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The Mesozoic (Age Of Cycads And Dinosaurs): Fact Sheet
1. Scan the fact sheets below. Set up a chart and find the most dominant form of plant and of animal in the key time periods given. Check the web sites for images.
2. Look at other references to find out what these dominant forms looked like.
3. Compare this chart with the information in the previous section: – Extinctions, the Geological Time Scale and the Glaciers sections beginning 600 million years ago.
4. State your findings. Defend your theory.
Mesozoic seas saw a rise and spread of another iteration of the cephalopods, the ceratites, eventually to be replaced by the ammonites. Coral reefs, which had been decimated by the Carboniferous extinction returned to prominence with the evolution of new groups of reef-building animals and algae. Modern bony fish evolved and began to compete with the ammonites as swimming predators.
Terrestrial animals saw the rise of major groups of tetrapods: dinosaurs and mammals and the later advent of birds. Tetrapods took to new environments in the air as well as a secondary return to the waters. Forests took on an increasingly modern look, with the evolution of cycads and cycadeoids, conifers and ginkgoes. Flowering plants finally appeared about 140 million years ago and have risen to become the dominant floral component of many areas of the planet.
The Cretaceous period, spanning the time interval from 144 to 65 million years ago, saw the final phases of the opening of the Atlantic Ocean, as well as the northward migration of India toward its collision with Asia during the Tertiary period of the Cenozoic era. The last of the major forms of plant life, the angiosperms, appeared near the beginning of the period. Dinosaurs continued their diversification, with the increasing dominance of the herbivorous ornithischians.
Life in Water
The bivalves recovered from the Jurassic extinctions and again became major reef-formers in the numerous shallow marginal seas that encroached onto the continents during the Cretaceous.
Baculites, a genus of straight-shelled cephalopods, was particularly abundant in the Cretaceous seas. Note the elaborate suture patterns in the fossil specimen below.
The plesiosaurs from the Jurassic continued their diversification. Ichthyosaurs, however, declined during the early Cretaceous before becoming extinct well before the end-of-the-Cretaceous extinction. Both the long-necked plesiosaurs, and the short-necked pliosaurs were present during the Cretaceous. Despite inevitable competition and changes in their diets as new prey evolved, plesiosaurs were doing well up until the extinction at the close of the Cretaceous.
To the list of swimming reptiles is added the mosasaurs, essentially swimming “lizards”. These ferocious predators reached lengths to 30 feet (9 meters). Mosasaurs are usually considered relatives of the monitor lizards, although some recent analyses suggest a closer link to snakes is a distinct possibility. Mosasaurs were predators of the Cretaceous seas. Mosasaurs apparently gave birth to live young and perhaps had some level of parental care for their young.
Life on Land
Perhaps the greatest change in the terrestrial life of the Cretaceous was the rise and diversification of the flowering plants, the angiosperms. The angiosperms, the last of the seed plant groups to evolve, appeared over 140 million years ago during the the beginning of the Cretaceous. All members of this group produce flowers.
The flowering plants most likely evolved from a Mesozoic gymnosperm group. The classical view of flowering plant evolution suggests early angiosperms were evergreen trees that produced large Magnolia-like flowers. However, this view has recently been contradicted.
The angiosperms underwent an adaptive radiation during the Cretaceous, and for the most part escaped the major extinctions at the end of the Cretaceous.
Crocodiles, which had been around since the Triassic, produced a massive form, Deinosuchus, during the Cretaceous. This marine crocodile was over 20 feet long, and dwarfs the largest modern crocodile.
The ornithischian dinosaurs during the Cretaceous evolved into a number of interesting groups. From bipedal ornithischians during the Jurassic, like Camptosaurus, the larger and more nimble ornithopods of the Cretaceous evolved. One of the first dinosaurs to be scientifically described was Iguanodon, a bipedal herbivore thought to have moved about in herds. Iguanodon was between 6 and 10 meters long and weighed around 5 tons.
