Permineralized Pollen Cones of Classostrobus minutus sp. nov. Provide Evidence of Pollinivory in the Extinct Conifer Family Cheirolepidiaceae during the Late Cretaceous

Premise of research. Cheirolepidiaceae were widespread and conspicuous elements across many Mesozoic floras. While the family exhibits an extensive fossil record, reproductive organs (especially those that are anatomically preserved) are scarce, making it difficult to develop a detailed understanding of their ecology and reproductive biology. Despite this, cheirolepidiaceous conifers have long been considered to have engaged in plant-insect interactions such as pollinivory and insect-facilitated pollination. Here we describe new anatomically preserved pollen cones assignable to Cheirolepidiaceae, one of which has in situ evidence of a plant-insect interaction. Methodology. We studied two permineralized pollen cones collected from the Upper Cretaceous Holz Shale Member of Silverado Canyon, California. The cones were studied using the cellulose acetate peel technique. Serial sections were studied using light microscopy and to produce three-dimensional reconstructions. Pivotal results. Classostrobus minutus sp. nov. is 2 mm long and 1 mm wide, has helically arranged microsporophylls with two abaxial pollen sacs, and contains Classopollis-type pollen. One specimen contains clusters of coprolites composed entirely of digested Classopollis grains. Conclusions. Classostrobus minutus adds to a sparse record of anatomically preserved pollen cones of Cheirolepidiaceae and is the first from the Late Cretaceous of North America. This species provides the only in situ evidence of pollinivory in Cheirolepidiaceae cones. These findings add additional evidence of insect interactions within Cheirolepidiaceae during the Cretaceous, a time hallmarked by transitions to angiosperm-dominated floras and their associated plant-pollinator interactions.