Feathers finest moments..

When we think of feathers we often associate them with cute and cuddly things, like this: 

Baby Albatross

or this: 

super cute duckling

or cool things like this: 

Soaring eagles 

or comfy things like this: 

mmmm bed...

but feathers are rarely thought of in association with this: 

dinosaurs? whaaaaat!?

But this last picture is what i think is the coolest! And the evolution of feathers in dinosaurs was definitely their finest moment! But feathers on dinosaurs were not for the mechanism of flight, so what were they used for? Many theories are argued today including sexual dimorphism (differences in colour between make and female), camouflage, sexual selection (the prettier you look the more likely to find a mate), and heat retention. However, to be associated with heat retention it also comes in combination with endothermy (warm bloodedness), but dinosaurs are commonly thought of as cold blooded animals. 

Feathers evolved first in Theropods. Theropods were carnivorous dinosaurs that ruled the Earth from the beginning of the Jurassic Period to the end of the Cretaceous. They were the dominant predators on every continent during this time, however, there was also some omnivore and herbivore theropods. They include the largest carnivores ever recorded, and also some very small theropods. These smaller groups included those that evolved feathers and are the predecessors of birds today (Brett-Surman et al, 2012).

Theropod fossils have been found in various locations world wide, however, none compare to the standard of the fossils found in North Eastern China, at the Lower Cretacious Yixian and Jiufotang Formations, that together make the Jehol Group. The Jehol Group shows an exceptionally wide range of fossils that are extremely well preserved, giving us almost an entire Cretacious ecosystem and further insight into the relationship between Theropods and birds. These fossils include Theropod dinosaurs, early birds, mammals, invertebrates and a diverse flora, including the oldest flowering plant. 

The Jehol group was deposited in freshwater lakes, with very low energy and no tidal influence, so the fossils were generally unbroken and well preserved. They are deposits of fine shales and sandstones, including segments of fine volcanic ash, or tuffs, that are occasionally crosscut with dykes and sills. The high levels of volcanic activity provided the ideal environment for fossil preservation as the dead organism would fall into the lake and rapidly be covered in volcanic ash falls, creating an anoxic environment (an environment with no oxygen present) around the organisms, which inhibited bacterial decay and also other burrowing organisms from disturbing the remains. The age of the Jehol group is somewhat debated, with different ages ranging from the Late Jurassic to Early Cretaceous depending on biostratigraphy dating or radiometric dating of the volcanic sediments but the general excepted age is in the middle early Cretaceous. 

Theropod fauna preserved here are very significant as they are some of the very few dinosaur fossils in which their feathers have been preserved to such a high standard. Feathers are rigid branched structures, formed around a hollow, keratin shaft and exhibiting a complex branched structure. Until recently, these were presumed to be an elongation of broad, flat scales, and that their evolution was driven through the selection for gliding or flapping flight. However, evidence from fossils in the Jehol Group has shown that early feathers were not elongated scales but rather started as a single filament-like structure, evolving to more complex branched structures and a true feather appearance at a later stage.

These fossils have become the verification of the hypothesis that birds are descendents of dinosaurs, specifically Theropod dinosaurs. This was already accepted, to an extent, as dinosaur bones and bird bones are so similar but now it is more tangible as their feather structure is also strikingly similar.

The Jehol group has a diverse range of Theropod fauna, with fossils showing filamentous structures, considered to be the precursors to true feathers. Also others that have feathers where a central rachis supports a branching pattern of barbs, which seem to be of a modern appearance. However these animals had clearly not yet breached the mechanism of flight as they lacked certain flight specialisations needed today such as, asymmetrical feathers and a large sternum.

That leaves the question of why feathers evolved if not for flight? Today bird feathers are known to have a diverse array of functions apart from flight, such as camouflage, sexual dimorphism, warmth and sexual selection, so one could postulate that these functions were the original use of early feathers. Let's look further into some of these theories... 

Sexual dimorphism has been postulated due to the evidence from the well preserved, and abundant fossils of the Confuciusornis sanctus in the Jehol Group. Some specimens show ornamental rectrices, which represent tail feathers, and some without, indicating a presence or absence of long tail feathers in the same species. Today we see long tail feathers showing sexual dimorphism in peacocks, where the males have the long and beautiful tail. Recovery of bone tissue from these fossils show what is known to be a unique bone tissue to reproductively active females, the medully bone. The Medully bone is a reservoir for egg-shell calcium and is the most oestrogen dependant of all bone types. This bone tissue was found in accordance with the absence of long tail feathers, therefore leading to the conclusion that early feathers were in fact utilized for sexual dimorphism, where a long tail in the dinosaurs was a sign of a male, just like peacocks today! 

