Evidence for a Cenozoic radiation of ferns in an angiosperm-dominated canopy Eric Schuettpelz1 and Kathleen M. Pryer Department of Biology, Duke University, Durham, NC 27708 Edited by Peter R. Crane, University of Chicago, Chicago, IL, and approved May 27, 2009 (received for review November 3, 2008) In today’s angiosperm-dominated terrestrial ecosystems, leptospo- that the bulk of polypod diversity arose following the rise of rangiate ferns are truly exceptional—accounting for 80% of the flowering plants (15). Subsequent divergence-time estimates Ϸ11,000 nonflowering vascular plant species. Recent studies have suggested that this pattern of recent diversification—in the shown that this remarkable diversity is mostly the result of a major shadow of angiosperms—might be echoed in other leptospor- leptosporangiate radiation beginning in the Cretaceous, following angiate orders (16). Thus, it appears that the remarkable diver- the rise of angiosperms. This pattern is suggestive of an ecological sity of leptosporangiate ferns on Earth today is not simply the opportunistic response, with the proliferation of flowering plants result of being adept at holding on in the face of angiosperm across the landscape resulting in the formation of many new domination. Rather, it seems that ferns may have somehow been niches—both on forest floors and within forest canopies—into able to capitalize upon it. which leptosporangiate ferns could diversify. At present, one-third One plausible explanation for the success of leptosporangiates of leptosporangiate species grow as epiphytes in the canopies of involves an ecological opportunistic response to the rise of angiosperm-dominated tropical rain forests. However, we know angiosperms (12, 15, 17). In such a scenario, the proliferation of too little about the evolutionary history of epiphytic ferns to assess angiosperms across the landscape and the ensuing establishment whether or not their diversification was in fact linked to the of more complex ecosystems would have resulted in a plethora establishment of these forests, as would be predicted by the of new niches into which leptosporangiate ferns could have ecological opportunistic response hypothesis. Here we provide diversified. But why were leptosporangiates able to flourish as new insight into leptosporangiate diversification and the evolu- other nonflowering vascular plant lineages floundered? In part, tion of epiphytism by integrating a 400-taxon molecular dataset their success may be linked to acquiring a unique photoreceptor with an expanded set of fossil age constraints. We find evidence that enhanced their sensitivity to light (in orienting leaves and for a burst of fern diversification in the Cenozoic, apparently driven chloroplasts) (18) and likely allowed them to better occupy the by the evolution of epiphytism. Whether this explosive radiation shady floors of angiosperm-dominated forests (15). Traits asso- was triggered simply by the establishment of modern angiosperm- ciated with the evolution of epiphytism—a capacity to reside on dominated tropical rain forest canopies, or spurred on by some an above-ground plant surface while not extracting water or other large-scale extrinsic factor (e.g., climate change) remains to nutrients from the host plant or the ground (19)—may also have be determined. In either case, it is clear that in both the Cretaceous played an important role. Desiccation tolerance has been doc- and Cenozoic, leptosporangiate ferns were adept at exploiting umented both in fern sporophytes and (especially) gametophytes newly created niches in angiosperm-dominated ecosystems. (20), and many epiphytic ferns also possess features (e.g., leathery leaves and thick cuticles) that allow them to withstand ͉ ͉ divergence-time estimates diversification ecological opportunistic dry conditions (21, 22). Some are even able to absorb water ͉ ͉ response epiphytes modern tropical rain forests directly into their leaves or stems (23), while others have specialized in the impoundment of leaf litter to form suspended hrough the 80 million years composing the Cretaceous soils (24). Tperiod (145.5–65.5 Ma; time scale follows ref. 1), the Earth’s With this suite of adaptations, leptosporangiate ferns have vegetation changed dramatically from a landscape populated by shown an extraordinary ability to colonize the canopies of gymnosperms and seed-free vascular plants to one dominated by modern, angiosperm-dominated, tropical rain forests (21, 25– angiosperms (2–8). As flowering plants rose to prominence, 29). Although these ferns account for just 3% of the world’s other vascular plant lineages were largely relegated to the vascular plant diversity, they comprise more than 10% of the sidelines, if not driven completely to