Chapter 14: Mesozoic Life



1. The Permian extinction wiped out almost 95% of all marine species.

2. During the Mesozoic, new fauna arose on both land and sea including new species of molluscs, sea urchins, crustaceans, and fish.

3. The Mesozoic also witnessed the advent of marine reptiles.

4. On land, the synapsids had been replaced by a great diversity of reptiles which included turtles, crocodiles, snakes, lizards and dinosaurs..

5. Flying reptiles and birds dominated the skies.

6. The earliest mammals came into existence but were generally small and confined to living in the underbrush.

7. Flowering plants came to dominate the land.


The Mesozoic Marine Revolution

1. Figure 14.37: In the marine realm, the void left behind by the demise of crinoids, blastoids, bryozoans and horn corals was filled by molluscs such as bivalves (clams) and gastropods (snails). Brachiopods were no longer dominant as in the Paleozoic.

2. The Jurassic and Cretaceous seas contained shell-crushing animals that may have been responsible for the disappearance of many species of brachiopods during this period. These shell-crushing predators included crabs, lobsters, sharks, rays, certain marine reptiles and some species of fish.

3. Starfish, which use suction tipped tube feet to pull clams apart, appeared in the Jurassic. Ammonites developed jaws capable of spearing or crushing prey.

4. Unlike brachiopods, however, clams and snails were better able to survive this hostile environment in being able to burrow into the substrate, thus escaping the deadly claws of predators.

5. The echinoderms underwent major changes during the Mesozoic when vulnerable stalked crinoids and blastoids were replaced by armored and burrowing echinoids (sea urchins). Giant stalked crinoids occupied the seafloor during the Jurassic, but largely disappeared by the Early Cretaceous.

6. Rugosid and tabulate corals were wiped out at the end of the Paleozoic and replaced by Mesozoic scleractinian corals. By Late Triassic time, scleractinians were building complex reef communites.

7. Figure 14.38: During the Cretaceous, scleractinian corals were displaced in some regions by reef-building rudistid clams. The rudistids were asymmetrical bivalves with one shell shaped as a cone anchored to a hard foundation (such as other rudistids) while the other shell was essentially a small hinged lid. Rudistids became extinct by the end of the Cretaceous and scleractinian corals and coralline algae returned to dominate Cenozoic reef communities.

8. The Cretaceous seas were also home to the inoceramids, huge bivalves with shells shaped like dinner plates that often grew in excess of 2 meters across. Some fossils of inoceramids have been found with symbiotic fish and other communities that lived in their shells. The inoceramids vanished at the end of the Cretaceous when the shallow seaways retreated.

9. Figure 14.40: Ammonoids, which were almost entirely wiped out during the Permian extinction, proliferated again during the Mesozoic. Triassic ammonoids generally had U-shaped sutures whereas the sutures of Jurassic ammonoids were much more complex and elaborate. Cretaceous ammonites included the straight-shelled Baculites and other forms with odd J-shaped, knotted, and spiral-shaped shells.

10. Figure 14.40: Another cephalopod group called belemnites were very common in the Mesozoic. Belemnites resembled modern squids and possessed rodlike internal shells that served as counterweights in keeping their bodies horizontal while they swam.

11. Figure 14.41: Mesozoic seas experienced a radiation of microscopic plants such as diatoms that secreted silica-rich shells. Coccolithophores were single-celled algae that secreted a series of button-shaped plates around them. After the plant died, these plates, or coccoliths, settled to the seafloor and accumulated to form chalk. Most of the worlds chalk deposits are made of solid microfossils.

12. Figure 14.42: New species of foraminifera (single-celled protozoans) replaced the Paleozoic fusulinids and fed on the microscopic plants. One group, the free-floating Globigerinids , first appeared in the Jurassic and underwent a huge evolutionary radiation in the Cretaceous. Globigerinids built their shells out of a spiral arrangement of bubble-shaped chambers from which long, fingerlike projections of protoplasm radiated. Once the forams died, their calcareous shells settled to the ocean floor.


Marine Vertebrates

1. Figure 14.44: Feeding on the variety of swimming ammonoids and fish of the Mesozoic seas were different kinds of marine reptiles ranging from short-legged, short-necked varieties resembling seals to long-necked plesiosaurs with whale-like bodies and broad, paddle-shaped fins. The neck-span of pleisosaurs could reach 17 meters in length.

2. Figure 14.44: The ichthyosaurs, or "fish lizards", had dolphin-like bodies, well-developed paddles, and a long bill with teeth for catching fish. Most ichthyosaurs were less than 3 to 5 meters long, but some reached 15 meters in length.

3. Figure 14.44: Mosasaurs, related to modern monitor lizards, had a flattened tail for underwater propulsion, large flippers, and nostrils located on the top of their heads in order to breath while swimming. The largest mosasaurs grew more than 14 meters in length.


