Tullimonstrum gregarium

''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.''

Description

''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. 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 which have been identified as melanosomes (containing the pigment melanin). Their form and structure is suggestive of a camera-type eye. ''Tullimonstrum'' possessed structures which have been interpreted as gills, and a possible notochord or rudimentary spinal cord.

History of discovery

Amateur collector Francis Tully 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. The species ''Tullimonstrum gregarium'' ("Tully's common monster"), as these fossils later were named, takes its genus name from Tully, whereas the species name, ''gregarium'', means "common", and reflects its abundance. 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. Since it appears to lack characteristics of the well-known modern phyla, some speculate that it was representative of a stem group to one of the many phyla of worms that are poorly represented today. Similarities with Cambrian fossil organisms were noted. Chen ''et al''. suggested similarities to ''Nectocaris pteryx''. 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.

Classification

The classification of ''Tullimonstrum'' has been an ongoing debate since the creature was first described. With many scientists presenting evidence of a vertebrate affinity, and others of an invertebrate affinity.

Arguments in favor of vertebrate affinities

In 2016 a morphological study showed that ''Tullimonstrum'' may have been a basal vertebrate, and thus a member of the phylum Chordata, with one study suggesting ''Tullimonstrum'' may be closely related to modern lampreys. This affinity was attributed based on pronounced cartilaginous vertebral structures known as arcualia, a dorsal fin and asymmetric caudal fin, keratinous teeth, a single nostril, and tectal cartilages like in lampreys. McCoy ''et al''. raised the possibility that ''Tullimonstrum'' belongs to the ancestral group of lamprey, but 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 in the eye 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. The dark pigments in the eye were chemically tested and found to be fossilized melanin, as opposed to ommochromes or pterins, which are ocular pigments used by many invertebrate groups. The authors admit that some arthropods have spheroid eyes which fossilize in a similar manner to vertebrate eyes, and that the ocular pigments of many invertebrate groups have been poorly investigated. Regardless, they contend that the dual melanosome morphology and presence of an RPE is a uniquely vertebrate trait.

In 2020, McCoy, Wiemann and colleagues used Raman spectroscopy to identify the molecular bonds present in the organic material preserved with ''Tullimonstrum''. Based on samples from multiple points in the body, they identified the organic material as representing the decay products of chordate tissues as opposed to the polysaccharide-based chitin (as is seen in arthropods), offering independent and rather unambiguous evidence for the interpretation that ''Tullimonstrum'' is a chordate or vertebrate. 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.

Arguments in favour of non-vertebrate affinities

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''.

Other identifications of soft-tissue structures were considered as being equally problematic. The supposed brain has no associated nervous tissue and is not connected to the eyes, and the purported liver was located under the gills as opposed to being further back as in other vertebrates. The "mouth" at the front of the proboscis was described as possessing gnathostome-like distinct tooth rows, despite lampreys having "tooth fields" on the interior of the mouth. This would necessitate the convergent re-evolution of grasping jaws.

An additional difficulty is that the thin and jointed proboscis is inconsistent with the feeding methods typically used for open-water vertebrates: Either ram or suction feeding. The gill pouches would have obstructed the flow of water even further.

Sallan ''et al''. note that stalked eyes, tail fins, and brains are also present in anomalocaridids, and that ''Opabinia'' also has a similar proboscis. Previously arthropod affinities were rejected under the presumption that other Mazon Creek arthropods are preserved in three-dimensions, with carbonization of the exoskeleton, but the arthropods are not actually preserved in that manner. They also suggested that molluscs convergently evolved complex camera-like eyes containing melanosomes, but proponents of the vertebrate interpretation argue that no molluscs are known that have or had melanosomes in two distinct forms.

Regardless, Rogers ''et al.'' (2019) demonstrated that certain squid (''Loligo vulgaris'') and cuttlefish (''Sepia officinalis {{redire ...