In this article we will discuss about the fossil anglosperms of a plant.
The fossil remains of true angiosperms are found only in the later geological periods and as a group they are more modern than other vascular plants. The fossil history of Angiosperms is too short to enable us to approach a definite conclusion. The palaeobotanical record of this group is very fragmentary and poorly understood.
The flowering plants are probably still in the initial stages of their expansion and the developmental trends are not clearly expressed in the fossil series. Their initial formative stages and their early history is still hidden in the bowels of the earth. Their geological record is incomplete.
According to Krishtofovich (1950) the new fossil forms are detected only when they begin to play a significant role and not when they are either just arising or occupying a very modest position. The fossil record consists mainly of wood and impressions of levels and leaf fragments with only sprinkling of seeds and fruits to aid in their determination.
One of the most remarkable phenomena in biological evolutionary history is the rapidity with which the angiosperms arose to a position of dominance in the plant world during the latter part of the Mesozoic era i.e., the Cretaceous period (about 200 million years ago).
According to Takhtajan (1958) the first angiosperms appeared in the early Jurassic flora or perhaps even at the close of the Triassic, and played only an insignificant role among the then dominant ferns, Ginkgoales, Cycadales, Bennettitales and Coniferales.
According to Core the angiosperms are relatively youthful as compared to other groups of plants, as their fossils have not been found earlier than the Cretaceous and their period of rapid expansion came after the period of dominance of gymnosperms and other lower plants.
According to Arnold (1947) it is hypothetically assumed that the angiosperm line took shape at some unknown time during the Mesozoic era and all the naked-seeded groups (the pteriodosperms the Cordaiales, the Coniferales, the Cycadophytes, the Gnetales) and even the ferns had at times been proposed as the possible precursors of the flowering plants.
Although the flowering plants are so prominently displayed in the rocks of the late Cretaceous and Tertiary, Botanist are quite ignorant about their origin and evolution. The main reason given is that we do not know any series of fossil forms connecting the flowering plants with lower groups.
Arber and Parkin (1907) tried to supply the missing link with the hypothetical “Hemiangiospermae” where the fructifications were very much like the cycadeoid “flower”.
This fructification was supposed to possess a primitive perianth of spirally arranged leaves, an androecium of indefinite stamens also spirally arranged and carpels which were open leaf-like structures with marginal megasporangia seated upon a dome-like receptacle. Since there is no proof of the existence of such a fructification, the workable phylogenies cannot be built upon hypothetical forms.
Fig. 20.1. Record of Angiosperms on earth.
The exact position in the geologic sequence at which the angiosperms can first be supposed to have originated or recognised is not certain. No angiosperm plant ever existed in the Paleozoic era and if there is any reference of its existence in any literature it can be rejected outright. Either the formations were wrongly determined or there was wrong identification of the plants.
The oldest known plant of definite resemblance to an angiosperm is Fercula granulifera discovered by Harris in the Rhaetic (late Triassic) rocks of Eastern Greenland. According to Arnold (1947) the leaves of this plant were 7-15 cm. long and 6-8 cm. broad and are usually forked at about the middle. The petiole was short and the apex acute.
The leaf had a well-developed midrib and secondary veins which usually forked before the margin is reached. Numerous stomata of dicot type were irregularly scattered slightly sunken in the lower surface. This leaf was different from any other known in the older Mesozoic and according to him if it were found in the Cretaceous or Tertiary it would have been placed among the dicots with certainty.
The probable Jurassic angiosperm is a piece of secondary wood, Homoxylon rajmahalense, a fossil from the Rajmahal Hills in Bengal to the northwest of Calcutta (India) and described by the Indian botanist Birbal Sahni (1932). Although its exact source is not known, evidences show that it belonged to lower Jurassic period.
The wood resembles to that of a gymnosperm in having no vessels, but the tracheid walls bear a mixture of scalariform and circular pits as found in the family Winteraceae as well as the homoxylous Magnoliaceae such as Trochodendron and Tetracentron.
