Dipteronia (Sapindaceae) from the Tertiary of North America and implications for the phytogeographic history of the Aceroideae.

The fossil record of Dipteronia, the sister genus of Acer, is reviewed based on diagnostic winged fruits from the Tertiary of western North America. Today the genus is endemic to eastern Asia with two extant species in central and southern China, but it is well represented in the Tertiary of western North America, ranging from the Paleocene to the Oligocene with the greatest number of occurrences in the middle to late Eocene. There are no known fossil occurrences outside of North America. The fossil fruits, assigned to the new species D. brownii sp. nov., are smaller than those of both living species and were tricarpellate as well as bicarpellate in contrast to the modern species, which are almost exclusively bicarpellate. The tricarpellate condition may be plesiomorphic for Dipteronia and perhaps Aceroideae. The area of origin for Dipteronia is unknown, but it seems likely to have been either Asia or North America, with the genus crossing Beringia in the Paleogene.

Flowers, fruits, seeds, and pollen of Landeenia gen. nov., an extinct sapindalean genus from the Eocene of Wyoming.

The new genus Landeenia is recognized on the basis of flowers, pollen, infructescences, fruits, and seeds from the middle Eocene of southwestern and northwestern Wyoming. Landeenia aralioides (MacGinitie) comb.nov. has cymose inflorescences with actinomorphic, bisexual flowers, a pentamerous calyx, about ten stamens, and a superior gynoecium of ∼18 carpels sharing a single style. The fruits are globose to oblate, loculicidally dehiscent capsules, with a persistent calyx, and contain flat, elliptical seeds that are surrounded by a small wing. Pollen removed from the anthers is tricolpate with finely striate sculpture. Although clearly dicotyledonous, the combination of characters found in Landeenia is not known in any modern genus. The familial affinities of the plant, though certainly not with the Araliaceae as previously thought, remain uncertain. However, the combination of characters is consistent with treatment as a member of the Sapindales. The fossil material is thus assigned to the rank of Sapindales-Incertae sedis.

Timing the eastern Asian-eastern North American floristic disjunction: molecular clock corroborates paleontological estimates.

Sequence data of the chloroplast gene rbcL were used to estimate the time of the well-known eastern Asian-eastern North American floristic disjunction. Sequence divergence of rbcL was examined for 22 species of 11 genera (Campsis, Caulophyllum, Cornus, Decumaria, Liriodendron, Menispermum, Mitchella, Pachysandra, Penthorum, Podophyllum, and Phryma) representing a diverse array of flowering plants occurring disjunctly in eastern Asia and eastern North America. Divergence times of putative disjunct species pairs were estimated from synonymous substitutions, using rbcL molecular clocks calibrated for Cornus. Relative rate tests were performed to assess rate constancy of rbcL evolution among lineages. Corrections of estimates of divergence times for each species pair were made based on rate differences of rbcL between Cornus and other species pairs. Results of these analyses indicate that the time of divergence of species pairs examined ranges from 12.56 +/- 4.30 million years to recent (<0.31 million years), with most within the last 10 million years (in the late Miocene and Pliocene). These results suggest that the isolation of most morphologically similar disjunct species in eastern Asia and eastern North America occurred during the global climatic cooling period that took place throughout the late Tertiary and Quaternary. This estimate is closely correlated with paleontological evidence and in agreement with the hypothesis that considers the eastern Asian-eastern North American floristic disjunction to be the result of the range restriction of a once more or less continuously distributed mixed mesophytic forest of the Northern Hemisphere that occurred during the late Tertiary and Quaternary. This implies that in most taxa the disjunction may have resulted from vicariance events. However, long-distance dispersal may explain the disjunct distribution of taxa with low divergence, such as Menispermum.

Phylogeny and evolution of the Betulaceae as inferred from DNA sequences, morphology, and paleobotany.

Phylogeny of the Betulaceae is assessed on the basis of rbcL, ITS, and morphological data. Based upon 26 rbcL sequences representing most "higher" hamamelid families, the Betulaceae are monophyletic, with Casuarinaceae as its sister group, regardless of whether the outgroup is Cunoniaceae, Cercidiphyllaceae, Hamamelidaceae, or Nothofagus. Within the Betulaceae, two sister clades are evident, corresponding to the subfamilies Betuloideae and Coryloideae. However, with only 13 phylogenetically informative sites, the rbcL sequences provide limited intra-subfamilial resolution. Internal transcribed spacer (ITS) sequences provided 96 phylogenetically informative sites from 491 aligned sites resulting in a single most parsimonious tree of 374 steps (consistency index = 0.791) with two major lineages corresponding to the two traditional subfamilies: Betuloideae (Alnus, Betula) and Coryloideae (Corylus, Ostryopsis, Carpinus, Ostrya). This arrangement is mostly consistent with those from rbcL and morphology and is greatly reinforced by analyses with the three data sets combined. In the Coryloideae, the Ostryopsis-Carpinus-Ostrya clade is well supported, with Corylus as its sister group. The sister-group relationship between Ostryopsis and the Carpinus-Ostrya clade is well supported by ITS, rbcL, and morphological data. Phylogenetic relationships among the extant genera deduced by these analyses are compatible with inferences from ecological evolution and the extensive fossil record.

Estimation of temperature and precipitation from morphological characters of dicotyledonous leaves.

The utility of regression and correspondence models for deducing climate from leaf physiognomy was evaluated by the comparative application of different predictive models to the same three leaf assemblages. Mean annual temperature (MAT), mean annual precipitation (MAP), and growing season precipitation (GSP) were estimated from the morphological characteristics of samples of living leaves from two extant forests and an assemblage of fossil leaves. The extant forests are located near Gainesville, Florida, and in the Florida Keys; the fossils were collected from the Eocene Clarno Nut Beds, Oregon. Simple linear regression (SLR), multiple linear regression (MLR), and canonical correspondence analysis (CCA) were used to estimate temperature and precipitation. The SLR models used only the percentage of species having entire leaf margins as a predictor for MAT and leaf size as a predictor for MAP. The MLR models used from two to six leaf characters as predictors, and the CCA used 31 characters. In comparisons between actual and predicted values for the extant forests, errors in prediction of MAT were 0.6°-5.7°C, and errors in prediction of precipitation were 6-89 cm (=6-66%). At the Gainesville site, seven models underestimated MAT and only one overestimated it, whereas at the Keys site, all eight models overestimated MAT. Precipitation was overestimated by all four models at Gainesville, and by three of them at the Keys. The MAT estimates from the Clarno leaf assemblage ranged from 14.3° to 18.8°C, and the precipitation estimates from 227 to 363 cm for MAP and from 195 to 295 cm for GSP.