Ultrastructure of Lower Cretaceous Angiosperm Pollen and the Origin and Early Evolution of Flowering Plants

Abstract

In the last decade significant new information has been gained about the early evolution of flowering plants through studies of Early Cretaceous angiosperm pollen and the pollen of living primitive flowenng plants. Although most recent palynological studies of extant primitive angiosperms have used both scanning and transmission electron microscopy, few ultrastructural studies of early fossil aneioSDerm nollen erains pvict Thic r.an»r â"¢r^ro,o^«to « * * i.. ^li. _-^.._^.-tu:-* Cretaceous angiosperm Fredericksburgian of Ol yumupouennes nugnesii. two att. ClavatipoUenites species, Asteropollis asteroides. Stephanocolpites Jredencksburgensis, Retimonocolpites dividuus, R.peroreticulatm, two aff. Retimonocolpites species, StellatopoUis barghoornii, and three species of Liliacidites. These grains were investigated using a technique that we have developed for working with single fossil pollen grains by which we are able light-mic] known ' 7^ r""-" e^«x*xa, oui^ii aa muse luui cunsiiLuic me DUiK oi inc earuesi kiiuwu yai^. 01 the tossil record of the flowering plants, and provides a much improved means of deUmiting Early angiosperm pollen form uur study also reveals that a close similarity exists between some Early Cretaceous angiosperm pollen and pollen produced by certain living primitive angiosperms. ClavatipoUenites hughesii, Asteropollis asteroides, and Stephanocolpites fredericksburgensis exhibit varying degrees of similarity at the ultrastructural level respectively to pollen of the extant angiosperm genera Ascarina. Hedyosmum. and Chloranthus. all three of which belong to the modem family Chloranthaceae. Pollen described under

Early Cretaceous (Albian) spores and pollen from the Glen Rose Formation of Texas and their significance for correlation of the Potomac Group

“…This is similar to but larger and more coarsely columellar than Zone I and II pollen compared to Clavatipollenites hughesii (Doyle et al 1975;Walker & Walker 1984), which also occurs in our samples (Plate 3, figure 2). Heimhofer et al (2007) reported similar pollen, as Clavatipollenites cf.…”

“…The boundary between Zones I and II was based primarily on the appearance of new species of spores and the first tricolpate angiosperm (eudicot) pollen. In Zone I Brenner reported reticulate-columellar monosulcates, including Clavatipollenites, which was first described from the Barremian of England (Couper 1958 considered such forms incertae sedis, but their angiospermous affinities were subsequently confirmed by studies of exine structure (Doyle et al 1975;Walker & Walker 1984) and association with angiosperm stamens and carpels Friis et al 1999Friis et al , 2011. Based on the data available then from other areas, he dated Zone I as Barremian or Aptian and Zone II as Albian.…”

“…The distinction between these two genera is unclear, and indeed Kemp (1968) considered them synonyms. Pierce (1961) defined Retimonocolpites as having reticulate sculpture, whereas Clavatipollenites was originally thought to have clavate or 'retipilate' sculpture (Couper 1958;Brenner 1963), but since the first scanning electron microscopic (SEM) studies (Doyle et al 1975;Hughes et al 1979;Walker & Walker 1984) it has been known that such grains have a reticulate (semitectate) tectum supported by columellae. Retimonocolpites is often used for forms in which the reticulum is more conspicuous than the columellae under light microscopy, and Clavatipollenites for those in which the columellae are more evident, but these two conditions form a continuum, and not surprisingly the line between the two genera has been drawn differently by different authors.…”

Cenomanian angiosperm leaf megafossils, Dakota Formation, Rose Creek locality, Jefferson County, southeastern Nebraska

“…For example, Crassidenticulum decun-ens shows close similarities to extant Chloranthaceae (considered one of the primitive families of Laurales by Takhtajan [1980]), although its precise combination of venational features is not known from any extant genus. This is not surprising, because dispersed pollen and flowers comparable to extant genera of Chloranthaceae occur in older rocks (Walker and Walker, 1984;Friis and others, 1986). Landonia provides a possible evolutionary link between extant primitive Laurales that produce inaperturate pollen (Amborellaceae and Monimiaceae) and more derived members of the alliance because it combines (1) the pinnate venation and generally isodiametric intercostal regions characteristic of Amborellaceae and Monimiaceae, and (2) the generally transverse and sparsely branched intercostal tertiary venation with admedial branches characteristic of Gomortegaceae and certain Hernandiaceae (Illigera).…”

“…During this time, the angiosperms began their adaptive radiation, and major extant lineages at the level of subclass, order, and family made their first appearance. (See, for example, Wolfe and others, 1975;Hickey and Doyle, 1977;Dilcher, 1979;Upchurch, 1984;Walker and Walker, 1984. ) Associated with this adaptive radiation were progressive and broad-scale ecological displacements, wherein cycadophytes, ginkgophytes, and other dominant groups of gymnosperms declined in relative abundance and diversity and were replaced in most habitats and geographic regions by angiosperms, especially at lower and middle paleolatitudes (Doyle and Hickey, 1976;Hickey and Doyle, 1977;Retallack andDilcher, 1981, 1985;Crane, 1987;Upchurch and Wolfe, 1987b).…”