The Handy Dinosaur Answer Book. Patricia Barnes-Svarney

Ladinian 234–227 Anisian 242–234 Late Rhaetian 210–205 Norian 221–210 Carnian 227–221

      What did the Triassic period signify?

      The Triassic period represented the time after the great Permian period extinctions. It also was important as a time of transition—when the old life of the Paleozoic era gave way to the more highly developed and varied form of life of the Mesozoic era. The Permian period extinctions wiped out most of the animals and plants on Earth (about 90 percent of all species), making the very early Triassic an eerie place, almost completely devoid of the abundant life that existed perhaps hundreds or thousands of years before. Certain flora and fauna still dotted the land, and eventually, after about 10 million years or more, life began to emerge in full force again. But it still took even longer for larger animals, coral reefs, and other specialized animals to recover or evolve after the extinction at the end of the Permian period.

CONTINENTS DURING THE TRIASSIC PERIOD

      Do Earth’s continents change positions?

      Yes, the continents continually change positions, but it takes them millions of years to shift and move great distances. Earth’s continents are actually part of the thick plates that make up the planet’s crust, all of various sizes and shapes. These plates fit together like a jigsaw puzzle. They do not move fast—only fractions of an inch to inches per year.

      What are continental drift and plate tectonics?

      The reason (or reasons) for Earth’s crustal movement is still somewhat of a mystery. The most accepted theory of plate movement is called continental drift, and the theory of its mechanism, plate tectonics. These theories suggest that the continental plates move laterally across the face of the planet, driven by the lower, more fluid mantle. At certain plate boundaries, molten rock from the mantle rises at a mid-ocean ridge (such as the Mid-Atlantic Ridge, a long chain of volcanic mountains that lie under the Atlantic Ocean); or its equivalent on land, the rift valley (such as the one in eastern Africa), the magma solidifying and moving away to either side of the ridge. At other plate boundaries, plates are pushed under an adjacent plate, forming a subduction zone, in which the crust sinks into the mantle again. And at other boundaries, plates just slip by each other, such as the San Andreas Fault in California, in which a part of the North American plate slides by the Pacific plate.

      But not everyone agrees on these theories. One reason is because, although the idea of moving plates seems sound, the mechanisms for developing plate tectonics is not fully understood. Therefore, some scientists believe in continental drift, but not plate tectonics. Many of these scientists believe that the reason that the continents shift is that Earth is actually expanding, causing a false illusion of movement (although no one can explain why or how Earth is expanding). Another hypothesis is called “surge tectonics,” in which the features of Earth’s surface are explained by the sudden surge of plate movement, as opposed to a constant flow by the steady movement of the mantle. And still others suggest that the continents have always been in the same positions.

      No one really can fully explain the reason for the continual movement of the plates. One thing is certain: the plates do move. Since the advent of Earth-orbiting observation satellites, scientists have been able to track the plates using sophisticated laser-ranging instruments that measure the minute movements.

      How did fossil evidence support the theory of continental drift?

      Scientists discovered the fossils of many identical-appearing species on widely separated continents. They had two theories for this. First, they theorized that separate species developed identically across the far-flung continents, a notion that was highly unlikely. The second theory was that the continents had been in contact with each other millions of years ago, and had somehow drifted apart.

      Hundreds of millions of years ago the seven continents were joined together as a supercontinent scientists now call Pangea. It later broke up into Laurasia and Gondwanaland during the Triassic, and eventually split apart even more (based on a U.S. Geological Survey map).

      For example, fossils found in South America were related to those in Australia and Antarctica. These landmasses were in contact sometime in the past, allowing species to roam freely, die, be buried, and become fossilized across these continents. The fossilized remains in the rock layers of the continents drifted with the landmasses, leading to widely separated—but nearly identical—fossils.

       When did scientists determine the jigsaw puzzle fit of the continents?

      The actual connection between the continental fit (the idea that continents fit together like the pieces of a jigsaw puzzle) was first proposed in 1858 by Antonio Snider-Pellegrini (1802–1885). Other scientists mentioned this idea for years afterward, but it was not until 1912 that German meteorologist and geologist Alfred Wegener (1880–1930) expanded the theory, suggesting that the continents at one time formed a supercontinent he called Pangea (or Pangaea). Wegener’s theory was not taken seriously until about the 1960s, when scientists believed they had finally worked out a mechanism (plate tectonics) for the movement of the continental plates.

      Who discovered seafloor spreading?

      Harry Hess (1906–1969), an American geologist and professor of geology at Princeton University, discovered seafloor spreading. Based on material brought up from the ocean floor during a drilling project, he determined that rocks on the ocean floor were younger than those on the continental landmasses. He also discovered rocks on the ocean floor varied in age: there were older rocks farther from the mid-ocean ridges and younger rocks around mid-ocean ridges. Hess proposed that the seafloor was spreading as magma erupted from Earth’s interior along the ocean’s mid-ocean ridges. The newly created seafloor slowly spreads away from the ridges, and later sinks back into Earth’s interior around deep-sea trenches.

      What magnetic evidence did scientists use to verify seafloor spreading?

      When molten lava is expelled from mid-ocean ridges, it cools, creating new ocean floor. And as the rock cools, specific minerals with magnetic properties line up with the prevailing magnetic field of Earth. This preserves a record of the magnetic field orientation at that particular point in time. Changes in the rocks’ magnetic field records, called magnetic anomalies, happen when Earth’s field reverses—or when the northern and southern magnetic poles change places—usually over hundreds of thousands of years. Scientists still do not know what causes these magnetic reversals, but it may have something to do with the giant convection currents in Earth’s interior.

      The theory of seafloor spreading was confirmed by measuring such magnetic anomalies in rocks on the ocean floor. Scientists discovered a symmetrical, striped pattern of magnetic anomalies on the ocean floor, spreading out on either side of the Mid-Atlantic Ridge. This ridge is a long volcanic mountain range that runs down the Atlantic Ocean seafloor between the continents of North America, Europe, Africa, and South America. The pattern and distribution of these stripes showed that the magnetic fields had reversed many times over millions of years— and only could have formed if the seafloor had been spreading apart over those millions of years.

      What is the