Transcript ray-finned
Chapter 13 Paleozoic Life History: Vertebrates and Plants Tetrapod Footprint Discovery • Tetrapod trackway – at Valentia Island, Ireland • These fossilized footprints – which are more than 365 million years old – are evidence of one of the earliest four-legged animals on land Tetrapod Footprint Discovery • The discovery in 1992 of fossilized Devonian tetrapod footprints – more than 365 million years old – has forced paleontologists to rethink – how and when animals emerged onto land • The Late Devonian trackway – has helped shed light on the early evolution of tetrapods • the name is from the Greek tetra, meaning four and podos, meaning foot – Based on the footprints, it is estimated • that the creature was longer than 3 ft • and had fairly large back legs Tetrapod Wader • Furthermore, instead of walking on dry land – this animal was probably walking or wading around in a shallow, tropical stream, • filled with aquatic vegetation and predatory fish • This hypothesis is based on the fact that – the trackway showed no evidence of a tail being dragged behind it • Unfortunately, there are no bones associated with the tracks – to help in reconstructing what this primitive tetrapod looked like Why Limbs? • One of the intriguing questions paleontologists ask is – Why did limbs evolve in the first place? • It probably was not for walking on land • In fact, many scientists think – aquatic limbs made it easier to move around – in streams, lakes, or swamps – that were choked with water plants or other debris Unanswered Questions • Presently, there are many more unanswered questions – about the evolution of the earliest tetrapods – than there are answers • During the 1990s, only a few Devonian tetrapods were known • Today, paleontologists have a more detailed knowledge – and are able to fill the gaps between the fish and amphibians – leading to more complete fish-amphibian phylogeny New Information from Fossils • As more paleoenvironmental and paleoecologic data and analyses – from a variety of sites • are made available, • A better understanding of the linkage – between morphological changes and the environment – is fast emerging. • New technologies now provide the means – to extract more and more detailed information – from the fossils Vertebrates and Plants • Previously, we examined the Paleozoic history of invertebrates, – beginning with the acquisition of hard parts – and concluding with the massive Permian extinctions – that claimed about 90% of all invertebrates – and more than 65% of all amphibians and reptiles • Now we examine – the Paleozoic evolutionary history of vertebrates and plants Transition from Water to Land • One of the striking parallels between plants and animals – is the fact that making the transition from water to land, – both plants and animals had to solve the same basic problems • For both groups, – the method of reproduction was the major barrier – to expansion into the various terrestrial environments • With the evolution of the seed in plants and the amniote egg in animals, – this limitation was removed, – and both groups were able to expand into all the terrestrial habitats Vertebrate Evolution • A chordate (Phylum Chordata) is an animal that has, • at least during part of its life cycle, – a notochord, – a dorsal hollow nerve cord, – and gill slits • Vertebrates, which are animals with backbones, are simply a subphylum of chordates Characteristics of Chordates • The structure of the lancelet Amphioxus illustrates the three characteristics of a chordate: – a notochord, a dorsal hollow nerve cord, and gill slits Phylum Chordata • The ancestors and early members of the phylum Chordata – were soft-bodied organisms that left few fossils – so little is known of the early evolutionary history of the chordates or vertebrates • Surprisingly, a close relationship exists between echinoderms and chordates – They may even have shared a common ancestor, – because the development of the embryo is the same in both groups – and differs completely from other invertebrates A Very Old Chordate • Yunnanozoon lividum is one of the oldest known chordates – Found in 525 MY old rocks in Yunnan province, China – 5 cm-long animal Spiral Versus Radial Cleavage • Echinoderms and chordates – have similar – embryonic development • In the arrangement of cells resulting from spiral cleavage, – cells in successive rows are nested between each other • In the arrangement of cells resulting from radial cleavage, – cells in successive rows are directly above each other – This arrangement exists in both chordates and echinoderms Echinoderms and Chordates • Both echinoderms and chordates have similar – biochemistry of muscle activity – blood proteins, – and larval stages • The evolutionary pathway to vertebrates – thus appears to have taken place much earlier and more rapidly – than many scientists have long thought Hypothesis for Chordate Origin • Based on fossil evidence and recent advances in molecular biology, – vertebrates may have evolved shortly after an ancestral chordate acquired a second set of genes • the ancestor probably resembled Yunnanozoon • According to this hypothesis, – a random mutation produced a duplicate set of genes – allowing the ancestral vertebrate animal to evolve entirely new body structures – that proved to be evolutionarily advantageous • Not all scientists accept this hypothesis and the evolution of vertebrates is still hotly debated Fish • The most primitive vertebrates are fish – and some of the oldest fish remains are found – in Upper Cambrian Deadwood Formation, • northeastern WY – Here, phosphatic scales and plates of Anatolepis, • a primitive member of the class Agnatha – have been recovered from marine sediments. Fish • All known Cambrian and Ordovician fossil fish – have been found in shallow nearshore marine deposits, • while the earliest nonmarine (freshwater) fish remains – have been found in Silurian strata • This does not prove that fish originated in the oceans, – but it does lend strong support to the idea Fragment of Primitive Fish • A fragment of a plate from Anatolepis cf. A. heintzi from the Upper Cambrian marine Deadwood Formation of Wyoming • Anatolepis is one of the oldest known fish Ostracoderms — “Bony Skinned” Fish • As a group, fish range from the Late Cambrian to the present • The oldest and most primitive of the class Agnatha are the ostracoderms – whose name means “bony skin” • These are armored jawless fish that first evolved during the Late Cambrian – reached their zenith during the Silurian and Devonian – and then became extinct Geologic Ranges of Major Fish Groups Bottom-Dwelling Ostracoderms • The majority of ostracoderms lived on the seafloor • Hemicyclaspis is a good example of a bottomdwelling ostracoderm – Vertical scales allowed Hemicyclaspis to wiggle sideways • propelling itself along the seafloor • while the eyes on the top of its head allowed it to see predators approaching from above • such as cephalopods and jawed fish • While moving along the sea bottom, – it probably sucked up small bits of food and sediments through its jawless mouth Devonian Seafloor • Recreation of a Devonian seafloor showing: an acanthodian (Parexus) a ray-finned fish (Cheirolepis) – a placoderm (Bothriolepis) an ostracoderm (Hemicyclaspis) Swimming Ostracoderm • Another type of ostracoderm, • represented by Pteraspis – was more elongated and probably an active swimmer – although it also seemingly fed on small pieces of food it could suck up Evolution of Jaws • The evolution of jaws – was a major evolutionary advantage – among primitive vertebrates • While their jawless ancestors – could only feed on detritus • jawed fish – could chew food and become active predators – thus opening many new ecological niches • The vertebrate jaw is an excellent example of evolutionary opportunism – The jaw probably evolved from the first two or three anterior gill arches of jawless fish Evolutionary Opportunism • Because the gills are soft – they are supported by gill arches composed of bone or cartilage • The evolution of the jaw may thus have been related to respiration rather than feeding – By evolving joints in the forward gill arches, – jawless fish could open their mouths wider – Every time a fish opened and closed its mouth – it would pump more water past the gills, – thereby increasing the oxygen intake • The hinged forward gill arches enabled fish to also increase their food consumption – The evolution of the jaw for feeding followed rapidly Evolution of Jaws • The evolution of the vertebrate jaw – is thought to have occurred – from the modification of the first two or three anterior gill arches • This theory is based on the comparative anatomy of living vertebrates Acanthodians • The fossil remains of the first jawed fish are found in Lower Silurian rocks – and belong to the acanthodians, • a group of small, enigmatic fish – characterized by • • • • • large spines, scales covering much of the body, jaws, teeth, and reduced body armor Acanthodians: Most Abundant during Devonian • Although their relationship to other fish has not been well established, – many scientists think the acanthodians – included the probable ancestors of the present-day • bony and cartilaginous fish groups • The acanthodians were most abundant during the Devonian, – declined in importance through the Carboniferous, – and became extinct during the Permian Other Jawed Fish • The other jawed fish, the placoderms • whose name means “plate-skinned” – evolved during the Late Silurian • Placoderms were heavily armored, jawed fish – that lived in both freshwater and the ocean, – and like the acanthodians, – reached their peak of abundance and diversity during the Devonian Placoderms • The Placoderms exhibited considerable variety, – including small bottom dwellers – as well as large major predators such as Dunkleosteus, • a Late Devonian fish • that lived in the midcontinental North American epeiric seas – It was by far the largest fish of the time • • • • • attaining a length of more than 12 m It had a heavily armored head and shoulder region a huge jaw lined with razor-sharp bony teeth and a flexible tail all features consistent with its status as a ferocious predator Late Devonian Marine Scene • A Late Devonian marine scene from the midcontinent of North America featuring the giant placoderm, Dunkleosteus Age of Fish • Many fish evolved during the Devonian Period including – – – – the abundant acanthodians placoderms, ostracoderms, and other fish groups, • such as the cartilaginous and bony fish • It is small wonder, then, that the Devonian is informally called the “Age of Fish” – because all major fish groups were