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.

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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.

The study was covered by several media outlets including The New York Times and Smithsonian Magazine.

I have published an experimental study where bird carcasses were sub-aqueously buried and compacted in a hydraulic press in order to simulate burial. Contrary to previous studies that claimed that compaction could lead to the clumping of feathers to resemble simpler, more primitive structures, our results show that the sediment maintains the shape of the feather through compaction, meaning that the structure of fossil feathers can be interpreted without concern for this proposed burial bias. The paper was published in PalZ.

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Results of finch compaction experiment

 

Abstract

‘Exceptional fossils’ of dinosaurs preserving feathers have radically changed the way we view their paleobiology and the evolution of birds. Understanding how such soft tissues preserve is imperative to accurately interpreting the morphology of fossil feathers. Experimental taphonomy has been integral to such investigations. One such experiment used a printing press to mimic compaction, done subaerially and without sediment burial, and concluded that the leaking of bodily fluid could lead to the clumping of feathers by causing barbs to stick together such that they superficially resemble simpler, less derived, filamentous structures. Here we use a novel, custom-built experimental setup to more accurately mimic subaqueous burial and compaction under low-energy, fine-grain depositional environments applicable to the taphonomic settings most plumage-preserving ‘exceptional fossils’ are found in. We find that when submerged and subsequently buried and compacted, feathers do not clump together and they maintain their original arrangement. Submersion in fluid in and of itself does not lead to clumping of barbs; this would only occur upon pulling feathers out from water into air. Furthermore, sediment encases the feathers, fixing them in place during compaction. Thus, feather clumping that leads to erroneously plesiomorphic morphological interpretations may not be a taphonomic factor of concern when examining fossil feathers. Our current methodology is amenable to further improvements that will continue to more accurately mimic subaqueous burial and compaction, allowing for various hypothesis testing.

A reanalysis of the chemistry of feather fibers from the dinosaur Shuvuuia reveal that, contrary to a 1999 study that used antibodies to conclude keratin protein preservation, the fibers are 1) inorganic, 2) made of calcium phosphate (and therefore could represent the calcified shaft of a more complex, branching feather rather than simple hair-like filaments), and 3) were covered in cyanoacrylate glue which can adsorb antibodies and cause false positives. Immunochemistry is likely not an ideal tool to study fossils. The paper was published in Organic Geochemistry.

Saitta, E.T., Fletcher, I., Martin, P., Pittman, M., Kaye, T.G., True, L.D., Norell, M.A., Abbott, G.D., Summons, R.E., Penkman, K. and Vinther, J., 2018. Preservation of feather fibers from the Late Cretaceous dinosaur Shuvuuia deserti raises concern about immunohistochemical analyses on fossils. Organic Geochemistry.

Abstract

White fibers from a Late Cretaceous dinosaur Shuvuuia deserti stained positive for β-keratin antibodies in a 1999 paper, followed by many similar immunological claims for Mesozoic protein in bones and integument. Antibodies recognize protein epitopes derived from its tertiary and quaternary structure, so such results would suggest long polypeptide preservation allowing for sequencing with palaeobiological implications. However, proteins are relatively unstable biomacromolecules that readily hydrolyze and amino acids exhibit predictable instability under diagenetic heat and pressure. Furthermore, antibodies can yield false positives. We reanalyzed a Shuvuuia fiber using focused ion beam scanning electron microscopy, energy-dispersive X-ray spectroscopy, time-of-flight secondary ion mass spectrometry, and laser-stimulated fluorescence imaging, finding it to be inorganic and composed mainly of calcium phosphate. Our findings are inconsistent with any protein or other original organic substance preservation in the Shuvuuia fiber, suggesting that immunohistochemistry may be inappropriate for analyzing fossils due to issues with false positives and a lack of controls.

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Shuvuuia feather fibers consisting of calcium phosphate.

Furthermore, I am designing an experiment to quantify the relationship between material properties and the degree of calcification in various modern keratinous tissues.

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Psittacosaurus tail bristles