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Phylogeny and Origin of Angiosperms When, Where and from What Did Phylogeny and Origin of Angiosperms By the end of this Chapter, you are able to understand When, Where and from What Did the Angiosperms Arise? (1) What were their ancestors? (2) What is their antiquity? (3) What was the general nature and environment in which they originated? (4) Theories related to Phylogeny and Origin of Angiosperms Angiosperms (flowering plants) group arose from a yet-to-be-identified ancestral lineage and diversified to form over 350,000 species alive today. The angiosperm stem group probably diverged from other seed plants by the late Palaeozoic (Moldowan et al. 1994; E. L. Taylor et al. 2006; Lutzoni et al. 2018: (395-)365(-338) Ma). So how might the heterosporangiate strobilus with short internodes with ovules enclosed in a carpel that is the angiosperm flower today have evolved from the separate male and female strobili with ovules borne free on ovuliferous scales of most gymnosperms, living and fossil? Is the flower a monoaxial structure, as in most cycad strobili, or is it polyaxial, as in a pine cone, or are the flowers of some groups monoaxial and those of other groups polyaxial? The latter idea might suggest that angiosperms are polyphyletic (see Friis et al. 2011: pp. 141-144 for a summary of such ideas), and X. Wang and Wang (2010) and Z.-J. Liu and Wang (2015) indeed suggest that angiospermy may have arisen more than once and Fu et al. (2017/2018) toy with the idea. Indeed, the envelopment of the seed to produce a fruit-like structure is likely to have happened independently in a number of groups like Bennettitales, Doyleales, and angiosperms (Rothwell & Stockey 2010, 2016 and references; see also Friis et al. 2011; Tomlinson 2012 and angio-ovuly). Pollen tubes in Pinales/Gnetales and in angiosperms also probably evolved independently since at least some glossopterids had multiciliate male gametes (Nishida et al. 2004; c.f. Lee et al. 2011: cilia lost and regained?). When, Where and from What Did the Angiosperms Arise? The Cretaceous period, which was transitional from the Mesozoic to the Cenozoic era, is well represented by a succession of fossil floras from both hemispheres. These floras record a change from an Early Cretaceous world dominated by typically Jurassic ferns and gymnosperms, to a Middle Cretaceous landscape in which the angiosperms had become characteristic and world wide in distribution. This transformation of vegetation brings us face to face with a major problem: that of early angiosperm evolution. The problem has several facets, which may be viewed in terms of the following questions: (1) What were their ancestors? (2) What is their antiquity? (3) What was the general nature and environment in which they originated? (4) Did they actually assume dominance with "bewildering suddenness" during the Cretaceous? Despite renewed interest, attested by the observations and inferences of numerous writers during the past few years (e.g., Arnold, 1947; Bailey, 1948; Camp, 1947; Chaney, 1947; Just, 1947, 1948; Mason, 1947, Stebbins, 1950; Suessenguth 1950' Thomas, 1947; Whitehouse, 1950), thes~ fundamental questions have not been satisfactorily answered. Additional data have been presented, but to judge from the diversity of conclusions which have appeared, the solution might seem no more certain today than when Darwin (1879, pp. 20-22), in writing to Hooker, referred to the problem of early angiosperm evolution as "an abominable mystery." The angiosperms appeared suddenly in Cretaceous age about 65 million years back. Charles Darwin described this sudden appearance of angiosperms in lower or upper Cretaceous as an ‘abominable mystery. When angiosperms appeared for the first time in lower or upper Cretaceous, they were full fledged like the trees and the herbs of today. In support of this view Prof. Knowlton advocates in his ‘Plant of the past’, “from the time of their appearance they did not progress at all due to their full-fledged appearance in the Cretaceous”. The fossil records of the angiosperms also support their appearance full-fledged in lower or upper Cretaceous. The fossils of that age are so characteristic and modem in appearance that most of them can be referred unmistakably to living families, general and even to some species. Prof. Knowlton stated in his book, ‘Plant of the past’ “if a student of present day trees and shrubs, could have wandered over the hills and vales in those days, he would have found himself quite at home among the trees and shrubs growing there”. The forms of cycads and conifers, which long dominated the universe were already pushed background and the earth had become infact the earth of flowering plants. Charles Darwin has called this sudden appearance of angiosperms as an “abominable mystery”. However, some workers do not agree with the doctrine of ‘abominable mystery’. According to H.H. Thomas (1936), the angiosperms of the past replaced many of older gymnosperms in asturine and marshy waters. Graud Eury (1906) believes that the angiosperms came into existence through mutation. Guppy (1919) however, supported the view of mutation. Prof. Bertrand