Cochise College Student Papers in Geology
Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
Tully Monster
Matt Wozniac
Spring 2006
Tully Monster
The Tully
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
juices.
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.
References
http://www.isgs.uiuc.edu/servs/pubs/geobits-pub/geobit5/geobit5.html
http://en.wikipedia.org/wiki/Tully_Monster
http://www.statefossils.com/il/il.html
http://www.museum.state.il.us/exhibits/symbols/fossil.html
http://www.ucmp.berkeley.edu/carboniferous/mazon.html