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Iridescence is responsible for some of the most striking visual displays in the animal kingdom. Now, thanks to a new study of feathers from almost 100 modern bird species, scientists have gained new insights into how this colour diversity evolved.

Two of the fossils sampled for the study with the fossil melanosomes found in each fossil (scanning electron microscope images). Scaniacypselus to the left and Primotrogon to the right. Melanosome shape varies in the different samples, and is indicative of colour. The sample furthest to the left was predicted as iridescent [Credit: Jakob Vinther & Fiann Smithwick]

Iridescence refers to the phenomena where colour changes when an object is viewed from different angles. Birds produce this varying coloration in their feathers by using nanoscale arrays of melanin-filled organelles (melanosomes) layered with keratin. In this form of structural colouration, the shapes of melanosomes together with the thickness of keratin layers determine what colour is produced.

While melanosome morphology has previously been used to predict colour in fossil animals, melanosome variation in iridescent feathers has not been analysed on as large a scale until this study.

As reported in the journal Evolution, a team of University of Bristol researchers used scanning electron microscopy to quantify melanosome extracts from the feathers of 97 species of modern birds with iridescent plumage, taken from the collections of the Zoological Museum of Copenhagen.

The study showed that iridescent feathers contain the most varied melanosome morphologies of all types of bird coloration sampled to date. Unlike black, grey and brown feathers that always contain solid melanosomes, iridescent feathers can contain melanosomes that are hollow and/or flattened.

Scaniacypselus fossil (above) compared with its modern day equivalent, the Plume-toed Swiftlet [Credit: Fossil: Jakob Vinther & Fiann Smithwick/Daniel Field]

"We found that melanosomes in modern iridescent feathers are more diverse in shape than those found in grey, black or brown feathers combined (that also contain melanosomes)," said lead author Klara Nordén, who conducted the study during her undergraduate years at Bristol's School of Earth Sciences. "It is already known that structural colouration is responsible for 70 per cent of the colour variability in birds. These two facts might be coupled -- birds evolved varied forms of melanosomes to achieve ever greater diversity in colour.

"I wanted to find out if we could improve current predictive models for fossil colour based on melanosome morphology by including all types of melanosomes found in iridescent feathers."

Dr Jakob Vinther, co-author of the study and a leading researcher in the field of paleocolour at Bristol's School of Biological Sciences, had already collected the perfect fossil samples to test the new model on.

"We had sampled Scaniacypselus, related to modern tree swifts, and Primotrogon, ancestor to modern trogons. These groups are iridescent today and have flat and hollow melanosomes. Did their 48-million-year-old ancestors from Germany also have iridescent plumage?"

Primotrogon fossil (above) compared with its modern day equivalent, the Narina Trogon [Credit: Fossil: Jakob Vinther & Fiann Smithwick/Daniel Field]

Interestingly, the model predicted that Primotrogon probably was iridescent, but it used solid rather than hollow melanosomes, unlike its modern descendants.

"This demonstrates how we now have the tools to map out the evolution of iridescence in fossil lineages," said Klara, who is now a PhD student at Princeton University. "It opens the door to many new discoveries of dazzling displays in fossil birds and other dinosaurs."

The current study focused on mapping out how melanosomes vary in iridescent feathers. Further avenues of research might examine why birds utilise such diversity of melanosome types in iridescent feathers. These insights could ultimately enhance our understanding of why fossil birds or dinosaurs might have used such morphologies, revealing something about their behaviour.

Source: University of Bristol [December 10, 2018]

A team of scientists led by Jingmai O'Connor from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP), Chinese Academy of Sciences, reported the first occurrence of medullary bone in Enantiornithes, the dominant clade of birds during the Cretaceous.

Main slab of Pengornithid Enantiornithine, preserved in three-dimensions unlike most compression fossils  from the Jehol Biota. Scale bar is one centimetre [Credit: Jingmai O'Connor]

Medullary bone is a bone tissue unique to birds today. It is present only in females about to lay eggs and forms in the empty spaces within the skeleton. This bone tissue serves as a reservoir for calcium needed to form the eggshell. Medullary bone has been reported in a variety of non-avian dinosaurs including Tyrannosaurus rex, ornithopod dinosaurs like Tenontosaurus, and several sauropods (huge long-necked dinosaurs) including Mussasaurus. It has also been identified in pterosaurs, which are flying reptiles closely related to dinosaurs.

Since the first report of medullary bone in a Mesozoic fossil in 2005, this tissue has attracted great interest because it links birds and dinosaurs. However, the presence of this bone tissue in pterosaurs and non-avian dinosaurs is perplexing. Non-avian dinosaurs were so large and their eggs so small that they shouldn't have required medullary bone. Since pterosaurs laid soft-shelled eggs, they also shouldn't have required medullary bone.

Cross section of the femur bone viewed under polarized light showing the thick layer of medullary bone within the medullary cavity [Credit: Jingmai O'Connor]

Some reported instances of medullary bone are probably actually bone pathologies causing abnormal growth. However, in this report, IVPP scientists, working together with Mark Norell from the American Museum of Natural History and Greg Erickson from Florida State University, argued that no previous description of medullary bone in a Mesozoic reptile was well supported.

The new report is the best support for medullary bone in the Mesozoic so far since it was found throughout the entire preserved skeleton, suggesting it was part of a system-wide process rather than a local pathology. However, the authors concede that scientists still know too little about medullary bone to confirm, without additional evidence (e.g., association with a nest or eggs), that the fossilized individual with this tissue was reproductively active.

In light of the currently available evidence, medullary bone might have been an entirely avian feature even in the Mesozoic. It evolved as a result of the thinned, hollow bones in birds, which lightened the skeleton for flight, as well as their increased egg size.

The findings were published in Nature Communications.

Source: Chinese Academy of Sciences [December 05, 2018]