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What is a Tully monster and why should we care?

For over half a century, the Tully monster (Tullimonstrum gregarium) has puzzled paleontologists due to its bizarre anatomy, making it incredibly difficult to classify. This peculiar animal lived approximately 300 million years ago. Francis Tully discovered it in the 1950s while fossil hunting at the Mazon Creek Lagerstätte in Illinois.

The Tully monster’s average length was around 15 centimeters. Its unique soft-bodied nature made it an enigma among fossils, sparking decades-long debates among researchers about its proper taxonomic classification.

Recently, a hypothesis emerged suggesting that the Tully monster was a vertebrate. Tully was possibly related to cyclostomes, a group of jawless fish like lampreys and hagfish. If this were true, it could fill a crucial gap in the evolutionary history of early vertebrates. However, supporters and opponents have met this hypothesis with both support and opposition over the years. This has left the scientific community divided.

Research team has a breakthrough on Tully nonster study

Now, a research team from the University of Tokyo and Nagoya University in Japan has made a groundbreaking discovery using 3D imaging technology. 

“We believe that the mystery of it being an invertebrate or vertebrate has been solved,” said Tomoyuki Mikami. Tomoyuki is a doctoral student in the Graduate School of Science at the University of Tokyo at the time of the study and currently a researcher at the National Museum of Nature and Science. 

“Based on multiple lines of evidence, the vertebrate hypothesis of the Tully monster is untenable. The most important point is that the Tully monster had segmentation in its head region that extended from its body. This characteristic is not known in any vertebrate lineage, suggesting a nonvertebrate affinity.”

The team studied more than 150 Tully monster fossils and over 70 other diverse animal fossils from Mazon Creek. Next, they created a detailed color-coded 3D maps using a laser scanner to highlight the fossils’ surface irregularities. Additionally, researchers employed X-ray micro-computed tomography to examine the creature’s proboscis. A proboscis is an elongated organ in its head.

What the study revealed about Tully monsters

The research, published in the journal Paleontology, revealed that the features previously used to argue that the Tully monster was a vertebrate were not consistent with those found in vertebrates. This finding strongly suggests that the Tully monster was not a vertebrate. This revelation added a new layer of complexity to the ongoing debate.

While the exact classification of the Tully monster remains uncertain, the next stage of the investigation will focus on determining its true taxonomic position. The creature could potentially belong to a group of nonvertebrate chordates. These possibilities include the lancelet, or a protostome group. A protostome is a diverse collection of animals that include insects, roundworms, earthworms, and snails – with radically modified morphology.

The perplexing case of the Tully monster highlights the challenges scientists face in unraveling the dynamic history of Earth. The vast array of organisms that have inhabited it over time make this task difficult. As researchers develop new technologies and research methods, they may emerge with more definitive answers.

Details regarding the Tully monster’s place in the evolutionary tree are still missing. For now, though, this enigmatic creature continues to captivate the scientific community and remind us of the complexities of life on our planet.

“There were many interesting animals that were never preserved as fossils,” Mikami said. “In this sense, research on the fossils from Mazon Creek is important because it provides paleontological evidence that cannot be obtained from other sites. More and more research is needed to extract important clues from Mazon Creek fossils to understand the evolutionary history of life.”

What Earth was like 300 million years ago

Around 300 million years ago, Earth was in the late Carboniferous to early Permian period. Several unique features characterized this time in Earth’s history:


During this period, Earth’s continents were coming together to form a single supercontinent called Pangaea. This massive landmass included almost all of the present-day continents. A vast ocean called Panthalassa surrounded it.


The late Carboniferous and early Permian periods experienced a generally warm and humid climate due to high levels of atmospheric carbon dioxide. However, the climate began to transition towards a more seasonal and arid environment towards the end of the Carboniferous period. This shift was partly because of the formation of Pangaea, which altered global ocean circulation and wind patterns.


The Carboniferous period is famous for its extensive coal deposits, which formed from the remains of vast swampy forests that dominated the landscape. The plant life was predominantly made up of ferns, horsetails, club mosses, and seed-producing plants called gymnosperms. As the climate became more arid in the early Permian, these swampy forests gradually gave way to more drought-resistant plants, such as conifers and other gymnosperms.


A diverse range of animal life inhabited Earth 300 million years ago. In the oceans, there were various marine invertebrates, such as trilobites, brachiopods, and mollusks, as well as early fish, including sharks and bony fish. On land, the first reptiles were evolving from their amphibian ancestors, and insects were becoming more diverse and abundant. The Carboniferous period also saw the rise of large, dragonfly-like insects called Meganeura, which had wingspans of up to 2.5 feet (75 cm).


The atmosphere during the late Carboniferous period had much higher oxygen levels than today, reaching up to 35%. This high oxygen concentration enabled insects and other invertebrates to grow much larger than their modern counterparts. The elevated oxygen levels also supported the growth of vast forests that ultimately contributed to the formation of extensive coal deposits.

In summary, the formation of the supercontinent Pangaea characterized Earth 300 million years ago. Earth had a warm and humid climate that transitioned to a more arid environment. It was also home to diverse plant and animal life, and a unique atmospheric composition with high oxygen levels.


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