present during this time period Cartilaginous Fish • Cartilaginous fish, – class Chrondrichthyes, – represented today by • sharks, rays, and skates, – first evolved during the Early Devonian – and by the Late Devonian, – primitive marine sharks • such as Cladoselache • were quite abundant Cartilaginous Fish Not Numerous • Cartilaginous fish have never been – as numerous nor as diverse – as their cousins, • the bony fish, – but they were, and still are, – important members of the marine vertebrate fauna • Along with cartilaginous fish, – the bony fish, class Osteichthyes, – also first evolved during the Devonian Ray-Finned Fish • Because bony fish are the most varied and numerous of all the fishes – and because the amphibians evolved from them, – their evolutionary history is particularly important • There are two groups of bony fish – the common ray-finned fish (subclass Actinopterygii) – and the less familiar lobe-fined fish (subclass Sarcopterygii) • The term ray-finned refers to – the way the fins are supported by thin bones that spread away from the body Ray-Finned and Lobe-Finned Fish • Arrangement of fin bones for (a) a typical ray-finned fish (b) a lobe-finned fish – Muscles extend into the fin – allowing greater flexibility Ray-Finned Fish Rapidly Diversify • From a modest freshwater beginning during the Devonian, – ray-finned fish, • which include most of the familiar fish • such as trout, bass, perch, salmon, and tuna, – rapidly diversified to dominate the Mesozoic and Cenozoic seas Lobe-Finned Fish • Present-day lobe-finned fish are characterized by muscular fins • The fins do not have radiating bones – – – – but rather articulating bones with the fin attached to the body by a fleshy shaft allowing a powerful stroke and making the fish a powerful swimmer • Three orders of lobe-finned fish are recognized: – coelacanths – lungfish – and crossopterygians Coelacanths • Coelacanths are marine lobe-finned fish – that evolved during the Middle Devonian – and were thought to have gone extinct – at the end of the Cretaceous. • In 1938, a fisherman caught a coelacanth – in the deep waters off Madagascar, – and several dozen more have been caught since then • in Madagascar and in Indonesia Lungfish Fish • Lungfish were fairly abundant during the Devonian, – but today only three freshwater genera exist, – one each in South America, Africa, and Australia • Their present-day distribution presumably – reflects the Mesozoic breakup of Gondwana • The “lung” is actually a modified swim bladder – that most fish use for buoyancy in swimming • In lungfish, this structure absorbs oxygen, – allowing them to breathe air – when the lakes or streams in which they live become stagnant or dry up. Lungfish Respiration • When the lakes become stagnant and dry up, – the lungfish burrow into the sediment to prevent dehydration – and breathe through their swim bladder – until the stream begins flowing or the lake fills with water • When the water is well oxygenated, – however, lungfish rely upon gill respiration Amphibians Evolved from Crossopterygians • The crossopterygians are an important group of lobefinned fish – because amphibians probably evolved from them • However, the transition to amphibians – is not as simple as once portrayed • Among the crossopterygians – the rhipidistians appear to be the ancestral group • These fish, reaching over 2 m in length, – were the dominant freshwater predators – during the Late Paleozoic. Amphibian Ancestor • Eusthenopteron, – – – – a good example of a rhipidistian crossopterygian, had an elongated body that enabled it to move swiftly in the water, as well as paired muscular fins that may have been used for moving on land • The structural similarity between crossopterygian fish – and the earliest amphibians is striking – and one of the most widely cited transitions – from one major group to another Rhipidistian Crossopterygian and Eusthenopteron Fish/Amphibian Comparison • Similarities between the crossopterygian lobefinned fish and the labyrinthodont amphibians • Their skeletons were similar Comparison of Limbs ulna radius humerus • Comparison of the limb bones – of a crossopterygian (left) and an amphibian (right) • Color identifies the bones that the two groups have in common Comparison of Teeth • Comparison of tooth cross sections show – the complex and distinctive structure found in – both crossopterygians (left) and amphibians (right) Paleozoic Evolutionary Events • Before discussing this transition – and the evolution of amphibians, – we should place the evolutionary history of Paleozoic fish – in the larger context of Paleozoic evolutionary events • Certainly, the evolution and diversification of jawed fish – as well as eurypterids and ammonoids – had a profound effect on the marine ecosystem Defenseless Organisms • Previously defenseless organisms either – evolved defensive mechanisms – or suffered great losses, possibly even extinction • Recall that trilobites – experienced extinctions at the end of the Cambrian, – recovered slightly during the Ordovician, – then declined greatly from the end of the Ordovician – to final extinction at the end of the Permian Extinction by Predation • Perhaps their lightly calcified external covering – made them easy prey – for the rapidly evolving jawed fish and cephalopods • Ostracoderms, – although