The Age of Dinosaurs

1. Figure 14.45: The Early Mesozoic lowlands were shaded by a tree canopy of ginkoes and other primitive conifers while underneath resided cycads and ferns.

2. By the Middle Triassic, advanced reptiles began to replace the relict synapsid-amphibian fauna that had dominated the Paleozoic.

3. One group of reptiles included the ancestors of lizards and snakes in addition to the ichthyosaurs and plesiosaurs.

4. Figure 14.46: Another group consisted of the archosaurs that included crocodiles, birds, dinosaurs, flying reptiles and many archaic reptiles.

5. The earliest dinosaurs appeared in the Triassic and were small, bipedal animals less than 1 meter in length. By late Triassic time, primitive archosaurs were replaced by a great radiation of dinosaurs.

6. Figure 14.47: The dinosaurs are divided into two main groups, the Saurischians and the Ornithischians.



The Saurischians

1. The saurischian (lizard-hipped) dinosaurs included all the carnivorous theropod dinosaurs and also the huge sauropods such as Brontosaurus. The largest sauropod in the Triassic reached 9 meters in length.

2. Most sauropod bones come from the Upper Jurassic Morrison Formation of the western United States, corresponding to an environment of well-drained uplands and plains rather than swamps.

3. Figure 14.50: These fossils indicate that by Jurassic time, sauropods had evolved to gigantic proportions. The Aptosaurus reached 23 meters (75 feet) in length and weighed 27,000 kg while Brachiosaurus and Ultrasaurus were even larger.

4. Although sauropods probably consumed huge quantities of ferns, their long necks allowed them to also easily reach the tops of conifers and ginkoes.

5. The other major group of saurischians, the theropods, preyed upon the herbivorous dinosaurs. Theropods included the 12 meter long Allosaurus of Late Jurassic time as well as the vicious Tyrannosaurus of the latest Cretaceous (Figure 14.54) which stood over 6 meters high and weighed up to 6,000 kg.

6. Other theropods such as Compsognathus (Compys of Jurassic Park fame) were the size of chickens.

7. Figure 14.52: Deinonychus was only about 3 meters long but was a ferocious preditor. It had enormous claws on the hind feet for disemboweling prey and a long stiffened tail for balance and agility while running.

8. Figure 14.53: Birds may be the descendants of theropod dinosaurs. The oldest known bird (Archaeopteryx), discovered in Germany in 1861, resembles Compsognathus in many ways. The feather impressions of Archaeopteryx (page 42 of book), however, distinguishes it as a bird although it had teeth and a theropod skeleton.


The Ornithischians

1. Figure 14.47: The ornithischian (bird-hipped) dinosaurs were all herbivorous and included the armored stegosaurs, turtlelike ankylosaurs, duck-billed hadrosaurs and horned ceratopsians (triceratops). The ornithischians had a toothless beak and well-developed cheeks for holding vegetation while chewing.

2. Figure 14.47: The Jurassic Stegosaurus was over 9 meters long, weighed 2,700 kg, and was covered with amored plating and had a spiked tail.

3. Figure 14.54: Armored dinosaurs of the Cretaceous period included the Ankylosaurs that had a heavy shield of armor like that of a turtle as well as a tail club. Ankylosaurs reached 10 meters in length.

4. The hadrosaurs (duck-billed dinosaurs) had broad tooth plates suitable for grinding vegetation. Some species had a distinctive crest and nasal passage looped back through the hollow crest, allowing it to produce deep, booming or honking calls. Hadrosaurs were generally around 9 meters long but could reach 15 meters in length.

5. Figure 14.54: The ceratopsians (horned dinosaurs) lived primarily during the Cretaceous and included the Protoceratops and Triceratops. Ceratopsians had various combinations of nose and eye horns as well as neck shields (with or without spiked edges) and complex tooth plates for chewing fibrous vegetation. Triceratops was the largest of the ceratopsians and was about 8 meters long and weighed up to 5,400 kg.



Figure 14.47: Flying reptiles included the pterosaurs, a close relative of the dinosaurs. Early pterosaurs were the size of birds, but by the Cretaceous, certain species had aquired wing spans of 7.5 meters. The great pterosaur Quetzalcoatlus had a wing span of 11 to 12 meters, larger than that of a small airplane. Pterosaurs possibly hunted by gliding across the Cretaceous seaways plucking belemnites and fish from the surface of the water.



Advent of Flowering Plants

1. A major change in the world's vegetation occurred in the mid-Cretaceous with the emergence of angiosperms (flowering plants). Unlike gymnosperms, which rely on wind to carry pollen to the naked seeds, angiosperms have developed flowers to attract pollinators like insects and birds.

