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Source: National Geographic |
Transitional fossils are the bugbear of creationism. Every fossil ever found, although so obviously intermediate between its parent generation and its offspring, and so obviously a sample of the evolutionary status of its particular branch at that point in time, never-the-less has to be vigorously dismissed as 'not transitional' by creationists.
It's a cardinal article of creationist doctrine that there are no transitional fossils, therefore, that no matter how obviously transitional, it can't be transitional. It mustn't be allowed to be transitional because to admit to transitional fossils is to admit to evolution.
So it is particularly pleasing to be able to present yet another example of an undoubtedly transitional fossil preserved in amber in amazing detail. It is particularly pleasing too that this is a transitional fossil of that iconic class of animals, dinosaurs.
Readers will no doubt recall how creationist frauds such as Ken Ham and Kent Hovind like to tell their dupes that dinosaurs and humans co-existed, so the vast amount of data showing how the small number of dinosaurs that survived the mass extinction at the end of the Cretaceous evolved into today's birds is a major embarrassment for them, and that has to be a good thing.
This 99 million year-old transitional fossil provides yet more evidence. It is an exquisitely preserved tail of a small hatchling dinosaur, and it is covered in the down feathers that can be seen on any avian hatchling. It was found in an amber market in Myanmar and has been investigated by a team of Chinese and Canadian scientists led by Lida Xing of the China University of Geosciences, Beijing, China.
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| Photomicrographs of DIP-V-15103 Plumage (A) Pale ventral feather in transmitted light (arrow indicates rachis apex). (B) Dark-field image of (A), highlighting structure and visible color. (C) Dark dorsal feather in transmitted light, apex toward bottom of image. (D) Base of ventral feather (arrow) with weakly developed rachis. (E) Pigment distribution and microstructure of barbules in (C), with white lines pointing to pigmented regions of barbules. (F–H) Barbule structure variation and pigmentation, among barbs, and ‘rachis’ with rachidial barbules (near arrows); images from apical, mid-feather, and basal positions respectively. Scale bars, 1 mm in (A), 0.5 mm in (B)–(E), and 0.25 mm in (F)–(H). |
Highlights
- The first non-avialan theropod fragments preserved in amber are described
- Vertebral outlines, curvature, and plumage suggest a source within Coelurosauria
- Branching structure in the feathers supports a barbule-first evolutionary pattern
- Iron within carbonized soft tissue suggests traces of original material are present
Summary
In the two decades since the discovery of feathered dinosaurs [1, 2, 3], the range of plumage known from non-avialan theropods has expanded significantly, confirming several features predicted by developmentally informed models of feather evolution [4, 5, 6, 7, 8, 9, 10]. However, three-dimensional feather morphology and evolutionary patterns remain difficult to interpret, due to compression in sedimentary rocks [9, 11]. Recent discoveries in Cretaceous amber from Canada, France, Japan, Lebanon, Myanmar, and the United States [12, 13, 14, 15, 16, 17, 18] reveal much finer levels of structural detail, but taxonomic placement is uncertain because plumage is rarely associated with identifiable skeletal material [14]. Here we describe the feathered tail of a non-avialan theropod preserved in mid-Cretaceous (∼99 Ma) amber from Kachin State, Myanmar [17], with plumage structure that directly informs the evolutionary developmental pathway of feathers. This specimen provides an opportunity to document pristine feathers in direct association with a putative juvenile coelurosaur, preserving fine morphological details, including the spatial arrangement of follicles and feathers on the body, and micrometer-scale features of the plumage. Many feathers exhibit a short, slender rachis with alternating barbs and a uniform series of contiguous barbules, supporting the developmental hypothesis that barbs already possessed barbules when they fused to form the rachis [19]. Beneath the feathers, carbonized soft tissues offer a glimpse of preservational potential and history for the inclusion; abundant Fe2+ suggests that vestiges of primary hemoglobin and ferritin remain trapped within the tail. The new finding highlights the unique preservation potential of amber for understanding the morphology and evolution of coelurosaurian integumentary structures.
Xing, Lida et al.
A Feathered Dinosaur Tail with Primitive Plumage Trapped in Mid-Cretaceous Amber
Current Biology , Volume 0 , Issue 0 DOI: http://dx.doi.org/10.1016/j.cub.2016.10.008
© 2016 Elsevier Ltd. Published open access
Unlike the usual fossilisation where remains are literally petrified by (normally) hard structures being replaced by minerals and becoming embedded in rock as the sediment turns to rock oer time, amber fossils are quite different. Amber is the hardened remains of resin which oozes from natural cracks in the bark of pine trees forming blobs and larger accumulations in suitable depressions. It is extremely sticky when fresh and quite capable of trapping insects, and, as in this case small vertebrates.
We can be sure of the source because the vertebrae are not fused into a rod or pygostyle as in modern birds and their closest relatives, instead, the tail is long and flexible, with keels of feathers running down each side.
Specimens trapped in amber are quickly sealed and any bacteria of fungal spores on them are deprived of oxygen so decay is limited. The tissues are impregnated with resin rather than being replaced by it so fine structure is preserved - something that only being covered quickly in fine silt could do with the normal fossilisation. In this case, so well preserved are the feathers that pigmentation can be seen. Additionally, by the time any weight of sediment builds up above amber it is relatively hard and resists to the compression and flattening seen in normal sedimentary fossils, so 3D structure is also well preserved. It's amazing to see all the details of a dinosaur tail - the bones, flesh, skin, and feathers - and to imagine how this little fellow got his tail caught in the resin, and then presumably died because he could not wrestle free.
And what do we see in this preserved tiny dinosaur tail?What we see is a bony vertebrate skeletal tail covered with skin from which are growing feathers indistinguishable from those of bird chicks. Had it not been for the fact that this is a long vertebrate tail, the specimen might well have been mistaken for a fossilised bird. It is undoubtedly that of a feathered dinosaur showing that feathers were present in the pre-avian and proto-avian ancestors of birds.
Creationists will undoubtedly perform all manner of intellectual gymnastics to deny these facts. In fact, before she rage-quit a Facebook group yesterday, one such creationist simply claimed it must be a forgery because transitional dinosaur-birds never existed. Mind you, she insisted that all fossils are forgeries
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