Palm pollen and the fossil record
Previously published descriptions of the pollen morphology of the Palmae are reviewed and discussed. The earliest macro fossil records for palms are summarised, while a more detailed review is given of the fossil records of palmlike pollen. Selected literature relating to pollen sharing some similarities to palm pollen in other monocotyledonous families are briefly reviewed, and the fossil pollen records for these families are examined. A brief chronological account of earlier systematic treatments of the palms is provided, as well as an outline of the systematic treatment of the family used in the present account. The pollen morphology of 1150 collections, representing 765 species of palms,f rom all but seveno f the currentlyr ecognisedg enera,h asb eene xamýined, as well as dispersedp alm-likef ossil pollen from the middle Eoceneo f the Isle of Wight, and of Java. Iii silit pollen of fossil palm flowers from the Messel oil shales (Germany)a re describedP. ost meiotic tetrad stageh asb eens tudiedf or representatives peciesin all subfamiliese xceptingt he PhytelephantoideaeP.o llen morphologyo f both recenta nd fossil pollen is describedf rom light, scanning electrona nd, selectivelyf rom transmissione lectronm icroscopy,w hile tetrad resultsa re from light and scanninge lectronm icroscopy.F ull detailso f preparation methods,t erminologya nd databaseus sedf or pollen morphological,f ossil and tetrad studies are given. Seventeena perturet ypes,p lus numerouss ubtypesa, nd twelve exine types with numerous subtypes are identified. The aperture types are shown to be broadly separablein to two groupsw hich are associatedw ith either simultaneous (tetrahedralt etrads)o r successive(t etragonalt etrads)m icros porogenessi. In generalt heset wo groups supportp resents ystematico pinion regardingt he subfamilies.S uccessivem eiosisi s dominanti n subfamiliesC alamoideaea nd Nypoideaew hile, with somer are exceptionss, imultaneousm eiosisp redominates in the remainingf our subfamiliesC: oryphoideaeC, eroxyloideaeA, recoideaea nd PhytelephantoideaeP.o llenu ltrastructurei s treatedi n detail only for simple tectate exines where it is important for further definition. Six types and a number of subtypesa re described.T he systematicd istributionso f aperturea nd exine types are summarisedA. trend towardsl arger pollen is noted, with the smallest pollen occurring in the least specialised subfamily, the Coryphoideae, while very large-sized pollen are characteristic of subfamily Phytelephantoideae. Monosulcate, disulcate and zonosulcate pollen are described from fossil material and closest affinities with recent palms suggested. Pollen morphology of recent palms is summarised and discussed, and compared with pollen of selected monocotyledonous families. The bearing of pollen data on recent palm systematics is considered at various levels from subfamily to species. Angiosperm pollen evolution is re-considered and evolutionary pathways for palm pollen aperture types and exine types are suggested. In the light of recent pollen morphology for the family the fossil record of palm pollen is re-evaluated. Some widely accepted affinities are challenged while previously unconsidered affinities are suggested, particularly for the mid- Cretaceous. The need is emphasized for future fossil pollen studies to look critically for pre Late Cretaceous palm-like monosulcates, which would be more informative of the early history of the family than the apparently highly evolved, easily recognisable Spini. -onocolpiles of the Late Cretaceous. Palaeogeography, environment and distribution of fossil records and depositional environment are discussed. Probable evolutionary pathways of pollen morphology, including pollen tetrad data which suggests imultaneousra thert han successivem eiosisa s the plesiomorphics tate,o ffer further evidencet hat the palmsa re indeeda n ancient group, and support the hypothesis that the palms may have originated in South America and Africa (West Gondwana)in the Late Jurassic or early Cretaceous, prior to the complete separation of these continents.