The Troodontids are a group of small-to-medium-sized therapod dinosaurs closely related to the birds. They are often referred to as feathered dinosaurs, though they are certainly not the only feathered non-avian dinosaurs. The group are quite diverse, and appear to have followed a variety of lifestyles; at least one species appears to have evolved the ability to fly independent of the birds. It is quite likely that the Troodontids were one of the most numerous dinosaur groups in the Late Cretaceous, but that they have not been as widely preserved as more robust groups.
The September 2011 edition of the online journal PLoS ONE contains a paper by a team lead by Lindsay Zanno of the Field Museum of Natural History in Chicago and the Biological Sciences Department at the University of Wisconsin-Parkside, describing a new Troodontid, Talos sampsoni, from the Upper Cretaceous Kaiparowits Formation of Utah, North America. Talos is described from an incomplete skeleton comprising most of the pelvis and legs, as well as some vertebrae and part of one forearm. It is estimated that the complete skeleton would have been about 2 m in length.
Reconstruction of Talos sampsoni from the available parts of the skeleton. Drawing by Scott Hartman.
An unusual feature of this specimen is an injury to the second digit of the left foot. This injury appears to have been quite severe, so that the toe, while healed, is quite badly deformed. This sheds light on how the digit was used in life, since if this had been a weight bearing toe the an injury of this sort would have required the animal to modify its gait considerably, leading to distortion of other bones of the foot. Since this is not the case it is assumed that the toe did not bear weight, and must therefore have had some other function most probably as a weapon, as is seen in the (related) raptors and some species of modern birds. Injuries in such a digit would not be uncommon, but would not greatly endanger the animals survival. This feature has been seen previously in a number of Troodontids, though this is the first time palaeontologists have been able to confirm the toe is non-load bearing.
Reconstruction of Talos sampsoni, with an enlarged view of the foot. Image by Jorge Gozales of the Utah Museum of Natural History.
Troodontids are closely related to the 'raptor' dinosaurs (Dromaeosaurs), which are noted for their enlarged toe claws, but this appears to be a derived feature in the Troodontids; the earliest forms did not have it, and it appears to have developed in one lineage within the group.
There are over twenty species of described Troodontid, mostly from the Late Cretaceous. They are best known from Asia, though Talos joins a small number of North American specimens, and there are two named European species. It is quite possible that they were more widespread.
One possible species, Koparion douglassi, has been described from the Upper Jurassic, based upon isolated teeth from the Morrison Formation of Utah, though the designation of this as a Troodontid as at best controversial. Another, as yet undescribed, specimen has apparently been found in the Morrison Formation of Wyoming and is being held at the Wyoming Dinosaur Center while it awaits formal description. This specimen, WDC DML 001, is said to be substantially complete and clearly identifiable as a Troodontid.
An Early Cretaceous Troodontid has been described from Utah. Germiniraptor suarezarem was described from an upper jaw found in the Cedar Mountain Formation in a 2010 paper in PLoS ONE by a team lead by Phil Senter of the Department of Biological Sciences at Fayetteville State University.
Early Cretaceous Troodontids are also known from Asia. A common Early Cretaceous Troodontid from China appears to have been Sinovenator changii; this was originally described in a paper in the journal Nature by a team led by Xing Xu of the Chinese Academy of Sciences, based upon two specimens from the Yixian Formation, but Xu and Mark Norell of the American Museum of Natural History later reported that there are hundreds more undescribed specimens in a paper in the Geological Journal. Sinoventor was a chicken-sized dinosaur with feathers, long legs and a raised second digit on each hind foot, which had led to speculation that this was used as a weapon in a way similar to the related raptors; speculation which appears to have been justified by the discovery of Talos sampsoni.
A reconstruction of Sinovenator changii by Scott Hartman.
Jinfengopteryx elegans was described from a single, 55 cm, largely intact, well-feathered specimen from the Huajiying Formation of Hebei in eastern China. It was originally described as a bird, though is now considered to be a small Troodontid.
Jinfengopteryx elegans, a bird-like Troodontid from the Lower Cretaceous of China.
Another Early Cretaceous Troodontid from China is Mei long, named from a small (40 cm) probably juvenile specimen found in a position which suggests it had been sleeping, bird-like, with its head tucked under one fore-limb at the time of its death. Mei long was discovered in Liaoning Province in eastern China and described by Xing Xu or the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences and Mark Norell of the American Museum of Natural History in a paper in the journal Nature.
