Coniferopsid tree trunks preserved in sabkha facies in the Permian (Sakmarian) Community Pit Formation in south-central New Mexico, U.S.A.: Systematics and palaeoecology

We describe the anatomy of silicified tree trunks found in the lower Permian (Sakmarian) Community Pit Formation (Hueco Group) near Las Cruces, south-central New Mexico, U.S.A. The fossils occur as allochthonous assemblages within facies interpreted as supratidal sabkha deposits in a dominantly shallow marine carbonate-siliciclastic succession, and probably represent remains of forests that fringed a seasonally dry coast. Although several hundred specimens were collected, only ten specimens, interpreted as derived from near the crowns of large trees, showed sufficiently good preservation of pith and primary vasculature for full description. The dominant taxon is Macdonaldodendron giganticus gen. et sp. nov., which shows a broad, non-septate pith containing sclerotic nests, an endarch, non-sympodial vasculature, helically arranged, triangular leaf bases embedded in the secondary xylem near the pith, and whorls of more than seven plagiotropic branches at some levels; it is interpreted as an evergreen walchian conifer. Somewhat rarer are two species of Giblingodendron gen. nov., which show a broad, non-septate pith, with sclereid cells, a mesarch, non-sympodial vasculature, and helically-arranged leaf traces that are occluded at the first ring boundary; this genus is interpreted as a deciduous coniferopsid, and probably, a voltzian conifer. One further incompletely preserved specimen is of Cordaixylon–Mesoxylon type and comprises a septate, homocellular pith, non-sympodial vasculature and, helically arranged leaf traces; it is probably cordaitalean. Macdonaldodendron lacks tree rings, whereas the two Giblingodendron species show marked tree rings, suggesting that the two genera employed varied phenological strategies and/or had different edaphic preferences in this seasonally dry region of western Pangaea. The fossil assemblage also demonstrates the diversity of early Permian tropical coniferopsid forests and challenges the view that late Palaeozoic conifers were exclusively small trees and shrubs. •Lower Permian silicified tree trunks are described from New Mexico, U.S.A.•The fossils represent remains of forests that fringed a seasonally dry coast.•Taxa include walchian conifers and other conifers/cordaitaleans.•Findings shed light on ecology of early Permian tropical dryland forests.