Cruralispennia, the Opposite Opposite Bird

Despite maintaining a continuous list of new maniraptor studies, I have not been very inclined to write entire articles about dinosaur news. After all, everyone else already blogs about them! However, I have come to the conclusion that this assumption is not completely correct. Some of the papers from last year that I found most interesting barely received any popular press. As a result, I have decided that I'm going to start blogging occasionally about maniraptor news, time permitting (but I'm in the middle of working on my Master's, so don't expect too much).

Fossil birds in particular get little attention in the blogosphere (or anywhere else) compared to other dinosaurs, except from Andrea Cau, Mickey Mortimer, and Matt Martyniuk, so it seems appropriate to start with one. I'll discuss one of the first new dinosaurs described this year, the enantiornithine Cruralispennia multidonta.

The name is somewhat clunky; the etymology section in the paper implies that "donta" is Latin for teeth... Even as someone who has never been educated in Latin, that gives me pause. I imagine they had the Latin "dens" or the Greek "odus" or "odon" in mind. Yet behind all that is quite an unusual and fascinating dinosaur.

The holotype of Cruralispennia, from Wang et al. (2017).

Cruralispennia hails from the Early Cretaceous Huajiying Formation in China, the oldest formation from which we have found pygostylian (short-tailed) avialans and home to other spectacularly-preserved early birds such as Eoconfuciusornis, Eopengornis, and Archaeornithura. As its species name suggests, Cruralispennia had a whole lot of teeth, specifically in its lower jaw. As preserved, the holotype preserves at least fourteen lower teeth. Even though most enantiornithines had teeth, this is more than almost all other known enantiornithines except maybe Eopengornis.

Enantiornithes translates to "opposite birds", so called because whereas modern birds have a socket in their coracoid bone where the scapula (shoulder blade) connects to it, most enantiornithines have a socket in their scapula that the coracoid fits into instead. As it happens, Cruralispennia doesn't have this feature, though other details of its anatomy suggest that it is an enantiornithine. However, it has some characteristics that are not only atypical of enantiornithines, but are in fact more similar to those of modern birds, hence the title of this post!

The pygostyle, a fusion of the tail vertebrae at the tip of the tail in short-tailed birds, is short and stubby in Cruralispennia. This is not at all normal for most groups of Mesozoic avialans, which generally have longer, rod-shaped pygostyles, but it is widely found in one specific clade: the euornithines (modern birds and anything more closely related to them than enantiornithines)! In euornithines, the pygostyle supports a mobile fan of tail feathers that functions in steering and braking during flight, and pygostyle shape has been correlated with tail feather structure in modern birds, so one might expect Cruralispennia to have had a (presumably convergent) tail fan as well.

It doesn't. Though the holotype preserves feathers on its tail, it doesn't have large rectrices at all, instead just having short fuzz much like the condition in Eoenantiornis or female(?) Confuciusornis. Perhaps it evolved a blunt pygostyle for a different, undetermined reason from euornithines. Or, speculatively, maybe only some individuals had rectrices? That is the case in Confuciusornis, after all. As is typical in paleontology, we need more specimens!

Photographs and schematics of fossil avialan pygostyles, from Wang et al. (2017). (A) is Cruralispennia, (B) and (C) are other enantiornithines, (D-F) are euornithines, and (G) is Confuciusornis.

Another way in which Cruralispennia is more similar to euornithines than to other enantiornithines is in its growth rate. Modern birds grow unbelievably fast, most reaching adult size in a matter of months or even weeks. This was also the case in some Mesozoic euornithines. Most enantiornithines, on the other hand, took several years. Lines of arrested growth (essentially annual growth rings) are visible when you cut into their limb bones. The describers of Cruralispennia looked at the bone histology of the holotype's humerus, and they found... no growth rings at all, despite the fact that it appeared to have stopped growing. Like most euornithines, Cruralispennia was essentially an adult by the time it celebrated its first birthday.

One last oddity of Cruralispennia that I would like to highlight is its feathers. It is these that its genus name (which translates to "shin feather") refers to. The feathers on its legs and the leading edges of its wings are quite unusual in their structure. Each feather appears to be a narrow, solid sheet for most of its length, but there are short individual filaments that stick out at the tip. The describers gave them another somewhat clunky-sounding name: Proximally Wire-like [feathers] with a Filamentous Distal Tip (PWFDTs). This exact type of feather has not been found in any other kind of dinosaur (living or extinct), though they remind me of the "paintbrush-like" feathers in scansoriopterygids. It's difficult to say what these feathers were used for, but the describers point out that narrow feathers are useful for display without impeding flight too much.

Photographs and schematic of PWFDTs in Cruralispennia, from Wang et al. (2017).

That took longer than I expected. However, Cruralispennia deserved the attention, as I'm certain everyone now agrees.

Reference: Wang, M., J.K. O'Connor, Y. Pan, and Z. Zhou. 2017. A bizarre Early Cretaceous enantiornithine bird with unique crural feathers and an ornithuromorph plough-shaped pygostyle. Nature Communications 8: 14141. doi: 10.1038/ncomms14141