Troodontids and All That Jazz
This post originally appeared on a mailing list discussion by yours truly back in 2000, here. I am posting it here in order to reintroduce the topic and begin elaborating on it, as I have a few/many comments on the topic of troodontids, deinonychosaur phylogeny, bullatosaurian phylogeny, their teeth, the nature of carnivore "guilds," and the general topic of teeth and tooth-based taxa and what to do about them (which pretty much sums up my research work).
The post begins below.
A few taxa were based on teeth, and these are generally referred to as nomina dubia, but as Currie, Rigby, and Sloan (1990) showed, are diagnostic individually, and different genera can show differences based solely on dentition. Koparion douglassi from the (LJ) Morrison Formation is one such example, based largely on the bulbous base of the tooth and low height/length aspect ratio. Pectinodon bakkeri was synonymized with Troodon formosus based on extreme similarities in that they fell within the same expected range of morphology for the same jaw, by Currie et al. (1990). Ciski and Grigorescu (1999) suggest that Pectinodon may be a distinct species of Troodon, T. bakkeri, based on another set of teeth from Romania, including another suggestion that Euronychodon is a troodont. I have no diagnostic opinion of Saurornithoides/Troodon asiamericanus and S./T. isfarensis.
Other troodont taxa are based on pedal elements only, such as Borogovia gracilicrus and Tochisaurus nemegtensis, but then again, these pes differ markedly from those of Troodon, Saurornithoides, and even Sinornithoides (pers. ob.). Short pedal toe 2 in Borogovia with the relatively strait claw is strange and unexpected, but also found in the deinonychosaur (sensu recens) Adasaurus. It's a troodont, though. But unlike other known troodont pes, Tochisaurus had a very short mtII which, given the average relative length of digit II to the metatarsal, would have prevented the toe from touching the substrate, and making it functionally didactyl, as in ostriches, hence the name (toch' in Mongolian refers to ostriches).
The taxa represented by more complete material including parts of several regions of the body, include Saurornithoides, Troodon, Sinornithoides, Byronosaurus, and another troodont from Ukhaa Tolgod. These taxa differ from each other in more significant ways, based largely on the more complete material, and are the main diagnostic tools for the group. A complete skeleton is known only for Sinornithoides youngi, and proportions are similar to ornithomimosaurs in many respects, especially the skull with Pelecanimimus.
The skulls of Byronosaurus, Troodon, and Saurornithoides are less complete, but are still complete enough to compare, and all are relatively more derived and closer to each other than to Sinornithoides, and this suggests that Sinornithoides is more basal. The skull is longer snouted with a deeper snout and less triangular profile in the last group, and the form of the pes is not indicative of any derived form. The pelvis is propubic with a derived, oviraptorosaurian-like anterior pubic process with the ilium, so that the pubis is slightly offset under the acetabulum then curves forward. Very short ischium with no elongate post obturator process in Sinornithoides and Saurornithoides, as far as I can see. Long tail lacks stiffening structures except for possible craniocaudally long chevrons in the distal half. Manus of the dromie/caenagnathid form, with a shorter third finger than second, and semilunate does not contact the third metacarpal [could be wrong on a few cases]. Long, long necks, very slender. Coracoids are intermediate between the ovate ornithomimosaur form and the everted rectangular form of oviraptorosaurs, closer to the latter than the former. Sinornithoides preserved furcular splints, or true clavicles, whereas the condition appears to be unknown in all other troodonts.
Taxonomy suggests that Koparion, Sinornithosaurus, and Byronosaurus are successive outgroups of the group formed by Saurornithoides and Troodon. Borogovia and Tochisaurus could lie anywhere along the line. Ornithodesmus has been hypothesized as a troodont, and it too, may lie anywhere, as a comparison of sacra has not been done so far on troodonts, and I am unaware of the full condition in Sinornithoides.
Hope this helps, and I would enjoy corrections on any errors or any additions that could be made to this.
Data comes from Russell and Dong, 1994; Currie, 1987, 1990; Barsbold, 1976; Makovicky et al., 1999; Chure, 1994; Currie et al., 1990; Russell, 1970; Osmólska, 1988; Osmólska and Kurzanov, 1992; and Glut, 1997.
Current research (as of 2006, in order to show this comment is current):
A new troodontid, Mei long, has refocused the evolution of troodontids towards the base of the avian divergence within dinosaurs, near dromaeosaurids. Remarkably tiny, Mei contains a skull much like Byronosaurus but with many other peculiarities of the jaw bone and braincase that make it rememble an Archaeopteryx-mimic. Even more recently, skulls of some alvarezsaurids (such as Shuvuuia and some more peculiar skulls from Liaoning in China and Ukhaa Tolgod in Mongolia) show a similar morphology that may have conceptualized the ur-bird and close relatives, from alvarezsaurids, troodontids, and dromaeosaurs like Microraptor. This new animal and another, described as a bird (Jinfengopteryx) show that the origin of troodontids was complex and peculiar, and that their evolution was to increase in size and diversify. The largest troodontid was also one of the latest surviving, "Troodon" or Stenonychosaurus.
More work to come.