An Ultimate Guide to Tullimonstrum: The Tully Monster

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NameTullimonstrum DietCarnivorous
Name MeaningTully monsterLength0.7 m (2.30 ft)
EraPaleozoic EraCarboniferous PeriodLocationUSA (North America)
ClassificationEumetazoa, ParaHoxozoa, & Bilateria 

Tullimonstrum Pictures

Artist reconstruction of tullimonstrum in a white background
Artist reconstruction of tullimonstrum in a white background | Entelognathus via Wikimedia Commons CC A-SA 4.0

The Tullimonstrum

Gage Beasley's Tully Monster Stuffed Plush Toy
Gage Beasley’s Tully Monster Stuffed Plush Toy

In the vast and awe-inspiring tapestry of Earth’s history, hundreds of thousands of distinct creatures have existed. 

Amongst these creatures, many mysterious animals have emerged, bewildering paleontologists and captivating the imaginations of enthusiasts worldwide. 

These creatures have been discovered and identified over the years to ensure accurate classification. Among these enigmatic fossils, one is an exceptional riddle: the Tullimonstrum, affectionately known as the “Tully Monster.” 

As unique as it is, this creature is classified by experts as a Bilaterian, a creature with a left and a right side that mirror each other, making them bilaterally symmetrical.

This unique creature was initially discovered in a fossil bed called Mazon Creek Formation in 1955. 

This fossil bed is located in Illinois, USA, primarily in Grundy, Will, and Kankakee counties. 

Soft-bodied fossil deposit of tullimonstrum of Illinois
Soft-bodied fossil deposit of tullimonstrum of Illinois | James St. John via Wikimedia Commons CCA 2.0

These fossil-rich deposits are renowned for their exceptional preservation of ancient organisms, offering valuable insights into the ecology and biodiversity of the Pennsylvanian period, approximately 309 to 307 million years ago. 

The creature, Tullimonstrum gregarium, was named by Francis Tully, the amateur fossil collector who first discovered the fossil in 1955. 

The genus Tullimonstrum combines Tully’s last name and the Latin word monstrum, meaning monster or strange creature. 

The species name gregarium refers to the fact that these fossils are often found in groups, suggesting the ancient animals’ gregarious or social behavior. 

This article covers more unique aspects of this creature’s life, physical features, social behavior, etc. 

Keep reading to discover more.

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Gage Beasley’s Prehistoric Shirt Collection
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Gage Beasley’s Prehistoric Plush Collection

Physical Characteristics

Gage Beasley Prehistoric's Tullimonstrum Tully Monster Size Comparison Chart
Gage Beasley Prehistoric’s Tullimonstrum Tully Monster Size Comparison Chart

As a small creature, the Tullimonstrum reached between 3.1 and 14 inches. 

The Tullimonstrum had an elongated and tubular body, tapering towards both ends. 

Its overall shape has been likened to that of a cylinder or a soft-bodied worm. 

Its body was segmented, displaying repeated units along its length, a feature commonly seen in certain groups of organisms. 

While it is difficult to tell from the fidelity of preservation of the fossils of the creature’s soft body, it had a pair of vertical ventral fins at the end of its body. 

Incredibly, the fossil record of the Tullimonstrum also preserved some of its soft tissues.  

Gage Beasley Prehistoric's Tullimonstrum Tully Monster Concept
Gage Beasley Prehistoric’s Tullimonstrum Tully Monster Concept

The lack of a hard portion in the fossil suggests that the animal did not have organs made of calcium carbonate, chitin, or bone. 

These internal structures included a notochord, a flexible, rod-like structure found in some primitive chordates. 

The presence of a notochord suggests that the Tullimonstrum was likely a chordate, a group that includes vertebrates.

According to experts, this creature possessed a proboscis, a long, slender, and flexible structure that extends from the front of the creature’s body. 

The function of the proboscis has been a subject of much debate among paleontologists. 

Some believe that the proboscis was used as a feeding organ. 

It has been proposed that the Tullimonstrum was a predator that employed its elongated proboscis to probe soft sediments and reach into crevices to capture small, soft-bodied prey, such as worms and small invertebrates. 

Fossil of tullimonstrum
Fossil of tullimonstrum | Ghedoghedo via Wikipedia CC BY-SA 3.0

Another hypothesis proposes that the proboscis served as a sensory organ. 

The creature might have used it to detect changes in the environment’s water current, vibrations, or chemical cues to locate potential prey or avoid predators.

Located at the proboscis’ base, the Tully Monster had large, stalked eyes facing forward. 

These eyes were well-developed, indicating that the creature likely relied on vision to navigate its environment and locate prey. 

