While feathers were once thought to have been restricted to birds,
and then to some other closely related theropod dinosaurs,
it is now apparent that primitive feathers, or at least filaments similar to them, were much more
widespread. One of the most notable examples is a small feathered dinosaur named Kulindadromeus.
First described in 2014, Kulindadromeus was not a theropod, the branch of Dinosauria birds
belong to, but an ornithischian. This clade included a number of herbivorous dinosaurs,
including the hadrosaurs, a variety of armored dinosaurs, and the horned ceratopsids.
Although ornithischians were until recently thought to have been unambiguously scaly,
fossils of Kulindadromeus also preserve feathers on much of its body. This makes
this otherwise unassuming little creature one of the most important dinosaurs ever discovered.
Kulindadromeus’s fossils were first discovered in the Siberian Chernyshevsky District,
and it is now on both its flag and coat of arms. All Kulindadromeus fossils have been
found in two bonebeds there, in a layer of the Ukureyskaya Formation laid down between 168
and 166 million years old. This is at the boundary of the Middle and Late Jurassic.
Like the fossils found in nearby Chinese sites, such as the famous Yixian Formation,
Kulindadromeus’s fossils were exquisitely preserved by volcanic activity. The fossils
of hundreds of different Kulindadromeus individuals have been found in the bone beds,
suggesting this small dinosaur lived in large herds. Kulindadromeus was very small, only about
one and a half meters long. While it was covered with feathers in life, Kulindadromeus’s skeleton
lacked the obvious striking features of many of its more famous cousins.
It was a biped with short arms and a short skull that ended in a small beak. Its teeth were typical
of a herbivore, although it may have eaten the occasional insect or even small lizard or mammal.
Unlike most other ornithischians, Kulindadromeus’s antorbital fenestra, an opening in the skull
between its nostrils and eyes, was smaller than its maxillary fenestra, another opening in the
skull, but in the jaw. Besides its fenestra, there were other ornithischians who had a
similar size and shape as Kulindadromeus. Many of them were formerly called “hypsilophodonts”,
although it is now clear they did not form a true clade with each other. Instead, they represent
what the first ornithischians were like. and although they are perhaps the most over-looked
group of dinosaurs, they survived to the very end of the Mesozoic era sixty-six million years ago.
In Kulindadromeus’s case, it was found to be the sister taxa to the clade Cerapoda,
which is the clade formed by the hadrosaurs, ceratopsians, and pachycephalosaurs.
Indeed, the first cerapodan is thought to have been very similar to Kulindadromeus.
Basal ornithischians were built for speed and Kulindadromeus was no exception. With its slender
hips and more muscular feet, Kulindadromeus would have been fast even by their standards. Such speed
would have been vital when it was spotted by predators as it lacked other means of defense.
Of course, what Kulindadromeus is famous for is not its skeleton but its integument, or outer
covering. It had three varieties of feathers. The most common type were simple filaments similar
to the stage 1 feathers seen in many theropods. However, the feathers on Kulindadromeus’s thighs
and arms were more complex. They consist of six to seven filaments that emerge from the same plate,
and likewise resemble a type feather seen in theropods. Finally, on the lower leg there
are bundles of ribbon-like feathers, which haven’t been found in any other dinosaur.
While feathers are famously used for flight, their original purpose is thought to have
been for thermal insulation. Dinosaurs like Kulindadromeus were endothermic,
producing their own body heat, which their feathers helped them retain. Without them
it would have required a lot more energy for them to maintain the same internal body temperature.
Kulindadromeus wasn’t entirely covered in feathers but also possessed three types of scales.
Its tail was covered in arched, plate-like scales. With the exception of those at the base of the
tail, most these scales overlap with each other. Its lower legs were instead covered in small,
overlapping hexagonal scales while its hands and feet were covered in even smaller round scales.
These last two types of scales are very similar to those found on bird feet today.
The previous oldest known example of a dinosaur with these bird-like foot scales was the humped
carnosaur Concavenator. Whether or not the foot scales of Kulindadromeus and birds are homologous,
meaning inherited from a common ancestor, is still unclear. If they are, then the foot scales
of the long-necked sauropods and some other ornithischians diverged from their ancestral
condition. Before Kulindadromeus was found, there were no ornithischians persevered with unambiguous
feathers. While quill-like structures had already been found in the ornithischians Tianyulong and
Psittacosaurus, it was debated whether or not they were actually feathers or the result of convergent
evolution. Since Kulindadromeus’s filaments bear a much closer resemblance to those of theropods,
it is much more likely they were actually inherited from the same common ancestor.
Kulindadromeus was also the most extensively feathered ornithischian at the time of its
discovery. Psittacosaurus only had quills on its tail while the rest of its body was scaly. The
Tianyulong holotype specimen preserves feathers on its neck, back, and tail. However, the feathers
preserved in another Tianyulong specimen are much more widespread, making Kulindadromeus’s feathers
look sparse in comparison. Notably, the feathered ornithischians are spread throughout the clade.
As mentioned before, Kulindadromeus was close to the last common ancestor of the ceratopsians
and hadrosaurs. While Psittacosaurus was an early ceratopsian, Tianyulong was a heterodontosaurid.
Heterodontosaurids are generally held to be some of the most basal ornithischians.
Even if their feathers were merely convergent with those of theropods,
this suggests these filaments were present in the first ornithischians. Furthermore,
the closest relatives of the dinosaurs, the pterosaurs, also had filaments called pycnofibers
which are very similar to the feathers of Kulindadromeus and many theropods. This suggests
that feathers may not only predate the theropods, but even the first dinosaurs as a whole.
Of course, not all ornithischians were fluffy like Kulindadromeus, or even had feathers.
Large patches of scales have been found covering much or even most of the bodies of dinosaurs like
the hadrosaurs. However, with the exception of Psittacosaurus, all dinosaurs found with evidence
of a primarily scaly integument were very large. Today, similarly sized mammals have very little
hair. This is due to their lower surface area to volume ratio, resulting in a greater need
to shed heat rather than retain it. While the largest mammals have ended up with bare skin,
if Kulindadromeus’s mix of feathers and scales was the norm, large dinosaurs had another option. Once
a dinosaur grew so large that its feathery coat became a liability, scales could easily replace
them on any given part of the body. This could potentially be accomplished via a single mutation.
The discovery of Kulindadromeus has had a profound impact on how dinosaurs are reconstructed,
suggesting feathers were the norm among small dinosaurs and their relatives.
Even if its feathers were the result of convergent evolution, Kulindadromeus’s integument provides
evidence that other small ornithischians were also covered in fluff. Thank you for watching.
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