Botanical Journal of the Linnean Society, 2004, 146, 385–398. With 1 figure © 2004 The Linnean Society of London, Botanical Journal of the Linnean Society, 2004, 146, 385–398 385 1464 385398 Original Article AGE OF MONOCOT GROUPS T. JANSSEN and K. BREMER *Corresponding author. Postal address from 01/2005: Albrecht-von-Haller-Institute für Pflanzenwissenschaften, Abt. Spezielle Botanik, Untere Karspüle 2, 37073 Göttingen, Germany. E-mail: janssen@mnhn.fr The age of major monocot groups inferred from 800 + rbcL sequences THOMAS JANSSEN 1 * and KÅRE BREMER 2 1 Museum National d’Histoire Naturelle, Département de Systématique et Evolution, USM 0602 Taxinomie et collections, 16 rue Buffon, 75005 Paris, France 2 Department of Systematic Botany, Evolutionary Biology Centre, Norbyvägen 18D, SE-752 36 Uppsala, Sweden Received August 2003; accepted for publication June 2004 Phylogenetic research on monocots has been extraordinarily active over the past years. With the familial interre- lationships being sufficiently understood, the question of divergence times and crown node ages of major lineages comes into focus. In this study we present the first attempt to estimate crown and stem node ages for most orders and families of monocots, based on rbcL sequence data and comprehensive taxon sampling. From our analysis it is obvious that considerable monocot diversification took place during the Early Cretaceous, with most families already present at the Cretaceous–Tertiary boundary. Araceae, Arecaceae and Orchidaceae are among the oldest families with crown node ages reaching back into the Early Cretaceous. We comment on possible error sources and the neces- sity for methodological improvement in molecular dating. © 2004 The Linnean Society of London, Botanical Jour- nal of the Linnean Society, 2004, 146, 385–398. ADDITIONAL KEYWORDS: Cretaceous – dating – fossils – NPRS – phylogeny – Tertiary. INTRODUCTION Monocots constitute an angiosperm clade of out- standing economic and ecological importance. The Angiosperm Phylogeny Group (APG II, 2003) recog- nizes 81 families in ten orders (with two families unplaced to order). Distribution of monocots is world- wide, with some families predominantly in open tem- perate habitats (e.g. grasses, Poaceae) and others with important species diversity in the tropics (e.g. orchids, Orchidaceae and palms, Arecaceae). Phylogenetic research on monocots has received much interest recently, fostered by three international congresses. Chase et al. (2000) proposed a first phylo- genetic tree, including all orders, based on a large data set comprising three DNA regions ( rbcL, atpB and 18S rDNA) but with comparatively limited taxon sam- pling. Meanwhile, a detailed treatment is available for several orders (Les, Cleland & Waycott, 1997; Fay et al., 2000; Kress et al., 2001; Vinnersten & Bremer, 2001; Bremer, 2002; Caddick et al., 2002a, b). With the results of the available studies taken together, we have at present a rather well supported phylogenetic hypothesis for the whole clade that is fairly well resolved down to family level. This provides us with a solid basis for research on age, biogeography and evolution of this group. Only a few studies are available to date on age inference including monocots. Wikström, Savolainen & Chase (2001) analysed a data set of 560 angiosperms (rbcL, atpB and 18S rDNA) using nonparametric rate smoothing (Sanderson, 1997). Bremer (2000) determined the ages of major monocot lineages, calculating mean branch lengths in a phylog- eny of rbcL sequences. Both studies also addressed the issue of age calibration using evidence from the fossil record. Givnish et al. (2000) estimated ages for several groups of commelinids using ndhF sequences. Recently, some detailed studies comparing different dating methods have become available for Liliales (Vinnersten & Bremer, 2001) and Poales (Bremer, 2002). These studies notwithstanding, the node ages