Subsequently Yarmolenko (1939) described two new species of Homoxylon – one from the Lower Jurassic deposits of Western Tian-Shan and the other from the Lower Cretaceous deposits of the eastern slope of the Urals. Sahni and Yarmolenko referred the species of the genus Homoxylon to the primitive vesselless angiosperms of the type Tetracentron, Trochodendron and Winteraceae.
The two other Indian botanists Hsu and Bose (1952) carried on further investigations and said that since in the structure of its wood, Homoxylon resembles with Bennettitales.
The most positive evidence of the existence of pre-Cretaceous angiosperms is pollen found in coal of Jurassic age in Scotland (Arnold 1947). The pollengrains bear three longitudinal grooves similar to those of the genus Nelumbo (family Nymphaeaceae).
Accompanying them are also pollengrains resembling those of Castalia of the same family Nymphaeaceae. According to Taklitajian (1958) also the remains of angiosperms begin to the found in Jurassic deposits being represented only by pollengrains. They have been recorded from the Jurassic coals of Brora, Scotland.
They were of two types, monocolpate, resembling the pollengrains of water lilies and tricolpate, resembling the pollengrains of the Lotus (family Nymphaeaceae). Analogous types of such pollengrains were also recorded in the lower Jurassic deposits in north-western Scania (Southern Sweden).
One of the two types of pollengrains recorded there is the magnolioid type, but it is also possible to refer it to the microspores of gymnosperms, among which typical monocolpate forms also occur (Erdtman). The other type of the pollengrains recorded there belong to the tricolpate form, which is found only in the dicots and are not met with in the gymnosperms and ferns.
Takhtajian is of opinion that the discovery of tricolpate type of pollengrains in the Jurassic deposit, is a positive evidence of the existence of angiosperms in the Jurassic period. According to him, since the tricolpate type is derived from the monocolpate type, the latter must have arisen much earlier, probably at the close of the Triassic period. The Triassic origin of the angiosperms is therefore entirely possible.
The examples given above indicate that there were plants present during the Jurassic period which at least resembled angiosperms and exhibited angiospermic characteristics. In fact angiosperms are first recognised with certainty in the Jurassic, but they do not become important elements of the flora until the Cretaceous.
The decipherable history of the flowering plants (angiosperms) begin with the Lower Cretaceous, for it is not until this period that we find them extending in unbroken sequence to the present (Arnold).
Some of the oldest angiosperms have been reported from the Lower Cretaceous Kome beds of Western Greenland where they are interpreted by leaf impressions. Out of them Populus primaera is probably the oldest leaf to be referred to a modern angiosperm genus.
Slightly younger or of almost equal age are Potomac beds of Maryland containing the most extensive Lower Cretaceous angiospermic plants, e.g., Ficophyllum, Quercophyllum, Juglandiphyllum, Nelumbites, Menisdermites, Sapindopsis etc. One of the early Cretaceous flowering plants in England is represented by a petrified wood in the Lower Greenland.
There are about five genera of dicot woods described from this period which possess vessels and rays typical of living angiosperms. The early Cretaceous angiosperms indicate the antiquity of some families of the flowering plants such as Nymphaeaceae, Menispermaceae, Salicaceae, etc.
The occurrence of monocots in the early Cretaceous in direct association with dicots proves that as far as the fossil evidence is concerned the two groups monocots and dicots are of almost equal antiquity.
Until the end of the Lower Cretaceous the angiosperms were scanty and everywhere in minority, not exceeding 25% of all species. This scanty occurrence of angiosperms until the end of the Lower Cretaceous is explained by Takhtajian due to the conditions of their habitat.
The Lower Cretaceous and Jurassic angiosperms lived mostly on the mountains where the conditions of their fossilization were unfavourable. The remains of leaves of the plants on mountains are pulverized during the dislocation strata and do not usually reach the area of accumulation.
In the mid-Cretaceous, the constitution of the vegetation of the earth suddenly changed and the angiosperms appeared, as though suddenly, in very large numbers and varieties and spread all over the world with astonishing speed.
