The Evolution of the early Mesozoic Plants

[Music] the transition from the permian to triassic saw the demise of glossitarites as well as other more ancient groups of plants from the paleozoic however the triassic brought further modernization of the flora as well as the appearance and diversification of new groups of ferns and seed plants relatively little is known about the plants of the early triassic but rich floras of the late triassic for example from greenland and south africa contain new kinds of ferns and many new groups of seed plants much research needs to be done to better understand these ancient seed plants including clarifying how they relate to each other and their possible living relatives many new kinds of seed plants come into the fossil record for the first time during the triassic studies of the moltino flora of south africa show a range of different reproductive structures for these plants many of them are difficult to understand in terms of the living plant groups to which they may be closely related especially prominent among the triassic seed plants are chorustosperms which are particularly diverse in the southern hemisphere bennett italians are another important group of seed plants of appear for the first time in the triassic their leaves are superficially similar to those of living cycads but the reproductive structures of the two groups are quite different those of psych8 are simple cones while those of bennett italians are often flower-like some bennett italian's flowers produced only pollen or seeds and a few were bisexual cycads and ferns continue to diversify through the triassic this period also sees the first reliable evidence of the ginkgo lineage conifer-like leafy shoots are common in triassic floras some of these plants are clearly related to living conifers but the relationships of others are much less certain various groups of plants during the triassic may have had conifer-like foliage but very different kinds of reproductive structures this simple plant is known from several locations in the southern hemisphere including triassic rocks in the antarctic peninsula hexagona colon was a flat bodied ortalos liverwort similar to many liverwort species alive today it lacked leaves and roots and anchored itself with fine hairs or rhizoids which formed tufts along its underside like modern liverworts hexagonal colon did not have any internal vessels for conducting water as a result the plant was restricted to damp habitats most plant fossils are formed by the sporophyte or spore producing phase in the life cycle the intervening phase known as the gametophyte is typically much smaller and is rarely found as a fossil in mosses and liverworts this situation is reversed the smaller sporophyte grew on the female gametophyte where it released spores the spores grew into new gametophyte individuals at the beginning of the triassic earth's vegetation was emerging from the greatest of all mass extinctions many plants died out but plura may have benefited from the catastrophic change fossils show that it grew right across the world appearing in a wide variety of different habitats left vacant by the disappearance of competing plants pleromea belonged to the lycophytes a group that included the giant club mosses that formed the coal swamps of the carboniferous period although pleuromaya was a tree it was built on a smaller scale it had a single unbranched trunk topped by a tuft of grass-like leaves its root system consisted of four bulb shaped lobes connected to rootlets that fanned out through the soil floramia reproduced with spores from cones some species produced several cones but many had just one on top of the stem this fern is similar to cladophlebus from the jurassic the segment of the frond called pinucles are about one centimeter long with rounded tips these pinules have a primary vein that runs to their tips as well as veins orientated sideways that form a distinct network this type of net ventilation is also characteristic of the primitive seed plant launch opterisk which lived during the carboniferous and permian also known as dictiophylum this common fossil plant is often used as a geological marker helping identify layers of rock that were formed at a given point in time some thomatopterous species straddle the triassic jurassic boundary but many fossils appear only at the point where the jurassic period began thamatopteris can be identified by the complex network of veins in its long fronds the veins divide and rejoin forming a mesh on the leaf surface dicroidium belonged to a group of plants called the chorustosperms which were seed plants of uncertain relationship found mainly in the southern hemisphere they had fern-like leaves but produced seeds instead of spores dicroidium and other choristosperms evolved in genduana which was the southern landmass of the supercontinent of pangaea during the triassic a fully grown dicroidium would have been beyond the reach of ground-based herbivores but all choristosperms would have been browsed by animals at some stage in their lives these browsers included the triassic dysinodont lystrosaurus whose fossils have been found in the same rocks as choristosperm remains in antarctica the earliest fossils of cycads date back to the permian period but they became more common at the donning of the triassic typically shaped like squat palm trees cycads such as dioid decarpidium reproduce by growing cones individual plants are either male or female the cones of male cycads release pollen grains which fertilize the female cones the seeds then develop inside the female cone ginkgos are not only living fossils they form part of an ancient group of flowerless seed plants called the gymnosperms which also include conifers and cycads they have a fossil history stretching back more than 200 million years during this time evolution has created countle s variations on the theme of fan-shaped leaves with its highly divided leaves baira looks very different from its living relative the well-known ginkgo or maidenhair tree a shrub or a small tree stachytaxis was one of the most abundant conifers during the late triassic but died out along with its relatives in the jurassic it was evergreen and had narrow leaves arranged in opposing ranks their narrow width made the leaves resistant to drought in much the same way as the needles of today's conifers stachytaxis produced seeds using cones pollen from male cones was spread by the wind and fertilized the female seed bearing cones the pollen granules were spherical and lacked the