extinction. Today, angio- epiphytic species (see Table S1 and ref. 30). Unfortunately, we sperms account for about 96% of vascular plant diversity, know too little about the evolutionary history of epiphytic whereas nearly all of the 12 remaining major vascular plant leptosporangiates to assess whether or not their diversification lineages comprise just a few—or perhaps a few hundred— was in fact linked to the establishment of angiosperm-dominated species [supporting information (SI) Table S1]. Leptosporangi- tropical rain forests, as would be predicted by the ecological ate ferns are the only exception. Although not as diverse as opportunistic response hypothesis. There is not even a consensus flowering plants, this group comprises more than 9,000 living as to how many times epiphytism has arisen within leptospor- species—4 times the number of extant species in all other angiates. In this study, we combine the best-sampled molecular nonflowering lineages combined. dataset for ferns to date with an expanded set of age constraints Leptosporangiate ferns originated near the start of the Car- from the fossil record to obtain a more complete picture of boniferous period (359.2 Ma) (9, 10)—about 200 million years leptosporangiate diversification. We then reconstruct habit before the evolution of angiosperms (11). Based on the fossil record, this group of ferns is thought to have undergone 3 successive radiations (12–14): an initial radiation in the Carbon- Author contributions: E.S. and K.M.P. designed research; E.S. performed research; E.S. iferous, giving rise to 6 now-extinct families; a second radiation analyzed data; and E.S. and K.M.P. wrote the paper. in the late Paleozoic and early Mesozoic, resulting in several The authors declare no conflict of interest. families with extant representatives; and a third radiation be- This article is a PNAS Direct Submission. ginning in the Cretaceous, primarily within what is now referred 1To whom correspondence should be addressed. E-mail: [email protected]. to as the ‘‘polypod’’ clade. An analysis combining fossil and living This article contains supporting information online at www.pnas.org/cgi/content/full/ data confirmed the timing of this third radiation, demonstrating 0811136106/DCSupplemental. 11200–11205 ͉ PNAS ͉ July 7, 2009 ͉ vol. 106 ͉ no. 27 www.pnas.org͞cgi͞doi͞10.1073͞pnas.0811136106 Downloaded by guest on September 24, 2021 across the resulting phylogenetic chronogram (timetree) to more resulting in 2 extant lineages). For the osmundaceous ferns fully understand the evolution of epiphytism and the timing of (Osmundales; node 1) (see Fig. 1), diversification was delayed epiphytic radiations. This allows us to recognize what factors until near the Triassic/Jurassic boundary (199.6 Ma) (see Table may have been responsible for epiphytic diversification— S4), the time at which the earliest crown group fossils appear (see ultimately providing further insight into the leptosporangiate Table S2). For filmy ferns (Hymenophyllales; node 4), we success story. estimate the initial divergence yielding the 2 major extant lineages—hymenophylloids and trichomanoids—to have oc- Results and Discussion curred somewhat later, in the Lower Jurassic (185.1 Ma). But Leptosporangiate Phylogeny. Phylogenetic analysis of our 3-gene although the trichomanoids (node 5) began to diversify soon dataset yielded a well-resolved and well-supported evolutionary thereafter (147.3 Ma), the hymenophylloids (node 18) did not framework (80% of the nodes received maximum likelihood begin their diversification until the Eocene (41.9 Ma). Gleichen- bootstrap support Ն 70%), while also providing an unprece- ioid ferns (Gleicheniales; node 32) are rather exceptional, having dented picture of relationships across leptosporangiate ferns. begun to diversify before the end of the Paleozoic (262.2 The tree topology is presented in Fig. S1; branch lengths, support Ma)—only 14.2 Ma after their inferred origin. For the schizaeoid values, and a thorough discussion of relationships are provided ferns (Schizaeales; node 43), we again see a substantial lag elsewhere (31). between origin (264.6 Ma) and crown group diversification (218.4 Ma); however, this latter date is still much older than the Leptosporangiate Diversification. Divergence-time estimates, re- oldest crown group fossils for this clade (see Table S2). Based on sulting from the integration of our molecular phylogeny (see Fig. our estimates, crown group diversification for both the heteros- S1) with age constraints from the fossil record (see Table S2), porous ferns (Salviniales; node 50) and the tree ferns (Cya- suggest that there was very little accumulation of extant fern theales; node 55) began in the Lower Jurassic (186.8 Ma and diversity in the Permian, Triassic,