PENNSYLVANIAN CONIFEROPHYTE FORESTS IN GROWTH POSITION REPRESENT PROBABLE SABKHA ENVIRONMENTS IN TROPICAL WESTERN PANGEA

We report Pennsylvanian coniferophyte forests preserved in growth position from near Socorro, New Mexico, USA. At the most prolific horizon, more than 60 silicified tree stumps, 20-60 cm diameter, occur at five localities, scattered over 2 km², in the lowermost Tinajas Member (Atrasado Formation). Anatomical studies reveal that the trees had septate piths and wood with low uniseriate (rarely biserate) rays and uniseriate tracheid pits, suggesting affinity with cordaitaleans and/or conifers. This is consistent with the walchian and, possibly, voltzialean conifer adpressions in underlying shales. The stumps are rooted in, and entombed by, microbial limestone beds that include quartz spherules and gypsum pseudomorphs. The limestone beds occur within a succession of bedded gypsum above the marine Amado Limestone – a unit of early Kasmovian age. The setting is interpreted as a coastal sabkha formed under a Mediterranean-type climate. As the trees are shallowly rooted, unevenly spaced, and the tree-rings are wide and complacent, we argue that coniferophytes grew in waterlogged soils around freshwater ponds (in which microbial limestone developed) – and were partly buffered against rainfall seasonality. However, vascular traces truncated at the first ring boundary imply that trees were deciduous, dropping leaves or branches during the dry season. Similar fossil forests in comparable facies at three other horizons in the Tinajas Member indicate that this coastal coniferophyte forest was widespread. These ecosystems may have served as the source of conifer-bearing vegetation that repeatedly replaced Coal Forests in central and eastern Pangaea during drier glacial phases. Thus, our discovery has important implications for unraveling the dynamics of Pennsylvanian tropical biomes.