This article provides a summary on Views Regarding the Time of Origin of Earliest Angiosperm.
We have to depend almost entirely upon fossil evidence for the determination of the first traces of angiosperms. Unfortunately, due to lack of proper fossil records, palaeobotanical studies have not been able to solve the mystery about the earliest angiosperms.
There are several views regarding the time of origin of the earliest angiosperms:
i. Some botanists suggested that the angiosperms originated in the early Mesozoic or even the late Paleozoic i.e. about 250 million or more years ago, and that they underwent extensive diversification by the Aptian-Albian stages of the lower Cretaceous period. However, there is no dependable fossil record from the pre-Cretaceous period.
ii. The best available evidence from the fossil record indicates that the angiosperms originated during the early cretaceous period about 130 to, 135 million years ago.
In the older Cretaceous sediments, the angiosperm fossil records show that, the vegetation during this period was dominated by the gymnosperms and ferns and it is not until the late part of the Cretaceous that the angiosperms became dominant.
This view is supported by Taylor on the basis of his studies of fossil pollen, which indicate the origin of the angiosperms during the very early Cretaceous or the latter stages of the Jurassic period.
(a) Pre-Cretaceous Angiosperms:
The appearance of monocot and dicot angiosperms in the fossil record without “transitional” forms has led workers to search for pre-Cretaceous angiosperms. Several authors have also hypothesized the possible mechanisms of the evolutionary change. Many of the purported pre-angiosperm ancestors have “angiosperm” leaf characters (net-like venation pattern), which had arisen independently in several clades.
i. Sanmiguelia is a herbaceous plant of the Late Triassic, whose leaves although appearing “palm-like” lack the midvein and petiole features of true palms. This enigmatic plant has been allied by some workers with the cycads, whereas others have allied it with pteridosperms.
ii. Furcula is also a Late Triassic genus, which shows angiosperm-leaf characteristics along with that of other groups, which include a forked midrib (primary vein) from which secondary veins arise. Intercostal veins between these anastomose to form a reticulate pattern that delimits poorly defined areoles. Stomata’s are syndetocheilic, which is also an angiosperm character.
iii. Pannaulika, a Late Triassic leaf, is the only known specimen having brochidodromous secondary veins with tertiary veins forming loops outside the secondary loops, which is also an angiospermic character.
iv. Marcouia is another Late Triassic pinnate leaf, which is palmately compound with four-five pinnae, that are basally united. Secondary veins originating from a well-defined midrib dichotomize and anastomose. This genus probably does not have any relation to angiosperms.
v. Monocolpate pollen grains having an outer exine, which is differentiated into a well-defined tectum supported by columellae, characteristic of angiosperms, have been described from the Late Triassic-Early Jurassic.
The leaf impressions from Triassic and Jurassic deposits, which have been attributed to angiosperms, are disputable. Similarly, the pollen from the pre-Cretaceous deposits, are also very problematic and in most cases have proved to be pollen grains of gymnosperms.
Even some deposits of this period, which were considered to be pollen grains of angiosperms at one time, have been suggested to be the unicellular green algae Tetraedron.
Pollen sequences have clearly shown a pattern of increasing diversity in the cretaceous from primitive types in older strata to more derived types in younger sediments. The first monosulcate angiosperm pollen grains, characteristic of the primitive dicotyledons and the monocotyledons, appear in the Barremian stage of the lower cretaceous.
Tricolpate pollen grains, characteristic of the more advanced dicotyledons were first reported from slightly younger Aptian rocks. The increasing diversity of pollen through the cretaceous indicates that angiosperms underwent much diversification during this period.
Further, recent investigations have shown that double fertilization, anastomosing leaf venation, reduction of the male gametophyte, tetrasporic mega gametophyte with free nuclei serving as eggs, and a feeder in the embryo, the primary characters of angiosperms, occurs sporadically in Ephedra, and a welwitschian-type reproductive structure (Archaestrobilus) found in the Late Triassic of Texas, which has been interpreted to have been a precursor to angiosperms.
Thus, there is little doubt that angiospermic features originated from a gymnosperm ancestor. Recently, however, a fossil of Archaefructus liaoningensis, the world’s oldest flower, from a lake deposit in China, in which limestone’s and volcanic ashes accumulated, exhibiting the presence of closed carpels was found, revealing that angiosperms were present in the Late Jurassic.
Pollen and pollen- bearing organs were not found in the specimen. Whether Archaefructus flowers were unisexual or bisexual, also remains unknown.
(b) Upper-Cretaceous Angiosperms:
The upper Cretaceous period is marked by the abundance of genera and families of angiosperms. The floras of the Vancouver Island, Amboy clays of New Jersey, the basin of the Rocky Mountain system, Dakota sandstone near Kansas, Nebraska and Minnesota, etc.
Were particularly rich in angiospermous genera, which altogether account for about 700 to 750 forms, of which most are preferable to extant genera. Several fossil forms including abundant fruits and seeds have been described subsequently from the Tertiary floras of Europe and Western United States, the valaughnian deposits of Southern France and Northern California.
Angiosperms steadily and progressively occupied all vegetated areas of the world from Albian to Senomanian times, none of which could however be classified in recent families. Angiosperms appear to be comparatively rare up to Albian times, apparently forming only small populations with smaller number of individuals.
Occurrence of angiospermous woods, leaves, pollen, etc. became considerably more numerous from Aptian-Albian times onwards, and became widely distributed throughout the world at the close of the Albian period or towards the middle of the Cretaceous period during which, they appeared in great diversity of form and quickly became dominant.
Most of these Cretaceous angiosperms belong to the extant genera, and there are representatives of both more or less primitive forms, as well as the highly evolved ones.