Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
monster was a soft-bodied, invertebrate, marine animal which is an animal that
has no shell and no backbone, and lived in the ocean. It had an elongate,
segmented body that tapered at both ends. At the front was a long snout ending
in a "jaw" with eight tiny "teeth." At the other end was a tail and two fins. Two eyes on stalks projected out sideways near the front of the body. Judging
from the streamlined shape, flexible body, and maneuverable fins, it's likely
the Tully monster was an active swimmer. Perhaps, like a modern squid, it
hovered near the sea bottom. The Tully monsters' "jaws" and apparent swimming
abilities suggest that they attacked other marine animals such as jellyfish and
shrimp, perhaps piercing their prey with their "teeth" and sucking out the
The Tully Monster is said to be unique to Illinois during the Pennsylvanian geologic time period about 300 million years ago, in fact the Tully Monster is considered to be the state fossil of Illinois. The first Tully Monster was discovered by amateur collector Francis Tully in 1958. He took the strange creature to the Field Museum, where none of the staff could identify it. Curator Eugene Richardson gave it a proper scientific name in 1966, dubbing it Tullimonstrum gregarium, meaning "Tully's common monster."
The species name gregarium means common. This refers to the fact that Tully Monsters are fairly common fossils in the Mazon Creek deposits. More recently they have also been found in open-pit coal mines in central Illinois.
The Mazon Creek deposits are located in Will and Grundy Counties. They are some of the most important fossil deposits in North America because the soft parts of many organisms are preserved. The deposits contain the remains of both plants and animals. Some of the organisms lived in the ocean; others were washed in from the nearby shore. The material is preserved in concretions of ironstone. More than one hundred Tully Monsters have been found all around the Mazon Creek in Illinois.
The area of the Mazon Creek fossils is riddled with coal seams. The coal seams are composed of alternating layers of coal and shale. The shale beds above and below the coal seams range in depth from twenty to sixty feet and represent a period when the sea advanced and flooded the swamplands in which the forests grew. The different strata piled on top of each other indicate that the sea level rose and fell a corresponding number of times in order to create the alternation between coal and shale. Because of the abundance of coal in the area, coal strip mining is common, in which the top layer of shale is cut away in order to expose the coal. The shale that is cut away is deposited in spoil heaps close to the strip mines. It is from these deposits that most of the fossils belonging to Mazon Creek are found in nodules of siderite, or iron carbonate. Because the nodules are found in dumps, it is not possible to know the relation of the fossils to each other in terms of position in the shale and therefore it is not possible to precisely know the relative time span. The nodules are made from sediment carried down by water, burying the organisms and hardening around them. The sediment was cemented with iron carbonate to form a hard nodule distinct from the shale around it. The nodules range in size from a fraction of an inch to more than twelve inches long. The average size of a nodule is five inches in diameter lengthwise and two inches in diameter. Because of the mode in which the fossils were formed, the fossils are mainly impressions or incrustations of organisms. An impression is formed when a plant part falls into water and the air spaces in the plant are gradually filled with water. The organism thus becomes waterlogged and sinks to the bottom where it is buried by sediment. The sediment then hardens around the organism, forming a nodule. The organism may then rot away, leaving an impression of itself in the sediment that hardened around it. This cavity may then be filled with coal, calcite, siderite, or any other form of sediment.