300-million-year-old ‘Tully Monster’ may not be the creature scientists thought it was

The mysterious “Tully Monster,” a 15-centimeter-long, stalk-eyed  creature (artist’s concept above) that swarmed the seas of what is now Illinois more than 300 million years ago, was a vertebrate and a close relative of lampreys. At least that’s what scientists concluded 3 years ago. An even more recent study seems to have confirmed that classification. But a new analysis could shake up this strange animal’s family tree.

This new effort focused on the eyes of Tullimonstrum, whose informal name honors the paleontologist who first discovered it. They homed in on melanosomes, microscopic, pigment-containing structures that often bind to metals such as zinc and copper, which possibly serve as antioxidants. Such structures were widely thought to be present only in the eyes of vertebrates, hence a previous team’s 2016 classification.

But analyses of modern-day invertebrates such as the common octopus (Octopus vulgaris) and the European squid (Loligo vulgaris) reveal that melanosomes can be found in invertebrate eyes, too, the scientists report today in the Proceedings of the Royal Society B. Not only that, the melanosomes in invertebrate eyes appear to attract and bond to metals differently from those in vertebrate eyes. Whereas melanosomes in vertebrate eyes sport higher concentrations of zinc than their invertebrate counterparts, those in the eyes of the invertebrates they tested contain higher proportions of copper, especially the Cu⁺¹ form of the element. Interestingly, the researchers note, fossil melanosomes from the eyes of Tullimonstrum contain little zinc compared with the melanosomes of vertebrate fossils found in the same rocks, and they also contain substantial amounts of Cu⁺¹.

Together, the findings suggest Tullimonstrum could have been an invertebrate. And similar analyses could help scientists classify other mysterious creatures from the past, the researchers suggest.