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The Mesozoic Era of the Phanerozoic Eon:

251 to 65.5 million years ago


Lasting little more than half the duration of the Paleozoic, this was a spectacular time. The generalized archosaurian reptiles of the Triassic gave way to the dinosaurs, a terrestrial megafauna the like of which the Earth has not seen before or since. While dinosaurs dominated the land, diverse sea-reptiles ruled the oceans, and invertebrates, especially ammonites, were extremely diverse. Pterosaurs and later birds took to the sky. Mammals however remained small and insignificant. Climatic conditions remained warm and tropical worldwide. The supercontinent of Pangea broke up into Laurasia and Gondwana, with different dinosaurian faunas evolving on each. During this era modern forms of corals, insects, new fishes and finally flowering plants evolved. At the end of the Cretaceous period the dinosaurs and many other animals abruptly died out, quite likely the result of an asteroid impact and associated extensive volcanism (acid rain).

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The supercontinent Pangea divides into Laurasia in the north and Gondwana in the south. The climate is hot and tropical worldwide. On land, the dinosaurs reign supreme. In the oceans are various kinds of marine reptiles, as well as ammonite and belemnite molluscs and many other invertebrate groups. Plants include ferns and gymnosperms. Mammals are small and insignificant, but probably numerically common.

The Mesozoic Era lasted more than 180 million years. During this time, many modern forms of plants, invertebrates, and fishes evolved. On land, dinosaurs were the dominant animals, while the oceans were populated by large marine reptiles, and Pterosaurs ruled the air. For most of this period, the climate worldwide was warm and tropical, and shallow seas covered low-lying landmasses. At the beginning of the Mesozoic, all of the world's continents were joined into the supercontinent of Pangea, which rifted into Laurasia in the north and Gondwanaland in the south. By the end of the era most of continents had separated into their present form.

The Mesozoic Era is divided into three periods, each lasting many millions of years: the , , and . The saw the emergence of many modern invertebrate groups, and on land the archosaur reptiles replaced the therapsids. In the oceans Ichthyosaurs such as Shonisaurus became as large as whales. The was the height of the dinosaur era, with giants such as Brachiosaurus, Stegosaurus, etc, and mammals tiny and shrew-like. Distinctive plants like ferns, Cycads, Bennettitales, and conifers characterized the landscape. During the period, the first flowering plants appeared, birds and fish diversified, and new types of dinosaurs appeared. The climate cooled and unique dinosaurs evolved on different continents.

The Mesozoic era came to an end with the great terminal extinction event known as the .

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Some of the main outlines of Mesozoic climate are matters of general agreement, but almost no one is very satisfied with the explanations for what has been observed. Here's the usual story:

The Triassic, particularly the first half of the Triassic, was dry and highly seasonal, with particularly large annual temperature variations in the vast continental interior of Pangea, the world-spanning continent of the Triassic. Low sea levels probably exaggerated these temperature extremes. Water acts as a heat sink -- it takes much more heat to warm a cup of water than it does to warm a cup of rock. Water also circulates, so that heat doesn't build up in one place. The net result is that water tends to stabilize temperatures. Land areas near the ocean are warmed or cooled by winds which pass over the ocean and by rains from evaporated ocean waters. It is generally agreed (a) that the low sea levels of the Triassic contributed to temperature extremes in the interior of Pangea and (b) that the interior of Pangea probably included huge areas of desert.

During the Jurassic, sea levels began to rise, probably due to an increase in sea-floor spreading. This caused flooding of large areas of the continents. As a result, the deserts began to retreat, and continental temperatures stabilized. Pangea also began to break up into smaller units, which brought more land area in contact with the ocean. The presence of nearby oceans also increased humidity, so that climates worldwide became wetter as well as warmer.

During the first half of the Cretaceous, this process continued. In addition two climate trends which began in the Jurassic became quite pronounced in the Cretaceous. The mechanism for these events is not fully understood. First, the temperature gradient from North to South became almost flat -- much more so than would be predicted from ocean circulation models. In other words, average temperatures were about the same everywhere on Earth, from the poles to the equator. Second, average temperatures were much higher than today, probably by about 10C°. Higher CO2 (carbon dioxide) levels certainly played a part, but the paleoclimate data do not match theoretical predictions.

