Can preserved proteins reveal paint-by-numbers plumage?

by Mary Caperton Morton
Thursday, October 12, 2017

Some of the most compelling dinosaur fossils are those found with clearly defined feathers. Feathers may seem fragile and unlikely to be preserved, but in fact they’re composed of durable keratin, one of the toughest natural proteins. Additionally, some fossil feathers unearthed are speckled with tiny black dots, which, according to different studies, could be remnants of either bacteria or melanosomes. Melanosomes are organelles that produce and store melanin, the main source of pigment in feathers. If melanosomes are indeed preserved in some ancient fossils, they could reveal information about dinosaur coloration and plumage — and represent further evidence of preserved proteins in dinosaurs.

Fossil feathers highlight one of the biggest challenges for molecular paleontology: proving that structures or tissues are endogenous and not modern or bacterial contamination. Melanosomes “are identical in size and shape to bacteria,” and “both bacteria and melanosomes can be present in the same microscopic viewing field,” says Alison Moyer, a molecular biologist at Drexel University in Pennsylvania. That means scientists “can’t assume that structures are melanosomes and then use that data to make further assumptions about the animal’s appearance and lifestyle.” The good news, she says, is that researchers have the capability to identify melanosomes; the bad news is that the technique is complicated, time-consuming and expensive.

In modern bird feathers, melanosomes are encased in keratin. In 2016, Yanhong Pan of the Chinese Academy of Sciences and colleagues published a study in Proceedings of the National Academy of Sciences in which they differentiated melanosomes encased in keratin from similar looking bacteria. “That should be the gold standard for melanosome studies,” says Johan Lindgren, a molecular paleontologist at Lund University in Sweden. “But too often people skip that step and go straight to coloration.” For molecular paleontology to move forward with confidence, more standards are needed, Lindgren says. “The field is so new that we haven’t developed common standards that all scientists use,” he says. “Each research team has their own standards, which can range from simply looking at the morphology to full-blown molecular comparisons.”

The stronger the standards, the better the evidence will be, says Mary Schweitzer, a molecular paleontologist at North Carolina State University. People accept that scientists can recover proteins when they relate to color but not that scientists can recover proteins when it relates to soft tissues, like collagen, she adds. It “highlights the unequal application of standards.”

As to whether we’ll ever have a complete picture of dinosaur plumage in full color, Lindgren says, “never say never.” However, melanin is only one factor determining coloration, with other pigments and structurally derived colors — those generated by microscopic surface structures that create iridescent colors — also playing a role. “It’s hard to know how much information can be preserved,” he says. “True dinosaur coloration may be lost to time.”

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