Perhaps no other dinosaur has inspired as many plush toys, tattoos, and awe as Tyrannosaurus rex. Sporting six inch long serrated teeth T. rex was a meat-eating machine. Tyrannosaurs ranged from 10-14 meters in length and weighed in at an estimated 5-7 tons, and were restricted to North America during the latest part of the Cretaceous period.
A smaller carnovore that precedes T. rex in time is Albertosaurus. This smaller version of T. rex reached lengths of between 7 and 8.5 meters and weighed in at a mere 2 tons. It ranged North America in the time interval just prior to the rise of T. rex.
The flying reptiles of the Cretaceous were the pterodactyls, short tailed descendants of the long tailed Jurassic forms. The largest of the pterodactyls was Quetzalcoatlus, named after the Aztec feathered serpent god. Quetzalcoatlus northropii had a wingspan of 11 meters and may have weighed over 200 km.
Cretaceous birds continued to expand their environmental range. Hesperornis was a flightless swimming bird that lived in the shallow seas of the Cretaceous. There were other Cretaceous birds that flew. Large, flightless birds, like the ostrich, would not appear until the Tertiary period.
Cretaceous mammals were still quite small and minor components of the fauna. Their significance would change with the Cretaceous mass extinction, from which they would emerge as co-rulers of the land (with the avian dinosaurs).
The Jurassic period spanned the time interval from 208 to 146 million years ago. The major events of the Jurassic were the explosive adaptive radiation of dinosaurs and the evolution of birds (sometimes referred to as the avian dinosaurs), the diversification of the cycads, seed ferns and the “cycadeoids”, and the continued breakup of Pangaea.
The opening of the Atlantic Ocean between Africa and North America created new shallow-water environments, as did the global rise of sea-levels that occurred during the Jurassic. As you might expect, marine life responded with a burst of adaptation and the appearance of new forms.
Life in Water
The separation of Pangaea caused the formation of new oceans, such as the Atlantic. It also led to a global rise in sea-level. Consequently there were many more environmental spaces open for exploitation. Since the Triassic extinction was not nearly as calamitous as the Permian one, life rebounded much quicker.
Dinoflagellates and coccoliths became major elements of the oceanic phytoplankton. The bivalves, specifically the rudistids, began to play a larger role in reef formation.
Coral reefs expanded into these new ocean environments.
The cephalopods again produced a new coiled, chambered form, the ammonites. Suture patterns of these forms were even more elaborate than those found in the Triassic ceratites. The belemnites were straight-shelled cephalopods with elaborate suture patterns.
The ichthyosaurs had been virtually unaffected by the Triassic extinction. During the Jurassic they reached their peak of diversity and prevalence ecologically. Whereas the triassic forms had resembled swimming lizards, Jurassic and later forms took on increasngly fish-like shapes.
In addition to ichthyosaurs, Jurassic seas also contained long-necked plesiosaurs. Plesiosaurs evolved at the end of the Triassic, most likely from nothosaurs, and diversified during the Jurassic, persisting until the terminal Cretaceous event that closed the Mesozoic era.
While most of us think of plesiosaurs as having long necks, there are also short-necked forms. Unlike the ichthyosaurs, which swam in a fishlike manner, plesiosaurs swam by using their paddle-like limbs in a manner similar to that of a sea lion or seal. Plesiosaurs did not give birth to live young, but instead crawled ashore to lay eggs as turtles do.
Life on Land
The Jurassic terrestrial environments saw an increase in gymnosperm groups such as the cycads, ginkgoes, and cycadeoids. Reptilian groups also became more adept at flying, while the dinosaurs grew in size with such giants as Apatosaurus and Brachiosaurus, and ferocious predators like Allosaurus. The first birds appeared, as did the most famous fossil bird, Archaeopteryx.
The Jurassic was a time of increasing diversity for the cycads and a similar-looking group, the cycadeoids. The cycadeoids, sometimes known as the Bennettitales, produced leaves that superficially resembled cycad leaves.
However, details of the reproductive structures indicate the two were not closely related. During the Mesozoic, leaves of these groups were so common that paleobotanists refer to the era as the age of cycads.
The modern cycads are a pale remnant of the dominant plants of the Mesozoic, with today only 160 species in 11 genera, confined to the tropics. They retain some fern-like features, notably pinnate leaves and circinate vernation. However, they (usually) produce cones of non-photosynthetic reproductive structures.
