The "Tully Monster" was probably not a vertebrate, scientists have said, contradicting the conclusions of two other high-profile studies published last year. They hope to encourage more researchers to take on the challenge of finding out where the mysterious creature belongs on the tree of life.

The Tully Monster is an iconic 300 million year-old, soft-bodied fossil from the Mazon Creek fossil beds in the USA. It was first discovered by amateur palaeontologist Francis Tully in 1958, but many other specimens have been found since then.

The creature continues to fascinate and puzzle scientists, as they struggle to define what type of animal it was.

In 2016, there seemed to be a breakthrough, as two studies published in the journal Nature suggested that the Tully Monster was a vertebrate. The first one even suggested that the weird creature could be related to lampreys.

However, the findings of both papers were based on conflicting arguments and in new research now published in the journal Palaeontology, an international team of scientists has shown that their interpretations are potentially flawed.

Dr Ivan Sansom, Senior Lecturer in Palaeobiology at the University of Birmingham and one of the authors, told IBTimes UK:

"It is very problematic to interpret soft body fossils, as soft tissues can be poorly preserved, where they are preserved they will be squashed and deformed. There are a number of examples of contentious interpretations of soft bodied organisms in the fossil record and the Tully Monster is a classic example of that. In the two Nature studies, a number of issues arose with the interpretations, and some of the features described didn't ring right with us so we looked at whether some of these stood up to scrutiny".

Not a vertebrate

The scientists argue that the two studies that seemingly settled the Tully Monster debate cannot definitively classify it as a vertebrate, least of all a lamprey. They say that important preservational, functional and anatomical issues were ignored.

"In one of the studies published last year, the scientists described the Tully Monster as having an early spinal column and a brain without a skull. These are internal structures which are not preserved in any of the fish or lampreys found at Mazon Creek. On top of that, it doesn't have have some of the features that are preserved in all other fish fossils such as the otic capsule - the inner ear", lead author Lauren Sallan, from the University of Pennsylvania, explained.

"So Tully had features it shouldn't have had as a vertebrate, but it is missing basic vertebrate traits that are universally preserved in fishes at the site. This makes it hard to consider it a vertebrate".

Tully monster
One of the mysterious Tully Monster fossils. Paul Mayer, The Field Museum

Sallan and her team also show that the Tully Monster could not have functioned as reconstructed.

"For a fish to function, it has to get water over its gills. Typically the way that happens is with a large mouth drawing water to pass over the gills – the mouth tends to be larger than the gills. If you don't have that, there are other solutions are put in place. Lampreys for example have a specialised cartilage running down their throats, which enables them to feed but also provides water to the gills. In the Tully Monster, there isn't the room for such cartilage and the mouth is very narrow in comparison to the gills and is at the end of a proboscis - a trunk-like feature", Sansom explained.

The team also says that the first study presenting the Tully Monster as a vertebrate is based on a flawed phylogenetic analysis. It focused on a small number of vertebrate species, instead of including a larger number of diverse species, such as molluscs, to make the appropriate comparisons with the Tully Monster.

Sallan and her colleagues now call for more research to be conducted, to pierce the secrets of the creature and to describe it more accurately.

"The Tully Monster has been sitting there for more than fifty years and it became famous for evading description. No one had yet considered it as a vertebrate, so this explains why these studies were widely discussed", Sallan commented.

"The problem here is that the scientists found traits that appeared to be those of vertebrates, if they made sense from a functional, preservational or developmental point of view. The evidence we have suggests it was not a vertebrate. We want to encourage scientists to take another look at the Tully Monster and resolve this aged-old mystery".