It turns out that Cretaceous troodontid dinosaurs had asymmetrical ears. This makes them like owls, which also have asymmetrical ears. But not all owls have asymmetrical ears and, what’s more, the story of ear asymmetry in owls is itself a pretty remarkable one….
Before getting distracted by godwits, I was talking about troodontids and their asymmetrical ears (and this itself came as a distraction, as beforehand I was talking about the evolution of blood-feeding in birds). The irresistible comparison that comes to mind is of course with owls, as owls also have asymmetrical ears (though not all of them do). The fact that troodontids evolved asymmetrical ears immediately falsifies the long-cherished notion that owls are unique among vertebrates in this respect. The asymmetry seen in owls differs, however, from that of troodontids in being restricted to the external ear; it is never present in the middle or internal ear.
Oh yeah, before I start, I want to say a big thank you to everyone who is reading, visiting and/or making comments. Sorry if I have yet to leave a response to a comment you’re left. Sorry also if I owe you an email: I was already struggling to keep up with them, and since I’ve joined scienceblogs things have gone nuts. Anyway, if you like what I’m doing, please do tell your friends. Tetrapod Zoology seems to be growing in importance, and it would be nice to see it grow that little bit more: when the site first went live it was rated 60th in terms of number of hits, whereas today it is rated 28th. Right, back to those owls.
As mentioned, we begin with the remarkable fact that the ear asymmetry present in owls is not the same among all owls that have it: instead it turns out that owls are markedly different in the sort of asymmtry they possess. In some owls (including Eurasian eagle owls Bubo bubo* and most Strix species), the asymmetry is exhibited in the position of the external ear opening in the skin, in others (including some Asio species and Pseudoscops) it is the shape of the external ear opening that is asymmetrical, and in others (including the Tawny owl S. aluco and Ural owl S. uralensis) it is the skin folds around the ear openings that are asymmetrical. Actual asymmetry in the bony part of the ear has only been reported for the Ural owl, Great grey owl S. nebulosa, Tengmalm’s owl Aegolius funereus and Northern saw-whet owl A. acadicus (Norberg 1977, 2002). Several species combine various of these different types of asymmetry. Asymmetry is not present in Athene, Ninox or Otus [adjacent pic of a Tengmalm's owl borrowed from here].
* Among Bubo owls, Norberg (2002) could only find asymmetry in B. bubo: it hasn’t been reported in other Bubo species. However, further study is needed to determine whether B. bubo is really unique in this respect.
These markedly different types of asymmetry indicate that it didn’t evolve once early on in owls, but at least five times, and perhaps as many as seven times. This makes things even more remarkable.
Why have asymmetrical ears? In simplified terms, it allows owls to better pinpoint objects that they are locating using aural cues. While all tetrapods that hear use interaural time delays to help pinpoint sound sources, owls with asymmetrical ears are also able to work out the elevation of a sound source, a pretty neat trick given that it allows them ‘to capture prey in total darkness using acoustic cues alone’ (Nishikawa 2002, p. 246). Norberg (2002) suggested that this specialized pin-pointing presumably evolved as owls first became expert at locating prey on the ground, and then on ground-living prey that inhabit dense forest, or hide in dense ground vegetation or under snow cover.
Here we come to the speculation I talked about at the start. If troodontids also have asymmetrical ears, does this mean that they were also specialized for the pinpointing of concealed prey based on acoustic cues? If so, then (combined with the fact that their hearing was probably very sensitive anyway) this is neat evidence supporting the idea that they were locating small mammals and other animals in undergrowth or leaf litter. The caveat here is that, as usual with dinosaurs, we don’t have a sample size good enough to verify that this asymmetry was present across populations and species. I’ve only read about it in Byronosaurus and don’t know if it’s present in other taxa, which is why I’m especially intrigued by Castanhinha & Mateus’ (2006) statement that it is present in some troodontid taxa (i.e., more than one). They also concluded that ear asymmetry in troodontids ‘can be an analogy resulting from convergent evolution between troodontids and [owls], used for 3D directional acoustics’. So, we await the full paper with great interest. How incredibly cool.
What is ‘the irony’ I mentioned in the title? This is a reference to the fact that a group of peculiar European fossils from the Upper Cretaceous of Romania have been identified at times as belonging either to troodontids, or to owls. I just find it vaguely amusing that troodontids and owls turn out to have an unusual genuine similarity after all, albeit it in just one feature. The fossils concerned – dubbed Heptasteornis andrewsi and Bradycneme draculae – turn out to be neither troodontids nor owls, thus explaining the hilarious title of a paper that Gareth Dyke and I published on this material in 2004 (Naish & Dyke 2004). Heptasteornis is in fact an alvarezsaurid: a species of which is depicted in the adjacent Luis Rey image. I was going to talk about these Romanian fossils here as well but I have to stop there. In a bizarre twist of fate, the specific name for Bradycneme brings us back to that vampire theme again.
Oh well. So much for the shorter posts.
Coming very soon: those posts on vampires, Australian mega-cats, real live giant sea serpents, confessions of a quadrupedal hominid, those lost tree frogs, rhinogradentians, terrifying sex organs of male turtles, tortoises tortoises tortoises, and lots more (I did list loads of other stuff in the original version of this post but accidentally deleted them when I had to do an emergency edit on Jan 31st. Let me know if you have a copy of the original post).
Refs – –
Castanhinha, R. & Mateus, O. 2006. On the left-right asymmetry in dinosaurs. Journal of Vertebrate Paleontology 26 (Supp. 3), 48A.
Makovicky, P. J., Norell, M. A., Clark, J. M. & Rowe, T. 2003. Osteology and relationships of Byronosaurus jaffei (Theropoda: Troodontidae). American Museum Novitates 3402, 1-32.
Naish, D. & Dyke, G. J. 2004. Heptasteornis was no ornithomimid, troodontid, dromaeosaurid or owl: the first alvarezsaurid (Dinosauria: Theropoda) from Europe. Neues Jahrbuch fur Geologie und Palaontologie, Monatshefte 2004, 385-401.
Nishikawa, K. C. 2002. Evolutionary convergence in nervous systems: insights from comparativand phylogenetic studies. Brain, Behavior and Evolution 59, 240-249.
Norberg, R. A. 1977. Occurrence and independent evolution of bilateral ear asymmetry in owls and implications on owl taxonomy. Philosophical Transactions of the Royal Society of London B 280, 375-408.
– . 2002. Independent evolution of outer ear asymmetry among give owl lineages; morphology, function and selection. In Newton, I., Kavanagh, R., Olsen, J. & Taylor, I (eds.) Ecology and Conservation of Owls: Proceedings of the Owls 2000 Conference. CSIRO Publishing (Collingwood, Victoria, Aus.), pp. 329-342.