Study: Ichthyosaur Fins Were Built Like Fiberglass

An ancient sea reptile propelled itself with fins reinforced by a fiberglass-like mesh, a new study finds.

In fiberglass, thin strands of glass are mixed with plastic to provide strength. The toughened material, which can be molded into any shape, is commonly used in light-body cars, airplanes and boats.

Researchers recently discovered that a similar mesh covered the fins of ichthyosaurs, dolphin-like marine reptiles that lived during the age of dinosaurs.

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Instead of glass, though, their fins were coated with the protein collagen. Present in animal bone and connective tissues, this fibrous protein helps give skin its elasticity, and its breakdown with age causes wrinkles.

Theagarten Lingham-Soliar of the University of KwaZulu-Natal in South Africa and Gerhard Plodowski of the Senckenberg Research Institute in Germany examined fossilized soft-tissue from a well-preserved specimen of an ichthyosaur species called Stenopterygius quadricissus.

The researchers found evidence for layered sheets of collagen fibers in the skin of the dorsal and tail fins. The collagen would have kept the fins rigid, allowing the creatures to glide through the water like torpedoes.

The finding, to be detailed in an upcoming issue of the German science journal Naturwissenschaften, is an example of convergent evolution, in which animals not closely related develop similar characteristics.

Stiff fins and a streamlined body shape also evolved in fast-swimming tuna, dolphins and sharks.

Convergent evolution

In a previous study, Lingham-Soliar showed that a great white shark's skin contains large amounts of collagen, arranged in sheets that lie at slightly different angles to one another — an arrangement virtually identical to that found in Stenopterygius' fins.

In the great white, the collagen mesh enveloped the animal's entire body, but it was thicker around the tail and dorsal fins.

"We showed that the support given by collagen in sharks is every bit, if not stronger, than bone," Lingham-Soliar said in a telephone interview.

The dorsal fin and tail flukes of dolphins are similarly stiffened with collagen, but the fibers are arranged more randomly than in sharks, Lingham-Soliar said.

"A large and stiff back fin deflects a large body of water effectively and pushes an animal through the water," Lingham-Soliar explained.

Tuna also have stiff dorsal and tail fins, but their fins are supported by bony spines.

Like hunchbacked dolphins

Early ichthyosaurs were serpentine, and looked like snakes with fins, but their bodies gradually became more streamlined and fish-shaped as they evolved.

Later species resembled hunchbacked dolphins, and even had dorsal fins on their backs.

Ichthyosaurs first appeared about 250 million years ago, slightly earlier than dinosaurs, and disappeared just before them.. They ranged in length from 2 feet to more than 40 feet, but most were around 9 feet long. At their peak abundance during the Jurassic period, they were the ocean's top predators.

The ichthyosaurs' body shapes suggest they were fast swimmers, but until now, it was not known what kept their fins rigid. Scientists suspected collagen; the new study is the first to confirm it.

"Collagen is one of the most effective materials for toughening something, so from that point of view, it's not a surprise" Lingham-Soliar told LiveScience. "Where one is surprised is to find good soft tissue in an animal that is so, so very old — 200 million years old — and to actually find the collagen fibers in the [fin] structures. That becomes near miraculous."

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