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The Silurian Portal

Introduction

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Selected article on the Silurian world and its legacies

Phoronids (sometimes called horseshoe worms) are a phylum of marine animals that filter-feed with a "crown" of tentacles, and build upright tubes of chitin to support and protect their soft bodies. Most adult phoronids are 2 cm long and about 1.5 mm wide, although the largest are 50 cm long. The bottom end of the body is an a flask-like swelling, which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, little hairs on the sides of the tentacles draw food particles to the mouth, which is inside and slightly to one side of the base of the lophophore. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus, which is outside and slightly below the lophophore.

As of 2010 there are no indisputable body fossils of phoronids. There is good evidence that phoronids created trace fossils found in the Silurian, Devonian, Permian, Jurassic and Cretaceous periods, and possibly in the Ordovician and Triassic. Phoronids, brachiopods and bryozoans have collectively been called lophophorates, because all use lophophores to feed. Most researchers now regard phoronids as members of the protostome super-phylum Lophotrochozoa. The relationships between lophotrochozoans are still unclear. Some analyses regard phoronids and brachiopods as sister-groups, while others place phoronids as a sub-group within brachiopoda. (see more...)

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

Evolutionary thought, the conception that species change over time, has roots in antiquity. With the beginnings of biological taxonomy in the late 17th century, a new anti-Aristotelian approach to modern science challenged traditional essentialism. Naturalists began to focus on the variability of species; the emergence of paleontology with the concept of extinction further undermined the static view of nature. In the early 19th century, Jean-Baptiste Lamarck proposed the first fully formed theory of evolution.

In 1858, Charles Darwin and Alfred Russel Wallace published a new evolutionary theory that was explained in detail in Darwin's On the Origin of Species (1859). Unlike Lamarck, Darwin proposedcommon descent and a branching tree of life. The theory was based on the idea of natural selection, and it synthesized a broad range of evidence from animal husbandry, biogeography, geology, morphology, and embryology.

The debate over Darwin's work led to the rapid acceptance of the general concept of evolution, but the specific mechanism he proposed, natural selection, was not widely accepted until it was revived by developments in biology that occurred during the 1920s through the 1940s. Before that time most biologists argued that other factors were responsible for evolution. The synthesis of natural selection with Mendelian genetics during the 1920s and 1930s founded the new discipline of population genetics. Throughout the 1930s and 1940s, population genetics became integrated with other biological fields, resulting in a widely applicable theory of evolution that encompassed much of biology—the modern evolutionary synthesis. (see more...)

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Selected image

Silurian proximal turbidite sedimentary rocks on the coast at New Quay, Ceredigion, Wales, UK.
Photo credit: AliV

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Did you know?

... that a fossil of the extinct sea scorpion Eurypterus (restoration pictured) was once thought to be a catfish?
... that the Paleozoic monoplacophoran Pilina unguis was twice as large as any known living monoplacophoran?
... that the extinct trilobite Bumastus (artist's rendition pictured) was named after its resemblance to large grapes?
...that Ornatifilum is likely to be the oldest known fossil fungus?
...that the Silurian Lau extinction event caused the Earth to temporarily return to a condition similar to how it was before multicellular life evolved?