Hagenia from the early Miocene of Ethiopia: Evidence for possible niche evolution?

Mario Coiro,Silvia Ulrich,Friðgeir Grímsson,Shirley A Graham,Alexandros Xafis,Ellen D Currano,Reinhard Zetter,Bonnie F Jacobs

doi:10.1002/ece3.7408

Abstract

Fossil pollen believed to be related to extant Hagenia abyssinica were discovered in the early Miocene (21.73 Ma) Mush Valley paleoflora, Ethiopia, Africa. Both the fossil and extant pollen grains of H. abyssinica were examined with combined light microscopy, scanning electron microscopy, and transmission electron microscopy to compare the pollen and establish their relationships. Based on this, the fossil pollen grains were attributed to Hagenia. The presence of Hagenia in the fossil assemblage raises the questions if its habitat has changed over time, and if the plants are/were wind pollinated. To shed light on these questions, the morphology of extant anthers was also studied, revealing specialized hairs inside the anthers, believed to aid in insect pollination. Pollen and anther morphology are discussed in relation to the age and origin of the genus within a molecular dated phylogenetic framework, the establishment of complex topography in East Africa, other evidence regarding pollination modes, and the palynological record. The evidence presented herein, and compiled from the literature, suggests that Hagenia was an insect‐pollinated lowland rainforest element during the early Miocene of the Mush Valley. The current Afromontane habitat and ambophilous (insect and wind) pollination must have evolved in post‐mid‐Miocene times.

Highlights

Mountains are home to a substantial proportion of biological diversity, especially at tropical latitudes (Spehn et al, 2011)
Fossil pollen believed to be related to extant Hagenia abyssinica were discovered in the early Miocene (21.73 Ma) Mush Valley paleoflora, Ethiopia, Africa
Both the fossil and extant pollen grains of H. abyssinica were examined with combined light microscopy, scanning electron microscopy, and transmission electron microscopy to compare the pollen and establish their relationships

Summary

| INTRODUCTION

Mountains are home to a substantial proportion of biological diversity, especially at tropical latitudes (Spehn et al, 2011). Hagenia abyssinica is recognized in Africa under a number of local names, kosso in Amharic, mlozilozi or mdobore in Swahili, and as African redwood in English (Orwa et al, 2009) It has become increasingly rare, endangered by the escalating destruction of forest lands due to human population growth. Cliffortia L., Acaena L., Margyricarpus Ruiz et Pav., Polylepis Ruiz et Pav., Poterium L., Poteridium Spach, and Sanguisorba L.), by Pérez de Paz (2004) and Chung et al (2010) These studies show that H. abyssinica pollen is unique within the tribe and identified using combined light microscopy (LM) and scanning electron microscopy (SEM). The combined use of LM and SEM provides a uniquely diagnostic suite of characters that makes Hagenia the perfect pollen type to be discovered in paleopalynological samples These would allow for documentation of the origin and evolution of the genus, together with its phytogeographic history. Along with the Hagenia pollen, additional Sanguisorbeae (studied with combined LM and SEM) and Rosaceae fossils are used to calibrate divergence ages within the family/tribe and that of the African genus Hagenia

| MATERIAL AND METHODS

Findings

| DISCUSSION