Diverse early dwarf mistletoes (Arceuthobium), ecological keystones of the Eocene Baltic amber biota.

Abstract

Extant dwarf mistletoes (Arceuthobium M. Bieb., Viscaceae) are hemiparasites with complex roles in nature. They are one of the most severe pests in northern hemisphere conifer forests, but they also enhance the structural complexity and species diversity of the forests. Here, we describe the first pre-Miocene macrofossils of dwarf mistletoes. The fossils from Eocene Baltic amber provide new insights into the morphological evolution of the Arceuthobium lineage and its paleobiogeography. The amber inclusions were investigated with light microscopy and compared with extant Viscaceae and to historic descriptions of lost Baltic amber fossils with affinities to Viscaceae. Six fossil species of the Arceuthobium lineage, A. johnianum comb. nov., A. mengeanum comb. nov., A. conwentzii sp. nov., A. groehnii sp. nov., A. viscoides comb. nov. and A. obovatum sp. nov., occurred in source forests of Baltic amber, representing the oldest macrofossil evidence of dwarf mistletoes. They share morphological features of their bracts, internodes, fruits, and stomata with extant Arceuthobium. Differences from extant dwarf mistletoes, such as the perianth merosity, the nonfusion of squamate bracts and presence of oblanceolate expanded leaves, indicate their affiliation to an ancient lineage of the genus. The occurrence of six species of dwarf mistletoes in a single amber deposit suggests Arceuthobium was a keystone taxon of the Baltic amber source area. As in extant conifer forests, they probably influenced the structural complexity of the forest, not only leading to more open woodlands but also increasing species diversity, at least at a microhabitat scale.