Thoughts on Kulindadromeus’ integrument
Thoughts on Kulindadromeus’ integrument
So Kulindadromeus‘ paper is out, and so is the description of it’s feather and “scale” types. As Matt Martyniuk pointed out long before, bird “scales” are actually stunted feathers, and since filamental integrument is known to be present elsewhere in archosaurs (pterosaur pycnofibrils and alligator “feather genes”), this adds a layer of complexity to our understanding on the evolution of not just feathers, but sauropsid integrument as a whole. Kulindadromeus thus is not just an example of a feathered dinosaur as far away from birds as possible phylogenetically, it is also a window to the proccess of the conversion of feathers into scales, and thus perhaps an example of a process that goes as far back as the earliest archosaurs.
Godeforit et al. 2014 describes six different types of integrument in Kulindadromeus: three are filamental structures, identifiable as “feathers”, while another three are “scales”:
- Simple, hair like filaments, up to three centimeters long, that cover the torso, neck and head (possibly also most of the tail; see below). Similar to the simplest theropod feathers, Tianyulong‘s fuzz and pterosaur pycnofibrils, these are most likely the “original” integrument for archosaurs as a whole.
- What appears to be the “type 3″ feathers in the famous feather evolution schematics (already known from a few theropods), composed of six or seven filaments projecting from a single base plate (the “scales branching into feathers” that were originally advertised). The filaments are as long as 1.5 centimeters, the plates are organised in an hexagonal pattern but not overlapping. They cover primarily the upper arm and thigh, which in life most likely would have looked quite fluffy.
- An unique type of “feather”, composed of bundles of six or seven ribbon-like structures, in turn composed as as much as ten parallel filaments up to 0.1 millimeters wide, as long as two centimeters. By far the most complex integrumental structures seen in the animal, they are found exclusively on the upper lower legs, and almost certainly were display devices, their rarity and location implicating a minimal, if any, role in thermoregulation.
- Small (less than a millimeter in cross section), round, non-overlapping scales, covering the hands and feet (including the digitigrade ankles). These are classical examples of reticulate scales (reticulae), seen in modern birds, most non-avian dinosaurs (including ornithopods like hadrosaurs) and pterosaur foot pads, and are proven to be “stunted” simple feathers (in the case of pterosaurs, stunted pycnofibrils), as evidenced by their prevalence in foot pads and other dystal areas of the limb, less in need of insulation and more in need of protection.
- Overlaping hexagonal scales, up to 3.5 millimeters in diameter. They are found only in the lower shins.
- Another unique integrumental structure, a series of overlapping, rectangular scales that cover the upperside of the tail (these are the only “scales” known outside of the limbs, and based on the decomposition patterns in other feathered dinosaurs like Sinosauropteryx, this may mean that the rest of the tail was covered by feathers). They are as long as two centimeters, having a thickness of less than 0.1 millimeter, and a largely smooth surface with the exception of a small spur projecting forwards, that covers the trailing edge of the preceding scale. They are arranged in five longitudinal rows, imbricated thanks to their overlapping.
Feather complexity in the limbs vs simple body feathers
The first thing you’ll notice about Kulindadromeus‘ feathers is a dichotomy between those covering the body and those covering the limbs. The body coat is composed by the most simple form of feather known, while the limbs (an specific areas of the limbs at that) are the ones to display the more complex feathers. Given the priority of the torso over the limbs in terms of insulation, as well as the extremely restricted location of the “ribbon bundle” feathers, it seems very plausible that the complex limb feathers didn’t evolve in response to thermoregulatory needs.
Instead, it seems more likely that Kulindadromeus‘ branched feathers were used for ornamental purposes, particularly the “ribbon bundle” feathers, whose complexity and restricted location is consistent with the tendencies seen in other flamboyant dinosaur feathers. Thus, branched feathers originated as display devices, and only latter were reapropriated into an insulatory role. Their absence in the other known feathered ornithischians like Tianyulong and Psittacosaurusseems to give credence to this idea: in both of these animals, simpler quills are known, forming a dense coat in the former, while branched feathers are nowhere to be seen, in spite of their inferred usefulness as insulation devices.
