A NEW CLUE FOR THE ORIGIN OF CONIFERS FROM THE BEEMAN FORMATION (KASIMOVIAN, NEW MEXICO)

A new fossil plant assemblage from south-central New Mexico contains the peculiar plant remains of foliated branches with a morphology characteristic of walchian-type conifers, but that bore ovuliferous cones known from Cordaitales (cf. Cordaitanthus). The morphology of these remains seems to affirm a long-standing origination hypothesis, which postulates that conifers arose from within the Cordaitales and involved a change in shoot structure (Rothwell 1982). These new remains strongly suggest that such shoot structure shifts occurred. The fossil flora is preserved in the Beeman Formation, exposed in the Sacramento Mountains. This formation is an approximately 200 m thick unit of terrigenous clastics and marine carbonates. Fusulinid and conodont biostratigraphy indicates that it is of Missourian-Virgilian (Late Pennsylvanian) age. The gymnosperm-rich flora discussed here is from the lower part of the formation, and thus of early/middle Missourian (i.e. early Kasimovian) age. The flora containing the new find is "mixed,", consisting of both typical Missourian-age wetland elements, principally pteridosperms and ferns, and xeromorphic taxa often found in association with climatic indicators of seasonal drought, such as conifers. These plants are intimately associated, indicating that they grew in close proximity, suggesting local landscape heterogeneity. The Missourian (Kasimovian) was the time of onset of more seasonally dry conditions across much of Euramerican Pangaea, thus improving the chances that long-existing, more drought-tolerant taxa entered areas of higher preservation potential. Since the new finds are younger than the oldest described conifers, the question arises whether the new finds represent descendants of taxa that split off from the remaining cordaitaleans – giving rise to a single conifer clade – or whether they represent one of multiple changes to a conifer-like shoot structure that may have occurred within the Cordaitales, but not necessarily one that led to any of the known conifer lineages.

Reference: Rothwell, GW, 1982. New interpretations of the earliest conifers. Review of Palaeobotany and Palynology, 37: 7–28.