Туллимонстр: загадочное животное из отложений Мазон-Крик

Extinct genus of soft bodied sea animals

Tullimonstrum, colloquially known as the Tully monster or sometimes Tully's monster, is an extinct genus of soft bodied bilaterian animal that lived in shallow tropical coastal waters of muddy estuaries during the Pennsylvanian geological period, about 300 million years ago. A single species, T. gregarium, is known. Examples of Tullimonstrum have been found only in the Essex biota, a smaller section of the Mazon Creek fossil beds of Illinois, United States. Its classification has been the subject of controversy, and interpretations of the fossil have likened it to molluscs, arthropods, conodonts, worms, tunicates, and vertebrates. 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. Based on the fossils, it seems this creature was a nektonic carnivore that hunted in the ocean’s water column. When Tullimonstrum was alive, Illinois was a mixture of ecosystems like muddy estuaries, marine environments, and rivers and lakes. Fossils of other organisms like crustacean Belotelson, the cnidarian Essexella, and the elasmobranch fish Bandringa have been found alongside Tullimonstrum.

Tullimonstrum probably reached lengths of up to ; the smallest individuals are about long. 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. The absence of a hard part in the fossil implies that the animal did not possess organs composed of bone, chitin or calcium carbonate. Their form and structure is suggestive of a camera type eye.

Amateur collector found the first of these fossils in 1955 in a fossil bed known as the Mazon Creek formation. He took the strange creature to the Field Museum of Natural History, but paleontologists were stumped as to which phylum Tullimonstrum belonged in. The species Tullimonstrum gregarium ("Tully's common monster"), as these fossils later were named, takes its genus name from Tully, The term monstrum ("monster") relates to the creature's outlandish appearance and strange body plan. The fossil remains "a puzzle", and interpretations liken it to a worm, a mollusc, an arthropod, a conodont, or a vertebrate. Others pointed to a general resemblance between Tullimonstrum and Opabinia regalis, although Cave et al. note that they were too morphologically dissimilar to be related.

The classification of Tullimonstrum has been an ongoing debate since the creature was first described, it also has many features not found in Cyclostomes (lampreys and hagfishes).

A second study, by Clements et al. (2016), came to the conclusion that Tullimonstrum was a stem vertebrate based on its eye anatomy. 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. These ocular pigments and their unique structure was interpreted to be a retinal pigmented epithelium (RPE), offering strong support that the bar organs were indeed eyes. In 2022, Wiemann and colleagues replicated these spectral signals in collaboration with independent laboratories using Fourier Transform Infrared spectroscopy. Comparable tissue signatures have been detected in preserved carbonaceous remains of a diversity of other animals.

In 2017 Sallan et al. rejected the identification of the Tully monster as a vertebrate. Firstly, they noted that even the presence of the two melanosome types is variable among vertebrates; hagfish lack them altogether, and extant sharks as well as extinct forms found in the Mazon Creek area, such as Bandringa, only have spheroid melanosomes. Additionally, the supposed notochord extends in front of the level of the eyes, which is not the case in any other vertebrate, although is seen in lancelets. Even if the structure was a notochord, the presence of notochords is not limited to vertebrates either. Further criticism was drawn towards the identification of the blocks of the body variously as gill pouches and muscle blocks (myomeres), despite the lack of differentiation in the structure of these blocks. In vertebrates, myomeres are also thinner, and extend along the whole length of the body rather than stopping short of the head. Meanwhile, the gill pouches of lampreys are paired extensions rather than segmented structures, and are usually embedded in a complex gill skeleton, neither of which is the case in Tullimonstrum. On a plot of trace metal signatures in the eyes of Mazon Creek fossils, Tullimonstrum is clearly distinct from both vertebrates (which have a higher concentration of zinc) and the eyespots of the putative cephalopod Pohlsepia (however, no evidence of melanosomes were found in Pohlsepia, and some studies deny its affinity as cephalopod) although it should be noted however that these signals are influenced by the fossilisation process. 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.