The oddball fossil that puzzled experts for almost 60 years is
probably an ancient fish akin to today's bloodsucking lampreys





T he worm-like creatures writhed in the dark waters, fins twitching
and eyestalks roving. Each one sported a long, pincher-tipped
proboscis lined with tiny, needle-like teeth. When paleontologists
found fossils of these ancient horrors trapped in stone, they named
them Tullimonstrum gregarium, or Tully monsters.


For roughly 60 years, no one could say for sure what the strange
beasts actually were. Paleontologist Eugene Richardson, who gave the
species its name in 1966, was so unsure of the creature's nature that
he wasn’t confident sticking it within any known lineage beyond
“animal.”

Now, an international team says they have at last cracked the mystery,
and their answer overturns every other theory offered to date.
Depending on who you asked, the Tully Monster could have been related
to ribbon worms, snails, eel-like protovertebrates called conodonts or
other ancient oddballs, like another nozzle-nosed creature called
Opabinia. But based on studies of more than 1,200 fossil specimens,
the researchers say the Tully Monster was really a vertebrate,
specifically, a type of fish akin to modern lampreys. If they're
right, the fossil changes what we know about the history of these
aquatic bloodsuckers.

“Instead of being a small, conservative lineage of bloodsucking fish,
lampreys are inferred to have undergone a dramatic diversification,
achieving some outlandish body plans and long forgotten modes of
life,” says University of Manchester paleontologist Rob Sansom.


Found by the dozens in the roughly 300-million-year-old rock of Mazon
Creek, Illinois, the Tully Monster was a tiny terror—the largest
specimens stretch a little more than a foot long. But they have an
outsized appeal to paleontologists, and have even been named the
official state fossil of Illinois.

For decades the prehistoric whatsit remained a frustrating enigma,
and was so weird that it even skirted the edges of myth. Some
cryptozoologists became enamored with the idea that the legendary Loch
Ness Monster was a supersized version of Tullimonstrum.

Yale University paleontologist Victoria McCoy says that she’s always
had a soft spot for the Tully Monster, in part because the creature
stood out as something very different from anything alive today. Given
that there are thousands of specimens from a time when the major
branches of animal life were already in place, she felt there was a
good chance of solving the mystery.

As it happens, the key was staring paleontologists in the face all
along. Although the weird eyestalks and flexible snoot are the most
obviously bizarre features of Tullimonstrum, paleontologists were
puzzled by what they previously interpreted as the “gut trace.” Other
animals from the same rock have gut contents that are preserved as
dark, mineralized sections, but the guts of the Tully Monster were
different. It appeared as a lightly colored, flattened structure that
ran from the eye stalks to the end of the tail.



That was odd, because the gut should not continue past the end of the
tail in both vertebrates and mollusks, McCoy noted. The pale line had
to be something else. While McCoy was reading up on other Mazon Creek
fossils, including fossil lampreys and hagfish, she realized that
these vertebrates had the same structure: a notochord. This is what
drew the mysterious creature into the vertebrate family tree.

“Lampreys are vertebrates,” McCoy says, “so the Tully was as well.”

From there, the stranger features of the Tully Monster started to fall
into place. In addition to a notochord, “the Tully Monster also has
large complex eyes, horny teeth, a tail fin with fin rays and a
tri-lobed brain,” McCoy says. These features aren’t always unique to vertebrates, but they nevertheless fit with the new identification.

Likewise, recent research on how animals like modern lampreys decay
showed that the worm-like “segments” of the Tully Monster are really
slightly decayed muscles that match up with those of early fish.
Suddenly all the pieces snapped into focus, allowing McCoy and her
co-authors to finally identify the Tully Monster, as they report this
week in Nature.

The result was “quite surprising and raises a lot of interesting
questions,” says Sansom, who was not part of the research team. While
the Tully Monster shows some traits in common with vertebrates, there
are still some “extraordinarily bizarre” parts of its anatomy that are
unlike any other vertebrate, he adds.

“There are currently no known mechanisms by which a jawless vertebrate
could develop eyes on stalks or jaws on a long proboscis,” Sansom
says, opening up mysteries about how the Tully Monster came to be.
Given that the lamprey fossil record is sparse, further surprises
might await in the rocks below.

For now, though, thanks to the efforts of McCoy and her colleagues,
another oddball finds its place on the Tree of Life, matched to the
greater vertebrate branch to which we also belong. But, as its
scientific name implies, the creature is still worthy of the title
"monster."

(Photos at website)


Read more: http://www.smithsonianmag.com/science-nature/what-tully-monster-scientists-finally-think-they-know-180958422/#EX64mHYh6IGOX61p.99
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