The creature’s mouth was also at the base of the proboscis, serving as a primary opening for ingesting and processing food. 

Although the soft tissues of the mouth have not been perfectly preserved in the fossil record, enough evidence exists to infer some characteristics. 

The eyes of tully monster were located on a rigid bar or stalk that extended from the sides of its body, enabling it to have a wide field of vision
The eyes of tully monster were located on a rigid bar or stalk that extended from the sides of its body, enabling it to have a wide field of vision | Nobu Tamura via Wikimedia Commons CC A-SA 4.0

The creature’s mouth likely had a wide gape, allowing it to engulf and consume relatively large prey items compared to its overall body size. 

Sharp, needle-like teeth within the mouth indicate a carnivorous diet, suggesting that the Tullimonstrum primarily fed on other small marine organisms, such as worms, crustaceans, and soft-bodied invertebrates. 

The presence of teeth in the Tullimonstrum’s mouth has led researchers to categorize it as a vertebrate or a jawed animal. 

However, its precise taxonomic position within the vertebrates remains a subject of ongoing research and debate.

Habitat and Distribution

The Tullimonstrum existed about 300 million years ago in the Pennsylvanian geological period of the Carboniferous period. 

As mentioned, this creature was first discovered in the Mazon Creek fossil beds of Illinois, United States, in the 1950s. 

The Tully Monster inhabited a vast and diverse marine ecosystem during the Carboniferous period, approximately 307 to 299 million years ago. 

At that time, much of the United States was covered by a shallow sea known as the Illinois Basin

This region featured an extensive coastal plain with numerous estuaries and lagoons, making it a highly favorable environment for various marine life forms. 

Tullimonstrum most likely lived in open sea water as a free-swimming carnivore and sometimes washed to the area close to the coast where it was preserved. 

It was not tethered to a benthic habitat or a hard surface, allowing it to swim freely.

Tullimonstrum fossils are distributed in the Mazon Creek fossil beds in northeastern Illinois, particularly in Grundy County. 

These fossil-rich deposits were formed in lagoonal environments characterized by slow sedimentation and anoxic conditions, favoring soft tissue preservation. 

3D model of tullimonstrum
3D model of tullimonstrum | Петр Меньшиков via Wikimedia Commons CC A-SA 4.0

The way the Mazon Creek fossils formed is peculiar

The animals were quickly buried in silty outwash once they passed away. 

Carbon dioxide was created in the surrounding sediments as the bacteria started to break down the plant and animal remnants in the mud. 

In the vicinity of the ruins, the carbonate and iron from the groundwater mixed to produce encrusting siderite nodules. 

The organism was entombed to prevent deterioration and preserve imprint or carbonaceous remnants. 

While most Tully Monster fossils have been discovered in Illinois, unconfirmed isolated findings have been reported in other locations, including Russia and England. 

Fossil of tully monster taken at the Museo di Storia Naturale di Milano
Fossil of tully monster taken at the Museo di Storia Naturale di Milano | Ghedoghedo via Wikimedia Commons CC A-SA 3.0

However, most Tullimonstrum fossils remain confined to the Mazon Creek region.

Behavior and Diet

Determining whether the Tullimonstrum was solitary or social is challenging due to the limitations of the fossil record. 

Fossilized Tullimonstrum specimens are often found preserved alone, which suggests a solitary lifestyle. 

However, this is insufficient evidence to conclude that the Tullimonstrum was entirely solitary. 

One sure social interaction would have been during the mating season. 

However, reconstructing the mating behavior of the Tullimonstrum is particularly challenging due to the need for more direct evidence. 

Reconstruction of the Tully Monster
Reconstruction of the Tully Monster | PaleoEquii via Wikimedia Commons CC A-SA 4.0

Fossilized eggs or nests have yet to be discovered, making it difficult to determine the nature of their reproductive strategy. 

Nonetheless, paleontologists have proposed several hypotheses based on related species and the creature’s anatomy.

Some researchers believe that the Tullimonstrum may have been oviparous, laying eggs in protected environments to ensure the survival of its offspring. 

Others suggest it could have been viviparous, giving birth to live young. 

In either case, it is possible that the Tullimonstrum exhibited minimal parental care, similar to many modern fish species. 

Various factors, including food availability, predation pressure, and mating opportunities, might have influenced this creature’s social behavior. 

Tullimonstrum swimming through seaweed
Tullimonstrum swimming through seaweed | Stanton F. Fink via Wikimedia Commons CC A-SA 2.5

Environmental changes, such as fluctuations in sea levels and temperature, could have also shaped its behavior and interactions with conspecifics.