The later Cretaceous story is more complex, and more controversial. Many researchers, but not a real consensus, believe that sea temperatures near the equator may have become a bit too warm by the Aptian-Albian, perhaps actually incompatible with ocean life. In addition, some data suggest that land areas near the equator were not jungle- or forest-covered, that plant diversity was low, and that these regions were arid despite being close to the sea. Deep ocean circulation may also have broken down. That is, water continued to circulate horizontally, but not vertically. The deep oceans weren't getting oxygen, and "black shales" appeared in the Aptian-Albian and High Cretaceous. These are large volumes of organic matter in the oceans which never completely decomposed because of lack of deep ocean oxygen. Still, the north-south temperature gradient remained very flat.

Things cooled off a little during the End-Cretaceous, but it's unclear how much or how regularly. The climate at the very end of the Mesozoic is particularly controversial.

Unfortunately, the data only match this story to a limited degree, and there are internal inconsistencies. Here are a few of the problem areas.

1) If temperature extremes in the Triassic were as great as general circulation models predict, one would expect rather hefty ice-build-up in at least some polar regions. Glaciers leave a rather distinct geological signature, and we simply don't have any evidence of Triassic glaciers or polar caps.

2) Conversely, there is evidence of the kind of rapid sea level changes associated with polar ice in the Mid-Cretaceous, which is rather hard to accept. Miller et al. (2003).

3) CO2 levels are usually invoked to explain Cretaceous warmth and the flat Cretaceous temperature gradient. This makes sense, since the very active mid-ocean spreading ridges might well have bee associated with out-gassing of CO2 from deep within the Earth. Unfortunately, the geology of the period and stable carbon isotope records, don't really support the idea as well as they might.

4) Even the most sophisticated quantitative models can't reconstruct the flatness of the Cretaceous temperature gradient. Either our temperature estimates are off, or some important factor is missing from the models. Since dinosaurs and semi-tropical vegetation are known from within 10° of the Cretaceous poles, the problem is likely to be with the theory. A recent study of a mid-latitude continental interior (in eastern Russia) -- far from the ocean in even Late Cretaceous times, suggest that temperatures were very even and that these regions were damp and non-seasonal even in the Mid-Cretaceous.

ATW041023. Ed. MM060921

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During the early , the broke up into the northern supercontinent and the southern supercontinent ; the Gulf of Mexico opened in the new rift between North America and what is now Mexico's Yucatan Peninsula. The Jurassic North Atlantic Ocean was relatively narrow, while the South Atlantic did not open until the following Period, when Gondwana itself rifted apart. The Sea closed, and the Neotethys basin appeared. Climates were warm, with no evidence of . As in the Triassic, there was apparently no land near either pole, and no extensive ice caps existed.

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The Mesozoic came after the Paleozoic. The Paleozoic Era ended with the Permian Period, which ended with a sort of general meltdown, sometimes called the "PT" or "End-Permian" extinction. We still aren't certain exactly what happened, but the fact that much of central Siberia turned into a sort of volcanic bubble bath for a few million years didn't help. This caused, bar none, the worst mass extinction in the last 600 My. Don't get this one confused with the "KT" extinction at the end of the Mesozoic -- the one which finished off the dinosaurs 200 My later. That was a sumo match by comparison. That is, it eliminated some very large and conspicuous folks very quickly, but it was all very quick and civilized.

The PT dragged on for at least a few hundred thousand years and resulted in the extinction of perhaps 98-99% of all species of animals. The survivors of the End-Permian radiated into a world that was rather empty, and the new life forms that evolved from those survivors were sometimes quite different from what had come before. For example, of all the therapsids (mammal ancestors) in the world at the end of the Permian, only a few cynodonts and dicynodonts were left. Not surprisingly, they multiplied like rabbits and spread out all over the world. As they did, they encountered environments and ecological challenges quite different from those in their South African (probably) home base. So, different populations evolved in different directions.

In addition, the great legion of their dinocephalian cousins had vanished completely, leaving those large-herbivore and carnivore jobs empty. Some of those jobs were filled by newly modified cynodonts and dicynodonts; but, some of those jobs were taken over by archosaurian reptiles, instead. So, not only were the surviving groups changed in composition, but the balance between them changed as well. Among the tetrapods, the newly expanded range of the archosaurs created an opening for the evolution of the archosaurian dinosaurs and pterosaurs, both of which appeared just a Dinilysia few million years after the PT extinction. These went on to drive the dicynodonts to extinction and reduce the cynodonts to a marginal population of small, furtive night-dwellers -- the mammals. A similar, but larger, set of vacancies in the marine job market created new opportunities for other major reptile lines, which evolved several different groups of specialized sea-going forms, including the sauropterygians (plesiosaurs and their kin), ichthyosaurs, and mosasaurs.

