The true nature of the “Tully monster” may once again be a mystery.

Just last year, some researchers declared that the extinct aquatic animal was a vertebrate, possibly a relative of today’s lampreys. Not so fast, says vertebrate paleontologist Lauren Sallan. Like a mismatched puzzle, the Tully monster lacks some vertebrate pieces and has others that are the wrong shape, Sallan and colleagues report in the March issue of Palaeontology.

Tullimonstrum gregarium didn’t get its monstrous name because of its size. Only about a foot long, the oddball creature, which lived about 300 million years ago, sported wide-set eyes like a hammerhead shark and a pincerlike mouth at the end of a long trunk. In the past, it’s been lumped in with everything from sea slugs to arthropods.

Most recently, in a paper published in Nature in 2016, paleontologist Victoria McCoy, who was then at Yale University, argued that the Tully monster was a vertebrate (SN: 4/30/16, p. 5). Analysis of more than 1,200 Tully monster specimens dug up from Mazon Creek, a fossil hotbed in Illinois, revealed that the fossils appear to have notochords, a sort of spinal rod associated with some vertebrates, McCoy and colleagues concluded.

The Tully monster’s place in the tree of life is still unclear, but scientists have proposed many placements for the animal: The creature has been called an arthropod (perhaps a relative of the extinct Opabinia), declared a lamprey relative and grouped with mollusks such as sea slugs.

But the length is all wrong to be a notochord, says Sallan, of the University of Pennsylvania. A notochord always tapers to a point behind the eyes and brain in vertebrates, she says. The structure identified as a notochord along the Tully monster’s back widens and then extends past the brain.

Sallan’s team tallies other strikes against the creature being a vertebrate. Notochords aren’t preserved in fossils of any of Mazon Creek’s known vertebrates — various types of fish — probably because conditions were too acidic, Sallan says. That raises the unlikely situation that this structure would be preserved in Tully monsters but not in fish. Previously, researchers have identified this feature as the creature’s gut, which Sallan says is a more convincing possibility than a notochord.

In addition, she says, the Tully monster fossils lack two other distinct features seen in the majority of vertebrate fossils found at Mazon Creek: an inner ear and skin pigment.

McCoy, now at the University of Leicester in England, stands by the conclusion that the Tully monster is a vertebrate and plans to publish a response. “They didn’t do any new specimen analyses or really provide any new data,” she says.

Sallan counters that the Tully monster’s true identity remains an open question. “Our paper is really a call for further examination of these animals, but with expertise from all different fields in order to weigh all the evidence.”

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