Scientists have long assumed that the drab dinosaur fossils they dig up were
once colorful creatures. This has been especially true for certain theropods,
those fast-running, feathered reptiles thought to be the ancestors of modern
birds. In fact, one theory to explain the origin of feathers is that they
originally arose not for flight, but to provide a colorful display for sexual
selection during mating or to startle predators or prey. But until this year,
apart from what existed in an artist’s imagination, paleontologists could
offer no evidence of colored dinosaurs. Now two research papers provide evidence
that the feathers were boldly bedecked with color.
The first report by a Chinese and British team of
scientists, published in the January 27, 2010 online issue of Nature,
finds that a 125 million-year-old, turkey-sized theropod fossil, Sinosauroptyrex,
had a tan and white-striped tail.
was first discovered in China in 1996 with a downy fringe of hairlike filaments,
“dino fuzz”, along its neck and backbone. At that time proponents of the
dinosaur-bird theory said the filaments were feather precursors and hailed them
as supporting evidence that dinosaurs evolved into birds. Opponents countered
they were merely part of a reptilian fringe of internal collagen fibers that
looked feathery because it had frayed-see ScienceWatch:
Birds(?) of a Feather?? (January/February
1999). The Chinese/British team, headed by Fucheng Zhang, Institute of
Paleontology and Paleoanthropology, Beijing, China, and Michael Benton,
University of Bristol, Bristol, UK, has now laid this criticism to rest by
showing that the filaments have the coloration typical of modern feathers.
They used scanning electron microscopy (SEM) to find
melanosomes, pigment-bearing organelles 1,000 times smaller than a human hair,
crammed into the filaments. SEM can examine surface topography at very high
magnifications of up to 500,000X. The team found two types of melanosomes,
rod-shaped eumelanosomes, which produce black and shades of gray, and
phaeomelanosomes, which are round and generate reddish-brown to yellow hues in
the feathers of modern birds. Dark bands along the tail were “absolutely
packed with phaeomelanosomes”, according to Benton and it led the team to
conclude that Sinosauroptyrex had a chestnut to reddish-brown striped
tail and a rufous back (see Figure 1 and title cartoon).
The second report, published in the online version of Science
on February 5, 2010, took the melanosome analysis one step further. This
Chinese/American team, headed by Qunguo Li, Beijing Museum of Natural History,
Beijing, China and Richard Prum, Yale University, New Haven, CT, examined a
well-preserved, 150 million-year-old fossil of a chicken-sized theropod (Anchiornis
huxleyi) with its crown and limbs covered in feathers.
The scientists sampled portions of feathers throughout the fossil. They then
measured the dimensions of the melanosomes using SEM and noted the distribution
and density of each morphological type. Next they used the skills of team member
Matthew Shawkey, a University of Akron, OH biologist, who has studied
melanosomes in living birds, and can make predictions of feather color from the
types of melanosomes found in the feather. The plumage coloration pattern they
generated for Anchiornis is a red-and-black crown with black-and-white
limbs, reminiscent of a modern pileated woodpecker (see Figure 2). The startling
color pattern exhibited by Anchiornis led Li et al. to
conclude that a signaling function for early feathers may have been the reason
why they first evolved.
According to Prum, “a more likely function for both the
crown and limb feathers of Anchiornis is communication or signaling. This
could have been in lots of contexts, including sexual display, territoriality,
etc. It could have been like modern redstarts, which use their bright wing and
tail patches to scare up insects, which [the birds] then seize in flight.”
In the world of early birds what may have mattered most is
not who you are but how you look.
Sinosauroptyrex