Paleoallium billgenseli gen. et sp. nov.: Fossil Monocot Remains from the Latest Early Eocene Republic Flora, Northeastern Washington State, USA

Abstract

Premise of research. Fossil inflorescences (scapes) producing both pedicellate flowers and sessile bulbils, both covered partially by a persistent spathe, are described from the latest early Eocene Republic flora of north-central Washington. They are associated with an individual specimen of a single bulb with attached roots, and two small flower buds that appear to represent the same plant. The morphology of these fossils closely resembles that of certain bulb-forming monocots, such as some species of the onion genus Allium and other members of Amaryllidaceae. Methodology. Compression-impression fossils preserved in a lacustrine shale were uncovered from the rock matrix to reveal morphological details and were photographed with LM. Specimens were compared morphologically with extant material of related plants, and resulting images were processed minimally with Adobe Photoshop. Pivotal results. Specimens demonstrate an organography that is quite similar to that of modern onions and related forms. To our knowledge, this is the first description of plants showing a combination of bulbils and florets (representing asexual and sexual reproduction) among Paleogene plants. It also represents one of few reports among the fossil record of monocot plants similar to members of Amaryllidaceae. Conclusions. Scapes bearing flowers and bulbils within a spathe similar to those of some modern Amaryllidaceae, associated flower buds, and a root-producing bulb indicate the presence of a distinctive monocot plant in the Republic flora of the latest early Eocene Okanogan Highlands, northeastern Washington. Along with other Republic plants with distinctive morphological features indicative of temperate floras (leaf dimorphism, possible hybridization), these fossils suggest that bulbil-and flower-bearing monocots with a combined asexual and sexual reproductive strategy were already well established among plants of Paleogene.

Complete chloroplast genomes shed light on phylogenetic relationships, divergence time, and biogeography of Allioideae (Amaryllidaceae)

“…This estimate of the crown age of Allioideae is similar to that obtained in a previous research (37.0 mya, 95% HPD = 27.8–44.5 mya) 6 . This result is also supported by the fossil genus Paleoallium , which is similar to extant Allium , recently reported during the Eocene 36 . Thus, we believe that this is the most reliable estimate of the divergence time for Allioideae to date.…”

Polyploidy promotes species diversification of Allium through ecological shifts

“…In addition, potential triploid bridges may provide an important genetic connection between distinct cytotypes given the high occurrence of odd polyploids (28.4%) in Allium , which can contribute to the stabilisation of newly generated rare cytotypes. Furthermore, asexual reproduction through bulbs or rhizomes and perennial life forms are common characters well established by the latest early Eocene in Allium (Pigg et al ., ), which may promote polyploid survival at early stages or during the establishment of populations (Van Drunen & Husband, ).…”

“…As available reliable fossils are limited for Allium and Amaryllidaceae, secondary calibrations were used for constraining the crown nodes of Allioideae and Amaryllidaceae, with the 95% highest posterior density (HPD) defined as 27.8–44.5 million years (Myr) and 42.0–61.7 Myr, respectively (Chen et al ., ). This is in temporal agreement with the age of the first described Allium ‐like fossil ( Paleoallium billgenseli ) from the early Eocene, dating back to 49.42 ± 0.54 million years ago (Ma) (Pigg et al ., ). We recalibrated the divergence time by defining the 95% HPD for the crown age of Amaryllidaceae as 49.4–76.7 Myr according to the age of the P. billgenseli fossil (Pigg et al ., ) and the estimated 95% HPD maximum age of Amaryllidaceae (Magallón et al ., ), with or without constraining the crown age of Allioideae as 27.8–44.5 Myr (Chen et al ., ), respectively.…”

“…This is in temporal agreement with the age of the first described Allium ‐like fossil ( Paleoallium billgenseli ) from the early Eocene, dating back to 49.42 ± 0.54 million years ago (Ma) (Pigg et al ., ). We recalibrated the divergence time by defining the 95% HPD for the crown age of Amaryllidaceae as 49.4–76.7 Myr according to the age of the P. billgenseli fossil (Pigg et al ., ) and the estimated 95% HPD maximum age of Amaryllidaceae (Magallón et al ., ), with or without constraining the crown age of Allioideae as 27.8–44.5 Myr (Chen et al ., ), respectively. Four independent runs with 200 million Markov chain Monte Carlo (MCMC) generations were performed for each analysis.…”

Tunicate bulb size variation in monocots explained by temperature and phenology

“…Time calibration was performed using penalized likelihood as implemented in treePL (Smith & O'Meara, ). The following calibration points were as follows: (a) a fossil dated between 48.88 and 49.96 MYA at the crown of Amaryllidaceae (Pigg, Bryan, & DeVore, ), (b) a fossil dated between 33.8 and 34 MYA placed at the crown of Alismataceae (Iles, Smith, Gandolfo, & Graham, ), (c) a fossil dated between 72.1 and 83.6 MYA placed at the crown of Zingiberales (Iles et al, ), (d) a fossil dated at 23.2 MYA at the crown of Asteliaceae, (e) a fossil dated between 14.5 and 16.2 MYA at the crown of Agavoideae, (f) a secondary calibration of 133–136 MYA at the split between Acorus calamus and the remaining monocots (Givnish et al, ), and (g) a secondary calibration of 136–139.35 MYA at the split between Amborella trichopoda and the remaining angiosperms (Magallón, Gómez‐Acevedo, Sánchez‐Reyes, & Hernández‐Hernández, ). One priming step followed by 10 cross‐validations was performed in order to obtain the appropriate smoothing parameter of 0.1.…”