armored, – would also have been easy prey – for the swifter jawed fishes • Ostracoderms became extinct by the end of the Devonian, – a time that coincides with the rapid evolution of jawed fish Late Paleozoic Changes • Placoderms also became extinct by the end of the Devonian, – while acanthodians decreased in abundance after the Devonian – and became extinct by the end of the Paleozoic Era • In contrast, cartilaginous and ray-finned bony fish – expanded during the Late Paleozoic, – as did the ammonoid cephalopods, – the other major predator of the Late Paleozoic seas Amphibians— Vertebrates Invade the Land • Although amphibians were the first vertebrates to live on land, – they were not the first land-living organisms • Land plants, which probably evolved from green algae, – first evolved during the Ordovician • Furthermore, insects, millipedes, spiders, – and even snails invaded the land before amphibians Land-Dwelling Arthropods Evolved by the Devonian • Fossil evidence indicates – that such land-dwelling arthropods as scorpions and flightless insects – had evolved by at least the Devonian Water to Land Barriers • The transition from water to land required that several barriers be surmounted • The most critical for animals were – – – – desiccation, reproduction, the effects of gravity, and the extraction of oxygen • from the atmosphere – by lungs rather than from water by gills Problems Partly Solved • Until the 1900s, the traditional evolutionary sequence had a rhipidistian crossopterygian, – like Eusthenopteron • evolving into a primitive amphibian – such as Ichthyostega. • At the time, fossils of these two genera – were all that paleontologists had to work with • Although there were gaps in morphology – the link between crossopterygians and these earliest amphibians – was easy to see. Problems Partly Solved • Crossopterygians already had a backbone and limbs – that could be used for walking – and lungs that could extract oxygen • The oldest amphibian fossils are found – in the Upper Devonian Old Red Sandstone of eastern Greenland – and belong to the genus Ichthyostega Oldest Amphibians • These amphibians, – had streamlined bodies, long tails, and fins • In addition, they had – – – – four legs a strong backbone a rib cage and pelvic and pectoral girdles, • all of which were structural adaptations for walking on land A Late Devonian Landscape • A Late Devonian Landscape • Ichthyostega was an amphibian that grew to a length of about 1 m • The flora was diverse, – consisting of a variety of small and large seedless vascular plants Evolution of Amphibians • The earliest amphibians – appear to have inherited many characteristics – from the crossopterygians – with little modification • The question still remained as to when animals made the transition – from water to land – and perhaps more intriguing, – why limbs evolved in the first place Evolution of Amphibians • Limbs probably were not for walking on land • Current thinking of many scientists – is that aquatic limbs made it easier – for animals to move around in streams, lakes, or swamps – that were choked with water plants or other debris • The fossil evidence that began to emerge in the 1990s – seems to support this hypothesis Transition to Amphibians • Panderichthys, – a large Late Devonian lobe-finned fish from Latvia – was essentially a contemporary of Eusthenopteron. • It had a large tetrapod-like head – with a pointed snout – dorsally located eyes – and modifications to the part of the skull • related to the ear region. • It lived in shallow tidal flats or estuaries Transition to Amphibians • Fossils of Acanthostega, – a tetrapod found in 360-million-year-old rocks from Greenland • reveal an animals that had limbs, – but was unable to walk to land Transition to Amphibians • Paleontologist Jennifer Clack points out – that Acanthostega’s limbs were not strong enough – to support its weight – and its rib cage was too small – for necessary muscles to hold its body off the ground • In addition, Acanthostega had gills and lungs – meaning it could survive on land but was more suited for water • Clack thinks Acanthostega used its limbs to maneuver around in swampy, plant-filled waters Transition to Amphibians • Fragmentary fossils from other tetrapods – living at about the same time as Acanthostega – suggest that these early tetrapods – may have spent more time on dry land than in water • The discovery of such fossils shows – that the transition between fish and amphibians – involved a number of new genera that are intermediate between the two groups – and fills in some of the gaps between the earlier postulated rhipidistian-crossopterygian-amphibian phylogeny Transition to Amphibians • In 2006, an exciting discovery – of a 1.2-2.8 m long – 375-million-year-old (Late Devonian) “fishapod” – was announced. • Tiktaalik roseae (“large fish in a stream”) – was hailed as an intermediary – between lobe-finned fish like Panderichthys – and the earliest tetrapod, Acanthostega. Tiktaalik roseae • This “fishapod” has characteristics of both fish and tetrapods – It has gills and fish scales – but also a broad skull, eyes on top of its head, flexible neck and large ribcage – that could support its body on land or shallow water, – and lungs • It has the beginning of a true tetapod forelimb – with functional wrist bones and five