At any rate, clearly, Dromaeosauridae was quite successful. But the dromaeosaurs constitute but one branch of a larger Deinonychosauria. Their sister group, the Troodontidae, is not only far less popular, but also poorly represented.
In general, troodontids seem to be derived from a Mahakala-like ancestor, and earliest members of both the Troodontidae and Dromaeosauridae differ only in subtle ways. Troodontids seem to represent a lither, more gracile type of dromaeosaur. Troodonts also evolved in a slightly different direction from their more famous cousins. While dromaeosaurs kept their recurved, serrated teeth and hypertrophied raptorial claw, troodontids reduced their tooth size, changed the serration pattern, and reduced the size (and changed the shape) of the raptorial claw. Clearly, they two families were doing different things.
This first picture shows the raven-sized, exquisitely preserved Mei long, a basal troodontid from (where else?) China, that was described in 2004. It died in a sleeping posture similar to modern birds, with its head tucked under one arm. The only real difference is that Mei long's lengthy tail is wrapped neatly around its body. The flexibility of the tail is another differentiating factor from dromaeosaurs--Velociraptor and its cousins would not have been able to wrap their tails in such a way given their unique tails, in which the caudal vertebrae were all joined together by long prezygapophyses, a feature that troodontids seem to lack.
As a troodontid, Mei displays unusual dentition, in that the teeth are small and tightly packed. Their curvature is not as extreme as that seen in dromaeosaurs, and the serrations are finer.
Surprisingly, more information on the anatomy of early troodontids can be gleaned from a more fragmentary skeleton. Sinovenator changiae was named in 2002. This chicken-sized troodontid actually helped re-establish the Troodontidae as a sister group to the Dromaeosauridae. Before its discovery, the only troodontid skeletal material was from derived members of the family like Troodon and Saurornithoides, and they displayed a non-retrovated pubis. For a long time, the troodontids were considered non-maniraptoran dromaeosaur mimics, and their true affinities were thought to lie closer to the Ornithomimosauridae (Holtz, Jr. placed troodontids, ornithomimids, and tyrannosaurs in a tri-radiate clade called "Bullatosauria," which has since been abandoned).
Sinovenator displayed clearly troodontid dentition, but had a dromaeosaurid retrovated pubis. Other features in the skull and axial skeleton link Sinovenator with primitive dromaeosaurs Microraptor and Sinornithosaurus, so trooodontids were clearly a sister group to their more famous cousins.
In 2005, yet another new primitive troodontid was discovered, although it wouldn't be recognized as such until earlier this year. Jinfengopteryx is known from a nearly complete skeleton, wonderfully preserved, including lots and lots of pennaceous feathers on the arms and tail. Unfortunately, its describers thought it was an archaeopterygian, despite its obvious troodontid features. Moreso than its basal characters, Jinfengopteryx is significant for demonstrating that troodontids had dromaeosaur-type feathers. This was, of course, predicted by troodontid phylogeny, but it's nice to have fossil confirmation.
The closely-packed, rostrally-placed teeth, long legs, short arms, and general shape of the pelvis were all quite clearly of troodontid origins, but Ji, et al. seemed unphased in their assignment of Jinfengopteryx to the Archaeopterygidae, even going so far as to publish a paper comparing the two "birds," in which they note, with some puzzlement, that the arms seem too short for flight, and that the teeth are wildly different from those of the urvogal. The pelvis, too, seemed divergent. Despite these glaring differences, it wasn't until Mahakala was described earlier this year that Turner, et al. finally placed Jinfengopteryx in the Troodontidae officially.
With all the primitive troodontids, known from virtually complete material, being dug out of the ground, you'd think the more derived members would be just as well known. Such is not the case, sadly, and derived troodontids are extremely rare and their remains tend to be fragmentary. The list of confirmed genera is extremely short: Sinornithoides, Byronosaurus, Troodon, and Saurornithoides (which may be a synonym of Troodon) make up the Late Cretaceous Troodontidae, the first half from Asia, and the second half from North America. There is also a problematic taxon named Borogovia that seems to have lost its sickle-claw altogether. These were not, though, diverse animals. And yet, they displayed some unique specializations which imply that troodontids were specialist hunters.
As Darren Naish so well describes, troodontids have asymmetrical ears, like owls. In addition, troodontids famously have the largest brains (compared to body size) of all dinosaurs. They compare to the ratios of ratite birds. In addition, troodontid eyes are unusually large, and their field of binocular vision was quite impressive. Taken together, these features suggest that troodontids were at least partially nocturnal, and that they hunted small prey. In 1998, Holtz et al. suggested that plants may have been at least part of Troodon's diet based on the denticle morphology of its teeth, but this idea is hard to test.
It's a shame that troodontids are so poorly known, because they are clearly specialized toward a particular lifestyle. With such incomplete material and the the rarity of troodontids in the fossil record, we may never know exactly what they were doing. Still, Troodontidae is a significant and fascinating group of deinonychosaurs that really should get more attention. I mean, Velociraptor is cool and all, but Troodon was an owl-like omnivore!
* Mahakala's diagnosis as a basal Dromaeosauridae is tenuous as best. It may simply be a basal Paravian.
Hat-tip to Neil, who has been bugging me for awhile to write a post about troodontids.