Organ reconstruction and systematic relationships of Late Cretaceous palm stems and roots

Abstract

Permineralized palm stems and adventitious roots are classified under specific organ-type genera (Palmoxylon or Rhizopalmoxylon) without affinities below the family level. Fragments of vegetative basal axes provide an incomplete picture of histological changes due to axial and radial differences in cell growth. However, stem and root anatomical descriptors were previously reported based on general and consistent internal structural features. Two root specimens from the Tarahumara Formation and six stem specimens from the Olmos Formation are included as terminals in a previously reported family-wide morphological and molecular character matrix at the genus level enriched with stem and root anatomical descriptors to elucidate their systematic relationships. Late Cretaceous vegetative basal axes from northern Mexico have systematic affinities to subfamily Coryphoideae. The root system of Palmoxylon polymorphum is included in its original stem description and this fossil species is inferred to have a sister relationship to Cryosophileae. Circumscription of Rhizopalmoxylon huepaciense is emended to include former R. teguachiense, and a systematic affinity to the syncarpous clade is inferred. Circumscription of Palmoxylon longum is also emended, to include former P. commune, P. enochii, P. fibrosum and P. kikaapoa, and a systematic affinity to tribe Borasseae is inferred. These findings highlight the utility of cladistic analyses for reconstructing fossil fragments of palm vegetative basal axes and for inferring their systematic relationships. They also support a Late Cretaceous divergence of Coryphoideae and raise questions about its biogeographic history.