Tullimonstrum gregarium* (Tully Monster) 13 8 10 14 12 11 9 1 Basic Fossil Taphonomy** 5 (1) Rapid burial of animal in undisturbed sediments that are low in oxygen inhibits the degradation of 7 6 soft tissues by bacteria. (2) Chemical interactions between the animal's body and the surrounding mud and seawater encourage the formation of minerals around the dead animal. Chemical changes caused by decay result in the “precipitation” of Iron minerals from the water surrounding the animal’s body. This results in the formation of a Siderite (Iron carbonate) “concretion” (3), or “nodule”. Interior “laminae” (layers) (4) form as the concretion experiences the changes of increased compaction and an altered chemical environment. This fossil is a “composite” impression (5), containing both a “cast”, or positive form (6), and a “mold”, or negative impression (7) of the animal. Reconstruction and Classification Stalked eyes (8) appear superficially arthropod-like, but are known to exist in other groups as well. 4 (9) Eyes appear to be “camera like” as are the eyes of chordates and cephalopods 2 Flexible proboscis (10) ending in a toothed claw (11) reminiscent of that of the Cambrian Opabinia Circular openings (12) appear similar to the gill pouches of hagfish and lampreys 3 Fossils exhibit radial bands (13). These have been interpreted as evidence of segmentation, but are currently viewed as muscle striations. * This animal, of unknown classification, is found only in the Mazon Creek Tail fin (14) appears to be vertically oriented as with Fossil Beds, of the Francis Creek Shale formation (Grundy County, fish and other chordates, but fossil evidence has Illinois), dating to approximately 307-309 MYA (Pennsylvanian). been interpreted differently at times. ** Taphonomy is the study of the process of fossilization. © www.taylorcustom.com.