This same story could be told about molluscs and echinoderms and even plants, which suffered much less than animals from the effects of the PT events. In each case, one or two of the big groups was completely eliminated, the rest were changed, and the old ecological balances of the Paleozoic were very thoroughly unbalanced. The entire Triassic, and most of the Jurassic, was spent getting all that sorted out. At the end of this period -- about the Late Jurassic and earliest Cretaceous -- there was another burst of evolutionary creativity associated with rising seas and relatively warm, equable climate throughout the world. Familiar examples include birds, placental mammals and angiosperm (flowering) plants. Again, even more fundamental changes were going on in the sea: rudist molluscs, new types of sharks, and planktonic foraminifera, and several new types of algae, to name but a few.

Both temperature and sea level reached maxima in Aptian-Albian time, or perhaps a little later. By this time, things were getting a bit too warm in the seas, and there was some climatic deterioration. The Late Cretaceous saw a remarkable evolution of smaller animals of all kinds, perhaps at the expense of the giants of earlier Mesozoic. So, for example, we find the first examples of modern lizards and snakes, and mammals that were probably primates.

Of course, all these vermin might have come to nothing if a small asteroid hadn't happened to land in Mexico, 65 Mya. But it did, and the cycle of disaster, evolution, dispersal, and recovery continued. Speaking of which, if you're still interested in that Nirvana CD, forget it. Sure, Cobain could have been the TS Elliot of the Twenty-First Century had he, likewise, taken a different path. But he didn't either, and its just no use pretending otherwise.

ATW050205. Text public domain. No rights reserved.

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It is easy to type "Mesozoic Reef Systems," just as it is easy to type the words "The History of the Asia." In both cases, it's a bit harder to say anything meaningful in a few words. We might try a few verbal pictures instead.

The Mesozoic began with the universal desolation of the end-Permian world. Most reef systems were devastated beyond recovery. The frothy and exhuberant dream castles of Late Permian calcareous sponge were were now in ruins -- crumbling blocks of lifeless rock, around which no fishes swam. Instead, there sprouted, here and there, the squat and flaccid mushroom shapes of pale stromatolites. These glowed a ghost-like green against the garish, toxic shades of fungal blooms which gnawed like ghouls opon the last decaying flesh of Permian life. The seas were weirdly clear. The rich planktonic rains of fusilinid forams, diatoms, and softer-bodied forms, uncounted and unknown, were gone. In deeper seas, the drifting galaxies of crystal radiolarian stars were swept away. All ocean life was strangled by anoxic waters reaching through unheard-of depths; and nothing lived that did not feed on death.

Almost two million centuries later it closed with a riot of shape and of form, leaving reefs made of corals and sponges and clams, leaving mountains of algae and snails, leaving brachiopods by the billion or more, leaving walls built by rudists on carbonate platforms with foraminiferan floors. For throughout the Triassic, Jurassic, Cretaceous, the oceans continued to rise. And as long as the waters continued to rise, the corals continued to grow. Like the rudists and algae and sponges and clams, they grew to the tops of their tropical seas, where the sun made a tropical glow.

While an interesting exercise, the attempt to deliver scientific information in blank verse suffers from certain unavoidable inefficiencies. We will therefore return to our regular bland diet of tasteless literary grits -- with but with an occasional metrical lapse for particular pieces and bits.


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The fossil record of Mesozoic annelids, like the fossil record of all annelids, is poor. We can only make a few, general remarks.

The end-Permian extinction more or less destroyed the entire Paleozoic benthic fauna. The Mesozoic benthic communities, developed an entirely new style, possibly (i.e., this is complete speculation) based on the very few anoxia-tolerant detritivores who would have flourished in the benthic carnage of the end-Permian. Whatever their origin, Mesozoic and Cenozoic benthic communities are dominated by infaunal (burrowing) deposit-feeders, rather than epifaunal suspension feeders. This was surely good for the annelids who are quite handy with low-oxygen, burrowing ways of making a living. Oligochaetes probably evolved in the Late Jurassic. However, they were unable to employ the usual annelid skills on land until the Late Cretaceous, when angiosperms began creating large quantities of humus, permitting the evolution of the oligochaete earthworms.


