Floral evolution in Chloranthaceae: Implications of a morphological phylogenetic analysis

Chloranthaceae have extremely simple flowers of three distinct types: unisexual and naked (Ascarina, male Hedyosmum), unisexual with three tepals (female Hedyosmum), and bisexual and naked with one stamen or a lobed androecium (Sarcandra, Chloranthus). Their long fossil record stimulated speculation that they represent an alternative prototype for the angiosperm flower, but molecular analyses exclude Chloranthaceae from the basal ANITA grade and imply that their flowers are reduced rather than primitive. However, Chloranthaceae may belong just above the ANITA grade, and if so, some of their features (such as ascidiate carpels) may be ancestral for angiosperms. Our morphological cladistic analysis of living and fossil Chloranthaceae confirms molecular evidence that Hedyosmum is sister to other Chloranthaceae and Ascarina is sister to Sarcandra and Chloranthus. The reduced perianth of Hedyosmum appears to be ancestral; its inferior ovary position may be ancestral or derived but had already evolved in related Barremian-Aptian flowers. "Clavatipollenites," appearing in the Barremian, resembles pollen of Ascarina, but it is known from stigmas of Couperites fruits, which may fall either inside or outside Chloranthaceae because of their anatropous ovule. It is equivocal whether the bisexual flowers of Sarcandra and Chloranthus are ancestral or derived. Analysis of extant taxa indicates that the lobed stamen of Chloranthus evolved by subdivision of one stamen, as in Sarcandra, but addition of Late Cretaceous androecia with free lobes supports fusion of three stamens. A tripartite Albian androecium might support stamen fusion, but its anomalous pollen sac orientation and pollen may mean it is not chloranthaceous. Floral reduction and a tendency toward wind pollination in Chloranthaceae may be consequences of spread into cooler regions (possibly favored by evolution of greater sun tolerance) and a factor in their early success.