A 345-million-year-old fossil, long presented as one of the oldest reptiles, actually concealed internal gills.
Westlothiana lizziae was discovered in Scotland and scientifically described at the end of the 20th century. With its elongated body and well-developed limbs, this animal looked like a small reptile. It therefore occupied an important place in discussions about the transition of the first four-limbed vertebrates toward a fully terrestrial life.
Westlothiana
Image from Wikimedia
A new study examined the fossil using X-ray imaging techniques capable of revealing details enclosed in the rock. The researchers thus observed anatomical structures that had previously been invisible. Some are located in the throat region and correspond to an internal gill apparatus.
This discovery significantly changes the animal’s profile. True amniotes, the group that today includes reptiles, birds, and mammals, are adapted to reproduction independent of water. The presence of gills in Westlothiana instead indicates that it retained characteristics inherited from older tetrapods.
The scans also revealed new details of the mouth and skull. Their combination places Westlothiana closer to the evolutionary branch preceding amniotes than to the first true reptiles. The animal therefore loses the iconic status that had been attributed to it.
Most interestingly, its anatomy nevertheless possessed several traits associated with terrestrial life. Its limbs and certain parts of its skeleton show that a body adapted to moving on land could appear while other characteristics remained linked to an aquatic life.
The evolution toward a terrestrial life therefore did not follow a single transformation in which all organs changed together. Different traits could have appeared at distinct times and become combined in unexpected ways in species that are now extinct.
Westlothiana lived during the Carboniferous, a period when terrestrial vertebrates were rapidly diversifying. Wet environments were widespread, and several lineages were exploring lifestyles situated between dependence on water and increasing independence on land.
The same imaging techniques can now be applied to other historical fossils. Specimens studied for decades sometimes contain structures still enclosed in the rock. Examining them could once again shift the boundary between ancient tetrapods and the first true amniotes.