The cycadeoids superficially resemble cycads in having recurrent leaf bases on soft-wood stems. Unlike the cycads, however, the cycadeoids have bisexual reproductive structures and structure of their stomata is more like those found in angiosperms and gnetophytes.
The ginkgoes, like the cycads, originated during the late Paleozoic and diversified during the Mesozoic. Beginning during the Jurassic the number of species increased to a maximum of 11 during the Cretaceous.
The Mesozoic saw the spread of other gymnosperms – true conifers, including forms similar to Metasequioia and Sequoia, as well as numerous types of pines. A wide variety of extinct gymnosperms were present, including Caytonia and Pentoxylon.
The flying reptiles of the Triassic underwent diversification during the Jurassic. The pterosaurs (literally winged-lizard) consisted of the earlier forms with long tails, like Rhamphorhynchus shown below, and the later forms that ruled the Cretaceous period, the pterodactyls. Pterosaurs had hollow bones, large brains and possibly some had endothermy.
The true giants of the dinosaur world were the sauropods. These quadrupedal, herbivores included Apatosaurus, Brachiosaurus, and Diplodocus. They ate numerous types of plants, often specializing in plants of a certain height. Since the teeth of sauropods are not adapted for grinding, this mechanical part of the digestive process was accomplished in their stomachs. Sauropod skeletons are often found in association with curiously rounded stones in the approximate position of the stomach. These “stomach stones” are known as gastroliths, and served a similar function in sauropods as bird gravel does in birds: to aid digestion of plant material not ground up enough in the mouth. Gastroliths are not unique to sauropods, as many other herbivorous dinosaurs and plesiosaurs have been found with stomach stones.
Apatosaurus, once much better known as “Brontosaurus”, was a herbivore that ranged from 21 to 26 meters long, and had an estimated weight of between 30 and 35 tons. In modern reconstructions of the the Apatosaurus environment the vegetation is not swampy but almost desert-like, and Apatosaurus is considered to have been a fully terrestrial animal.
Barchiosaurus (22 to 30 meters long; 30 to 80 tons weight) was a larger dinosaur than Apatosaurus, and for many years was considered the heaviest of the sauropods. Brachiosaurs were different from other sauropods in having their forelegs longer than their hind legs, as well as having their nostrils high on the forehead. The largest known doinosaur, Ultrasaurus, either belongs to the brachiosaurs or is in fact a large specimen of Brachiosaurus, according to some paleontologists.
The remaining saurishian dinosaurs were theropods, the bipedal carnivores exemplified by Allosaurus. Current thought is that there were several groups of theropods, and that one group, the carnosaurids contained the largest of the carnivores such as Allosaurus. Another theropod group was the coelurosaurids, which contained the birds and smaller carnivores such as ornithomimids, “raptors”, and the tyrannosaurs.
The carnosaurs, such as the Jurassic-aged Allosaurus, had long narrow skulls, strong forelimbs, and a variety of horns or knobs projecting from the skull.
The ornithischian dinosaurs became more significant elements of the dinosaurian faunas during the Jurassic, and especially so during the Cretaceous period. The ornoithischians are the so-called bird-hipped dinosaurs. However, the resemblance of hip structure between birds and this group is no longer considered evidence for the rise of birds from ornithischians, but as a result of convergent evolution.
A major group during the Jurassic was the stegosaourids. These quadrupedal animals sported spiked tails and large flat armor plates arrayed along the spine. These plates functioned for regulation of body heat. Stegosaurus was about 9 meters long and weighed about 2 tons. Even among dinosaurs it is noteworthy for its incredibly small brain.
The ankylosaurs were ornithischians that bore armor and spikes on their backs and may have defended themselves by using their tails (which often had armor balls at the tips) and by their dense back armor.
Another group of ornithischians was the ornithopods. Among the earliest ornithopods was Camptosaurus, a 17 foot long Jurassic dinosaur from Europe and North America. Camptosaurus reached Australia by the early Cretaceous. During the Cretaceous the ornithopods became much more diverse.
