Earlier we looked at the origin of feathers and the evolution of epidermal structures in dinosaurs, noting that embryo birds first develop primal buds (primordia) in the middle of their otherwise naked back. As we learned earlier, feathers are not elaborate scales, but develop from naked skin. We see this every time we pluck a chicken. We also learned that leg scales on birds are derived from feathers. Remember those 4-winged Mesozoic birds?
Today some further thoughts on the genesis of feathers.
Figure 1. Sinosauropteryx in lateral view on a primitive conifer. Despite the complete preservation of several specimens attributed to Sinosauropteryx, very few reconstructions (Fig. 1) have been made of it. Clinging to trees ultimately led to clinging to dinosaurs in dromaeosaurids. Like Limusaurus, Sinsauropteryx is off the main line of bird evolution.
Feathers are rarely preserved on dinosaur fossils.
One of the most primitive dinosaurs to preserve (admittedly very primitive) feathers is Sinosauropteryx (Figs. 1-3; Ji and Ji 1996) from the late Jurassic (with origins earlier in the Jurassic). It has short filamentous feathers running down its spine and around its throat and apparently nowhere else. This ‘mohawk haircut’- pattern could be due to the process of fossilization. Perhaps only those feathers on the parasagittal plane got preserved. However, from available evidence if the feathers were not restricted to the back, they did not stray very far from the spine at this stage. You don’t see feathers around the belly or legs in Sinosauropteryx (Fig. 2).
Figure 2. Sinosauropteryx fossil. As everyone knows, those are primitive feathers lining the spinal column and below the throat. Analysis indicates this is not the most primitive feathered theropod. Note the on/off appearance of the tail feathers indicating a decorative device: stripes!
Adding Sinosauropteryx to the large reptile tree
nests it with Limusaurus and both were basal to the much larger Sinocalliopteryx, which also had primitive feathers (Fig. 3). So Sinosauropteryx is not the most basal dinosaur with feathers or proto-feathers (contra Ji and Ji 1996). Unfortunately, more primitive theropods do not preserve feathers or scales. Scales do appear on later, larger dinosaurs of all sorts, not so much on the smaller, earlier dinos. Based on birds we can’t assume that small, early dinos had scales (contra Barrett et al. 2015). Rather, based on the appearance of primordia and feather-like structures on a wide variety of dinosaurs, feather primordia appears to precede scales, and perhaps many of these primordia ultimately became scales on larger dinos.
Figure 3. Sinocalliopteryx along with Limusaurus, Aurornis and Archaeopteryx to scale. Similar to Sinopteryx, but includes leg feathers here. Sinopteryx and Limusaurus are off the main line of bird evolution, which includes Haploceheirus and dromaeosaurs. Note the depth of the pelvis here compared to Scleromochlus (fig. 5).
Figure 4. Scales on the back of Scleromochlus forming a lumbar girdle for support during bipedal excursions. This taxon nests as a basal bipedal croc and thus a distant sister to basal bipedal dinosaurs.
The genesis of feather primordia appears to be correlated to bipedal locomotion and a long torso. Before a feather was a feather, or even a quill, it was something else more primitive.
When one looks
at the pattern of dorsal scalation in Scleromochlus (Figs. 4, 5), a basal archosaur, one gets the impression that it was wearing a kind of lumbar girdle to support the long lower back. Indeed, as a newbie biped, Scleromochlus would have used such support near the fulcrum of the large leverage arm created by its stance, its long dorsal region and short ilium. Nothing appears to be sticking out above the dermal layer here. All of the scales (or whatever they were) appear to in lines, like a weave.
Unlike ancestral rauisuchians and the more closely related and larger Erpetosuchus and Gracilisuchus, there were no dorsal parasagittal scutes on Scleromochlus. It was a small animal that lost these structures as it evolved to depend on speed, not armor, to defend itself from predators.
Figure 5. Scleromochlus, a basal crocodylomorph and an early biped in the archosaur line. Scleromochlus reinforced its long lower back with a dermal lumbar support or girdle. This is same area on a chicken embryo that first develops feathers. Compare torso length here to figure 3.
Primordia evolved into feathers only on the short torso basal dinos
Pre-dinosaurs are distinct from pre-crocs in many ways, but pre-dinos all have a shorter torso and a deeper pelvis (Fig. 3) reducing the leverage arm and the need for a reinforcing lumbar girdle. After the pelvis deepened and the torso shortened in early dinosaurs, the individual primordia of that old girdle were free to evolve into something else, in this case, something decorative.
Sinosauropteryx, with its dorsal line of feathery filaments extending from head to tail is one such example. When more feathers began to wrap around the body, that added insulation as a use. When wing feathers lengthened, the forelimbs began to flap to bring attention to those decorations. Later, wing feathers were co-opted for thrust and lift to enable flight.
But the genesis of feathers
still appears to be in the middle of the back, where primordia first appear on embryo chicks, replaying the old lumbar girdle innovation of Scleromochlus. The ornithischians, Tianyulong and Psittacosaurus had elongated primordia along their backs and tails indicating that this trait probably goes back to Herrerasaurus and Trialestes, no doubt in a smaller, more primitive state. With that small field of primordial scales on the lower back of an otherwise naked Scleromochlus (Fig. 5), the genesis of extradermal structures appears to extend to basal archosaurs.
Figure 6. Feathers, scales and scutes in the Archosauria.
If anyone can provide evidence for scales or any other dermal preservation in any Triassic or Early Jurassic dinosaur, please let us know of them.
If anyone has other thoughts on the origin of feathers, please share them. If the above scenario does not make sense, please tell us your thoughts.
Barrett PM, Evans DC, Campione NE 2015. Evolution of dinosaur epidermal structures. Biol. Lett. 11: 20150229. online
Ji Q and Ji S-A 1996. On the Discovery of the earliest fossil bird in China (Sinosauropteryx gen. nov.) and the origin of birds. Chinese Geology 233:30-33.