A 280-million-year-old fossil, classified as a member of the reptile group, that has baffled scientists for decades may have been partly forged, according to new examination of the remnants.

The discovery, published in the journal Palaeontology, has led the team to urge caution in how the fossil is used in future research.

Tridentinosaurus antiquus was discovered in the Italian alps in 1931 and was thought to be an important specimen for understanding early reptile evolution.

Its body outline, appearing dark against the surrounding rock, was initially interpreted as preserved soft tissues. This led to its classification as a member of the reptile group Protorosauria.

However, the new research reveals that the fossil renowned for its remarkable preservation is mostly just black paint on a carved lizard-shaped rock surface.

The purported fossilised skin had been celebrated in articles and books but never studied in detail.

The somewhat strange preservation of the fossil had left many experts uncertain about what group of reptiles this strange lizard-like animal belonged to and more generally its geological history.

"Fossil soft tissues are rare, but when found in a fossil they can reveal important biological information, for instance, the external colouration, internal anatomy and physiology," said Valentina Rossi of University College Cork, Ireland (UCC).

"The answer to all our questions was right in front of us, we had to study this fossil specimen in details to reveal its secrets – even those that perhaps we did not want to know," Rossi said.

The microscopic analysis showed that the texture and composition of the material did not match that of genuine fossilised soft tissues.

Preliminary investigation using ultraviolet (UV) photography revealed that the entirety of the specimen was treated with some sort of coating material, the researchers said.

Coating fossils with varnishes or lacquers was the norm in the past and sometimes is still necessary to preserve a fossil specimen in museum cabinets and exhibits, they said.

The team was hoping that beneath the coating layer, the original soft tissues were still in good condition to extract meaningful palaeobiological information.

The findings indicate that the body outline of Tridentinosaurus antiquus was artificially created, likely to enhance the appearance of the fossil. This deception misled previous researchers, and now caution is being urged when using this specimen in future studies.

The team behind this research includes contributors based in Italy at the University of Padua, Museum of Nature South Tyrol, and the Museo delle Scienze in Trento.

"The peculiar preservation of Tridentinosaurus had puzzled experts for decades. Now, it all makes sense. What it was described as carbonised skin, is just paint," said study co-author Professor Evelyn Kustatscher.

However, the fossil is not a complete fake. The bones of the hindlimbs, in particular, the femurs seem genuine, although poorly preserved, the researchers said.

The new analyses have shown the presence of tiny bony scales called osteoderms -- like the scales of crocodiles -- on what perhaps was the back of the animal, they added.

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