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The Silurian Portal

Introduction

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Selected article on the Silurian world and its legacies

A life restoration of Orthoceras.

The molluscs or mollusks, compose the large phylum of invertebrate animals known as the phylum Mollusca. Around 85,000 extant species of molluscs are recognized. Molluscs are the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. They are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat. The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct. Cephalopod molluscs, such as squid, cuttlefish and octopus, are among the most neurologically advanced of all invertebrates—and either the giant squid or the colossal squid is the largest known invertebrate species. The gastropods (snails and slugs) are by far the most numerous molluscs in terms of classified species. The scientific study of molluscs is called malacology. The three most universal features defining modern molluscs are a mantle with a significant cavity used for breathing and excretion, the presence of a radula, and the structure of the nervous system.

Good evidence exists for the appearance of gastropods, cephalopods and bivalves in the Cambrian period 538.8 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral Lophotrochozoa and of their diversification into the well-known living and fossil forms are still subjects of vigorous debate among scientists. (see more...)

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Selected article on the Silurian in human science, culture and economics

The history of paleontology traces the history of the effort to study the fossil record left behind by ancient life forms. Although fossils had been studied by scholars since ancient times, the nature of fossils and their relationship to life in the past became better understood during the 17th and 18th centuries. At the end of the 18th century the work of Georges Cuvier ended a long running debate about the reality of extinction and led to the emergence of paleontology as a scientific discipline.

The first half of the 19th century saw paleontological activity become increasingly well organized. This contributed to a rapid increase in knowledge about the history of life on Earth, and progress towards definition of the geologic time scale. As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology shifted to understanding evolutionary paths.

The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America. The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection, as demonstrated by a series of important discoveries in China near the end of the 20th century. There was also a renewed interest in the Cambrian explosion that saw the development of the body plans of most animal phyla. (see more...)

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Selected image

Outcrop of the Silurian Shawangunk Formation (rock in foreground) at crest of Kittatinny Mountains in High Point State Park, New Jersey. Facing northeast.
Photo credit: Jstuby

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Did you know?

...that the Burgsvik beds, a geological formation exposed on Gotland, Sweden, contain the only fossil euglenid ever discovered?
...that fossil Thelodont fish (depiction pictured) surprised scientists by showing that stomachs evolved before jaws?
...that eighty years on, scientists are still debating whether the Palæozoic fossils known as Chitinozoans (SEM image pictured) represent plants, animals or eggs?
... that Kulindroplax is the first known mollusk showing an unambiguous combination of exterior shells and a worm-like body?
... that the extinct bivalve subfamily Praenuculinae can be told apart from its sister subfamily by looking at teeth?