New petrified calamitaleans from the Permian of the Parnaíba Basin, central-north Brazil, part II, and phytogeographic implications for late Paleozoic floras

Abstract

Continuing palaeofloristic studies in the Northern Tocantins Fossil Forest, we describe two new calamitalean species from the Permian of the Parnaíba Basin (central-north Brazil). The fossils comprise axes of various sizes, preserved anatomically as siliceous petrifactions, and found in highly mature sandy fluvial deposits of the Motuca Formation. Based on anatomical and morphological characteristics, Arthropitys tocantinensis sp. nov. and Arthropitys barthelii sp. nov. are described. They share a small central pith cavity (extremely reduced in the latter), scalariform tracheid pitting, and prominent pitting of the ray parenchyma. However, they differ markedly in their branching system: the former having 3–12 branches per node either with or lacking secondary growth, the latter showing 2–17 branches without any secondary growth. However, in A. tocantinensis sp. nov., the presence of large woody branches supports a more complex architecture with at least three successive orders of branches. The extensive secondary tissue in both species is homogeneous; clear segmentation is only visible in the proximal wood of A. tocantinensis sp. nov., but completely absent in A. barthelii sp. nov. The growth architecture of these upright growing, self-supporting trunks are reconstructed based on sizable transverse and longitudinal preparations. Our results confirm that thick woody calamitaleans were elements of disturbed riparian vegetation and much more diverse in terms of anatomy and branching patterns than previously thought. They were well adapted to seasonally dry conditions and formed major plant constituents of Permian low-latitude Southern Hemisphere communities. Additionally, we report the first evidence of colonisation on arborescent calamitaleans by herbaceous sphenophyte axes from the Permian. One of the Arthropitys stems hosts at least 30 Sphenophyllum shoots of various ontogenetic stages, growing inside the destroyed pith, which was previously excavated by arthropod boring. Based on the distribution of key genera within late Paleozoic floras of Euramerica, Gondwana and Cathaysia floral realms cluster analysis and Jaccard Coefficient highlight the distribution of a “Mid-North Brazilian” phytogeographic Region during the early Permian.