Examination of the unique preservation and identification of enigmatic fossil tree specimens from Nova Scotia

Abstract

The geological and paleobotanical history of Nova Scotia is rich and diverse. Sites such as the Joggins Fossil Cliffs and the Sydney Coal Mines have been studied extensively, but studies on fossil tree specimens from other localities are lacking. This study examines two mineralized tree specimens from the Carboniferous Period, one preserved in marine gypsum from Miller Creek (Gypsum Tree - NSM022GF16.1) and one preserved in fluvial deposits from Cheticamp (Cheticamp Tree - NSM024GF22.1). Both specimens share a similar external morphology, which this study aims to examine with the following questions: 1) identify the specimens based on their vascular anatomy, 2) determine if they are taxonomically related, and 3) conduct a mineral analysis to determine the process of mineralization. An extensive literature review was conducted to determine applicable taxonomic groups and modes of preservations. Transverse and longitudinal thin sections were taken of both specimens, and were examined using optical microscopy and scanning electron microscopy (SEM). The Gypsum Tree has well preserved cell structures with pycnoxylic cell bands showing signs of soft tissue, mineralogical, and structural deformation. The Cheticamp Tree has poorly preserved cells in pycnoxylic cell bands and extensive mineralogical deformation, resulting in nodular growth around fragmented permineralized nuclei. Scanning electron microscopy results show that the Gypsum Tree is composed primarily of CaCO3 with pyrite inclusions. The Gypsum Tree is composed primarily of iron-rich dolomite with a small pocket of exquisitely preserved cells in a SiO2 inclusion. Based on the pycnoxylic arrangement of the secondary xylem, both specimens have been identified as belonging to the order Cordaitales. The Gypsum Tree was likely transported from a near-shore environment to a shallow marine depositional system before being buried and mineralized in marine gypsum. The Cheticamp Tree likely underwent moderate decomposition before being buried in an anoxic brackish environment, which facilitated the mineralization of dolomite nodules. In conclusion, this study found that the two fossilized tree specimens were taxonomically related, both belonging to the order Cordaitales. They were deposited in two different paleoenvironments, and underwent different levels of soft tissue, mineralogical, and structural deformation as they were mineralized. Keywords: fossil wood, Carboniferous, Nova Scotia, taphonomy, cordaites