The Triassic period lasted from 245 to 208 million years ago. Following the massive extinction at the end of the Permian, the survivors underwent adaptive radiatives as they diversified and began to reoccupy many of the now-vacated environmental roles.
Dinosaurs arose in the Triassic. Plateosaurus (larger) spies two Yaleosaurus (left) and a smaller Coelophysus behind. Cycads were a dominant vegetative type. The taller trees belong to the genus Bjuvia. Check images at http://seaborg.nmu.edu/earth.
Life in Water
Coral, decimated by Paleozoic extinctions, rebounded during the Triassic with the evolution of the scleractinian corals. Dinoflaellates made their first appearance as part of the phytoplankton.
A phytoplankton group that appeared at the end of the Triassic was the coccolithophorids, usually shortened to coccoliths. These microscopic creatures collect calcium carbonate from the seawater and use it to build their small “skeletons”.
The cephalopods recovered from the extinction of the goniatites and developed a remarkably similar group, the ceratites. These coiled, chambered animals had slightly more complex sutures than did the goniatites.
Bivalves (specifically pelecypods) became more abundant and important parts of reefs as the Triassic progressed, and would remain important parts of the marine fauna throughout the Mesozoic.
Life on Land
The Triassic was a period of transition for the world’s floras, as Paleozoic plants such as the calamites and lycopsids declined in favor of newer gymnosperm groups such as the “cycads”. During the Triassic up to 20% of the world’s flora consisted of cycads.
During the Mesozoic ginkgoes were worldwide in their distribution and important elements bin the gymnosperm forests that dominated the land.
Terrestrial faunas of the early Triassic were dominated by therapsids, evolutionary offshoots of the by-then extinct “sail-backs”. These therapsids divided into two groups: herbivores and carnivores. The therapsids of this time may have had some sort of “hair” as a body covering as well as some degree of endothermy. By the close of the Triassic therapsids had given rise to early mammals.
The ancestors of modern turtles appeared during the Triassic. Whereas modern turtles have lost their teeth, these Triassic turtles retained teeth on their jaws.
Another reptile group, the euryapsids, made the transition from land to sea during the Triassic. These swimming reptiles diverged into three groups during the Triassic: the ichthyosaurs (dolphin-shaped swimming reptiles), the placodonts (bulky, paddle-limbed reptiles that went extinct by the end of the Triassic), and the nothosaurs (streamlined, long-necked swimming predators).
The diapsids, the “reptiles” except for turtles and a few extinct forms, rose to prominence during the latter Triassic times. Diapsids, you recall, have two openings in the skull behind the eye socket. While the diapsids had originated possibly as far back as the late Carboniferous, they did not become major elements of the fauna until the Triassic. The major group of diapsids, the archosaurs (ruling reptiles) evolved in the later part of the Permian, and underwent a major adaptive radiation in the late Triassic.
The first true crocodiles also appeared during the Triassic. Several other reptile groups, such as the Triassic phytosaurs, adapted to the environment that crocodiles lived in. Due to evolutionary convergence, these animals took on an outer semblance of crocodiles, even though there were numerous anatomical differences.
One early archosaur was Euparkeria, a small, Triassic-aged, quadrupedal reptile that some scientists think may have been able to run short distances on its hind legs. Euparkeria specimens reach about 0.5 meters in length. The genus appears restricted to the Triassic of South Africa.
Another archosaur, at one time considered a dinosaur, was Herrerasaurus, from the Triassic of Argentina. This animal marked the transition between the archosaur stem group and the derived dinosaurs. In all but a few characteristics Herrerasaurus is a dinosaur, although a smallish one of 3-4 meters in length and a body weight estimated at around 300 kg.. Eoraptor, from the same age and area, was another archosaur with a mosaic of dinosaurian and nondinosaurian characteristics.
