Plant Fossils from the Pennsylvanian–Permian Transition in Western Pangea, Abo Pass, New Mexico

Abstract

Plant fossils are described from five stratigraphic levels spanning the Pennsylvanian–Permian boundary in Abo Canyon, New Mexico. Charles B. Read collected the fossils in 1940 and 1941; Read’s field notes cannot be located. A combination of Read’s bed numbering pattern, his notes in the collections, and collection taxonomic composition permits them to be placed in an oldest to youngest sequence. The youngest fossils, from the Abo Formation of early Permian age, anchor the collection stratigraphically. A collection labeled “Base of Red Magdalena” is most likely equivalent to the modern Bursum Formation, thus immediately below the Abo. The three remaining collections are from the Upper Pennsylvanian portion of the Atrasado Formation. All collections are dominated or codominated by plants typical of environments with seasonal moisture stress and record increasing moisture limitation through time. Conifers, Sphenopteris germanica, and mixoneurid odontopterids are common to abundant in pre-Abo Formation collections. These same collections also contain wet-substrate taxa, particularly calamitaleans and marattialean tree ferns, with arborescent lycopsids in the oldest collection. The middle three collections contain the remains of microconchids / spirorbids, snails, ostracods, and conchostracans closely associated with the plant remains, indicating brackish to marine salinities at the burial sites of the organics. The Abo Pass collections document Pennsylvanian–Permian floristic changes in the western Pangean equatorial belt, an important point of comparison to better studied floras from this same interval in central Pangea (eastern United States and Europe). Most of the plants in the western equatorial assemblages are the same as those of similar age from west central to central Pangea (Euramerica), indicating a widespread tropical biogeographic province at this time, but within which there were several distinct biomes.