Distribution of living Cupressaceae reflects the breakup of Pangea

Abstract

Most extant genus-level radiations in gymnosperms are of Oligocene age or younger, reflecting widespread extinction during climate cooling at the Oligocene/Miocene boundary [∼23 million years ago (Ma)]. Recent biogeographic studies have revealed many instances of long-distance dispersal in gymnosperms as well as in angiosperms. Acting together, extinction and long-distance dispersal are likely to erase historical biogeographic signals. Notwithstanding this problem, we show that phylogenetic relationships in the gymnosperm family Cupressaceae (162 species, 32 genera) exhibit patterns expected from the Jurassic/Cretaceous breakup of Pangea. A phylogeny was generated for 122 representatives covering all genera, using up to 10,000 nucleotides of plastid, mitochondrial, and nuclear sequence per species. Relying on 16 fossil calibration points and three molecular dating methods, we show that Cupressaceae originated during the Triassic, when Pangea was intact. Vicariance between the two subfamilies, the Laurasian Cupressoideae and the Gondwanan Callitroideae, occurred around 153 Ma (124-183 Ma), when Gondwana and Laurasia were separating. Three further intercontinental disjunctions involving the Northern and Southern Hemisphere are coincidental with or immediately followed the breakup of Pangea.

Fig. 1.

(Upper) Chronogram for 122 Cupressaceae species and 22 outgroups based on an alignment of >7,000 nucleotides of plastid DNA (144-taxon dataset). A geological time scale is shown at the bottom (48). Blue lines represent Cupressoideae restricted to the area of Laurasian continents. Red lines represent Callitroideae restricted to Gondwanan continents. Pink lines represent species occurring in Africa in and north of the Sahara. Yellow lines represent species occurring in Africa south of the Sahara. Gray hexagons represent calibration points. Gray bars represent 95% HPD intervals for nodes 1–10. Gray (run 1) and purple (run 7) normal distributions represent the posterior for the BEAST age estimate of node 7 when uniform or lognormal priors were applied to calibration points. Orange shading indicates the period of decreasing feasibility of floristic exchange between Laurasia and Gondwana. Divergence times of nodes 5, 6, 7, 8, and 10 overlap with the fragmentation of Pangea. (Lower) Maps show (A) a paleocontinent reconstruction at 150 Ma and (B) the current distribution of Callitroideae and Cupressoideae. The stippled circle in A emphasizes island chains between North and South America; Ath, Athrotaxidoideae; Cun, Cunninghamioideae; Seq, Sequoioideae; Tai, Taiwanioideae; Tax, Taxodioideae. Reprinted with permission from Ron Blakey, Colorado Plateau Geosystems.

Fig. 2.

AARs for Cupressaceae. (A) The six areas (“N,” “S,” “E,” “F,” “A,” and “U”) used in the analyses (Left) and the modeled biogeographic processes (Right). (B) Likelihood reconstruction without fossil lineages. (C–E) Likelihood reconstructions that include fossil taxa in the tree and assume alternative placements of three early Cupressaceae fossils (see SI Text for details). The AARs with the highest likelihood are shown as colored boxes at each node. Single-area boxes indicate an ancestor confined to a single geographic area; combined boxes indicate an ancestor with a distribution encompassing two or more areas; two boxes separated by a space indicate the ancestral ranges inherited by each of the daughter lineages arising from the node. For each node with alternative reconstructions (within log2 likelihood units of the maximum), the relative probability of the global likelihood for the optimal reconstruction is given. Fossil lineages are shown by a dashed line, indicating their extinct status. Area codes are explained in SI Text.