The Triassic saw the evolution of the first true dinosaurs, the ceratosaurians, or horned dinosaurs. These bipedal, carnivorous dinosaurs have skeletons with a number of bird-like features. Famous members of this group include Dilophosaurus and Coelophysis. Both occur in Triassic deposits in the North American Painted Desert. Coelophysis and similar dinosaurs are known from other Triassic rocks throughout Pangaea. Dilophosaurus was a slender 6-7 meters long dinosaur that weighed an estimated 300-400 kg. Coelophysis was less than 3 meters long and weighed a slight 15-30 kg.
The Permian period spanned the time interval from 286 to 245 million years ago. During the Permian the assembly of Pangaea was completed and a whole host of new groups of organisms evolved.
Life in Water
The fusulinid foraminiferans that had appeared during the Carboniferous continued their diversification. Trilobites were rarely encountered, although brachiopods and crinoids had some recovery of species diversity after the Carboniferous extinction. Marine environments were much reconstructed due to the Pangea-related tectonism and resulting uplift of the supercontinent.
Life on Land
At first glance it might appear that the only evolutionary changes of note were occurring on the land during the Permian. Part of this illusion results from the relative scarcity of Permian-aged marine sediments as well as the tendency of land-dwellers to view themselves as the pinnacle of evolution.
Plant life of the Permian took on an increasingly modern “look” with the rise of a number of gymnosperm (naked seeded) plants during the late Carboniferous and their diversification during the Permian.
The amniote lineage (the first truly terrestrial vertebrates that did not need to return to water to lay eggs), especially the fossils, often is classified based on the number of openings in the skulls. These openings allow for muscle attachment and have traditionally been used to separate the “reptiles” into several groups, including the anapsids, synapsids, and diapsids. Recent studies suggest that many animals in this group were not so strongly reptilian (such as ectothermy) as the obsolete term “mammal-like reptiles” implied.
Dimetrodon, shown above, was a member of the pelycosaurs, or non-mammalian therapsids. By the end of the Permian the therapsids had developed, a group possibly characterized by some degree of endothermy (warm-bloodedness). The early mammals of the Triassic period are possibly an offshoot or descendant group of the therapsids.
The Carboniferous, at least as seen in North American deposits, is really two distinct sets of conditions: marine dominated early Carboniferous environments characterized by shallow seas, followed in time by terrestrial “coal-swamp” environments of the late Carboniferous.
Coal Swamps and Glaciers
The Carboniferous period (360 to 286 million years ago) in Europe is better known in North America as the Mississippian period and the Pennsylvanian period. The regression of the Mississippian seas from North America provided a natural break in deposition that was not echoed in Europe.
The Carboniferous takes its name from the widespread occurrence of coal deposits formed during this timespan in Europe and North America. Coal is a sedimentary rock composed of plant debris that was deposited in a bog or swamp with little biological activity at its bottom.
Life in Water
The marine environments, following the rebound from the late Devonian mass extinction underwent changes, with crinoids becoming more dominant and diverse. The early Carboniferous is sometimes known as the “age of crinoids”. The primitive, armored Devonian fish were replaced by more “modern” ray-finned fish.
Fenestrate bryozoans were particularly common in the early Carboniferous seas. Archimedes, a corkscrew-shaped fossil, represents the secreted support of a colony of bryozoans that are usually no longer present. The so-called lacy bryozoans were among the first invertebrate groups to recover after the Devonian extinction.
Brachiopods became increasingly important animals. The spiriferids resumed or increased their ecological dominance during the early Carboniferous.
Corals were much restricted after the Devonian crisis and the large reefs of the Devonian were replaced with smaller reefs known as patch reefs. The role of corals in these new reefs was much reduced from what it had been in earlier times. Crinoids were also important contributors to the building of these small reefs, as were blastoids, a similar group of echinoderms.
Forminiferans a group of unicellular protozoans that date from the Cambrian, developed a new series of forms with foram “shells”. The fusulinids were a large group of foraminiferans that reached sizes of several centimeters in length.
Sharks and bony fish continued to diversify during the good times of the early Carboniferous, before the dropping sea levels of the late Carboniferous caused loss of habitat. The earliest bony fish to dominate the aquatic environment were the palaeoniscoids, a group that raged in time from the Carboniferous to the Triassic wjich somewhat resembled modern bony fish but had significant anatomical differences.