digits – and a modified ear region Tiktaalik roseae • Diagram illustrating how Tiktaalik roseae is a transitional species between lobe-finned fish and tetrapods Rapid Adaptive Radiation • Because amphibians – did not evolve until the Late Devonian, – they were a minor element – of the Devonian terrestrial ecosystem. • Like other groups that moved into new and previously unoccupied niches, – amphibians underwent rapid adaptive radiation – and became abundant during the Carboniferous and Early Permian Rapid Adaptive Radiation • The Late Paleozoic amphibians – did not all resemble the familiar • frogs, toads, newts, and salamanders – that make up the modern amphibian fauna • Rather they displayed a broad spectrum of sizes, shapes, and modes of life Carboniferous Coal Swamp • The amphibian fauna was varied Large labyrinthodont amphibian Eryops Labyrinthodonts • One group of amphibians was the labyrinthodonts, – so named for the labyrinthine wrinkling and folding of the chewing surface of their teeth • Most labyrinthodonts were large animals, as much as 2 m in length • These typically sluggish creatures – lived in swamps and streams, – eating fish, vegetation, insects, and other small amphibians Labyrinthodont Teeth • Labyrinthodonts are named for the labyrinthine wrinkling and folding of the chewing surface of their teeth Carboniferous Coal Swamp • Reconstruction of a Carboniferous coal swamp The serpentlike Dolichosoma Labyrinthodont Decline • Labyrinthodonts were abundant during the Carboniferous – when swampy conditions were widespread, – but soon declined in abundance – during the Permian, – perhaps in response to changing climactic conditions • Only a few species survived into the Triassic Evolution of the Reptiles — the Land is Conquered • Amphibians were limited in colonizing the land – because they had to return to water to lay their gelatinous eggs • The evolution of the amniote egg freed reptiles from this constraint • In such an egg, the developing embryo – is surrounded by a liquid-filled sac, • called the amnion – and provided with both a yolk, or food sac, – and an allantois, or waste sac Amniote Egg • The amnion cavity – surrounds the embryo. • The yolk sac – provides the food source • while the allantois – serves as a waste sac • The evolution of the amniote egg freed reptiles – to inhabit all parts of the land Colonization of All Parts of the Land • In this way the emerging reptile is – in essence a miniature adult, – bypassing the need for a larval stage in the water • The evolution of the amniote egg allowed vertebrates – to colonize all parts of the land – because they no longer had to return – to the water as part of their reproductive cycle Amphibian/Reptile Differences • Many of the differences between amphibians and reptiles are physiologic – and are not preserved in the fossil record • Nevertheless, amphibians and reptiles – differ sufficiently in • skull structure, jawbones, ear location, and limb and vertebral construction – to suggest that reptiles evolved from labyrinthodont ancestors by the Late Mississippian • based on the discovery of a well-preserved skeleton • of the oldest known reptile, Westlothiana, from Late Mississippian-age rocks in Scotland Earliest Reptiles • Some of the oldest known reptiles are from – the Lower Pennsylvanian Joggins Formation in Nova Scotia, Canada – Here, remains of Hylonomus are found • in the sediments filling in tree trunks • These earliest reptiles were small and agile – and fed largely on grubs and insects One of the Oldest Known Reptiles • Reconstruction and skeleton of Hylonomus lyelli from the Pennsylvanian Period – Fossils of this animal have been collected from sediments that filled tree stumps – Hylonomus lyelli was about 30 cm long Permian Diversification • The earliest reptiles are loosely grouped together as protorothyrids, – whose members include the earliest reptiles • During the Permian Period, reptiles diversified – and began displacing many amphibians • The success of the reptiles is partly because – of their advanced method of reproduction – and their more advanced jaws and teeth, • their tough skin and scales to prevent dessication, • and their ability to move rapidly on land Paleozoic Reptile Evolution • Evolutionary relationship among the Paleozoic reptiles Pelycosaurs—Finback Reptiles • The pelycosaurs, • or finback reptiles, – evolved from the protorothyrids • during the Pennsylvanian – and were the dominant reptile group • by the Early Permian • They evolved into a diverse assemblage – of herbivores, • exemplified by Edaphosaurus, – and carnivores • such as Dimetrodon Pelycosaurs (Finback Reptiles) • Most pelycosaurs have a characteristic sail on their back The herbivore Edaphosaurus The carnivore Dimetrodon Pelycosaurs Sails • An interesting feature of the pelycosaurs is their sail – It was formed by vertebral spines that, – in life, were covered with skin • The sail has been variously explained as – – – – a type of sexual display, a means of protection and a display to look more ferocious but... Pelycosaurs Sail Function • The current consensus seems to be – that the sail served as some type of thermoregulatory device, – raising the reptile's temperature by catching the sun's rays or cooling it by facing the wind • Because pelycosaurs are considered to be the group – from which therapsids