is of opinion that all the great groups of vascular plants (Pteridophyta, Gymnospermae and Angiosperms) not only arose quite independently of each other but also they originated simultaneously as far back in the Archian period (2000 million years old-oldest). There is a very considerable but scattered literature on the origin and phylogeny of angiosperms. The palaeobotanical evidence shows that there seems three possibilities as regards the origin of angiosperms. These possibilities are: 1. That the angiosperms are monophyletic in their origin but have had a very much longer history than at present known, perhaps stretching back into Palaeozoic times and with a whole series of missing links; 2. That the angiosperms are monophyletic but that the first and at present unknown group diverged quickly in terms of geological time, into a considerable number of different groups; 3. That the angiosperms are polyphyletic. According to Campbell, “both comparative morphology and the geological record indicate that the existing angiosperms represent a number of distinct phyla which cannot be traced back to a single ancestral type”. This statement shows that he does not believe in monophyletic origin of angiosperms. According to Thomas, the evolutionary tendencies detected in the three groups, i.e., Caytoniales, Bennettitales and Pteriodsperms furnish reasonable grounds for the idea that the angiosperms were derived from various pteridosperms early in the Mesozoic period. Parkin also argues for the monophyletic origin of the angiosperms. Most molecular phylogenetic analyses indicate that the living gymnosperms and angiosperms form two sister clades whose lineages separated some 300 million years ago (MYA). However, the radiation of the extant angiosperms dates from much more recently: most molecular clock estimates place this divergence between 180 and 140 MYA (Bell et al. 2005), while the earliest unequivocal fossilized angiosperm pollen, found in Israel, Morocco and southern England, dates from around 135 MYA (Willis and McElwain 2013). Molecular and fossil data therefore combine to suggest a paleotropical origin for the flowering plants in the late Jurassic/early Cretaceous. Accordingly, we may conclude the living angiosperms to derive from a stem lineage of perhaps some 140 MY in length, from which no other living groups are available for study A fundamental problem that must be dealt with is whether the angiosperms are monophyletic (i.e. a group consisting of all descendants derived from a single ancestor) or polyphyletic (i.e. a group that does not have a common ancestor). Major contributions to the problem of angiosperm origins (such as the interpretation of Caytonia or Leptostrobus) were made by paleobotanists who, like T. M. Harris, never undertook the special search for angiosperm ancestors, but thoroughly studied large regional taphofloras. This may witness the perfection of the fossil record. Studies of the Mesozoic gymnosperms have reached the stage when any discoveries of completely unknown extinct groups are unlikely, although further reinterpretations are to be expected. Thus, imperfection of the fossil record can no longer be the sole excuse for still unsolved problems of the angiosperm ancestry and phylogeny. These problems can be approached from several directions, such as (1) unpreconceived phylogenetic analysis of the living angiosperms, (2) a more realistic estimation of the early angiosperm diversity, not biased by unsound taxonomical practice of "leaf paleobotany", (3) revision of the Mesozoic gymnosperm morphology, (4) clarifying the ecological significance of angiospermy and paleoecology of angiospermization. Due to inadequate fossil records, the question of phyla of the angiosperms still remains unsolved. However, angiosperms are a natural group and contain characters, which make them unique from all other vascular plants. (a) Monophyletic Origin: i. As a group, the angiosperms have typically been viewed as being monophyletic. However, no definite fossil evidences are available in favour of the monophyletic origin. This view is based on the fact that present-day angiosperms show remarkable consistency in their characters, i.e. presence of sieve tubes in all, uniform staminal structure, characteristic endothedial layer of the anther wall, double fertilization, and formation of triploid endosperm, which are considered defining features of angiosperms and support the monophyletic grouping. ii. The monophyletic origin of angiosperms is also supported by Hickey & Doyle on the basis of their studies of mono-sulcate pollen. iii. Dahlgren believes that the ancestor of the present-day angiosperms was a gymnospermous member. (b) Polyphyletic Origin: i. Several phylogenists including Cronquist, Hughes, Games, Krassilov and Meeuse have argued that the angiosperms are polyphyletic i.e. dicots and monocots originated from different primitive stocks at different times, and attained their present status through parallel or convergent evolution. ii. The theory of polyphylesis is also supported by the fossil records of variety in perianth and the nature of carpel in both dicots and monocots.
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