One of the earliest hypotheses suggested that the Tullimonstrum was a carnivorous predator, using its elongated proboscis to snatch and immobilize prey. 

The creature’s mouth contained eight needle-like teeth, four in each jaw. 

Recent advancements in technology, such as synchrotron scanning, have allowed scientists to probe deeper into the fossilized remains of the Tullimonstrum

Through synchrotron scanning used to analyze the contents of a Tullimonstrum fossil, it was revealed that the dark carbonaceous material found alongside the fossilized remains represented remnants of the Tullimonstrum’s internal organs. 

Further analysis indicated the presence of melanosomes, organelles responsible for producing melanin, suggesting that the creature had pigmented tissues. 

Group of tullimonstrum swimming in the ocean
Group of tullimonstrum swimming in the ocean | dottedhippo via iStock

Based on these findings, the researchers proposed that the Tullimonstrum was most likely a soft-bodied organism with a notochord, similar to modern-day vertebrates. 

The presence of melanosomes also hints at the possibility of eyes or other light-sensing structures, possibly supporting the idea that the Tullimonstrum was an active predator.

Life Cycle

Despite the progress in understanding the Tullimonstrum’s classification, its life cycle remains poorly understood due to the rarity of fossilized soft tissues that could provide crucial clues. 

One hypothesis suggests that the Tullimonstrum’s peculiar adult form might result from drastic metamorphosis. 

According to this theory, the creature would have started life in a vastly different larval form before undergoing a radical transformation during development. 

The larval stage may have been planktonic or lived in a different habitat, which would explain the lack of intermediate fossils. 

The Tully Monster likely lived in coastal environments, possibly in estuaries or lagoons
The Tullimonstrum likely lived in coastal environments, possibly in estuaries or lagoons | Stocktrek Images via Getty Images

Another possibility is that the Tullimonstrum exhibited paedomorphosis, a phenomenon where an organism retains juvenile characteristics into adulthood

In this scenario, the adult Tully Monster would resemble the juvenile form of its ancestors. 

This hypothesis has been proposed to explain the creature’s unique morphology and the absence of clear transitional stages in the fossil record.

There is no direct evidence conclusive information about the reproductive behavior of the Tullimonstrum

Since the Tullimonstrum is known only from its fossilized remains, it is challenging to determine its reproductive method with certainty. 

The fossil record’s lack of soft tissue preservation makes it difficult to study the reproductive organs or any evidence of reproductive behavior. 

Fossil of Tully Monster
Fossil of Tully Monster | Field Museum Library via Getty Images

Therefore, the specific reproductive strategy of the Tullimonstrum, whether it was oviparous (laying eggs) or viviparous (giving birth to live young), remains unknown. 

Researchers may continue exploring and analyzing new fossil findings or utilize advanced imaging techniques to study the preserved specimens, potentially providing more insights into the Tullimonstrum’s reproductive biology and overall life cycle. 

However, until new evidence emerges, the reproductive behavior of the Tullimonstrum remains speculative and subject to further investigation.

Evolution and History

The first Tullimonstrum fossil was discovered in Mazon Creek, Illinois, United States, in the 1950s. 

These fossils were part of the Mazon Creek Lagerstätte, a fossil-rich deposit from the Pennsylvanian geological period. 

Initially, the specimens were thought to be jellyfish or worms due to their soft-bodied nature, but later research revealed a more complex and unique creature. 

A scientific illustrator creating a model of tullimonstrum
A scientific illustrator creating a model of tullimonstrum | Field Museum Library via Getty Images

This creature’s classification has been a subject of intense debate among scientists for many years. 

Despite the numerous Tullimonstrum fossils found at Mazon Creek, scientists struggled to determine the creature’s evolutionary relationships and classification. 

It remained an “enigma taxon,” defying conventional categorization for many years. Several theories were proposed, suggesting it could be related to mollusks, worms, arthropods, or even chordates.

Initially, it was described as a marine worm (phylum Annelida), but this interpretation was eventually dismissed. 

Over the years, researchers have proposed various classifications, including a mollusk (phylum Mollusca), a vertebrate (subphylum Vertebrata), a conodont (extinct microfossils), or an entirely unknown and extinct species. 

One of the most influential studies supporting the vertebrate theory was published in 2016, based on discovering preserved melanosomes (organelles involved in pigmentation) within the Tully Monster’s eyes. 

Tullimonstrum in a concretion from the mazon creek, Illinois
Tullimonstrum in a concretion from the mazon creek, Illinois | Mark A. Wilson via Wikimedia Commons Public Domain

This study suggested that the creature might have been a jawless fish or lamprey’s relative. 