evolved, – it is interesting that they may have had some sort of body-temperature control Therapsids: Mammal-like Reptiles • The pelycosaurs became extinct during the Permian – and were succeeded by the therapsids, • mammal-like reptiles – that evolved from the carnivorous pelycosaur ancestry – and rapidly diversified into • herbivorous • and carnivorous lineages Therapsids • A Late Permian scene in southern Africa showing various therapsids – Many paleontologists think therapsids were endothermic – and may have had a covering of fur – as shown here Moschops Dicynodon Therapsid Characteristics • Therapsids were small- to medium-sized animals – displaying the beginnings of many mammalian features • fewer bones in the skull because many of the small skull bones were fused • enlarged lower jawbone • differentiation of the teeth for various functions such as nipping, tearing, and chewing food • and a more vertical placed legs for greater flexibility, • as opposed to the sideways sprawling legs in primitive reptiles Endothermic Therapsids • Many paleontologists think therapsids were endothermic, – or warm-blooded, – enabling them to maintain a constant internal body temperature • This characteristic would have allowed them – to expand into a variety of habitats, – and indeed the Permian rocks • in which their fossil remains are found – have a wide latitudinal distribution Permian Mass Extinction • As the Paleozoic Era came to an end, – the therapsids constituted about 90% of the known reptile genera – and occupied a wide range of ecological niches • The mass extinctions – that decimated the marine fauna – at the close of the Paleozoic – had an equally great effect on the terrestrial population Losses Fewer for Plants • By the end of the Permian, – about 90% of all marine invertebrate species were extinct, – compared with more than two-thirds of all amphibians and reptiles • Plants, in contrast, – apparently did not experience – as great a turnover as animals did Plant Evolution • When plants made the transition from water to land, – they had to solve most of the same problems that animals did • desiccation, • support, • and the effects of gravity • Plants did so by evolving a variety of structural adaptations – that were fundamental to the subsequent radiations – and diversification that occurred – during the Silurian, Devonian, and later periods Major Events in the Evolution of Land Plants • The Devonian Period was a time of rapid evolution for land plants – Major events were – the appearance of leaves – heterospory – secondary growth – and emergence of seeds Marine, then Fresh, then Land • Most experts agree – – – – that the ancestors of land plants first evolved in a marine environment, then moved into a freshwater environment and finally onto land • In this way, the differences in osmotic pressures – between saltwater and freshwater – were overcome while the plant was still in the water • The higher land plants are divided into two major groups, – the nonvascular – and vascular plants Vascular Versus Nonvascular • Most land plants are vascular, – meaning they have a tissue system – of specialized cells – for the movement of water and nutrients • The nonvascular plants, – such as bryophytes • liverworts, hornwarts, and mosses – do not have these specialized cells – and are typically small – and usually live in low moist areas Earliest Land Plants • The earliest land plants • from the Middle to Late Ordovician – were probably small and bryophyte-like in their overall organization • but not necessarily related to bryophytes • The evolution of vascular tissue in plants was an important step – Because it allowed for the transport of nutrients and water • Probable vascular plant megafossils – and characteristic spores indicate • to many paleontologists – that vascular plants evolved – well before the Middle Silurian Features Resembling Present Land Plants • Sheets of cuticlelike cells • that is, the cells • that cover the surface • of present-day land plants – and tetrahedral clusters • that closely resemble the spore tetrahedrals of primitive land plants – have been reported from Middle to Upper Ordovician rocks – from western Libya and elsewhere Upper Ordovician Plant Spores and Cells • These fossils closely resemble spore tetrahedrals of primitive land plants Ancestor of Terrestrial Vascular Plants • The ancestor of terrestrial vascular plants – was probably some type of green alga • Although no fossil record of the transition – from green algae to terrestrial vascular plants has been found, – comparison of their physiology reveals a strong link • Primitive seedless vascular plants • such as ferns – resemble green algae in their pigmentation, – important metabolic enzymes, – and type of reproductive cycle Transitions from Salt to Freshwater to Land • Furthermore, the green algae are one of the few plant groups – to have made the transition from salt water to freshwater • The evolution of terrestrial vascular plants from an aquatic plant, • probably of green alga ancestry – was accompanied by various modifications – that allowed them to occupy – this new and harsh environment Vascular Tissue Also Gives Strength • Besides the primary function – of transporting water and nutrients throughout a plant, – vascular tissue also provides – some support for the plant body • Additional strength is derived from – the organic compounds lignin and cellulose, – found throughout a plant's walls Problems of Desiccation and Oxidation • The problem of desiccation – was circumvented by the evolution of cutin, • an organic compound • found in the outer-wall layers of plants • Cutin also provides additional resistance – to oxidation, – the effects of ultraviolet light, – and the entry of parasites Roots • Roots evolved in response to – the need to collect water and nutrients from the soil – and to help anchor the plant in the ground • The evolution of leaves – from tiny outgrowths on the stem – or from branch systems • provided plants with – an efficient light-gathering system for photosynthesis Silurian and Devonian Floras • The earliest known vascular land plants – are small Y-shaped stems – assigned to the genus Cooksonia – from the Middle Silurian of Wales and Ireland • Upper Silurian and Lower Devonian species are known from • Scotland, New York State, and the Czech Republic, • These earliest plants were – small, simple, leafless stalks – with a spore-producing structure at the tip (sporangia) Earliest Land Plant • The earliest known fertile land plant was Cooksonia – seen in this fossil from the Upper Silurian of South Wales • Cooksonia consisted of – upright, branched stems – terminating in sporangia • It also had a resistant cuticle • and produced spores typical of vascular plants • These plants probably lived in moist environments such as mud flats • This specimen is 1.49 cm long Earliest Land Plant • The earliest plants – are known as seedless vascular plants – because they do not produce seeds • They also did not have a true root system • A rhizome, • the underground part of the stem, – transferred water from the soil to the plant – and anchored the plant to the ground • The sedimentary rocks in which these plant fossils are found – indicate that they lived in low, wet, marshy, freshwater environments Parallel between Seedless Vascular Plants and Amphibians • An interesting parallel can be seen between seedless vascular plants and amphibians • When they made the transition from water to land, – Both plants and animals had to overcome the same problems such a transition involved • Both groups, – while successful, – nevertheless required a source of water in order to reproduce Plants and Amphibians • In the case of amphibians, – their gelatinous egg had to remain moist • while the seedless vascular plants – required water for the sperm to travel through – to reach the egg Seedless Vascular Plants Evolved • From this simple beginning, – the seedless vascular plants – evolved many of the major structural features – characteristic of modern plants such as • leaves, • roots, • and secondary growth • These features did not all evolve simultaneously – but rather at different times, • a pattern known as mosaic evolution Adaptive Radiation • This diversification and adaptive radiation – took place during the Late Silurian and Early Devonian – and resulted in a tremendous increase in diversity • During the Devonian, – the number of plant genera remained about the same, – yet the composition of the flora changed Early Devonian Plants • Reconstruction of an Early Devonian landscape – showing some of the earliest land plants Protolepidodendron\ Dawsonites / - Bucheria Early and Late Devonian Plants • Whereas the Early Devonian landscape – was dominated by relatively small, – low-growing, – bog-dwelling types of plants, • the Late Devonian – witnessed forests of large tree-size plants up to 10 m tall Evolution of Seeds • In addition to the diverse seedless vascular plant flora of the Late Devonian, – another significant floral event took place • The evolution of the seed at this time – – – – liberated land plants from their dependence on moist conditions and allowed them to spread over all parts of the land Seedless Vascular Plants Require Moisture • Seedless vascular plants require moisture – for successful fertilization – because the sperm must travel to the egg – on the surface of the gamete-bearing plant • gametophyte – to produce a successful spore-generating plant • sporophyte • Without moisture, the sperm would dry out before reaching the egg Seedless Vascular Plant • Generalized life history of a seedless vascular plant • The mature sporophyte plant produces spores – which upon germination grow into small gametophyte plants Seedless Vascular Plant • The gametophyte plants produce sperm and eggs • The fertilized eggs grow into • the spore-producing mature plant • and the sporophytegametophyte life cycle begins again Reproduction by Seed • In the seed method of reproduction, – the spores are not released to the environment • as they are in the seedless vascular plants – but are retained – on the spore-bearing plant, – where they grow – into the male and female forms • of the gamete-bearing generation Gymnosperms • In the case of the gymnosperms, • or flowerless seed plants, – these are male and female cones • The male cone produces pollen, – which contains the sperm – and has a waxy coating to prevent desiccation, – while the egg, • or embryonic seed, – is contained in the female cone • After fertilization, – the seed then develops into a mature, cone-bearing plant Gymnosperm Plants • Generalized life history of a gymnosperm plant • The mature