Fly fossils might challenge the idea of trilobites’ crystal eyes

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Fossil crane flies found in Denmark
have crystals in their eyes — individual, see-through mineral pieces where the living eyes’ lenses
once were.

Those little crystals of
calcium carbonate are renewing a fuss about more mysterious ancient animals,
the trilobites. Fossils of those extinct, shield-shaped invertebrates also have
crystalized mineral lenses in their eyes. There are no living trilobites, but since
at least the 1970s, scientists have been imagining how crystal lenses might have worked for the creatures when they were alive (SN: 2/2/74).
Now the crane fly researchers argue that crystal lenses, in crane flies as well
as in trilobites, are just quirks of fossilization.

Living crane flies don’t
have crystal lenses, the researchers note online August 15 in Nature. Neither do other known living
insects or any of the bigger group of jointed-legs animals, the arthropods, says
coauthor Johan Lindgren, a molecular paleontologist at Lund University of
Sweden. These animals sometimes grow tinier calcite crystals in their eyes or in
their rigid exoskeletons for strength, but not “one big crystal basically in
each individual lens,” he says.

In these fossilized crane fly eyes, dark, lacy networks outline the many pale spots where individual lenses once lay in the living fly.Johan Lindgren

In the new eye study,
Lindgren and colleagues focus on beautifully preserved crane fly specimens
of several ancient kinds. The fossils were found in 54-million-year-old
sediments in what was once a waterway in today’s
Danish peninsula of Jutland. Like modern crane flies, the fossil ones look a
bit like mosquitoes, but with longer legs.

One surprise in studying the
fossils was their eye-shade pigments. Several kinds of lab analyses identified
signs of eumelanin, a form of melanin, in the flies “despite the fact that
they’re not supposed to have it,” Lindgren says. Curious about living insects’
pigments, Lindgren and colleagues checked a modern species, the tiger crane
fly. They found eumelanin there too, adding to the case that ancient relatives
could have had it as well.

magnified fossil fly eye
Colors added to a highly magnified image of a fossil crane fly eye show hexagonal pieces found to contain lots of calcium (pink), indicating mineralized lenses. Rims contain abundant carbon (green) and other elements as expected from a screening pigment.J. Lindgren et al/Nature 2019

Vertebrates use forms of
melanin to screen their eyes from stray light that’s not focused by the lens. But
biologists had thought that while insects and other arthropods use melanins for
body coloring, among other functions, these animals just had a different kind
of eye-shading pigment, called ommochromes. Crane flies are the first of any arthropod group,
living or extinct, shown to have a melanin pigment in their eyes, Lindgren says.

So far, the evidence looks
“suggestive” of eumelanin in the extinct crane fly eyes, says biophysicist
Doekele Stavenga from the University of Groningen in the Netherlands, who
wasn’t involved in the study. He would like to see some more kinds of tests identifying
the fossil pigment.

Unlike with the pigment,
crystal lenses don’t show up in living crane flies — a result that doesn’t surprise Lindgren. “There are
only deficits of having rocks in your eyes,” he says. For one, crystals of
calcium carbonate have optical quirks that need just the right alignment with incoming
light to avoid double images. Some mollusks known as chitons have calcite-lensed eye spots that can pick up spatial
information
about looming predators (SN: 11/19/15).
 That’s not fancy vision, but it may be
enough for mound-shaped animals creeping around the sea floor instead of flying.

trilobite fossil
The elaborate eyes of this Coltraneia trilobite fossil, more than 350 million years old from Morocco, show up as curved arrays of little bumps.Guillermo Guerao/shutterstock

If crane
flies’ lenses fossilized into calcite lumps, the assumption that trilobite eyes
likewise calcified after death seems “more probable,” says Gerhard Scholtz, a zoologist specializing in arthropod
evolution at Humboldt University in Berlin. “I always had doubts about the
calcitic nature of trilobite eye lenses.”

But physiologist
Brigitte Shoenemann at
the University of Cologne in Germany is sticking with the idea that living
trilobites saw through lenses that were mainly calcium carbonate. One advantage
is the mineral’s power to strongly bend incoming light underwater, a help in collecting
and focusing ample illumination. Also, she says that trilobite eyes that
fossilized under a variety of conditions nonetheless show such lenses. She’s
prepared to accept that crane flies didn’t see through mineral crystals but is
not ready “to raise this singular finding, as excellent as it may be, to a
general principle touching trilobites.” Clearly, the debate goes on.  


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Originally posted by: Susan Milius

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