland, in the Broken River Province and the Burdekin Basin. Three taxa are recognized, Dameria hueberi gen. et sp. nov., the putative progymnosperm species Protopitys buchiana Goeppert 1850, and a taxon of uncertain affinities characterized by a Pitus type of wood but which differs from the latter genus by anatomical traits relative to its primary body. The possible occurrence of subtle growth rings in some of the woods from the Burdekin Basin and the conifer like traits of the Dameria wood are interpreted as ecological clues suggesting that these trees may have inhabited drier settings than the wetland habitats generally inferred for the lycopsids. This difference in habitats is suggested to explain the rarity of arborescent lignophytes compared to lycopsids in the Australian record of plants of early Mississippian age. The three taxa reported in this paper represent the earliest record of arborescent lignophytes in Australia after the global events that affected the vegetation at the Devonian–Carboniferous boundary and the extinction of Archaeopteris. They show that the successional pattern of arborescent lignophytes around the Devonian–Carboniferous boundary is comparable to that observed in Western Europe with the evidence of a diversification occurring as early as the middle Tournaisian. The palaeogeographic and stratigraphic distribution of Protopitys and Dameria indicates the occurrence of extensive floral exchanges between southern Laurussia and north-eastern Gondwana in the Late Devonian to Mississippian.