Male Peacock

Female peacock - called the peahen. 

Evidence for sexual selection or camouflage has been found too. Melanin containing organelles, called melanosomes, have been found and contain key information in determining the feather colour. Samples from one specimen indicated varying colours over the Theropods body, suggesting a pretty pattern for sexual selection or colour variation for camouflage.  Today feather colouration is used for display, communication, or camouflage in birds, whether that be for sexual selection or as a defence mechanism, early feathers may have also had these functions before they became a mechanism of flight.

The bird of paradise attracting its mate with pretty feather colours that even look like a smiley face, evolution - you rock! 

Feathers may have also been used in heat retention. Today, many birds depend on feathers for their great heat retention properties (like penguins!), which leads to the postulation that this is another trait in which feathers evolved for in the dinosaurs, however this is a heavily debated theory. Dinosaurs were supposedly cold blooded, ectotherms, and like ectothermic reptiles today would sun themselves to reach normal body temperatures rather than covering themselves with something warm, such as feathers. It has been postulated that the Theropods were in fact warm blooded endotherms, and the presence of these feathers are an indication of that.

Before going into this argument, what is endothermy really? Endothermy is defined by having elevated rates of metabolism, about 5 – 15 times greater than that of ectotherms. This is the main heat source and enables endotherms to maintain stable body temperatures. Most endotherms also have a greater stamina for physical activity than ectotherms with a higher rate of lung ventilation and oxygen consumption, while ectotherms are capable of short bursts of high energy, endotherms are able to sustain this energy, to levels well beyond that of ectotherms. Therefore, evolution towards endothermy was extremely valuable as it provides many more benefits than ectothermy.

So the postulation seems valid and plausible that evolution would move in this direction, however, the presence of feathers on Theropod fossils is not enough evidence to assume that the feathers function was to retain heat and just presume endothermy. Some extant birds today will sun themselves in the morning to warm in an ectothermic way to reach normal body temperatures. For example, the roadrunner, although having a fully feathered body, has a temperature drop at night in low ambient temperatures, causing its body temperature to drop by up to 4°C and then after sunrise it will raise its feathers to expose poorly feathered parts of its body and warm ectothermicaly to a normal body temperature.

True evidence of endothermy is difficult to come by in fossil taxa as it is almost exclusive to soft tissue anatomy, which is usually nonexistent in the fossil record. To prove endothermy researchers established signals of growth rates of fossils by assessing their bone types, because fast growth rates are somewhat associated with fast metabolic rates. However this theory was disproven due to flawed correlation of certain bone types with any particular growth rate. Another argument was oxygen isotopes in bone phosphate interpreted body temperatures, however due to uncertainties of changes in isotopes during fossilization process and flawed assumptions, this was too disproven. Several other methods have relied on evidence from brain size, heart size, skeletal posture and so on, but have also been disputed.

Evidence has been found to support the claim that they were in fact ectothermic animals due to the absence of respiratory turbinates (things that endotherms have). These are found in the nasal cavities of endotherms but the narrow nasal cavities of ectotherms mean there is simply not enough room for turbinates in the nasal cavity. With CT scans of the Theropod nasal cavity it was found that they were also too narrow to possess turbinates, and in addition had large sinuses, limiting space in the nasal cavity to an even further extent. This theory seems more credible, to date, and leads to the postulation ventilation rates were at ectothermic rates in Theropods.

What feathers actually evolved for is a heavily debated topic. However, my personal opinion is that feathers did evolve before flight for use in the many other functions that they provide today, as discussed above. I think that endothermy evolved after flight evolved, with selection for higher physical stamina for longer flights. However, in saying that, I think that the presence of feathers would have definitely had an impact on the thermoregulatory strategies of these animals and played an important role in the evolution of endothermy. Nevertheless, feathers were present in dinosaurs, which were predecessors of birds.. so in a way we are still living amongst dinosaurs (descendants)! 

Thanks for reading, 

Rach