Exceptional preservation of tiny embryos documents seed dormancy in early angiosperms

Abstract

The rapid diversification of angiosperms through the Early Cretaceous, between about 130 and 100 million years ago initiated fundamental changes in the composition of terrestrial vegetation and is increasingly well-understood based on a wealth of palaeobotanical discoveries over the last four decades [1][2][3][4][5] , and their integration with improved knowledge of living angiosperms 3,6 . Prevailing hypotheses, based on evidence from both living and fossil plants, emphasize that the earliest angiosperms were plants of small stature 7-12 with rapid life cycles 7,8,12,13 that exploited disturbed habitats 3,9,11,13,14 in open 3,9,11,13,14 , or perhaps understory conditions 15,16 . However, direct palaeontogical data relevant to understanding the seed biology and germination ecology of Early Cretaceous angiosperms are sparse. Here we report the discovery of embryos and their associated nutrient storage tissues in exceptionally wellpreserved angiosperm seeds from the Early Cretaceous. Synchrotron radiation X-ray tomographic microscopy (SRXTM) of the fossil embryos from many taxa reveals that all were tiny at the time of dispersal. These results support hypotheses based on extant plants that tiny embryos and seed dormancy are basic for angiosperms as a whole 17,18 . The minute size of the fossil embryos, and the modest nutrient storage tissues dictated by the overall small seed size, is also consistent with the interpretation that many early angiosperms were opportunistic, early successional colonizers of disturbance-prone habitats 2,15,16 .As part of a broader survey of Early Cretaceous angiosperm reproductive structures using SRXTM 19 we analysed the internal structure of mature seeds from about 75 different angiosperm taxa recovered from rich assemblages of angiosperm flowers, fruits and seeds in 11 mesofossil floras from eastern North America and Portugal that range in age from 6/17/16 -2

A Whole Plant Herbaceous Angiosperm from the Middle Jurassic of China

“…This is in line with the ecophysiological analyses of extant and fossil plants, which conclude that herbaceous habit may well have been adopted by early angiosperms (Stebbins, 1981;Taylor and Hickey, 1990, 1992, 1996Carlquist, 1996;Royer et al, 2010). This conclusion has been favored by recent studies on Early Cretaceous plants (Jud, 2015;Friis et al, 2015). Juraherba unequivocally confirms that early angiosperms have already adopted this strategy at least back to the Middle Jurassic (Figs.…”

On the Interpretation of Fossil Nuclei

“…Recently Friis et al [1] also reported nuclei in exquisite Early Cretaceous fossil seeds. Although their main conclusion that herbs are among early angiosperms is an old idea dated back to 1960s [1] and is supported by recent discovery of Jurassic herbaceous plant [6], their claim of nuclei deserves further scrutiny. Here, to shed more light on this issue, some plant mesofossils from the AlbianCenomanian (Cretaceous) of Kansas, USA, were studied using LM, SEM and TEM [7] [8].…”

“…Some researchers have published excellent papers regarding this aspect [2]- [5]. Recently Friis et al [1] also reported nuclei in exquisite Early Cretaceous fossil seeds. Although their main conclusion that herbs are among early angiosperms is an old idea dated back to 1960s [1] and is supported by recent discovery of Jurassic herbaceous plant [6], their claim of nuclei deserves further scrutiny.…”

“…In the studies of plant fossils, the exploration of nuclei is a difficult spot [1]- [5]. Some researchers have published excellent papers regarding this aspect [2]- [5].…”

Paisia, an Early Cretaceous eudicot angiosperm flower with pantoporate pollen from Portugal

“…The mesofossil assemblages are rich in angiosperm flowers and inflorescences as well as isolated fruits, seeds and stamens of angiosperms, in addition to a diversity of seeds related to the Bennettitales-Erdtmanithecales-Gnetales complex, conifer seeds and twigs of Cheirolepidiaceae as well as many fern fragments and megaspores (e.g. Friis et al 1994Friis et al , 2009Friis et al , 2010Friis et al , 2014bFriis et al , 2015aFriis et al , 2015bFriis & Pedersen 2014).…”