However, this hypothesis has also faced criticism, and further research is required to reach a definitive conclusion.

Interactions with Other Species

As an active predator or scavenger, the Tullimonstrum likely had various interactions with its prey. 

It could have pursued and captured small invertebrates or fish using its proboscis as a grasping tool if it was a predator. 

Alternatively, if it was a scavenger, the Tully monster may have fed on decaying organic matter, exploiting an ecological niche by cleaning up detritus in its environment. 

Both scenarios would have positioned the Tullimonstrum as a significant player in its ecosystem, impacting the population dynamics of its prey and potentially influencing the community structure.

An artist's rendering shows tullimonstrum that lived during paleozoic era
An artist’s rendering shows tullimonstrum that lived during paleozoic era | Sean McMahon via smithsonian magazine

As a soft-bodied creature, the Tullimonstrum might have faced predation by larger marine organisms of its time. 

It could have developed various adaptations to evade or deter predators, such as hiding in the sediment, camouflage, or swift swimming abilities. 

The Carboniferous Period was characterized by a high diversity of marine life, presenting numerous opportunities for interspecific competition. 

The Tullimonstrum likely faced competition with other predators or scavengers occupying similar ecological niches. 

Understanding how it coexisted with other species provides valuable insights into the mechanisms of resource partitioning and ecological balance during this ancient era.

Cultural Significance

Tullimonstrum odd fossil in an ironstone concretion found in an unrecorded site in the Mazon Creek area, Illinois, USA
Tullimonstrum odd fossil in an ironstone concretion found in an unrecorded site in the Mazon Creek area, Illinois, USA | James St. John via Wikimedia Commons CC A 2.0

The Tullimonstrum’s discovery in the Mazon Creek region sparked local folklore and legends surrounding this ancient creature. 

Tales of mysterious monsters lurking in the nearby riverbanks and forests captured the imaginations of the locals, adding to the cultural significance of the Tully Monster as a symbol of wonder and mystery in the region’s folklore. 

The creature’s bizarre appearance and enigmatic nature have inspired artists across various mediums.

From paleoartists attempting to reconstruct its appearance in lifelike illustrations to abstract painters seeking to capture its essence in surreal artworks, the Tully Monster’s cultural influence is evident in visual art. 

Illinois’s state fossil was formally declared Tullimonstrum gregarium in 1989. 

Its artwork is shown on U-Haul rental trucks from the state.

Tullimonstrum attacking a carbotubulus in late carboniferous Illinois
Tullimonstrum attacking a carbotubulus in late carboniferous Illinois | Prehistorica (Christian M.) via reddit

As an intriguing paleontological mystery, the Tully Monster has become a popular topic in educational programs and outreach activities. 

Teachers and educators use it as a tool to engage students in the fields of paleontology, biology, and geology. 

The Tully Monster’s allure encourages the younger generation to explore the wonders of the natural world and instills a sense of curiosity and wonder about prehistoric life. 

The creature’s fame has led to its inclusion in natural history museums and special exhibitions. 

These displays not only showcase the scientific aspects of the creature but also emphasize its cultural significance.


The Tullimonstrum, better known as the Tully Monster, is a prehistoric enigma that continues to captivate scientists and enthusiasts alike. 

Discovered in the Mazon Creek Formation in Illinois, USA, this unique creature defied easy classification, giving rise to years of debate among paleontologists. 

Its elongated tubular body, proboscis, and well-developed eyes made it an intriguing subject for artists, inspiring various artistic interpretations and becoming a local folklore symbol of wonder and mystery. 

The Tullimonstrum’s cultural significance extends to education and outreach, as it is a captivating tool to engage students in paleontology and natural sciences. 

While its precise evolutionary relationships remain elusive, the Tully Monster’s presence in the fossil record has a lasting impact on scientific understanding and artistic creativity, showcasing the power of ancient mysteries to inspire human curiosity and imagination.


Are there any other organisms similar to the Tullimonstrum in the fossil record?

The Tullimonstrum stands out as a unique and peculiar organism in the fossil record. 

No other creature shares its exact combination of features and morphology.

Was the Tully Monster a vertabrate or invertabrate?

The classification of the Tullimonstrum has been a subject of intense debate among scientists. 

Based on new findings, it was initially described as a marine worm; it has also been proposed to be related to vertebrates, such as jawless fish or lampreys. 

However, its precise taxonomic position as either a vertebrate or an invertebrate remains uncertain, and further research is needed to resolve this enigmatic creature’s classification definitively.


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