Examples of Tullimonstrum have been found only in the Essex biota, a smaller section of the Mazon Creek fossil beds of Illinois, United States.

This creature had a mostly cigar-shaped body, with a triangular tail fin, two long stalked eyes, and a proboscis tipped with a mouth-like appendage.

[2] Tullimonstrum had a pair of vertical, ventral fins (though the fidelity of preservation of fossils of its soft body makes this difficult to determine) situated at the tail end of its body, and typically featured a long proboscis with up to eight small sharp teeth on each "jaw", with which it may have actively probed 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.

[2] A transverse bar-shaped structure, which was either dorsal or ventral, terminates in two round organs[2][3] which are associated with dark material which have been identified as melanosomes (containing the pigment melanin).

Close examination revealed that the animal had a camera-like eye, with preserved lenses and the presence of cylindrical and spheroid melanosomes arranged in distinct layers.

While the authors admitted that the ocular pigments of many invertebrate groups have been poorly investigated, at the time of publication the presence of RPE and two distinct melanosome morphology is a uniquely vertebrate trait.

[4] In 2020, McCoy, Wiemann and colleagues used Raman spectroscopy to identify the molecular bonds present in the organic material preserved with Tullimonstrum.

[15] Sallan et al. note that stalked eyes, tail fins, and brains are also present in anomalocaridids, and that Opabinia also has a similar proboscis.

[15] Regardless, Rogers et al. (2019) demonstrated that certain squid (Loligo vulgaris) and cuttlefish (Sepia officinalis) species do in fact have two different melanosome forms which can decay to look like an RPE-like layer, similar to that observed in vertebrates and Tullimonstrum fossils.

The authors doubt that Tullimonstrum was a cephalopod (in the absence of other supporting traits), they argue that eye structure and chemistry alone cannot disprove invertebrate affinities.

Many vertebrate-like traits are also observed in tunicates (the larvae of which have pigmented eyes and tail fins), lancelets and acorn worms (both of which have gill openings and axial support structures), and the extinct vetulicolians.

Alternative classifications were discussed in detail, but Tullimonstrum could be a non-vertebrate chordate (due to its segmentation resembling the myomeres of Esconichthys apopyris, an enigmatic jawed vertebrate from Mazon Creek) or a protostome.

The first insights into the mechanisms of carbonaceous preservation in the Mazon Creek are provided as part of a large fossil data set[13]; however, the details are still subject of ongoing research.

[2] Many unique fossils have been found alongside Tullimonstrum like the sea anemone Essexella, the malacostracan Belotelson, the eurypterid Adelophthalmus mazonensis, horseshoe crabs, the elasmobranch fish Bandringa, and the coleoid cephalopod Jeletzkya.

[26][27] A 1966–1968 prank promulgated by paleontologist Bryan Patterson suggested that modern representatives could possibly be found in remote lakes of Kenya, known under the local name "Ekurut Loedonkakini".

[28][29] Richardson later recounted the story and published the original letters, poems, and doctored photos in a book under the pseudonym E. Scumas Rory.