Brachiopods suffered greatly during the end-Permian extinction. They were able to make a considerable come-back during the Late Triassic, but ultimately declined and were ecologically replaced by bivalves. Their fate may have been tied to substrate. The brachiopods of the Late Triassic resurgence were strongly associated with carbonate shelves, the classic reef environment of the Late Paleozoic and Early Mesozoic. The rise in sea levels during the Jurassic and Early Cretaceous drowned these platforms on a global basis. That is, the residents of the carbonate platforms gradually found themselves too deep in the water column for sunlight to sustain photosynthesis, and the shelf ecosystems collapsed. This permitted the bivalves to "mussel" their way in, as they were better adapted to the and unstable sand & mud sea bottoms within the new photic zone. In fact, with the evolution of the rudists, the bivalves were able to make their own quick and sloppy reefs on even the softest substrate.

As a consequence, the surviving Mesozoic brachiopods became off-shore specialists, occupying deeper-water and more cryptic environments in crevices and on submarine cliffs below the photic zone. Some developed poisonous tissues. The more robust and globose terebratulides such as Terebratella and probably some species of Tichosina were free on the substrate. A few of these developed semi-infaunal strategies.

Mesozoic brachiopods, like many other invertebrates, show considerable differentiation between Tethyan (tropical) and Boreal (subtropical and temperate) types in the Late Triassic and Jurassic. Also like many other invertebrates, these distinctions broke down in the Cretaceous when rising sea levels and flattened climate zonation homogenized most marine fauna.


Early Mesozoic bryozoans were largely cheilostomes and cyclostomes. During the Early Cretaceous, however, the cyclostomes declined while cheilostomes diversified. The reasons for this replacement is unclear. Both suffered massive extinctions in Maastrichtian time, possibly coinciding with the more general KT extinctions. The cheilostomes rebounded during the Cenozoic. The cyclostomes generally did not. McKinney & Taylor (2001).


Mesozoic cnidarians are mostly known from their greatest success story, the scleractinian corals. Several groups of scleractinians developed tight symbiotic relationships with photosynthetic zooxanthellae with a resulting huge boost to their productivity. The scleractinians suffered considerably from the drowning of the carbonate platforms on which their reefs were based during the Late Jurassic and Cretaceous. However, they recovered quickly after the KT extinctions.


The End-Permian extinction at the end of the Paleozoic Era took a heavy toll on the stemmed echinoderms. The blastoids became extinct at that time and the crinoids suffered heavy losses. In general, Paleozoic echinoderms were epifaunal suspension and detritus feeders. Like so many high school students, their strategy was to sit more or less stationary on the sea bottom with their mouths open and wait for food to come to them. In the Mesozoic and Cenozoic, the echinoderms became more like undergraduates -- still bottom-feeders, but now willing to dig for it (infaunal detritus feeders) or, if sufficiently pressed, to go and hunt for it (armored herbivores and carnivores).

This use of rather heavy armor runs counter to a general trend among Mesozoic life forms to shed heavy plates and to depend more on speed, or on other behavioral adaptations for survival. However, behavioral strategies depend on having the neural equipment to select a response and adapt it to local conditions. Echinoderms are poorly adapted for this sort of thing because they are attractive, but brainless. So as time went on, echinoderms, like other attractive but brainless organisms, were increasingly forced to rely on heavy make-up, intimidating ornament, and a thick skin. The surviving crinoids, for example, were articulates, with rounded, closely fitting armor plates, usually bearing elaborate ornamentation. Some also gave up sessile life, left their stems behind, and became motile. These swimming crinoids, the , are discussed briefly elsewhere.

However, for the most part, the old crinoid fauna simply died out. The future of the Echinodermata lay with the and . Echinoids are rare in Paleozoic faunas, but radiated extensively during the Mesozoic and Paleogene. Paleozoic, and even Triassic, urchins have no compound plates, and the interambulacral plates are constructed in many columns [1]. These earliest sea urchins are generally small and lack strong spine development -- characters which developed over the course of the Mesozoic.

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Sponges as a whole did well and slowly diversified until the very end of the Mesozoic. However, this general trend is made up of varying fates of different groups of sponges. Demosponges, particularly the genus and their relatives, and calcisponges recovered from the end-Permian extinction and dominated the reef fauna once more in many locations during the Late Triassic. However, they were gradually replaced by scleractinian corals. Hexactinnelids and some stromatoporoids continued as important frame builders for the coral reefs of Jurassic Europe. Demosponges and hyalosponges became more common in the Cretaceous. As sea levels rose, these sponges were sometimes able to thrive in regions which had become too deep for the corals. Mesozoic stromatoporoids (demosponges probably not related to the Paleozoic forms) were significant reef-builders in the Cretaceous. All types of reef-building sponges virtually disappeared at the KT boundary and never recovered.

