YSM Issue 90.1

evolutionary biology
FOCUS
classification have resolved lingering questions
about many previously problematic fossils, the
Tully Monster had largely resisted phylogenic
placement.
The mission begins
In 2015, McCoy was leading the Briggs lab
group’s annual project, on which all the researchers
in the group were collaborating.
“The Field Museum independently thought
that they would want someone to look at the
Tully Monster, so I independently met with
the Field Museum and curators and collection
managers at the Geological Society of America
meeting,” McCoy said. “We both kind of
simultaneously said we’d like to work on the
Tully Monster.”
The Field Museum, in Illinois, is the closest
major museum to Mazon Creek, where the
majority of Tully Monster fossils have been
unearthed. Its close relationship with local collectors
has helped it acquire a vast collection
of Tully Monster fossils—at over 2,000 specimens,
the largest in the world—which made it
the ideal location to work on the project, according
to Briggs.
It’s elemental, not elementary
Part of what helped the Yale team succeed
where others had failed was meticulous attention
to detail. The fossilization process, though
permitting long-term preservation, can make
morphological features difficult to decipher as
organisms decay and fossilize.
In total, the researchers examined around
1,200 Tullimonstrum specimens, organ by organ,
for general morphological features. As an
example, at the end of the Tully Monster’s proboscis,
the team counted and measured teeth
structures and studied how they were situated
in the Tully Monster’s claw structure. They also
looked at features such as whether the eye bar
sat at the top of the head or went through the
center of the body.
The second component of the research involved
studying features of preservation where
the Tully Monster was discovered. The fossils
were preserved as a discolored film on surrounding
rock where the organism had been
buried, and what appear to be morphological
features sometimes are merely discoloration
from the fossilization process.
This combined analysis of morphological
and preservational features in the Tully Monster
helped McCoy and the team conclude that
the Tully Monster had a notochord, a cartilaginous
skeletal rod and evolutionary precursor
to the spine that classified the fossil as vertebrate.
The researchers had noticed a long line
running down the middle of specimens, previously
identified as the gut of the organism.
While it was entirely possible that the line was
the fossil imprint of either a gut or a notochord,
examining preservation in conjunction with
morphology allowed the researchers to determine
which feature it was.
To further test the vertebrate hypothesis,
the researchers took specimen samples to Argonne
National Laboratory for synchrotron
analysis, a technique that allowed the team to
determine which elements in a sample were
enriched, what differences existed in the elemental
composition of different tissues, and
when they were preserved in the fossil. The
synchrotron data indicated that Tully Monster
eyes were often preserved in pyrite mineral, a
preservational feature shared only by the fish
—the other chordates—at the fossil site. This
was particularly strong evidence that the Tully
Monster was preserved similarly to fish, rather
than other proposed animal groups, such as
worms or mollusks.
The researchers also found that Tully Monster
teeth had a distinct composition from that
of the rest of the bifurcated, claw-like structure
at the end of its proboscis. This discovery also
pointed to chordate affinity, since arthropods,
another previously proposed position for the
Tully Monster, have teeth-like spikes made of
the same material as their claw. Fish teeth, on
the other hand, are biomineralized—minerals
were produced in those tissues to harden them,
making teeth tissue distinct from the rest of the
fish’s mouth.
The teeth were pyritized, or replaced with
iron sulfide, suggesting they were composed
of sulfur-rich material, but not biomineralized,
in contrast to most cartilaginous fishes,
like sharks, whose fossils typically contain
calcium or phosphate compounds. The sulfur-rich
soft tissue was likely made of keratin,
a sulfur-containing protein that is the main
component of fingernails. Interestingly, keratin
is also a component of hagfish and lamprey
teeth, which do not biomineralize, so the
researchers could conclude not only that the
Tully Monster was a chordate, but also that
it preserved similarly to soft-bodied fish like
lampreys and hagfish.
A fossil’s future
Future studies of the Tully Monster include
understanding its ecology and interactions
with its environment and other organisms of
the Carboniferous. It is unknown whether it
was a parasite (like modern lampreys) or a
scavenger, an ambush predator or one that
could sustain periods of continuous swimming.
Its many morphological oddities make
it particularly compelling to study, according
to McCoy.
Luckily for researchers, the Field Museum
recently acquired more Tullimonstrum fossils,
a collector’s gift that could provide researchers
the opportunity to explore and refine
their findings against the new specimens. An
important step, according to Briggs, is to examine
the new collection and see whether it
supports the team’s conclusions. While the
Tully Monster may be long dead and gone,
the story of its existence is sure to fascinate
generations to come.
ABOUT THE AUTHOR
ANDREA OUYANG
ANDREA OUYANG is a sophomore and prospective MCDB major in
Davenport College.
THE AUTHOR WOULD LIKE TO THANK Dr. Victoria McCoy and Professor
Derek Briggs for their time and enthusiasm in speaking about their work.
FURTHER READING
Clements, T. “The Eyes of Tullimonstrum reveal a vertebrate affinity.” Nature
532, 500-503, 2016.
Richardson, E.S. “Wormlike Fossil From the Pennsylvanian of Illinois.”
Science 151 (3706), 75-76, 1966.
www.yalescientific.org
December 2016
Yale Scientific Magazine
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