A New Baby Bird

This week, a [paper] came out describing a new baby enantiornithine. Enantiornithines are early birds that are closely related to the birds we see today, but part of a separate group. All enantiornithines went extinct at the end of the Cretaceous.

An enantiornithine. By S. Abramowicz.

This paper describes a fossil from the Early Cretaceous of Spain that preserves most of the skeleton. It is a remarkable specimen because it died around the time of birth. Because of its young age, it can give us a special glimpse of how the skeletons of enantiornithines developed in their lives.

Figure 1 from the paper showing the fossil. The head is up and the face is pointing to the right.

Of all the bones in this fossil, the sternum and the tail give us the most information into enantiornithine skeletons. The sternum is the large breastbone in birds that anchor the flight muscles. Enantiornithines also have a large sternum. This fossil shows that the sternum starts to ossify (or turn into bone) later than the other bones in the skeleton. It does this in a complicated pattern that is different from what we see in other enantiornithines and modern birds.

Figure 4g from the paper showing the ossification pattern of the sternum. It starts out as cartilage (grey) and it starts to ossify in the red, blue, and yellow sections. The bone grows out from there until the whole sternum is made of bone.

The tail in birds is usually fused into a bone called the pygostyle. In young birds, the vertebrae are still separate. This fossil has more separate vertebrae than the adult enantiornithines.

Figure 4 d and e from the paper showing a tail from an adult enantiornithine on the left and the baby on the right.

Both of these characteristics are slightly different than in other enantiornithines and in modern birds. This tells us that though very similar to modern birds on the outside, enantiornithines were developing their skeletons slightly differently. Ultimately, this could shed light on the different developmental strategies that we see in modern birds (how some can walk or fly at hatching and some take weeks or years to mature fully).

Figure 7 from the paper showing a reconstruction of the baby bird. The silhouettes show how big the baby would have been compared to a cockroach of the time. By R. Martín.

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.