Life on Land
The Lycophytes became even more significant elements of the world’s flora during the Carboniferous. These non-seed plants evolved into trees in the fossil genera Lepidodendron and Sigillaria, with heights reaching up to 40 meters and 20-30 meters respectively. Lepidodendron trunks can be over 1 meter in diameter. However, the stems are composed of less wood (secondary xylem) that usually is found in gymnosperm and angiosperm trees.
Lepidodendrids had a diamond-shaped “snakeskin” pattern produced by the leaf cushions arranged in a helix.. On the left is a lower magnification view of this type of pattern, showing the general features of many of these trees.
Pecopteris is the foliage of a tree fern, Psaronius, one of the most common fossil tree ferns. It reached a height of about ten meters (32 feet) and resembled a modern palm tree. Neuropteris the foliage of a seed fern, Pteridospermales, an extinct group of gymnosperms. Although their foliage resembled that of modern ferns, they reproduced by means of seeds. Modern ferns reproduce by means of spores.
During the late Carboniferous the Cycads, considered by paleobotanists to be closest to the gymnosperms, appear.
Cordaites, the name-bearer for the cordaitaleans, were large trees with strap-shaped, leathery leaves that often had parallel leaf veins. The cordaites had their reproductive structures and seeds arranged in cones.
The major animals on land during the Carboniferous were the amphibians (and their descendants such as the stem reptiles) and insects. Dragonfly fossils have been found with wingspans up to 75 cm.
With climate changes occurring during the late Carboniferous, the plants changed, as did the terrestrial animals. Glaciation in Gondwana has been blamed for this chanmge. One response that tetrapods made was to develop the amniotic egg. With the egg and resulting freedom from water for reproduction (to which amphibians to this day must return to lay their eggs) reptiles (sometimes known as stem-reptiles, could exploit drier environments further from water.
Hylonomus was one of the earliest reptiles. This quadrupedal (four-legged) stem reptile ate insects (which were prolific in the coal swamps). From this group of early reptiles evolved the pelycosaurs, the great reptiles of the Carboniferous-Permian times, and posibly the bipedal thecodonts of the permian-Triassic. Pelycosaurs included both herbivorous and carnovorous species. The group was characterized by an elaborate fin or sail on their back.
The Devonian world was one of major changes, both in the biological realm and the geological one. The two large landmasses, Gondwana to the south and Laurasia to the north began moving toward the formation (again) of a world supercontinent, Pangaea.
Mountain building events were occurring on the edges of each of these continents. On Laurasia, the collision of Eurpoe with Laurentia produced the beginnings of the Appalachian Mountains. Changes at the end of the Devonian brought an end to the time of extensive reef building.
The oldest jawed fish, the placoderms, the ostracoderms and acanthodian fish experienced an explosion of their fish groups.
Life in Water
The Silurian period (440 to 410 million years ago) saw a return to the moderate climates that had existed before the Cambrian and Ordovician glaciations. Eukaryotic life colonized terrestrial environments. Fish underwent an adaptive radiation with the jawed fish appearing and spreading into freshwater.
Corals diversified into a number of groups during the Silurian. Tabulate corals and rugose corals were major components of the new, larger reefs built during the Silurian through Permian. Rugose corals included the horn corals, while tabulate corals were colonial.
Crinoids had been present since the Ordovician (and possibly the Cambrian). With the flooding of continents during the Silurian, crinoids underwent an adaptive radiation and produced a number of interesting forms.
Icthyocrinus illustrates the three regions of the body that can be often found as isolated fossils. Caryocrinites ornatus shows details of the stalk and head region.
One arthropod group became more diverse during the Silurian and Devonian times, the Eurypterids, or sea scorpions became more diverse. They were among the major swimming predators of the Silurian-Devonian seas.
Fish continued to divesify, with the oldest jawed fish, the placoderms, joining the ostracoderms and acanthodian fish. themselves in both marine and freshwater environments.
Life on Land
Perhaps the most significant advance of life during the Silurian was the colonization of the land, first by plants and insects, and later by certain “fish” and their offshoots, the amphibians. The Silurian land was populated by early land plants as well as a variety of insects. Both plants and animals had a number of challenges when they moved from the water to land.
