Phylogenetic pattern, diversity, and diversification of eudicots

The implementation of explicit phylogenetic techniques to the study of rela tionships among angiosperms has led to the recognition of a major monophyle tic group, the eudicot clade, characterized by the production of tricolpate or tricolpate-derived pollen grains. Eudicots comprise nearly 75% of extan t angiosperm species (subclasses Hamamelididae, Caryophyllidae, Dilleniidae , Rosidae, and Asteridae, as well as the order Ranunculales in the Magnolii dae sensu Cronquist). Recent phylogenetic analyses, based on both morpholog ical data and molecular sequences, have begun to clarify higher-level phylo genetic relationships within the eudicot clade. The basalmost branch within the eudicots separates a small ranunculid clade, which includes the Ranunc ulales and Papaverales. The main group within the eudicots, here referred t o as the main eudicot clade, is formed by a basal grade of species-poor lin eages, mostly of "lower" Hamamelididae, and a large monophyletic group, her e referred to as core eudicots, which includes ca. 97% of eudicot species d iversity. 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 prev iously thought to be of dilleniid and rosid affinity. (2) The rosid clade ( ca. 39% of total eudicot species diversity) is composed mostly of taxa prev iously included in Dilleniidae and Rosidae, and includes a well-supported c lade that we term here the core rosids (ca. 24% of total eudicot species di versity). 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 di versity) consists of two large clades composed mostly of taxa previously as signed 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 broad ly defined Lamiidae (ca. 26% of total eudicot species diversity). Paleobota nical data first document the presence of early eudicots ca. 125 million ye ars 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 o rigin of eudicot lineages. Sediments of Albian age contain floral remains o f Platanaceae and probable Buxaceae, both of which fall within the species- poor lineages at the base of the main eudicot clade. In slightly younger se diments, 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 fr om the Turonian and Santonian-Campanian, respectively. The core rosids are represented by several flowers with affinities to Juglandales, Myricales, a nd 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 (includin g Actinidiaceae), from the Turonian-Campanian, document the presence of sev eral groups within the ericalean clade. The Asteridae s.l. are not securely represented in the Upper Cretaceous, and, to our knowledge, there is no re liable 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-d ocumented records of the Fabaceae, Asteraceae, Lamiales s.l., and Gentianal es, which together comprise ca. 45% of total eudicot species diversity, are found in uppermost Cretaceous (Maastrichtian) or Tertiary sediments. The t hree subfamilies of Fabaceae are well documented by flowers and fruits in t he Eocene, although the presence of pollen grains assigned to Caesalpinioid eae from Maastrichtian strata suggests that the family extends back into th e uppermost Cretaceous. The Asteraceae, Lamiales s.l., and Gentianales are known from the Paleogene based mostly on vegetative remains. The uneven dis tribution 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 r esult of relatively recent radiations that occurred during the second half of angiosperm evolutionary history. The evolutionary basis for the explosiv e diversification of specific eudicot clades-in terms of exceptionally high speciation rates, low extinction rates, or both-remains uncertain.