Troodontids and owls: oh, the irony (part I)

Apologies if you're here for the vampires. I'll come back to them soon, I promise, but in the meantime I got distracted...

Some biologists - and scientists from other fields - have been quite critical of the fact that people speculate, and speculate, and speculate about dinosaurs (and by 'people' I don't necessarily mean palaeontologists, as in fact most technical palaeontological literature is appropriately dry, boring and conservative). But let's be fair: how can anyone not try to imagine what these animals were like when they were alive? I will also say that informed (note: informed) speculations are often useful as, like it or not, scientists are often inspired to research certain areas because they want to test their own pet intuitions. One of the most speculated about groups of dinosaurs are the troodontids, a mostly Cretaceous group of maniraptoran theropods, best known from eastern Asia and western North America. They were not particularly big, the largest species reaching 3 m in length. One new detail - as yet little discussed and apparently mostly overlooked - has me speculating, despite best efforts [adjacent Greg Paul Saurornithoides image borrowed from the unofficial GSP gallery].

Troodontids are well known for a few reasons. The type taxon for the group, Troodon formosus, was named for an isolated tooth in 1856 and as such is one of North America's first named dinosaurs. Like their close kin the dromaeosaurs, troodontids were equipped with a raised sickle-claw on the second toe, though the claw in troodontids was proportionally much smaller than that of dromaeosaurs. In one peculiar troodontid - Borogovia gracilicrus* - this toe couldn't be raised far off the ground, and the claw was virtually straight.

* Its generic name is derived from the borogove, a creature from Lewis Carroll's Jabberwocky poem. How relevant.

Good troodontid braincases, first described in the 1960s and supplemented by more complete specimens described in the 70s and 80s, showed that these dinosaurs had enormous eyes and middle ear cavities, and relatively large brains. The big eyes obviously indicate good vision, and both enhanced nocturnal vision and enhanced diurnal vision have been argued for. Without the soft tissues, and details on rods and cones and so on, we of course can't say either way. The large ear cavities suggest that hearing was also acute, and an unusual structure termed the basisphenoid bulla may be involved here. Connected to various internal pneumatic cavities, the bulla is weird: elsewhere in theropods it's seen in the ostrich dinosaurs or ornithomimosaurs (and its presence here has led a number of workers to propose that troodontids and ornithomimosaurs are each other's closest relatives, a view not currently supported by most phylogenetic work). The fact that a large air-filled bony chamber was closely associated with the ear drum may have produced a dampening effect that aided the detection of low-frequency sounds (Currie 1985). This idea still requires testing but, to my knowledge, no one has come up with a better idea [adjacent image, showing skull of the Asian troodontid Saurornithoides, from amnh.org].

The brain size thing has led to all sorts of bizarre and misguided claims about how 'smart' these dinosaurs were: for a long, previous discussion of this subject see my blog post Dinosauroids revisited.

Given that the maniraptorans that surround troodontids in the phylogeny are now known to have had vaned, modern-type feathers we can safely assume that troodontids also would have been fully feathered. While this was, until recently, a reasonable assumption, what appears to be a fully feathered troodontid was finally described in 2005. Named Jinfengopteryx elegans, it's a small form (c. 55 cm long) from the Qiaotou Formation of Liaoning Province, China, and though identified by its describers as an archaeopterygid bird (Ji et al. 2005), it exhibits several features suggesting that it's more likely to be a troodontid. I included it in a parsimony analysis for part of my phd research and found it to be a basal troodontid [adjacent Greg Paul image, showing Troodon vs Parksosaurus, again borrowed from the unofficial GSP gallery].

Anyway, what with their big eyes, reasonably biggish brains, and hands and feet that are obviously suited for grabbing and dispatching small and mid-sized animal prey, it has often been imagined that troodontids stalked small mammals and other such prey, either in the undergrowth, or at dawn or dusk. Note that the larger species were quite probably capable of killing larger prey (and tooth morphology at least raises the possibility that troodontids might also have eaten plant material on occasion). I don't recall, however, anyone making a big deal out of the large size of the ear cavities. Obviously this raises the possibility that these theropods used an enhanced sense of hearing to help locate prey, which I suppose provides support for the idea that they located small mammals and such animals in undergrowth or leaf litter. A bit like big, long-legged flightless owls perhaps. But the ear cavities aren't unusual in just being big: particularly interesting is the fact that they are also asymmetrically positioned on the skull, with one being set higher up than the other.

Though this feature is not newly recognized (Makovicky et al. (2003) noted it in their description of the Mongolian troodontid Byronosaurus), what captured my imagination is the mention of this asymmetry in a recent study by Castanhinha & Mateus (2006). To date it has, so far as I know, only been published in abstract form. The fact that at least some troodontids have asymmetrical ears immediately brings another group of feathered theropods to mind: owls. Owls also have asymmetrical ears, though not all of them do. The fact that troodontids also 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 [adjacent image of Byronosaurus skull from the digimorph site].

I have to stop there for now. More on this subject in part II, to be posted soon.

Refs - -

Castanhinha, R. & Mateus, O. 2006. On the left-right asymmetry in dinosaurs. Journal of Vertebrate Paleontology 26 (Supp. 3), 48A.

Currie, P. J. 1985. Cranial anatomy of Stenonychosaurus inequalis (Saurischia, Theropoda) and its bearing on the origin of birds. Canadian Journal of Earth Sciences 22, 1643-1658.

Ji, Q., Ji, S.-A., Lu, J., You, H., Chen, W., Liu, Y. & Liu, Y. 2005. First avialian bird from China. Geological Bulletin of China 24, 197-203.

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.