Apart from their quick and sudden spread, all the Cretaceous angiosperms known to us belong to the present day families and even genera and are represented by more or less primitive forms such as Magnoliaceae, Nymphaeaceae and other related families as well as by forms which have progressed such as representatives of Fagaceae, Moraceae, Euphorbiaceae etc., (Takhtajian).
It is, therefore, clear that until the middle of the Cretaceous, angiosperms played only a subordinate role and were present only in comparatively small number of genera and species.
During the mid-Cretaceous period they had the opportunity to spread with rapid speed and within a short time they occupied vast areas of land. Simultaneously the old representatives of the Mesozoic era which dominated on the earth ceded their place to the new comers. The mid-Cretaceous is therefore regarded as the beginning of a new era in the history of world.
Towards the end of the Cretaceous period there was a widespread volcanic activity in Peninsular India. Preserved in the sedimentary deposits intercalated between the successive lava flows in found the earliest Tertiary flora of India known as the Deccan Intertrappean flora.
Its study was initiated by Sahni and continued by Rao, Mahabale, Surange. So far about 167 megafossil texa of the specific rank and a number of microfossils have been reported from various localities dotted over a vast area on the Deccan plateau (Surange). The flora consists of algae, fungi, bryophytes, ferns, conifers and angiosperms. Among angiosperms, the palms (monocots) are dominant.
Eocene sediment containing fossil plants is the Fuller’s Earth deposit in Barmer District, Rajasthan. The occurrence therein of such plants as Mesua and Garcinia reported by Lakhanpal and Bose and Cocos described by Kaul indicates that during early Tertiary times, the climate of this region was moist enough to support thick forests.
Extensive deposits of Miocene strata in Assam and South India have yielded rich collections of petrified woods strongly suggesting that during the Miocene epoch the vegetation of Eastern and Southern India was very rich consisting of such plants as members of the family Dipterocarpaceae growing in wet tropical climate. Similar plants also grew in the Siwalik regions along the foot-hills of the Himalayas.
Fossil Monocotyledons:
Our knowledge of the monocotyledones is based mainly on leaves that are often fragmentary and hardly identifiable, on petrified stems and on several large specimens of palm leaves. If the parallel venation of leaves is taken as the distinctive character of the Monocotyledones their presence is claimed in the Carboniferous period.
But since it is now known that such leaves are also characteristic of the great Paleozoic group Cordaites and other Gymnospermic plants as well as certain heterosporous Pteridophytes, this claim of the presence of monocots in Carboniferous period is no more considered as true.
The existence of monocots in the Paleozoic period has not been proved. As to the antiquity of monocots and Dicots, the historical evidence indicates that no definite Monocotyledones has been recorded from strata older than those in which typical Dicotyledones first appeared or vice versa.
There is also no historical evidence that the Monocots have ever been a dominant race, as Gymnosperms had been and as the dicots now are, although they do not appear to be so abundant now as they were during the tertiary.
According to Chamberlin, the only suggestion of paleobotany as to the origin of Monocots is that they are certainly a younger type than the Gymnosperms. When the claim for the Carboniferous Monocots is rejected, the question of their existence in the Jurassic period comes. This also rests upon the occurrence of certain grass-like forms resembling and suggesting of Monocots, but such an evidence cannot be taken as conclusive.
The possibility of the presence of Monocots during the Jurassic period rests not upon their positive discovery, but upon the fact that during the Cretaceous they were present in large numbers everywhere and this suggests of their long presence.
There is definitely no clear proof of the existence of Monocots in any strata earlier than the Cretaceous. Remains of Monocots are always subordinate to those of Dicots.
This however does not indicate that Monocots did not exist during the late Cretaceous and the Cenozoic, but the paucity is explained on the basis of habitat. Several Monocots such as various grasses inhabit the drier places where there are remote chances of their being fossilised.
The record of monocots can be considered in the three headings (Chamberlin):
I. Families represented during the Cretaceous.
II. Families whose earliest representatives are in the Tertiary.
III. Families only known since the Tertiary.