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The Mesozoic era was an extremely long period of time, which saw the rise and fall of successive "dynasties" of life. At least half a dozen succesive evolutionary communities or empires of land vertebrates (tetrapods) can be distinguished. Identifying them by characteristic large herbivores, these can be called the lystrosaur (Earliest Triassic [Induan]), kannemeyeriid- traversodontid (primarily Gondwanan, though this may be sampling bias) (Early [Olenekian] to Late (Carnian) Triassic], plateosaur-vulcanodontid (Late Triassic [Norian] - Early Jurassic), sauropod- stegosaur (Middle to Late Jurassic), iguanodont- nodosaur (Early to Mid Cretaceous), and ceratopsian-hadrosaur (Late Cretaceous - Laurasia only, Gondwana was predominantly Titanosaurid, with Abelisaurid carnivores) communities or "empires". In the sea one finds what could be perhaps termed the mixosaur- nothosaur (Mid Triassic), shastasaur (Late Triassic), ichthyosaur- plesiosaurid- rhomaleosaurid (Latest Triassic [Rhaetian] - Early Jurassic), ophthalmosaur- pliosaurid- metriorhynchid (Middle Jurassic-Early Cretaceous), and protostegid- elasmosaurid- mosasaur communities (Mid to Late Cretaceous).

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[1] The "tube feet" of sea urchins look like very short tentacles. You can see them moving between the spines of any live urchin. The tube feet are arranged in strips which run from the top of the urchin (apical disk) to the bottom. These strips are called ambulacral zones. The tube feet are connected to the urchin's internal water vascular system. The armor plates of the ambulacral zones are known, naturally enough, as ambulacral plates. The plates between the ambulacral zones are interambulacral plates. The ambulacral plates of many modern urchins are compound. That is, they are completely fused. The arrangement of ambulacral plates is an important tool of echinoid taxonomy. This is all explained, with figures and much additional information, at Skeletal morphology of regular echinoids from the Natural History Museum (London). The invertebrate part of the NHM site, unlike the awful dinosaur section, is an informative resource and worth browsing at length.

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Benton, MJ & DAT Harper (1997), Basic Paleontology. Longman, 342 pp.

Blake, DB (2000), The class Asteroidea (Echinodermata): Fossils and the base of the crown group. Amer. Zool. 40:316–325. WWW

Blake, DB & BS Kues (2002), Homeomorphy in the Asteroidea (Echinodermata); a new Late Cretaceous genus and species from Colorado. J. Paleontol. 76: 1007–1013. WWW.

Cox, B., RJG Savage, B Gardiner & D Dixon (1988), The Simon and Schuster Encyclopedia of Dinosaurs and Prehistoric Creatures : A Visual Who's Who of Prehistoric Life. Simon & Schuster, 312 pp.

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Creisler, B (2000) Mosasauridae Translation and Pronunciation Guide iNet.

Farinacci, A & R Manni (2003), Roveacrinids from the northern Arabian Plate in SE Turkey. Turkish J. Earth Sci. 12: 209-214. WWW.

Jenkyns, HC & PA Wilson (1999), Stratigraphy, paleoceanography, and evolution of cretaceous Pacific guyots: relics from a greenhouse Earth. Am. J. Sci. 299: 341–392.

Lehmann, C, DA Osleger, IP Montañez, W Sliter, A Arnaud-Vanneau & J Banner (1999), Evolution of Cupido and Coahuila carbonate platforms, Early Cretaceous, northeastern Mexico. GSA Bull. 111: 1010–1029. .

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McKinney, FK & PD Taylor (2001), Bryozoan generic extinctions and originations during the last one hundred million years. Paleontol. Elec.

Miller, KG, PJ Sugarman, JV Browning, MA Kominz, JC Hernández, RK Olsson, JD Wright, MD Feigenson & W Van Sickel (2003), Late Cretaceous chronology of large, rapid sea-level changes: Glacioeustasy during the greenhouse world. Geology 31: 585–588.

Tweet, J (2004) Thescelosaurus! iNet.

Williston, SW (1898) Mosasaurs, The University Geological Survey of Kansas, Volume IV, Paleontology, Part V, pp. 81-347

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