|Adorable Serikornis sungei by Emily Willoughby|
All this has happened in the last twenty years. Heck, nobody knew about Halszkaraptor until a few weeks ago.
|The holotype of Anchiornis huxleyi|
I'm not sure why I'm including specimen numbers in this essay, but I kinda like it.
|A much better specimen of Anchiornis huxleyi|
Also, the Tiaojishan Formation where Anchiornis is found turns out to be older than Archaeopteryx by up to 10 mya.
This paper caused a bit of a stir in the paleo community because the authors’ phylogenetic analysis actually removed Archaeopteryx from Avialae. Instead, Archaeopteryx is now in a group with Anchiornis and Xiaotingia, which is a sister group of Deinonychosauria. This large group, then, is the sister of Avialae. Xu et al. (2011) comment that basal birds like Sapeornis, Jeholornis, and Epidexipteryx (which they seem to think is a basal bird) are “more similar in general morphology to the oviraptorosaurs than to the archaeopterygids and basal deinonychosaurs.”
While the removal of Archaeopteryx from Avialae has not withstood subsequent testing, the idea that Anchiornis and Xiaotingia form a monophyletic group will come up again.
Godefroit et al. (2013a) posit that Eosinopteryx was more terrestrial than its cousins, both because of the featherless metatarsus and feet and its smaller wings. Interestingly, they note that Anchiornis and Eosinopteryx are very similar osteologically but clearly differ in terms of plumage. We may come back to this point later.
But it’s clear that nobody can agree who is actually related to whom because all of these near-birds are so much alike. Frustrating, right?
|A way, WAY better specimen of Anchiornis huxleyi|
Then, at the end of 2017, Foth & Rauhut took a closer look at the Haarlem Archaeopteryx fossil and realized that it’s probably not an urvogel, but an anchiornithid instead, which they named Ostromia crassipes. Furthermore, they actually give a name to this group of near-birds for the first time: Anchiornithidae. Their phylogenetic analysis does, in fact, place Anchiornis within the Anchiornithidae (contra Lefevre et al. 2017) which includes all the other near-birds I’ve been discussing plus Ostromia, of course. Interestingly, they find that Epidexipteryx is not just an avialan, but is one step higher than Archaeopteryx!
(you know, what we really need are more, and better preserved, scansoriopterygian fossils)
It’s also notable that Ostromia is the first anchiornithid to be found outside of China, although another one may be described before too long based on Scott Hartman’s SVP talk…
Beautiful feather impressions surround the whole body. Caihong’s wing feathers are much longer than Anchiornis and it appears to have an alula. Its leg feathers are also quite long, and like Anchiornis it has feathers on its metatarsus and toes. The authors note that:
Eosinopteryx brevipenna has been suggested to have reduced tail and hindlimb plumages, but specimens of Anchiornis huxleyi display variable plumages in terms of not only feather distribution, but also feather size and shape. Some closely related Tiaojishan theropods not differentiated by osteological features may need reassessment of their taxonomic status.
I'm not sure any of these Tiaojishan anchiornithids have been described as identical apart from plumage differences. The tail feathers of Caihong are particularly long and, like the basal troodontids Jianianhualong, appear to be asymmetrical; this is especially strange given that the wing and leg feathers are not. Like Serikornis, Caihong features a wide variety of feather types on its body. Its preserved melanosomes suggest that Caihong was mostly black, but featured hummingbird-like iridescence on its head, chest, and possibly the base of the tail. The only other iridescent dinobird I know of is Microraptor (Lu et al. 2012), but its melanosomes were not directly compared to hummingbirds. The implication of Hu et al. (2018) is that Caihong was colorful--moreso than its contemporaries, anyway.
|Rebecca Gelernter's Anchiornis|
Another oddity is that there apparently hundreds of specimens of Anchiornis available for study but every other Tiaojishan anchiornithid is known from a single specimen. What made Anchiornis so successful compared to its neighbors? How capable were these animals from an aerodynamic angle? Did they make frequent trips up tree trunks and then glide back down or between trees? The extreme feathering on the legs and feet of several anchiornithids doesn't seem terribly useful in a terrestrial context. Rebecca Gelernter restores Anchiornis in a hypothetical branch-climbing pose, but would it have been able to do this in life? Would any anchiornithid? Besides, with an apparently lack of carnivores to run from, would anchiornithids feel any pressure to get off the ground at all? How did they interact with the numerous pterosaurs and small mammals who were their neighbors?
Whatever the answers may be, I suspect that anchiornithids have much to tell us about the origin of birds, modern plumage, and (maybe) flight. And with such beautiful fossils coming out of Tiaojishan, the answers may be closer than we think.