Life and the evolution of continents

Life exists on a physically changing world, and these changes have both controlled the evolution of organisms and been recorded by their fossil record. Evolution operates rapidly on small populations, and so when a group of organisms becomes isolated through changes in the landscape around them, they quickly evolve to become different to their parent population. Organisms migrate across land bridges or along new seaways, as areas that were once isolated become accessible to one another. The migration of marsupial mammals such as possums into North America over the last 2 million years is a good example of this process. The analysis of the past distributions of organisms is known as paleobiogeography.

Plate tectonics drive changes to the map of the world

The continents and oceans change shape all the time, as crust is generated and modified by the forces of plate tectonics. New oceanic crust is formed at mid-ocean ridges where the mantle decompresses and melts, and as a consequence the oceans grow wider. Crust is consumed at destructive plate boundaries, where dense rock crust sinks back into the mantle. By this process oceans can become smaller or disappear altogether. Continental crust is increased in volume by the addition of island arc remnants and the sediments of the ocean floor. Continental collision joins these fragments together to form large masses, until the formation of new oceans pulls them apart.

The narrative of this evolving world map is well known for the last 200 million years, because it is recorded by the oceanic rocks of the modern sea floor. These rocks form like a conveyor belt, with the youngest rocks closest to the ridges and the oldest ones furthest away. Rock of decreasing age can be "stripped back" to reveal prior positions of the continents (Fig. 1.3). It is more difficult to reconstruct the position of oceans and continents older than 200 million years (which is only around the Triassic-Jurassic boundary), because too little oceanic crust of this age survives to produce an accurate map.

For older world maps, reconstruction is done by a variety of methods, but predominantly by tracing the latitude at which rocks cooled through the Curie point and "froze" into their minerals the direction of magnetic north. This technique, however, gives no measure of longitude, which has to be guessed from more qualitative types of data. One of the most useful of these is the distribution of the fossil remains of organisms.

□ Antarctica

□ India

■ Africa and Arabian Peninsula

HI Madagascar

■ Australia and New Guinea

| North America

□ Europe, Asia and Indonesia

■ South America

—- Equator

Fig. 1.3 Paleogeography: maps of the world for the last 200 million years.

Fig. 1.3 Paleogeography: maps of the world for the last 200 million years.

Fossil evidence for ancient continental distributions

During the Lower Palaeozoic, we know from paleomagnetic data that the continents were relatively small and widely dispersed. The landmass that now forms North America and parts of Scotland was close to the equator, while the area now forming Europe, Africa, and England was far away, at around 60° south. These island continents were surrounded by deep oceans which are now long vanished, but their position is recorded by an open marine animal, a type of colonial grapto-lite (Chapter 10), called Isograptus. This species lived only in the open ocean, and colonies were fossilized in the shales of the deep sea bed. These were sometimes preserved when the oceanic crust sank back into the mantle, scraped onto the over-riding continents as deformed strips of rock. A map of the modern distribution of Isograptus reveals their presence in these thin collisional bands, and an ancient map can be built by "tearing up" the modern continents along these bands (Fig. 1.4).

As the Lower Palaeozoic continued, these isolated continental fragments began to collide. One ofthe best studied collisions is that between Scotland and England, which happened during the late Silurian period (420 Ma). The line of collision runs east-west along the present Solway Firth, and the effects of the collision can be seen in the deep marine rocks preserved in the Southern Uplands, and in the seismic structure of the mantle beneath Scotland. Organisms that lived on either side of this ocean record its progressive closure as, first, deep marine, and then progressively more shallow-dwelling organisms became common to both sides of the seaway. The mixing of freshwater fish faunas of the latest Silurian age is the final sign that the ocean had gone.

Modern continents and mammals

The evolution of mammals coincided with, and was directly affected by, the break-up of a single giant continent, known as Pangea. The two most common groups of modern mammals -placental mammals (which gestate their young internally) and marsupial mammals (which bear tiny live young and nurture them in pouches) - are found across Pangea, and as the continent broke up they were able to migrate to all of the modern continents via land bridges. In South America, mammals have evolved independently for the last 60 million years, with little contact with the rest of the world apart from the intermittent migration of animals from North America, such as monkeys and rodents. The dominant mammals in South America were marsupials, with unusual species such as giant ground sloths and armadillos evolving. Many of these groups became extinct

Marsupials Southern Continents

Fig. 1.4 The distribution of Isograptus plotted on a modern map of the world, and a reconstruction of the ancient oceans in which they lived based on this distribution.

Ordovician continental distribution

Fig. 1.4 The distribution of Isograptus plotted on a modern map of the world, and a reconstruction of the ancient oceans in which they lived based on this distribution.

due to the migration of competitor placentals when the Isthmus of Panama formed, and this process of extinction was speeded up when hominids arrived a few thousand years ago.

Australia, New Zealand, and Antarctica split from the rest of Pangea during the Cretaceous period, and in turn split from one another during the early Cenozoic. The isolated faunas of Australia and New Zealand evolved independently, with both landmasses being dominated by marsupials.

Africa also became isolated from the rest of the continents late in the Cretaceous period and became a center of evolution for placental mammals, including groups that became predominantly marine, such as whales and sea cows. Elephants and other large grazers evolved here. Faunal exchanges with Asia began in the early Miocene, with cats arriving to become the dominant African predator, and apes and elephants migrating out of Africa to the north and east. The distinctive mammalian faunas of different modern continents are a product of Cenozoic continental break-ups and the consequent isolation of groups of animals.

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