plant bears both – male cones that produce spermbearing pollen grains – and female cones that contain embryonic seeds Gymnosperm Plants • Pollen grains are transported to the female cones by the wind • Fertilization occurs when the sperm moves through a moist tube growing from the pollen grain • and unites with the embryonic seed Gymnosperm Plants • producing a fertile seed • which then grows into a cone-bearing mature plant Gymnosperms Free to Migrate • In this way, the need for a moist environment – for the gametophyte generation is solved • The significance of this development • is that seed plants, • like reptiles, – – – – were no longer restricted to wet areas but were free to migrate into previously unoccupied dry environments Heterospory, an Intermediate Step • Before seed plants evolved, – an intermediate evolutionary step was necessary • This was the development of heterospory, – whereby a species produces two types of spores – a large one (megaspore) • that gives rise to the female gamete-bearing plant – and a small one (microspore) • that produces the male gamete-bearing plant • The earliest evidence of heterospory – is found in the Early Devonian plant – Chaleuria cirrosa, • which produced spores of two distinct sizes An Early Devonian Plant • Chaleuria cirrosa – from New Brunswick, Canada – was heterosporous, producing two spore sizes Spores of Chaleuria cirrosa • The two spore types of Chaleuria cirrosa – shown at about the same relative scale Evolution of Conifer Seed Plants • The appearance of heterospory – was followed several million years later – by the emergence of progymnosperms • Middle and Late Devonian plants • with fernlike reproductive habit • and a gymnosperm anatomy – which gave rise in the Late Devonian – to such other gymnosperm groups as • the seed ferns • and conifer-type seed plants Plants in Swamps Versus Drier Areas • Although the seedless vascular plants – dominated the flora of the Carboniferous coalforming swamps, • the gymnosperms – made up an important element – of the Late Paleozoic flora, • particularly in the nonswampy areas Late Carboniferous and Permian Floras • The rocks of the Pennsylvanian Period • Late Carboniferous – are the major source of the world's coal • Coal results from – the alteration of plant remains – accumulating in low swampy areas • The geologic and geographic conditions of the Pennsylvanian – were ideal for the growth of seedless vascular plants, – and consequently these coal swamps had a very diverse flora Pennsylvanian Coal Swamp • Reconstruction of a Pennsylvanian coal swamp – with its characteristic vegetation Coal-Forming Pennsylvanian Flora • It is evident from the fossil record – that whereas the Early Carboniferous flora – was similar to its Late Devonian counterpart, – a great deal of evolutionary experimentation was taking place – that would lead to the highly successful Late Paleozoic flora • of the coal swamps and adjacent habitats • Among the seedless vascular plants, – the lycopsids and sphenopsids – were the most important coal-forming groups – of the Pennsylvanian Period Lycopsids • The lycopsids were present during the Devonian, – chiefly as small plants, • but by the Pennsylvanian, – they were the dominant element of the coal swamps, – achieving heights up to 30 m in such genera as Lepidodendron and Sigillaria • The Pennsylvanian lycopsid trees are interesting – because they lacked branches except at their top Lycopsids • The leaves were elongate and similar to the individual palm leaf of today • As the trees grew, – the leaves were replaced from the top, – leaving prominent and characteristic rows or spirals of scars on the trunk Sphenopsids • The sphenopsids, • the other important coal-forming plant group, – are characterized by being jointed and having horizontal underground stem-bearing roots – many of these plants, such as Calamites, average 5 to 6 m tall • Living sphenopsids include the horsetail • Equisetum – or scouring rushes • Small seedless vascular plants and seed ferns – formed a thick undergrowth or ground cover beneath these treelike plants Horsetail • Living sphenopsids include the horsetail Equisetum Plants on Higher and Drier Ground • Not all plants were restricted to the coalforming swamps • Among those plants occupying higher and drier ground were some of the cordaites, – a group of tall gymnosperm trees – that grew up to 50 m – and probably formed vast forests A Cordaite Forest • A cordaite forest from the Late Carboniferous • Cordaites were a group of gymnosperm trees that grew up to 50 m tall Glossopteris • Another important non-swamp dweller was Glossopteris, the famous plant so abundant in Gondwana, – whose distribution is cited as critical evidence that the continents have moved through time Climatic and Geologic Changes • The floras that were abundant – – – – – during the Pennsylvanian persisted into the Permian, but because of climatic and geologic changes resulting from tectonic events, they declined in abundance and importance • By the end of the Permian, – the cordaites became extinct, – while the lycopsids and sphenopsids – were reduced to mostly small, creeping forms Gymnosperms Diversified • Gymnosperms – with lifestyles more suited to the warmer and drier Permian climates – diversified and came to dominate the Permian, Triassic, and Jurassic landscapes