A Fluffy Double-Feature

This week, two articles were published that discussed feathers in two different dinosaurs. We’ll start with the cooler one…. uhhhh… I mean…. the…. one with better preservation. Yes, that’s it!

The first [article] described a bird fossil in amber, the third one from Myanmar that has been recently described. It is of an enantiornithine, an extinct lineage of toothed birds from the Cretaceous, and it’s spectacular. Most of the animal is preserved because it’s trapped in amber and many of the feathers are preserved in detail.

Figure 6c from the paper showing the 99 million year old enantiornithine foot in amber. Behold its beauty! Scale bar is 5 millimeters.

The authors wrote a thorough report of each part of the specimen, along with descriptions of the feathers found on each portion of the body. By CT scanning and examining it under dissecting microscopes, the authors were able to see both bone and feather morphologies. The morphologies indicated that the specimen was a juvenile. The feathers show that enantiornithines were precocial at hatching. Precocial means that they were able to walk around, and potentially even fly, from the day they hatched (like a chicken or a brush-turkey). Baby birds that need a lot of care before they can manage by themselves are altricial. This new specimen, along with other enantiornithines, are pointing to most enantiornithines being precocial. They are also known to be mostly arboreal (tree-dwellers). The combination of precocial and arboreal is not something that modern birds are doing: the precocial birds of today are ground-dwellers and the altricial birds of today are tree-dwellers. This means that enantiornithines were superficially similar to modern birds, but living different sorts of lifestyles than what we see today and this could have impacted the places they could live in and the body-shapes they had.

The graphical abstract from the paper showing the amber chunk with the fossil (bottom), the CT scan (middle), and a line drawing interpretation (top).

The second [article] was about tyrannosaurids. This group contains Tyrannosaurus rex, Gorgosaurus, Tarbosaurus, and a few other large-bodied theropods that are known for their large heads and tiny arms. There has been an ongoing debate on whether or not they had feathers covering their bodies. This debate originated because we know feathers were present on a lot of other theropods, including on the most basal members of the group, like Dilong. The issue is that we’ve never found a larger bodied tyrannosaur with feathers preserved on it.

An illustration of Dilong by P. Sloan.

To address this question, the authors examined fossilized skin impressions of several specimens of this group. They found that scales covered parts of the neck, abdomen, hips, and tail and concluded that most of these large-bodied tyrannosaurids were covered in scales. If feathers were present, they would have been limited to the back of the animal. There are many hypotheses (testable scientific ideas) out there about why these big tyrannosaurids lost their feathers, but I’m not going to address those here.

Figure 1b from the article showing a piece of fossilized skin from T.rex. You can see the outline of each scale.

The main point I want to make about this paper, and I’m going to quote my undergraduate mentor (Dr. Tom Holtz) here, the absence of evidence is not evidence of absence. That means just because we haven’t found feathers preserved on big tyrannosaurids, does not mean they didn’t have them. The conditions needed for feather preservation are very specific, and the places where we find these big tyrannosaurids are not the same types of places that preserve feathers. So maybe T. rex had feathers and they just weren’t preserved. Maybe T.rex didn’t have any feathers. Maybe it had feathers as a baby and lost them as an adult. Maybe it had feathers in some places on its body. For now, we don’t really know. We might never know. And that’s ok because science is a process of continuous discovery and interpretation. We’ll just have to keep digging.

Birds in Amber – Part 2

Last week, a new [discovery] made headlines – a bird tail preserved in amber. You may remember that in July I talked about a pair of bird wings [trapped in amber]. This specimen comes from the same country, Myanmar, and is from the mid-cretaceous (around 99 million years ago).

tail-rc-mckeller-royal-saskatchewan-museum

The specimen. By R.C. McKeller and the Royal Saskatchewan Museum.

The specimen is a partial tail. It has an estimated 8 full vertebrae and one partial one and because of their shapes, the authors think that these are from the middle or end portion of the tail. Their shapes also indicate that the tail may have had more than 25 vertebrae total. The whole tail specimen is only around 35 mm long, so they think it was a juvenile.

fig-1e

Figure 1E from the paper showing the CT scan of the vertebrae.

Modern birds and their closest extinct relatives have short tails and pygostyles. A pygostyle is a fused section of tail vertebrae that support the tail feathers. Because this new specimen has a long tail, we know that it belonged to a non-avian coelurosaur.

peregrine_pygostyle-eyton-1867

A bird skeleton (Peregrine falcon) showing the pygostyle in green. By Eyton 1867.

Let’s detour for a moment and talk about feathers. Modern bird feathers have an assortment of shapes and functions. Down feathers, what baby birds are initially covered with, have tufts of rami (ramus – singular, means ‘branch’) that are used to insulate the bird. As the bird grows, it develops body feathers. These are symmetrical. They have a central shaft (called a rachis), and branching rami on each side. Each ramus has barbules (little hooks), and each barbule has little hooks so that the rami can stick together, like Velcro. We have specimens of dinosaurs that show each of these stages, and we know how feathers grow in living birds, so we understand their evolution and development very well.

fig-4c

Figure 4c from the paper showing feather development. Blue is barbs, purple is the rachis, red is the barbules. The circled one is what the new specimen has.

This specimen has feathers with a short rachis, a central ramus with branches, and branches with barbules. Not quite a modern feather, but not a very simple feather either. It’s intermediate. If the whole tail had these feathers, though, the authors do not think this animal would have been able to fly. The feathers are darker on the top of the tail (probably a chestnut brown) and white or pale on the bottom of the tail.

close-up-of-feathers-l-xing

A closeup of the feathers of the specimen. Photo by L. Xing.

Altogether, this specimen represents a juvenile coelurosaur, with adult feathers, that probably could not fly. Hopefully we will continue to find specimens in amber so that we can better understand these dinosaurs from Myanmar.

Bird Wings in Amber!

This week an exciting new article was [published]: two bird wings trapped in amber! The specimens were found in Myanmar.

map of myanmar

Myanmar is in Southern Asia.

Amber is what happens when tree sap is fossilized. Anything that was stuck in the tree sap while it was a liquid remains preserved in the amber forever.

Phil ant

Two ants fighting in amber. Photo by Phil Barden and Dave Grimaldi.

The sediments are mid-Cretaceous in age, therefore from the Mesozoic. The fossils are 2 partial wings and are thought to be from enantiornithine birds. These birds are an entirely extinct radiation of early birds that still have teeth.

elliot Enantiornthine

Elliot the enantiornithine from Dinosaur Train (from PBS).

Usually, enantiornithines and other early birds are preserved entirely flat. Even though we sometimes get feather impressions, we lose a lot of information because the specimens are so squished.

parabohaiornis

Parabohaiornis, an enantiornithine preserved flat (from Wikipedia).

These new specimens are preserved in 3D! And the part of the wing that got stuck is completely intact.

image j

Figure 1j from the paper showing one of the wings.

From the bone proportions, we know these two specimens were juveniles. The preserved feathers are very developed, indicating that these juveniles were precocial (able to move around well as soon as they hatch). Because the feathers are so well preserved, we can see what color they were (brown and pale/white, probably in bands across the wing). Lastly, one of the specimens has claw marks within the amber, showing that the bird was still alive when it got trapped. This specimen is so rare and incredibly preserved that it will help answer a lot of questions about feather morphology and preservation in other enantiornithines and early birds in general. Because of these new fossils, we know that the modern layout of feathers was already present 100 million years ago.

e and f

Images 1e and 1f from the paper showing the details of the feathers.

web_Bird by Cheung Chung-tat

Artist rendition of the bird getting trapped (by Chung-tat Cheung)