#9 of Our Top Stories of 2016: Feathers on a Bird or Dinosaur Tail?
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The following was originally published on December 9, 2016:
A section of a 99-million-year-old feathered tail discovered in a chunk of amber in Myanmar belongs to a juvenile dinosaur, a coelurosaur, not a bird. The media are certain of it. The actual evidence is more ambiguous. National Geographic pounds the pulpit for a feathered dino:
First Dinosaur Tail Found Preserved in Amber
The tail of a 99-million-year-old dinosaur, including bones, soft tissue, and even feathers, has been found preserved in amber, according to a report published today in the journal Current Biology.
While individual dinosaur-era feathers have been found in amber, and evidence for feathered dinosaurs is captured in fossil impressions, this is the first time that scientists are able to clearly associate well-preserved feathers with a dinosaur, and in turn gain a better understanding of the evolution and structure of dinosaur feathers.
The semitranslucent mid-Cretaceous amber sample, roughly the size and shape of a dried apricot, captures one of the earliest moments of differentiation between the feathers of birds of flight and the feathers of dinosaurs.
The presence of articulated tail vertebrae in the sample enabled researchers to rule out the possibility that the feathers belonged to a prehistoric bird. Modern birds and their closest Cretaceous ancestors feature a set of fused tail vertebrae called a pygostyle that enables tail feathers to move as a single unit.
The reason that they claim it is a dinosaur and not a bird is because this is supposedly from a "long-tailed" individual. Most birds with bony tails are short-tailed, meaning they have fewer vertebrae in their tail compared to dinosaurs. Under ten vertebrae, perhaps only six or seven, would be the norm for a bony-tailed bird.
So what do they think about the number of vertebrae in this amber fossil? They say the living individual "likely" had more than 25 vertebrae.
But this is a matter of inference, because, as the paper admits, the soft tissue in the fossil is so undifferentiated from the bone that it is very difficult to tell what is tissue and what are vertebrae. As the paper puts it in technical language:
SR X-ray μCT scanning of DIP-V-15103 revealed that soft tissues have a density insufficiently different from the partially replaced skeletal elements to permit X-ray imaging and virtual dissection of osteology alone. Consequently, many diagnostic and comparative osteological details remain obscured.
At most, they can see clear evidence of only two vertebrae in this tail. From that they extrapolate that it had more than 25:
However, two vertebrae are clearly delineated ventrally (Figures 1F-1H). Extrapolating lengths of these vertebrae, the preserved tail section contains at least eight full vertebrae and part of a ninth.
So when they extrapolate from the two visible vertebrae, they get nine. That would certainly not preclude the creature from being a bird. In fact, some fossil birds had up to 20 to 23 vertebrae. As another paper states:
Archaeopteryx retained an ancestral caudal vertebral count of between 20 and 23. The next most basal bird, Jeholornis, from the Jiufotang Formation of China and dated at approximately 120 million years old, was also long-tailed, and had 22 caudal vertebrae that are nearly identical to those of Archaeopteryx.
If it did have up to 20 to 23 vertebrae, it could still be a bird. Yet again it's not clear exactly how many vertebrae this tail had because too much of it is missing.
The paper goes on:
Even with the skin adpressed to the bony surface, no features other than the grooved ventral sulci of two centra are clearly visible. This lack of topography suggests that the vertebrae lack prominent neural arches, transverse processes, or hemal arches. Therefore, the preserved segment is only a small mid to distal portion of what was likely a relatively long tail, with the total caudal vertebral count not reasonably less than 15, and likely greater than 25.
So they claim that the shape of the vertebrae suggests that what they are seeing is from the middle of the tail, and the actual tail was much longer. They admit that most features are not "clearly visible." Given that you can barely distinguish soft tissue from bone in this fossil, their inferences go too far.
Indeed while researcher Ryan McKellar tells Science Daily that "the feathers definitely are those of a dinosaur not a prehistoric bird," the paper itself is more circumspect, saying "it can likely be excluded from the long-tailed birds" (emphasis added). The Supplemental Information that goes with the paper includes the notable statement:
[T]here is a distinctive ventral groove on the caudal centra of the specimen, which is widely distributed among non-avialan theropods but which has yet to be reported in avialans (though the possibility of its presence in the two known long-tailed birds Archaeopteryx and Jeholornis cannot be excluded).
As a final note, one could argue that the age of the fossil (about 99 Ma) suggests that this individual lived long after known long-tailed bony-tailed birds. According to this paper, the most recent long-tailed bony-tailed bird lived about 122 million years ago.
That would be a very weak argument, however, since evolutionists regularly tolerate chronological gaps in the fossil record much greater than 20 million years. In fact they do this in their dinosaur-to-bird hypothesis.
After all, the dinosaurs that they claim supposedly gave rise to birds lived about 125 million years ago, but true birds like Archaeopteryx lived as far as about 150 million years ago. That's a major time gap. So if one were to propose that perhaps the long-tailed birds known from ~122 million years ago survived until ~99 million years ago, you would not be extrapolating fossil lineages any more than they already do.
In any case, all praise is due to the researchers for discovering and presenting this beautiful fossil with its unmistakable feathers. Probably no known feathers this old have ever been preserved in such gorgeous detail.
But unfortunately we have so little material, and the bone is so difficult to distinguish from the soft tissue, that any strong claims about this tail should be greeted with skepticism. This paper will not be the last word on the nature of the animal that sported this wonderful tail.
Photo credit: Ryan McKellar/Royal Saskatchewan Museum via Science Daily.