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Phylogenetic Pattern, Diversity, and Diversification of Eudicots

Susana Magallon, Peter R. Crane and Patrick S. Herendeen
Annals of the Missouri Botanical Garden
Vol. 86, No. 2 (Spring, 1999), pp. 297-372
DOI: 10.2307/2666180
Stable URL: http://www.jstor.org/stable/2666180
Page Count: 76
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Phylogenetic Pattern, Diversity, and Diversification of Eudicots
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Abstract

The implementation of explicit phylogenetic techniques to the study of relationships among angiosperms has led to the recognition of a major monophyletic group, the eudicot clade, characterized by the production of tricolpate or tricolpate-derived pollen grains. Eudicots comprise nearly 75% of extant angiosperm species (subclasses Hamamelididae, Caryophyllidae. Dilleniidae, Rosidae, and Asteridae, as well as the order Ranunculales in the Magnoliidae sensu Cronquist). Recent phylogenetic analyses, based on both morphological data and molecular sequences, have begun to clarify higher-level phylogenetic relationships within the eudicot clade. The basalmost branch within the eudicots separates a small ranunculid clade, which includes the Ranunculales and Papaverales. The main group within the eudicots, here referred to as the main cudicot clade, is formed by a basal grade of species-poor lineages, mostly of "lower" Hamamelididae, and a large monophyletic group, here referred to as core eudicots, which includes ca. 97% of eudicot species diversity. Within the core eudicots, three distinct groups can be recognized. (1) The caryophyllid clade (ca. 6% of eudicot species diversity) includes the Caryophyllidae as traditionally defined and a few additional taxa previously thought to be of dilleniid and rosid affinity. (2) The rosid clade (ca. 39% of total eudicot species diversity) is composed mostly of taxa previously included in Dilleniidae and Rosidae, and includes a well-supported clade that we term here the core rosids (ca. 24% of total eudicot species diversity). Among the taxa in the core rosid clade are the Fabaceae, Rosaceae, Linales, and Cunoniaceae, as well as some families of Violales, and the "higher" Hamamelididae. (3) The asterid clade (ca. 50% of eudicot species diversity) consists of two large clades composed mostly of taxa previously assigned to Asteridae, and additional members of Rosidae and Dilleniidae. One of these large asterid clades is dominated by the Asterales s.l. (ca. 17% of total eudicot species diversity), while the other corresponds to a broadly defined Lamiidae (ca. 26% of total eudicot species diversity). Paleobotanical data first document the presence of early cudicots ca. 125 million years before the present (Barremian-Aptian boundary, Lower Cretaceous), prior to the major diversification and ecological radiation of angiosperms. Well-preserved floral remains and other fossils provide a minimum age for the origin of eudicot lineages. Sediments of Albian age contain floral remains of Platanaceae and probable Buxaceae, both of which fall within the species-poor lineages at the base of the main eudicot clade. In slightly younger sediments, the taxonomic diversity of eudicots increases considerably. Basal taxa in the core eudicots are represented by Hamamelidaceae and by several flowers of broad saxifragalean affinity in Turonian-Campanian strata. Among taxa within the rosid clade, the Capparales and Myrtales are documented from the Turonian and Santonian-Campanian, respectively. The core rosids are represented by several flowers with affinities to Juglandales, Myricales, and Fagales in the Santonian-Campanian. Flowers with possible affinities to Hydrangeaceae, from the Coniacian-Santonian, represent the basalmost group within the asterid clade, and flowers of broad ericalean affinity (including Actinidiaceae), from the Turonian-Campanian, document the presence of several groups within the ericalean clade. The Asteridae s.l. are not securely represented in the Upper Cretaceous, and, to our knowledge, there is no reliable Cretaceous record for any member of the Lamiidae s.l. Although nearly all of the main eudicot clades are represented by at least one of their included lineages in the Upper Cretaceous, the earliest well-documented records of the Fabaceae, Asteraceae, Lamiales s.l., and Gentianales, which together comprise ca. 45% of total eudicot species diversity, are found in uppermost Cretaceous (Maastrichtian) or Tertiary sediments. The three subfamilies of Fabaceae are well documented by flowers and fruits in the Eocene, although the presence of pollen grains assigned to Caesalpinioideae from Maastrichtian strata suggests that the family extends back into the uppermost Cretaceous. The Asteraceae, Lamiales s.l., and Gentianales are known from the Paleogene based mostly on vegetative remains. The uneven distribution of species diversity among the major clades of eudicots, and the fact that the most species-rich groups are known only from relatively young fossils, suggests that a significant portion of eudicot diversity is the result of relatively recent radiations that occurred during the second half of angiosperm evolutionary history. The evolutionary basis for the explosive diversification of specific eudicot clades-in terms of exceptionally high speciation rates, low extinction rates, or both-remains uncertain.

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