The mysterious Tullimonstrum gregarium – a 307-million-year-old creature unearthed from coal quarries in Illinois – is one of the world’s most controversial fossils. This 10-centimeter-long (4 inch) animal had a soft, striped body, a large tail, a big trunk-like proboscis with a claw-like jaw filled with little sharp teeth, and a stiff bar with dark, oval-shaped blobs at either end, presumably eyes on a stalk. In the last five decades, researchers trying to understand this peculiar anatomy have linked it to groups as disparate as mollusks, worms, and vertebrates.
Two recent studies conclude that this so-called "Tully monster" is a vertebrate. In work published last month, researchers found that it was a close relative to jawless fish called lampreys. They reinterpreted the light-colored structure present length-wise in the fossils as a notochord – a skeletal rod that’s found in all the members of Chordata, the phylum we and all other vertebrates belong to.
A study published in Nature this week focused on the Tully monster’s eyes: Their structural details are like that of vertebrate eyes.
"When a fossil has anatomy this bizarre it's difficult to know where to start, so we decided to look at the most striking feature – the stalked structures with dark blobs," University of Leicester’s Thomas Clements said in a statement. Using techniques ranging from scanning electron microscopy to mass spectrometry, the team examined the dark elliptical patches at the ends of the transverse bar (imagine a hammerhead shark) in 12 Tullimonstrum specimens uncovered from the Carboniferous Mazon Creek Lagerstätte.
The Tully monster fossil with “meatball” and “sausage” melanosomes. University of Leicester
The eyes of the Tully monster, they found, have a light-sensitive retina made of layers of hundreds of thousands of melanosomes – cells that produce and store the pigment melanin. And the melanosomes come in two shapes too: spheres or "meatballs" and cylinders with rounded ends or "sausages" (pictured above).
Many animals have melanin in their eyes. "It stops light from bouncing around inside the eyeball and allows the formation of a clear visual image," Clements explained. But having melanosomes of two distinct shapes arranged in layers is a trait that’s only found in vertebrates.
The Tully monster’s melanin is the oldest fossil pigment known. Furthermore, its vision was probably pretty good: The little monster had a camera-style eye capable of image formation.