Plants divide into two large groups: vascular plants that contain lignified conducting cells, and the nonvascular plants, which do not. The earliest vascular plants had no roots, leaves, fruits, or flowers. Cooksonia is a typical early vascular plant. It was less than 15 cm tall, with stems that dichotomously branched (stem divides into two ewqual branches). Many scientists now consider “Cooksonia” an evolutionary grade rather than a true monophyletic taxon.
The Ordovician period (505 to 440 million years ago). Diversification occurred among the survivors of extinction. Corals become dominant reef-building animals during the Ordovician, bryozoans and algae were also dominant elements of the reef building biota. Trilobites had survived the end of the Cambrian. The Ordovician is noteworthy because of the moves some green algae made toward the shoreline, and possibly onto land, becoming the first plants. Interesting animals of the period include the conodonts (thought to represent early vertebrates) and graptolites. The first fish also evolved.
Bryozoans are a group of organisms sometimes referred to as “moss animals”. They form colonies, often consisting of millions of individuals.
Brachiopods were present but after the Cambrian extinction, they underwent an adaptive radiation in the Ordovician. Brachiopods have bilaterally symmetrical shells, which make them among the most common marine fossils in Paleozoic rocks.
Mollusca increased in significance in the Ordovician faunas. A type is a cephalopod. Nautiloids resemble, somewhat, their living distant relatives the chambered nautilus and squids.
Gastropods, another class of the phylum Mollusca, also become more prevalent in the Ordovician seas. Ordovician deposits yield snails, as well as large, sedentary gastropods such as Maclurites.
Perhaps the most prfound advance during the Ordovician was the development of plants from an ancestral group of green algae. Fossil and biochemical evidence indicates plants are descended from multicellular green algae. Various green algal groups have been proposed for this ancestral type, with the Charophytes often being prominently mentioned.
Cladistic studies support the inclusion of the Charophytes (including the Coleochaetales) as sister taxa to the land plants. Algae dominated the oceans of the precambrian time over 700 million years ago. Between 500 and 400 million years ago, some algae made the transition to land, becoming plants by developing a series of adaptations to help them survive out of the water.
The vertebrates evolved during the Cambrian period. The oldest group of fish, the ostracodemrs, first appeared during the Cambrian. The Ordovician. was a time of great transgression of the seas onto the continents, producing a great many habitats in which new forms of life, such as fish, could develop. The exact appearance of the Ordovician fish in unclear. What we do have is fragments of hard coverings of soft tissue.
The Cambrian Explosion
The Cambrian period, the first in the Paleozoic era, spans the time between 544 and 505 million years ago. During what is termed the Cambrian Explosion, living things developed hard structures.
The early Cambrian was a wild time for animal evolution. During a span of time perhaps as short as 5 million years numerous body plans developed, including the three animal body plans that exist today. All major animal phyla, save the Bryozoans, have their first appearance during the Cambrian. One of the dominant animal groups that appeared was the trilobites. These now-extinct members of the Phylum Arthroopoda became the dominant animals in many Cambrian marine environments. Another major change was the evolution and spread of the archaeocyathids. They were important reef-building organisms, along with some calcareous algae.
The Burgess Shale Fauna (in British Columbia)
The organisms range from the prokaryotic cyanobacteria to eukaryotic green and red algae, to sponges, brachiopods, priapulids, annelids, and many different arthropod groups, as well as echinoderms and possibly one of the first chordates.
Check images at http://www.carleton.ca/~tpatters/teaching/intro/cambrian/cambrianex20.html.
One of the most common Burgess animals is the arthropod Marella. These animals were small, and exhibited the jointed appendages characterizing the arthropods. These delicate looking arthropods are referred to as “lace crabs”. The quality of preservation of these fossils is quite extraordinary. On many specimens dark stains are interpreted as body fluids that have leaked after burial.
