Mystery of prehistoric, alien-like tully monster deepens once more

Tully monsters haunted Earth's oceans 300 million years ago and left behind such bizarre fossils that researchers haven't even agreed on whether these strange creatures had backbones. Now, more than 60 years after the strange creature's discovery in 1958, a new investigation using 3D lasers finds that the Tully monster was likely an invertebrate, but not everyone is convinced. 

This alien-looking evolutionary oddball ​​— only found in the Mazon Creek fossil beds in Illinois — had a soft body, eyes on stalks, and a claw-like appendage coming out of its face. Other aspects of its anatomy, however, are open to interpretation.

The latest research, published April 16 in the journal Palaeontology, is one of a slew of studies attempting to classify the creature.

Most recent Tully monster (Tullimonstrum gregarium) studies argue that it's either a vertebrate relative of modern cyclostomes (including lampreys and hagfish) in the chordate group or an unknown invertebrate. Now, researchers in Japan think they've cracked the case, with the help of a 3D laser scanner.

"We believe that the mystery of it being an invertebrate or vertebrate has been solved," first author Tomoyuki Mikami, a doctoral student at the University of Tokyo and a researcher at the National Museum of Nature and Science in Tokyo, said in a statement. "Based on multiple lines of evidence, the vertebrate hypothesis of the Tully monster is untenable."

Related: Ancient 'Tully monster' was a vertebrate, not a spineless blob, study claims 

The researchers scanned more than 150 Tully monster fossils to create color-coded 3D maps of the animal's anatomical structures. They also X-rayed one well-preserved proboscis — the claw-like appendage — to examine the creature's teeth.

The results suggested that features previously used to argue Tully monsters are near cyclostomes taxonomically, including their teeth and gill pouches, were misinterpreted. The teeth analyzed in the new study had bulging bases — unlike cyclostome teeth, which are thinner at the base. The authors said what appeared to be gills was actually just segmentation in the body.

Most convincing of all, the team claims, is segmentation found on the creature's head. "This characteristic is not known in any vertebrate lineage, suggesting a nonvertebrate affinity," Mikami said. 

Victoria McCoy, an assistant professor at the University of Wisconsin-Milwaukee, is not convinced, however. McCoy led a 2016 study placing Tully monsters near cyclosomes and a 2020 study that found Tully monster tissues were made up of proteins like those of vertebrates — and not chitin, like those of invertebrates.  

"It didn't change my mind about what the Tully monster was," McCoy told Live Science. "But it is new information, and that definitely advances our understanding." 

McCoy was excited by the researchers' application of 3D imaging but had "minor technical quibbles with some of their conclusions." She argued that cyclostomes are very diverse in the fossil record and thus some species could have had the bulging-based teeth documented in the study. She also noted that the anatomy of animals preserved in Mazon Creek separates and shifts, which could account for the appearance of segmentation.

"The real-life morphology gets changed a lot during fossilization," McCoy said. "If you have a thousand specimens, any one feature might be preserved a hundred different ways."

Deciding where the Tully monster belongs is significant because the species is so unusual that it will expand the diversity of whatever group it ends up in, changing the way we think about that group.

The latest study also puts forward a potential compromise for the discrepancies: that Tully monsters could possibly be nonvertebrate chordates like modern tunicates or lancelets. For now, however, the Tully monster's place on the evolutionary tree remains unclear.