sacks or wing-like scales that pine pollen uses to drift in the wind the triassic was a time of great change for conifers in this period most of the earlier conifers of the paleozoic were dying out and new ones emerged and diversified into the conifers that survive until the present day the small tree or shrub voltsia belonged to one of the early now extinct groups but the structure of its cones shows that it had links to today's forms voltsia's male cone had pollen cases arranged in a spiral around the central core the seed producing scales of the female cones were fused together this was a continuation of the evolutionary trend in which cones developed originally from dwarf shoots the voltsia sandstone or northeast france where it is a common find rich and well-preserved middle jurassic flora found in yorkshire in northern england jurassic plants are known from many parts of the world but fossils from several localities in northern england have told us much of what we know about jurassic plants this so-called yorkshire jurassic flora has been studied since the earliest days of paleontology and attempts to reconstruct extinct plants from the different dispersed parts preserved in yorkshire flora have been especially successful ferns and conifers are common in many floras from yorkshire but cycad-like leaves are also prominent some of these leaves were certainly produced by true cycads but some are bented italian leaves or may belong to other kinds of plants that are distantly related to cycads bennett italians for example may be closer to living netales than to other groups of living seed plants another interesting group of extinct jurassic seed plants are the catonylenes they have leaves with four leaflets all of which have net venation similar to that of glossitarites katonia leans were originally thought to be related to angiosperms but later research showed that they lacked the key reproductive specializations characteristic of this [Music] group the evolution of land plants and insects had been closely linked since the devonian but during the meso zoi see several orders of insects still living especially flies and beetles diversified rapidly many of these insects probably fed on plants or on decaying plant matter but some may also have been involved in pollination by the jurassic it is probable that insects were already involved in the transfer of pollen from one plant to another bringing a new dimension to plant reproduction and a new potential for specialization insects play an important role in the pollination of living cycads and nettles there were also jurassic bennett italians with both pollen producing and ovule producing parts such bisexual flowers were probably insect pollinated this is one of the most diverse and widespread genera of jurassic ferns some fronds were vegetative with leaflets that had broad lobes to collect light for photosynthesis other fronds with more slender leaflets were fertile with small clusters of spore producing sacs at the end of the leaflets the detailed structure of the sporangia shows that these fossil ferns were related to the modern day tree fern dixonia at least some of the mesozoic coniopterus fronds also came from tree ferns although others may have grown on plants that were much smaller fossil casts of aquizotite stems witty clear ribs running along their length and more widely spaced crossbars are frequently found in the jurassic they are particularly common in rocks that formed along riverbanks or the shores of lakes where the plants probably grew at the water's edge they look very similar to the stems of living acquisitum but many are larger than any living species this fossil fern is usually found as fragments of divided leaf segments with slender leaflets attached occasionally complete leaves are found with several leaflets that appear to radiate from the end of a stock the fronds were probably produced by an underground creeping stem the modern day genus metonia belongs to the same family it has similar leaves and also bears similar clusters of spore producing organs in the jurassic period matone ac ferns were widespread today they are restricted both ecologically and geographically just like members of the metonia sea such as phlebopterus dictiophylum had leaf segments that radiated from the end of a stock which was attached to an underground creeping stem however this fern is distinctive in the way in which the veins formed a meshwork across the leaflets the veins and the structure of the spore producing bodies suggest that dictiophylum belonged to the family diptradaceae to dao this family only grows in southeast asia during jurassic times dictiophylum was widespread but it declined and eventually became extinct later in the jurassic tadit's leaves are common in jurassic floras these fertile leaves often have clusters of spore bearing sacs on the underside of the leaflets that are similar to those on modern day members of the fern family osmonday sea the most similar living fern is todia from australasia in south africa because the sporangia are slightly different in their detailed structure the jurassic leaves have been given a different name to deets the vegetative leaves without sporan gia are called cladophlebus both extinct and modern day members of the fern family osmandae sea to which clatiflebus belongs have non-fertile vegetative leaves and fertile that have spore bearing sacs as seen in the living royal fern these two types of frond can look quite different and their fossils are often found separately paleontologists therefore give them different genus names cladophlebus for the vegetative fronds with distinctive triangular leaflets and to deets for the fertile fronds the distinctive small tongue-shaped leaflets of clookia ferns are common in jurassic floras the fertile leaves are particularly distinctive they have quite large spore bearing sacs that are born singly on the underside of the leaflets this is quite different from most other jurassic ferns where the sporangia occur in a series of clusters on the pinuals members of this genus are most closely related to the modern day climbing ferns of the family shiz which grow today in tropical and subtropical areas leaves that are divided into a series of elongate leaflets and attached to a central axis are characteristic of many mesozoic fossil floras they are similar to the leaves of modern day cycads and some but not all are actually cycad remains various genera have been identified based on the shape and veining of the leaflets and how they are attached to the roches