Eocene "Chusquea" fossil from Patagonia is a conifer, not a bamboo.

Peter Wilf

doi:10.3897/phytokeys.139.48717

Abstract

Chusquea oxyphylla Freng. & Parodi, 1941, a fossilized leafy branch from the early Eocene (52 Ma), late-Gondwanan Laguna del Hunco biota of southern Argentina, is still cited as the oldest potential bamboo fossil and as evidence for a Gondwanan origin of bamboos. On recent examination, the holotype specimen was found to lack any typical bamboo characters such as nodes, sheaths, ligules, pseudopetioles, or parallel leaf venation. Instead, it has decurrent, clasping, univeined, heterofacially twisted leaves with thickened, central-longitudinal bands of presumed transfusion tissue. These and other features allow confident placement in the living Neotropical and West Pacific disjunct genus Retrophyllum (Podocarpaceae), which was recently described from the same fossil site based on abundant, well-preserved material. However, the 1941 fossil holds nomenclatural priority, requiring the new combination Retrophyllum oxyphyllum (Freng. & Parodi) Wilf, comb. nov. No reliable bamboo fossils remain from Gondwana, and the oldest South American bamboo fossils are Pliocene. Chusquea joins a growing list of living New World genera that are no longer included in Paleogene Patagonian floras, whose extant relatives are primarily concentrated in Australasia and Malesia via the ancient Gondwanan route through Antarctica.

Highlights

In 1941, the legendary Argentine botanists Joaquín Frenguelli and Lorenzo R
LH4 lies in the middle of the full 170 m stratigraphic section of the Tufolitas Laguna del Hunco at Laguna del Hunco (Wilf et al 2003) and is confidently dated to ca. 52.2 Ma using several 40Ar-39Ar dates and paleomagnetic data from strata intercalated with the fossil quarries; in particular, an 40Ar-39Ar age on sanidine of 52.22 ± 0.22 Ma was analyzed from a tuff only 40 cm above quarry LH4 (Wilf et al 2003, 2005, 2017a)
The holotype is identifiable as the flip-leaved, podocarpaceous conifer genus Retrophyllum; it matches precisely the distichous fossil foliage form of Retrophyllum spiralifolium, which was described recently from a suite of 82 specimens collected from both Laguna del Hunco, including quarry LH4, and the early middle Eocene Río Pichileufú site in Río Negro Province (Wilf et al 2017b)

Summary

Introduction

In 1941, the legendary Argentine botanists Joaquín Frenguelli and Lorenzo R. Parodi of Museo de La Plata (Frenguelli and Parodi 1941; Burkart 1967; Riccardi 2017) described a compressed leafy-shoot fossil from northwest Chubut Province, Argentina. Peter Wilf / PhytoKeys 139: 77–89 (2020). Under Chusquea Kunth, a diverse New World bamboo genus (Clark 1989, 1997a; Fisher et al 2014; Wysocki et al 2015). The assemblage, once thought to be Miocene in age, is constrained to the early Eocene (ca. 52.2 Ma; Wilf et al 2003, 2017a); it has remained a subject of intensive study for many decades (e.g., Romero and Hickey 1976; Fidalgo and Smith 1987; Romero et al 1988), over the past ca. 15 years (for summaries see, e.g., Wilf et al 2009, 2013, 2019)

Methods

Discussion

Conclusion