First cycad seedling foliage from the fossil record and inferences for the Cenozoic evolution of cycads.

Boglárka Erdei,M Patrick Griffith,Kirk R Johnson,Mario Coiro,Ian Miller,Vickie Murphy

doi:10.1098/rsbl.2019.0114

Abstract

The morphology of the early ontogenetic stages of cycad foliage may help resolve the relationships between extinct to extant cycad lineages. However, prior to this study, fossil evidence of cycad seedlings was not known. We describe a compression fossil of cycad eophylls with co-occurring fully developed leaves of adult specimens from the early Palaeocene (ca 63.8 Ma) Castle Rock flora from the Denver Basin, CO, USA and assign it to the fossil genus Dioonopsis (Cycadales) based on leaf morphology and anatomy. The new fossil seedling foliage is particularly important because fully differentiated pinnate leaves of adult plants and the eophylls belong to the same species based on shared epidermal micromorphology, therefore, increasing the number of morphological characteristics that can be used to place Dioonopsis phylogenetically. Significantly, the seedling fossil has a basic foliage structure that is very similar to seedlings of extant cycads, which is consistent with a cycadalean affinity of Dioonopsis. Nevertheless, the set of morphological characters in the seedling and adult specimens of Dioonopsis suggests a distant relationship between Dioonopsis and extant Dioon. This indicates that extinct lineages of cycads were present and widespread during the early Cenozoic (Palaeogene) coupled with the subordinate role of extant genera in the Palaeogene fossil record of cycads.

Highlights

The evolution of cycads is a topic that fascinates an increasing number of researchers and avocational cycad enthusiasts
Based on the gross morphological similarity of the fully differentiated leaves, Dioonopsis has traditionally been assigned to the lineage leading to the extant cycad genus, Dioonopsis and extant cycads (Dioon) [15,16], which lacks a fossil record
A matrix including 15 leaf morphological and cuticular characters, 10 characters modified from Martinez et al [20] and five new characters, was scored for all extant genera of cycads and Dioonopis based on new character observations from the Castle Rock fossils and data from previous matrices and observations [21]

Summary

Introduction

The evolution of cycads is a topic that fascinates an increasing number of researchers and avocational cycad enthusiasts. Fossils that show the early ontogenetic stages of plants may preserve important morphological characters to facilitate our understanding of the evolution of and relationships between extinct groups. These early stages, especially seedlings, are rarely fossilized. The leaves of two ontogenetic stages, the eophylls (the first foliage) and the fully differentiated pinnate leaves of adult plants, are documented by macromorphological and epidermal features and assigned to the extinct genus, Dioonopsis. Based on the gross morphological similarity of the fully differentiated leaves, Dioonopsis has traditionally been assigned to the lineage leading to the extant cycad genus, Dioon [15,16], which lacks a fossil record. The new fossil seedling sheds light on the evolutionary relationship between Dioonopsis and extant cycads (Dioon)

Material and methods

Results and discussion

Nomenclature