The Origins of Multicellularity
The oldest accepted prokaryote fossils date to 3.5 billion years; Eukaryotic fossils to between 750 million years and possibly as old as 1.2-1.5 billion years. Multicellular fossils, purportedly of animals, have been recovered from 750Ma rocks in various parts of the world. The first eukaryotes were undoubtedly Protistans, a group that is thought to have given rise to the other eukaryotic kingdoms. Multicellularity allows specialization of function, for example muscle fibres are specialized for contraction, neuron cells for transmission of nerve messages.
The Archean Eon encompasses the time from the formation of the earth until 2.5 billion years ago. The rocks formed during this eon are the most ancient rocks known, up to 3.96 billion years old. Perhaps the biggest development during the Archean was the first appearance of life. The earliest forms of life were simple prokaryotic cells. Ontario has parts of the Precambrian shield exposed-eg along the St. Lawrence River at Brockville.
Layers in the stromatolites are alternating biogenic and sedimentologic in origin. Stromatolites become more common in the Proterozoic and decline during the Cambrian. Modern stromatolites are found in marine environments where the presence of herbivorous :grazers” is limited.
Simple, prokaryotic cells still dominated the world’s environments until the evolution of simple eukaryotes approximately 1.5 – 1.2 billion years ago. With the appearance of eukaryotes comes the development of sexual reproduction, which greatly increased the variation available for natural selection.
A major enbvironmental change, initiated by living things, was the development of oxygenic photosynthesis. This led to increasing oxygen levels during later Proterozoic. Geologists refer to the “great iron crisis” when the rising levels of oxygen in the world’s oceans caused the formation or iron oxide (Fe2O3), often preserved as the banded iron formation (an important commercial source of iron).
Some problematic fossils, thought by some paleobotanists to be multicellular algae, have been found in rocks approximately one billion years old. However, the multicellular algae are usually classified based on their pigments, which commonly are not preserved in the fossils. During Vendian times the Ediacaran organisms thrived on the sea floor.
The Major Mass Extinctions of the Phanerozoic
The Mesozoic was the time of the beginning of the breakup of Pangea about 225-200 million years ago. It eventually fragmented into the modern continents. This breakup had profound consequences for living things, as had the earlier formation of Pangaea during the later Paleozoic. The breakup continues today as the continue to drift!
Mesozoic seas saw a rise and spread of another cephalopods, the ceratites, eventually to be replaced by the ammonites. Coral reefs, which had been decimated by the Carboniferous extinction returned to prominence with the evolution of new groups of reef-building animals and algae. Modern bony fish evolved and began to compete with the ammonites as swimming predators.
Terrestrial animals saw the rise of major groups of tetrapods: dinosaurs and mammals and the later advent of birds. Tetrapods took to new envirponments in the air as well as a secondary return to the waters. Forests took on an increasingly modern look, with the evolution of cycads and cycadeoids, conifers and ginkgoes. Flowering plants finally appeared about 140 million years ago and have risen to become the dominant floral component of many areas of the planet.
The Mesozoic ended with the great mass extinction that eliminated nearly 75% of all species, including dinosuars, swimming and flying reptiles, and ammonites. The causes of this extinction have long been speculated on and are not presently conclusively settled.
The Paleozoic Era means “time of ancient life” and spans between 544 and 245 million years ago. The Paleozoic is the first of three eras within the Phanerozoic Eon (the time of visible life). While life originated during the Archean and increased in complexity during the earlier Proterozoic, the Paleozoic Era is marked by the spread of animals with hard preservable parts such as shells and exoskeletons.
This led to the Cambrian Explosion, the sudden appearance of a stupendous array of animal life, much of which is not closely related to modern forms. Despite extinctions at various times, the Paleozoic is notable for the increasing modernization of life. By the end of the Paleozoic, almost all major groups of life had developed.
During the Paleozoic several major advances in life occurred. The Cambrian Explosion is the first with evolution of plants from some group of green algae. The Ordovician had plants move from water onto land, paving the way for vertebrate animals to follow. The first vertebrates, amphibians, were little more than legged fish, although their future descendants would rule the land as reptiles, the first truly terrestrial vertebrates.
The Paleozoic ended in the greatest mass extinction event in world history. During this massive die-off nearly 96% of all marine species became extinct.