Feather evolution

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Senckenberg Museum, Frankfurt, Germany – Psittacosaurus

The evolution of dinosaur integument and particularly feathers is a highly studied topic in vertebrate paleontology. I, too, am interested in feather evolution, but I try to approach the topic from the perspective of taphonomy.

I was a part of a team that used laser stimulated fluorescence to study the tail bristles on Psittacosaurus. We found that the morphology of the bristles, particularly the internal morphology, resembled bristles in modern birds whose structure and development is radically different from ‘true’ feathers. Thus, the filaments of non-avian dinosaurs were likely highly diverse and could have arisen multiple times in their evolution using shared fundamental developmental pathways.

The paper was published in Palaeontology:

Mayr, G., Pittman, M., Saitta, E., Kaye, T. G., & Vinther, J. (2016). Structure and homology of Psittacosaurus tail bristles. Palaeontology59(6), 793-802.

Abstract

Figure-4-A-the-'beard'-of-the-turkey-Meleagris-gallopavo-Galliformes-uncatalogued

Feather-like bristles in modern birds

We examined bristle-like appendages on the tail of the Early Cretaceous basal ceratopsian dinosaur Psittacosaurus with laser-stimulated fluorescence imaging. Our study reveals previously unknown details of these structures and confirms their identification as integumentary appendages. For the first time, we show that most bristles appear to be arranged in bundles and that they exhibit a pulp that widens towards the bristle base. We consider it likely that the psittacosaur bristles are structurally and developmentally homologous to similar filamentous appendages of other dinosaurs, namely the basal heterodontosaurid Tianyulong and the basal therizinosauroid theropod Beipiaosaurus, and attribute the greater robustness of the bristles of Psittacosaurus to a higher degree of cornification and calcification of its integument (both skin and bristles). Although the psittacosaur bristles are probably homologous with avian feathers in their origin from discrete cell populations, it is uncertain whether they developed from a follicle, one of the developmental hallmarks of true feathers. In particular, we note a striking resemblance between the psittacosaur bristles and the cornified spine on the head of the horned screamer, Anhima cornuta, an extant anseriform bird. Similar, albeit thinner keratinous filaments of extant birds are the ‘beard’ of the turkey, Meleagris gallopavo, and the crown of the Congo peafowl, Afropavo congensis. All of these structures of extant birds are distinct from true feathers, and because at least the turkey beard does not develop from follicles, detailed future studies of their development would be invaluable towards deepening our understanding of dinosaur filamentous integumentary structures.

The project was highlighted by the University of Hong Kong and the Senckenberg Museum.

I was also a coauthor on the following paper in Palaeontology that criticized the interpretation of filamentous structures in non-avian dinosaurs and ichthyosaurs as preserved collagen fibers, highlighting that such filaments in dinosaurs are primitive feathers:

Smithwick, F. M., Mayr, G., Saitta, E. T., Benton, M. J., & Vinther, J. (2017). On the purported presence of fossilized collagen fibres in an ichthyosaur and a theropod dinosaur. Palaeontology60(3), 409-422.

Abstract

Since the discovery of exceptionally preserved theropod dinosaurs with soft tissues in China in the 1990s, there has been much debate about the nature of filamentous structures observed in some specimens. Sinosauropteryx was the first non-avian theropod to be described with these structures, and remains one of the most studied examples. Despite a general consensus that the structures represent feathers or feather homologues, a few identify them as degraded collagen fibres derived from the skin. This latter view has been based on observations of low-quality images of Sinosauropteryx, as well as the suggestion that because superficially similar structures are seen in Jurassic ichthyosaurs they cannot represent feathers. Here, we highlight issues with the evidence put forward in support of this view, showing that integumentary structures have been misinterpreted based on sedimentary features and preparation marks, and that these errors have led to incorrect conclusions being drawn about the existence of collagen in Sinosauropteryx and the ichthyosaur Stenopterygius. We find that there is no evidence to support the idea that the integumentary structures seen in the two taxa are collagen fibres, and confirm that the most parsimonious interpretation of fossilized structuresthat look like feather homologues in Sinosauropteryx is that they are indeed the remains of feather homologues.

I have published a description of feathers in non-avian dinosaurs with a particular focus on primitive traits in such feathers as well as the evolution of contour feathers – revealing a novel, extinct morphotype. Such observations promise to change the way we depict dinosaurs, particularly paravians, and understand their biology, and part of the project involved advising a commissioned piece by scientific illustrator Rebecca Gelernter. A poster presentation for this project was given at the 2016 annual meeting of the Palaeontological Association, available through Researchgate. The paper was published in Palaeontology:

Saitta, E. T., Gelernter, R. & Vinther, J. 2017. Additional information on the primitive contour and wing feathering of paravian dinosaurs. Palaeontology.

Figure9 (Full Page, smaller file).jpg

Anchiornis reconstruction I advised scientific illustrator Rebecca Gelernter on showing the novel fossil contour feather type described in the paper

 

Abstract

Identifying feather morphology in extinct dinosaurs is challenging due to dense overlapping of filaments within fossilized plumage and the fact that some extinct feather morphologies are unlike those of extant birds or those predicted from an ‘evo-devo’ model of feather evolution. Here, we compare a range of dinosaur taxa with preserved integumentary appendages using high-resolution photographs to better understand fossil feather morphology and gain insight into their function and evolution. A specimen of the basal paravian Anchiornis possesses contour feathers disarticulated from the plumage, revealing a novel feather type: a ‘shaggy’, open-vaned, bifurcated feather with long barbs attached to a short rachis, which is much simpler than the contour feathers of most extant birds. In contrast, it is likely that the contour feathers of Sinosauropteryx were simpler than those seen in Anchiornis; a ‘tuft’ morphology of multiple barbs connected at their bases (e.g. via a shared follicle), but lacking a rachis, is tentatively inferred. However, conclusive morphological descriptions await the discovery of isolated Sinosauropteryxcontour feathers. Paravian wing feathers also show potentially plesiomorphic traits. Comparison with Confuciusornis suggests that Anchiornis wing feathers were at least partially open-vaned. Combined with the interpretation of Anchiornis contour feathers, this suggests that differentiated barbicels are relatively derived compared to pennaceous feathers and the appearance of wings. ‘Shaggy’ contour feathers probably influenced thermoregulatory and water repellence abilities, and, in combination with open-vaned wing feathers, would have decreased aerodynamic efficiency. Simplified, open-vaned wing feathers were also observed on the oviraptorosaur Caudipteryx, consistent with, but not necessarily diagnostic of, its suggested flightlessness. Taken together, these observations have broad implications for how we depict a wide variety of dinosaurs and how we view the function and evolution of feathers.

Furthermore, I am designing an experiment to quantify the relationship between material properties and the degree of calcification in various modern keratinous tissues. This project is being advised by Dr. Emily Rayfield.

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Psittacosaurus tail bristles