Tiny rhizomorphic rooting systems from the early permian abo formation of New Mexico, USA

Premise of research. Extant Isoetes species, which all develop small pseudoherbaceous habits, are the only living remnants of the once diverse clade of rhizomorphic lycopsids, which included trees that grew to towering heights of 50 m. Although the rhizomorphic lycopsids evolved a range of diverse body plans in the Paleozoic, it is thought that the evolution of the small pseudoherbaceous habit, with small rooting systems similar to modern Isoetes species, was a late event in the clade’s history, occurring in the Mesozoic. Here we describe small fossilized rooting systems from the early Permian Abo Formation of New Mexico, increasing our knowledge of the diversity of small Paleozoic rhizomorphic rooting systems.

Methodology. Ten fossilized rooting systems are described from a slab collected from the early Permian Abo Formation of New Mexico.

Pivotal results. Here we report 10 rooting systems interpreted as those of rhizomorphic lycopsids due to the presence on each of a large number of isotomously branched rootlets radiating from a central rhizomorph and two associated microphyll leaf impressions. Because of the fossils’ tiny size (the largest rhizomorph is only 1.5 cm in diameter, and the largest rooting system, including rootlets, is only 6 cm in diameter), we interpret these fossils either as juvenile plants or as adult morphologies with a small rooting system. Given the paucity of mature rhizomorphic lycopsids in the Abo Formation and the limited fossil record of juvenile rhizomorphic lycopsids from any geological period, we predict that these are most likely to be adult plants with small rooting systems.

Conclusions. The small size of the specimens described here increases our knowledge of the diversity of small rhizomorphic rooting systems in the Paleozoic before the radiation of the modern Isoetes growth habit during the Mesozoic.