Туллимонструм (Tullimonstrum) — водное беспозвоночное, найденное в среднокаменноугольных отложениях (возраст около 310 млн лет) дельты реки Мейсон-Крик в Пенсильвании (США). Рядом с древнейшими отложениями медуз, мальков рыб и икры было найдено необычное животное. Вторая часть названия состоит из латинского monstrum — «чудо», «диво». У этого водного беспозвоночного широко расставленые глаза на стебельках и длинный хоботок на голове, который заканчивается челюстями, удлинённое тело без конечностей и ромбовидный хвостовой плавник.

 Tullimonstrum probably reached lengths of up to 35 centimetres (14 in); the smallest individuals are about 8 cm long.

 The Tully Monster had a pair of fins not unlike a cuttlefish at the tail end of its body, and possibly vertical fins as well (though the fidelity of preservation of fossils of its soft body makes this difficult to determine), and a long proboscis with eight small sharp teeth with which it may have probed actively for small creatures and edible detritus in the muddy bottom. It was part of the ecological community represented in the unusually rich group of soft-bodied organisms found among the assemblage called the Mazon Creek fossils from their site in Grundy County, Illinois. The absence of hard parts in the fossil implies that the animal did not possess organs composed of bone, chitin or calcium carbonate. There is evidence of serially-repeated internal structures. Its head is poorly differentiated. A transverse bar-shaped structure, which was either dorsal or ventral, terminates in two round organs which are associated with dark material similar to the pigmentation often found in eyes. Their form and structure is suggestive of a camera-type construction.

 There is insufficient evidence to align Tullimonstrum to the crown or even stem group of any phylum. Since it lacks any characters of the well-known modern phyla, it has been speculated that it is a stem group to one of the many phyla of worms that are poorly represented today. Similarities with Cambrian fossil organisms such as Vetustovermis and Opabinia have also been noted.

 Tullimonstrum was probably a free-swimming carnivore that dwelt in open marine water, and was occasionally washed to the near-shore setting in which it was preserved.

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