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Abstract Background and aimsPalm fossils are often used as evidence for warm and wet palaeoenvironments, reflecting the affinities of most modern palms. However, several extant palm lineages tolerate cool and/or arid climates, making a clear understanding of the taxonomic composition of ancient palm communities important for reliable palaeoenvironmental inference. However, taxonomically identifiable palm fossils are rare and often confined to specific facies. Although the resolution of taxonomic information they provide remains unclear, phytoliths (microscopic silica bodies) provide a possible solution because of their high preservation potential under conditions where other plant fossils are scarce. We thus evaluate the taxonomic and palaeoenvironmental utility of palm phytoliths. MethodsWe quantified phytolith morphology of 97 modern palm and other monocot species. Using this dataset, we tested the ability of five common discriminant methods to identify nine major palm clades. We then compiled a dataset of species’ climate preferences and tested if they were correlated with phytolith morphology using a phylogenetic comparative approach. Finally, we reconstructed palm communities and palaeoenvironmental conditions at six fossil sites. Key resultsBest-performing models correctly identified phytoliths to their clade of origin only 59 % of the time. Although palms were generally distinguished from non-palms, few palm clades were highly distinct, and phytolith morphology was weakly correlated with species’ environmental preferences. Reconstructions at all fossil sites suggested that palm communities were dominated by Trachycarpeae and Areceae, with warm, equable climates and high, potentially seasonal rainfall. However, fossil site reconstructions had high uncertainty and often conflicted with other climate proxies. ConclusionsWhile phytolith morphology provides some distinction among palm clades, caution is warranted. Unlike prior spatially restricted studies, our geographically and phylogenetically broad study indicates phytolith morphology may not reliably differentiate most palm taxa in deep time. Nevertheless, it reveals distinct clades, including some likely to be palaeoenvironmentally informative.
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Palm phytoliths in subarctic Canada imply ice-free winters 48 million years ago during the late early Eocene
Abstract Background and AimsPhytoliths are microscopic siliceous structures produced in specific tissues by many plant families. The morphological features of phytoliths are diagnostic for many plant taxa and, given their inorganic composition, often become part of the fossil record. We used phytolith remains from lacustrine sediments to document the conclusive presence of Arecaceae (palms) in subarctic Canada during the late early Eocene (48 Ma). MethodsPalm phytoliths and aquatic microfossils were extracted from lacustrine mudstones in a drill core taken from the Giraffe kimberlite pipe locality using a combination of acid and oxidation treatments under low heat. Light microscopy and scanning electron microscopy were used to identify, examine and image the microfossils. Key ResultsSpherical echinate-shaped palm phytoliths with cone-shaped surface tubercles, likely belonging to the tribe Trachycarpeae (subfamily Coryphoideae), were uncovered in 45 strata over a 37-m section of core. We further document in situ linear arrays of phytoliths, or stegmata, from partially decomposed palm foliage. Additionally, four aquatic organisms, largely restricted to warm subtropical and tropical localities today, were also uncovered in the same strata harbouring the palm phytoliths. ConclusionsThe presence of palm phytoliths allows inference of a warm regional climate during the late early Eocene, with mean cold-month temperatures above freezing despite prolonged winter darkness. This conclusion is supported by the presence of multiple warm-water aquatic organisms that grew extensively in the maar lake. Our findings will help to document the extent and timing of perennial ice formation in the northern hemisphere during the Cenozoic. Finally, the discovery of stegmata documents that this morphological trait had evolved by early Eocene.
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- Award ID(s):
- 1940070
- PAR ID:
- 10580175
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Annals of Botany
- ISSN:
- 0305-7364
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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