A Late Pennsylvanian coniferopsid forest in growth position, near Socorro, New Mexico, U.S.A.: Tree systematics and palaeoclimatic significance

• We describe a Pennsylvanian fossil forest from New Mexico.
• The oldest conifers in growth position are documented.
• Findings shed light on enigmatic dryland ecosystems.

The systematics and palaeoclimatic significance of silicified tree-stumps found in growth position in the Late Pennsylvanian (Kasimovian) Atrasado Formation, near Socorro, New Mexico, U.S.A. are described. Two taxa are documented. Macdonaldodendron giganticus (3 or 4 stumps) comprises a broad, non-septate pith containing plate-like sclerotic nests, a non-sympodial vasculature of ~ 30–35 endarch cauline bundles, pycnoxylic secondary xylem characterised by irregular growth interruptions, and leaf traces that pass through multiple seasons of growth. It is interpreted as a large, evergreen walchian conifer tree. Giblingodendron aridus (10 stumps) comprises a non-sympodial vasculature of ~ 60–100 mesarch cauline bundles, pycnoxylic secondary xylem showing regular growth rings, and leaf traces apparently occluded at the first ring boundary. The abundant new material allows the diagnosis of this species to be emended, with the pith of relatively juvenile Giblingodendron specimens shown to be distinctly septate, a feature that is absent in relatively mature specimens. The systematic position of this small, deciduous coniferopsid, which bore profuse leaves directly on the stem, remains uncertain. It shares characteristics with dicranophylls, voltzian conifers, cordaitaleans and other enigmatic coniferopsids. The trees are rooted in, and entombed by, microbial carbonates associated with primary gypsum, suggestive of a sabkha environment formed under semi-arid coastal conditions. In contrast, the stumps show subtle growth rings or growth interruptions, which indicate more sub-humid tropical conditions characterised by erratic wet–dry seasonality, and imply that trees may have established during a short-lived pluvial phase when the region was wetter. The Socorro fossil forest is important for two reasons. It contains the oldest examples of unequivocal conifers in growth position, and provides a glimpse of the enigmatic coniferopsid ecosystems that are inferred to have covered much of seasonal tropical Euramerica, but are rarely preserved due to taphonomic megabias.