How are the Mighty Fallen 1

Contents Updated: Monday, September 13, 1999

• Greatest Puzzle
• Table: Genera surviving the Late Cretaceous extinctions

Greatest Puzzle

"Learn to see in another’s calamity the ills which you should avoid."

The extinction of the dinosaurs is the greatest of all titillating puzzles according to writer, John Noble Wilford.

Something unusual certainly happened at the end of the Cretaceous Period. Many genera of all kinds died off. Any terrestrial creature weighing more than 50 pounds as an adult became extinct; sea organisms of all sizes were devastated, including many minute sea creatures like the foraminifera; many of the sea bottom filter feeders such as bivalves disappeared; only about 30 per cent of sponges remained.

But not all living things were equally affected:

• marine genera were badly affected but not freshwater ones;
• tropical plants suffered more than those in more northerly climes;
• pterosaurs died but not birds;
• most mammals survived but few marsupials;
• squid survived but their relatives the ammonites died;
• dinosaurs died but not crocodiles ;
• mammals, which were all small, and small lizards mostly survived but even the smallest dinosaurs died out.

Though the Cretaceous mass extinction was bad, the earlier Permo-Triassic mass extinction, when 70 per cent of all known types of animal became extinct, was worse.

The Permian extinction is explained by the theory of plate tectonics. Several continents that had been slowly drifting separately around the globe crunched together creating the supercontinent of Pangaea. Most marine organisms live on the continental shelves but, where the continent buckled together, these were thrust up into mountain ranges leaving their inhabitants stranded. The mass of the new supercontinent also suppressed the welling up of magma at the mid-oceanic plate boundaries. Lacking the buoyancy of the underlying magma, the mid-ocean ridges then settled down under their own weight, increasing the depth of the oceans. Sea level fell as much as 200 feet, draining much of the remaining continental shelf and disposing of many more marine species. On land, species diversity was reduced because previously isolated species now met and only the fittest creatures were to survive the furious competition between them.

There is no such explanation for the extinctions at the end of the Cretaceous. By then the supercontinent of Pangaea had began to break up again and the continents were drifting apart slowly moving to their present positions. Alternative theories put forward to explain the Cretaceous extinctions often ignore the variety of species extinguished and cannot be credible. In our search for the truth behind the tragedy we shall examine some of these hypotheses, credible or not.

Genera Surviving the Late Cretaceous

Blog Back

Here you can give short responses and suggestions. Considered contributions, criticisms and discussion can be made privately via email[†]