Form and Function: A Tribute to Adolf Seilacher

Professor of Geology and Geophysics Adolf Seilacher.

 

Dr. Adolf Seilacher, of Yale University and the University of Tübingen, has spent his career investigating form and function in fossils. Displayed here are a few of the many Yale Peabody Museum specimens that he has studied.

Seilacher’s exceptional ability to elucidate concepts is seen in his detailed specimen drawings in this exhibit.

Photograph © Wolfgang Gerber. All rights reserved. Used with permission.

Innovative Adaptations

 

Scyphocrinites sp.
Upper Silurian, Morocco
YPM 202267
Purchased for the Yale Peabody Museum by Dr. Seilacher

 

Most of the great diversity of form that we see in the animal kingdom is the result of adaptation. An adaptation is a trait or group of traits that increases an organism’s chances of survival. Adaptation allows organisms to function well in the environment in which they live. However, the range of variation in form is limited by the basic body plan of the organism (jellyfish cannot develop legs and run around like a beetle) and by the materials available (such as the properties of a shell). Dr. Seilacher often uses unusual extinct organisms as examples to unravel the different factors that influence the evolution of form.

Natural selection favors variations that make an animal more likely to survive. Over the course of geologic history, some animals evolve to flourish in a very narrow range of conditions. When these conditions change extinction could result.

Tow-net Feeding

 

Illustration of Scyphocrinites sp.
(YPM 202267) by Dr. Seilacher.
Courtesy of SEPM (Society for Sedimentary Geology).

 

Many ancient crinoids, relatives of modern starfish, lived attached to the seabed by a long stalk (hence their popular name “sea-lilies”). Modern crinoids are free-living on the seafloor. Some unusual ancient crinoids, like Scyphocrinites, floated in the ocean with a water- and air-filled buoy. The arms were used in feeding in the same way that a net towed by a fishing trawler filters fish from the water. By casting an open mesh behind it, Scyphocrinites trapped food in the passing water. The arrangement of the branching arms was optimized for catching the maximum amount of food.

Rudists as Bivalvian Dinosaurs

 

Bournonia bournoni d'Orbigny
Late Cretaceous, France
YPM.6920

 

Rudists are extraordinary clams from the Mesozoic that showed pronounced adaptations. Modern clams generally have 2 equal-sized shells (valves) held together by a flexible ligament. The 2 valves of rudists, in contrast, are dramatically different in size and shape. Most rudist families either lost or greatly reduced the ligament through the course of evolution. Like modern corals, rudists probably had symbiotic photosynthetic algae, which helped create ideal conditions for skeletal growth.

Rudists as Bivalvian Dinosaurs

 

Toucasia patagiata (White)
Early Cretaceous, Texas
YPM.35763

 

Toucasia, an early rudist, lived attached to the sea floor in thickets. The larger valve grew in an elongated spiral form. In this Yale Peabody Museum specimen the attached valve is below, and the free valve (or “lid”) at the top.

Rudists as Bivalvian Dinosaurs

 

Requienia ammonia Nathorst
Early Cretaceous, France
YPM.35724

 

Requienia was not permanently attached, but lived in soft-bottom sediments. Unlike in the thin-shelled Toucasia, the larger valve was thicker to provide weight to anchor it in the sediment.

Adaptation in Brachiopods

 

Richthofenia cf. communis
Late Permian, Italy
YPM.35760

 

Brachiopods generally have two more-or-less equal-sized valves, although their symmetry differs from that of clams. Like rudists, some aberrant Permian brachiopods took the two-valve body plan to an extreme.

Adaptation in Brachiopods

 

Prorichthofenia sp.
YPM.19122
Permian, Texas

 

Richthofenia lived with its larger cone-shaped valve in the sediment. The smaller valve functioned like a lid. Its lifestyle is referred to as a “sediment sticker.” As you look into the cavity of Prorichthofenia, you can see the small lid-like free valve.

Adaptation in Brachiopods

 

Prorichthofenia sp.
Permian, Texas
YPM.50237

 

Some brachiopods evolved from sediment stickers to become reef-dwelling forms with spines cemented to a hard surface, as in Prorichthofenia.

Adaptation in Brachiopods

 

Leptodus nobilis americanus Girty
Permian, Texas
YPM.16401

 

Here, several reef-dwelling Leptodus form a cluster. The attached valve of these bizarre brachiopods was shaped like a shoehorn and the free valve only partially covered the soft body of the organism, which may have allowed photosynthetic algae to flourish.

Epizoans Provide Evidence of Host Biology

 

Ceratites semipartitus
Middle Triassic, Germany
YPM.200327

 

Since ammonoids are extinct, we cannot observe them directly. Dr. Seilacher used “hitch-hikers” on the ammonoid shell to interpret its lifestyle. This ammonoid is covered with epizoans (animals that attach themselves to the surface of other animals). These epizoans include bivalves, brachiopods, gastropods and serpulid worms.

Commonly, epizoans attach to shells of dead animals as they lie with one side on the sea floor. In this case epizoans encrust both sides of this shell, indicating that they attached to the shell while the ammonoid was alive and swimming. Most of the encrusters show a preferred orientation, that facilitated feeding and respiration. The orientation of the encrusters on the host indicates that the ammonoid did not jet quickly through the water, but floated and swam slowly near the seabed.

Epizoans Provide Evidence of Host Biology

 

Titanosarcolites sp.
Late Cretaceous, Habana Formation, Cuba
YPM.14892

 

This is a partial specimen of Titanosarcolites, a rudist that reached about 20 inches (over a half a meter); rudists could be as much as 6 feet (2 meters) tall!. This specimen has been cut in half and polished to show circular canals in the shell that are thought to have harbored photosynthetic algae.