pseudotennis leaves were produced by true cycads they have unbranched veins and leaflets that were attached to the sides of the roches although fossilized examples are not as abundant as some other cycad leaf fossils such as nilsonia they are very widespread pseudotennis plants occurred in both tropical and temperate floras examples have even been found in permian rocks making them among the oldest known cycad fossils caedonia is the seed-bearing organ of a group of extinct plants called kadonialas which were abundant in subtropical areas during the jurassic several seeds were enclosed in the protective helmet-shaped seed-bearing structures that were arranged in two rows on either side of a central axis the enclosure of the seeds within the cupule superficially resembles how today's flowering plants bear their seeds the leaves produced by the plant also had features in common with flowering plants particularly their meshvenation for many years it was thought that catonia might have been a direct ancestor of the flowering plants although this idea has now been rejected androstrobus is the name given to pollen-producing cones that are found together with cycad leaves such as pseudopteni they are thought to be part of the same plant their spirally arranged structure is thought to be made by modified leaves each of which has an upturned scale at the end the scales overlap each other to provide protection for the numerous pollen sacs on the underside of the sporophylls the bennett italians were a characteristic group of jurassic plant s many species had a stocky trunk up to two meters high with cycad-like leaves and distinctive flower-like reproductive structures a particular type of seed-producing flower known as williamsonia had a central dome-shaped structure to which numerous stocked seeds separated by scales were attached the entire flower was surrounded by a protective layer of bracts often these bracts are the only part that has been fossilized several aspects of the complex structure of williamsonia suckers that these plants may have been related to the ancestors of flowering plants however the exact relationship between bennett italians and flowering plants is still unclear these commonly found plant fossils look very similar to the leaves of modern day cycads however the detailed cellular structure of zemites particularly the small breathing pores on the underside of the leaves is very different it is now known that these cycad-like fossils are from plants that also bore the flower-like reproductive structures that are characteristic of all the bennett italians in fact the majority of cycad-like leaves found in jurassic floras are likely to be a type of bennett italian rather than true cycads the genus zomites is a good indicator of warm jurassic climates because it occurred only in tropical or subtropical conditions many bennett italians produced fronds that were only divided once the different types of these plants can be distinguished from each other by the style of the leaflets in contrast to zomites animosamites had broader pine that were not constricted at the base like zomites they also had the same type of breathing pores on the underside that characterize all bennett italians animosomites were probably more tolerant of cooler climates because fossilized remains of the leaves have been found in siberia the bennett italian plants that bore william sonia are also thought to have borne pollen producing flowers that are known as weltersha these flowers were made up of groups of long leaf-like bracts that had pollen sacs on their upper surface in some species there were also small stuck bodies on the upper surface that may have secreted a nectar-like substance to attract pollinating insects the brax radiated from a cup-shaped base giving it a flower-like appearance weltersha has never been found directly attached to a plant so it is hard to know exactly how the flowers were born they are however almost always found associated with williamsonia as well as with their leaves known as tillaphilum and there is little doubt that they belong to the same plants this enigmatic genus was once thought to be related to ginkgo but it is now classified in its own extinct group the zacanowski aleenes the leaves look superficially like clusters of pine needles except for the fact that they fork several times they are fused at the base to form a short shoot the plants that bore these leaves produce seeds and loose canelli case structures called leptostrobus zacanowski aliens mainly seem to have favored warm or hot humid conditions identifiable by their distinctive leaves ginkgos were widespread and abundant in jurassic times but went into decline during the late cretaceous period today there is just one living species ginkgo biloba which is only found growing in the wild in the mountains of china this species is probably one of the best examples of a living fossil in the entire plant kingdom just like its fossilized cousins the beautiful fan-shaped leaves with many fine veins are easy to recognize many different species of fossilized ginkgos have been identified by the shape and number of the lobes on the leaves and the shape of the leaf as a whole sometimes they are found together with small seeds that are attached to the ends of stocks these seeds and seed stocks are very similar to those produced by the living ginkgo in other species there were more complex clusters of seeds that are known as carcinia the leaves of most conifers are either short and scaly or slender needles both types have a single vein along their length however some jurassic conifers such as pidozamites had broader leaves with several veins running along their length it was originally thought that pidosamite sleeves belonged to cycads but studies of the structure of their breathing pores on the underside of leaves suggest that they were conifers the remains of conifer shoots and cones are common in jurassic floras and can often be assigned to the same families as living conifers eletides for example has seed cones that are very similar to the living china fur in that they both had three to five small seeds attached to each seed producing scale however cunninghamia has much larger and straighter leaves than the shoots that bore these jurassic cones cunninghamia is found growing today in china and vietnam in conditions that are probably similar to the warm climates eletide seems to have favored in the jurassic

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Length: 21min 1sec (1261 seconds)

Published: Thu Jun 24 2021