2. Flowering plants also had an advantage over gymnosperms in that angiosperms grew and regenerated more quickly and could quickly recover from dinosaur browsing.

3. Angiosperm evolution was also more rapid in that specialized fruits and flowers would attract only certain insects or birds, ensuring that pollen was only transfered to other plants of the same species, thus speeding up mutations.

4. Important pollinators today such as moths and bees have their earliest fossil record in the Late Cretaceous, suggesting that the emergence of flowering plants lead to their appearance in the insect world.

5. By the late Cretaceous, dinosaurs and angiosperms formed a complex community.



Early Mammals

1. Figure 14.55: It is generally believed that mammals descended from the last of the synapsids in the Late Triassic.

2. Throughout much of the Mesozoic, mammals remained mouse-sized and lived primarily in the underbrush, coming out primarily at night.

3. The archaic egg-laying mammals of the Late Triassic and Jurassic are survived today by the platypus and spiny anteater of Australia and New Guinea that still lay soft-shelled eggs.

4. Figure 15.43: By the mid-Cretaceous, mammals had diverged into two main groups that still survive today. The Marsupials (pouched mammals) include oppossum, kangaroo, wallaby, Tasmanian devil and koala. Placental mammals include almost all familiar mammals including bats, whales, dogs, sheep, apes and ourselves.

5. By the Late Cretaceous, the early marsupials and placentals were well established but didn't exceed the size of a housecat. The evolutionary explosion of mammals, however, was yet to come.



The Late Cretaceous Extinctions

1. The end of the Cretaceous witnessed the second biggest mass extinction in the Phanerozoic, possibly killing off half of all life on earth. Organisms doomed to extinction included many species of microplankton, brachiopods, fish, and land plants. All of the ammonites (only the chambered Nautilus survives today) and dinosaurs disappeared .

2. Survivors of the K/T extinction included mammals, lizards, snakes, crocodiles, amphibians and many plants.

3. For over a century, wild speculations about what killed the dinosaurs included theories relating to climatic change, vegetation change, competition with mammals, disease and supernova explosions. Any explanation of dinosaur extinction, however, must also explain the demise of other life forms. Explanations such as vegetation change, disease or competition with mammals are therefore not satisfactory.

4. Figure 14.56: Detailed examination shows that organisms did not all suddenly vanish at the very end of the Cretaceous. Ammonites were in decline long before the K/T boundary. Planktonic forams show a stepwise pattern of extinction across the K/T boundary that correlated with a gradual transition in the oxygen isotopes in the ocean, indicationg a slow cooling in global climate.

5. The apparently abrupt disappearance of many species at the K/T boundary may only be an artifact of a major unconformity in some cases.

6. Many scientists claim that dinosaurs were dying out long before the K/T boundary with as few as 10 species surviving to the end.

7. The Late Cretaceous was also a time when the inland seaways retreated, reducing the area of shallow seas that were home to many marine organisms.


The Impact Hypothesis

1. In 1978, a group of scientists including Walter and Luis Alvarez, discovered a high iridium concentration in a marine clay layer at the K/T boundary. Since iridium is in exceedingly low concentrations at the earth's surface, thay attributed the high iridium content of the clay to an extraterrestrial source.

2. Since the original discovery, iridium anomalies have been found at K/T boundary sites all over the world in both marine and terrestrial sediments.

3. Some scientist explain the iridium anomalies as du to an aasteroid impact at the K/T boundary. The impact supposedly caused a huge cloud of dust to form worldwide causing catastrophic cooling of the earth. The cooling and darkness caused most plants to die and ultimately led to the collapse of the food chain and the demise of many animals.

4. Figure 14.58: In 1981, tiny spherules were found in the boundary layer and were thought to be droplets of basalt that were melted by the heat of the impacting asteroid.

5. Possible impact sites were proposed at a variety of places. Recently, a possible impact site was found in the subsurface of the northern part of the Yucatan Peninsula.

6. A number of scientists, however, dispute the impact theory and have come up with alternative explanations for the iridium anomalies, spherules and supposed impact structures.


The Volcanic Hypothesis

1. Recent studies of volcanism in Hawaii have shown that mantle-derived volcanoes erupt small quantities of iridium within the vapor and ash, thus providing an alternative explanation for the K/T iridium anomaly.

2. Figure 14.59: The end of the Cretaceous was a time of extensive volcanism, including the eruption of massive amounts of flood basalt (Deccan traps) in India and Pakistan . The Deccan eruptions covered over 10,000 square kilometers and erupted over 10,000 cubic km of lava.

3. Huge volumes of flood basalt also erupted in southern Brazil during the Late Cretaceous.

4. Such massive volcanic eruptions must have pumped huge amounts of volcanic vapors and ash into the atmosphere, causing global cooling as well as changing the chemistry of the oceans.