This trend may have indeed been the norm for the whole of Dinosauria: in theropods as derived as Maniraptora, we often see simpler body plumage, while the limbs provide “wings” composed of more elaborate feathers. In Kulindadromeus, both the forelimbs and hindlimbs possess branched feathers, and we know that hindwings were probably present in theropods as basal asConcavenator (see Matt Martyniuk’s discussion on it’s pedal scutes), offering thus a further insight into the evolution of the wing.
“Scales”: stunted simple feathers, stunted dorsal scutes, and something else?
Kulindadromeus‘ “scales” are every bit worthy of discussion as the filamental feathers, unique as they are and as insightful into the evolution of archosaurian integrument. Kulindadromeus‘ presents clear reticulae in the feet and hands, “scales” that are proven to be stunted simple feathers/pycnofibrils, and seen most often in the foot pads of pterosaurs and theropods, as well as more extensively in the limbs of birds and across the whole body in hadrosaurs, ankylosaurs, sauropods and ceratopsians. Kulindadromeus, therefore, showcases an intermediary state between the inferred fully-fuzzy early ornithischians and (mostly?) featherless, reticulae covered derived forms: it’s hands and feet, less in need of insulation and in need of protection as the animal digs and forages on the ground, have stunted the dystal feathers into round, non-overlapping “scales”, while the rest of the body remains fluffy and warm. As seen in birds like owls, however, reticulae are easily reconverted into filamental feathers, so ornithischians most surely switched between reticulae and true feathers countless times across their evolutionary history.
More perplexing, however, are Kulindadromeus‘ scutes. These structures, thin and clearly keratinous, are not osteoderms like the “scutes” of crocodiles and ankylosaurs, representing a form of scale unique among sauropsids. Forming flat, overlapping surfaces, they are most easily comparable to the pedal scutes of birds, and indeed may share a similar origin. The other feathered ornithischians, Tianiyulong, Psittacosaurus and possibly Triceratops, all possess long, rigid quills across the back and tail upperside; given that the former’s a non-neornithischian and the latter two are ceratopsians, these structures must have been an ancestral condition for ornithopods according to phylogenetic bracketing, since ceratopsians are closer to ornithopods than heterodontosaurids are.
Could thus the scutes be simply stunted dorsal quills, like avian pedal scutes are stunted flight feathers? This seems most likely the case; phylogenetic bracketing aside, both structures are too similar to have their similarities dismissed, both being flat, overlapping “scales” derived from rigid quills. Equally interesting is the presence of a vestigial spur in Kulindadromeus‘ dorsal scutes, further implying their derivation from quills.
Of interest are the final group of “scales”, the hexagonal, overlapping structures in the lower shins. Are these stunted feathers as well? Reticulae seem to develop from both unbranched feathers and down, so Kulindadromeus‘ reticulae probably evolved from the branched limb feathers. Are these then true scales, a relic from Kulindadromeus‘ distant non-feathered ancestors? Only time will tell, though personally I don’t think so; we can’t even tell if archosaurs outside of ornithodira inherited scales that aren’t modified feathers, since alligators have the “feather genes”.
Kulindadromeus offers a powerful insight into the schematics of integrument evolution in not just dinosaurs, but archosaurs as a whole. It’s numerous types of feathers and “scales” pretty much confirm trends suspected for decades, and may even clarify several processes previously unanswerable. More research will be needed, but the evidence we have clearly is helpful in our general understanding.
- Pascal Godefroit, Sofia M. Sinitsa, Danielle Dhouailly, Yuri L. Bolotsky, Alexander V. Sizov, Maria E. McNamara, Michael J. Benton & Paul Spagna, 2014, “A Jurassic ornithischian dinosaur from Siberia with both feathers and scales”
- V.R. Alifanov & S.V. Saveliev, 2014, “Two new ornithischian dinosaurs (Hypsilophodontia, Ornithopoda) from the Late Jurassic of Russia”
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