Reconstruction of Mei long by Natural History Illustrator Julius Csotonyi.
A second species of Early Jurassic Troodontid has also been found in this roosting position. Sinorthoides youngi was discovered in the Ejinhoro Formation in Inner Mongolia and described in a paper in the Canadian Journal of Earth Sciences by Dale Russell of the Department of Marine, Earth and Atmospheric Sciences at North Carolina State University and Dong Zhimeng of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences. It was about a meter long.
A CAT Scan of Sinornithoides youngi.
Another Early Jurassic species from China was Sinusonasus magnodens, described from a partial skeleton found in the Yixian Formation in Lioaning Province in a paper in the journal Acta Geologica Sinica by Xu Xing and Wang Xiaolin of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences. Sinusonasus was about a meter long with enlarged teeth in its upper jaw.
There is also an Early Cretaceous Troodontid from Mongolia, specimen SPS 100/44, which has not been formally named as it is a partial skeleton lacking sufficient distinguishing features to rule out its being a juvenile of another species. SPS 100/44 was discovered at the southern Goby Desert Khamareen Us location by Sergei Mikhailovich Kurzanov of the Paleontological Institute of the Academy of Sciences of the USSR during the 1979 Soviet-Mongolian Paleontological Expedition, and described in a paper in the journal Acta Palaeontologica Polonica by a team lead by Rinchin Barsbold of the Institute of Geology in Ulaanbaatar.
The group reached the peak of its diversity in the Late Cretaceous, and appears to have been going strong right up to the KT Boundary, though unlike the closely related birds they do not appear to have survived into the Tertiary.
A number of North American forms have been collected together under the generic name Troodon, though it is probable that this refers to a range of distinct species known from fragmentary skeletons. The names Paranychodon, Richardoestesia, Pectinodon, Polyodontosaurus, Steynonychosaurus and Saurornithoides have also been used for Late Cretaceous Troodontids, though it is not clear that any of these represent distinct forms as described. Zanno et al. observe that the North American Troodontids are in need of a comprehensive reclassification.
That said, a 'typical' specimen of Troodon was a two-and-a-half meter long, bird-like dinosaur weighing about 50 kg, with a raised claw on the second toe of each hind foot, forward facing eyes, and a large brain. No specimen with feathers has been found, but it is thought likely to Troodon did have plumage, given its similarity to species known to be feathered.
Another dubious form is Archaeornithoides, named from a fragmentary skull found in the Djadokhta Formation in Mongolia. Archaeornithoides was described in a paper in the journal Nature by Andrzej Elzanowski of the Max Planck Institut fur Biochemie and Peter Welnhoffer of the Bayerische Staatssammiung fuür Paläontologie und historische Geologie. It is an extremely bird-like form, but it is difficult to say whether it is genuinely a separate species from the material available.
Also from Mongolia comes Borogovia gracilis, a Troodontid known only from a single pair of partial hindlimbs, which imply an animal roughly 2 m in length. As with Archaeornithoides it is difficult to see if Borogovia genuinely represents a separate species from the available material. Borogovia was described in a paper in the journal Acta Palaeontologica Polonica by Halszka Osmólka of the Polska Akademia Nauk.
A third dubious Mongolian Troodontid from the Late Cretaceous is Tochisaurus nemegtensis, described from a single bone, the metatarsus (a bone in the foot).
A better described Mongolian species is Byronosaurus jaffei, known from two partial adult skeletons, both with skulls and two juvenile skulls. Byronosaurus was about 150 cm long and about 50 cm high; it probably weighed no more than 4 kg. The two juvenile specimens were found in an Oviraraptorsaur nest; they were probably prey items, but there has been some speculation that this may represent brood parasitism, as in modern cuckoos or cow-birds. Byronosaurus was described in a paper in the journal Science by a team lead by Mark Norell of the American Museum of Natural History.
Another Late Cretaceous Mongolian Troodontid is Saurornithoides mongoliensis. This is a particularly large species, estimated to be 2-3 m long and to weigh between 23 and 54 kg, described from a single specimen from the Djadochta Formation found during one of Roy Chapman Adams's trips to Mongolia, and described in a paper in the American Museum Noviates by Henry Fairfield Osborn of the American Museum of Natural History. Saurornithiodes had binocular vision and an enlarged second toe claw on each foot.
A second species of large, Late Cretaceous, Mongolian Troodontid was originally placed in the genus Saurornithiodes, but is now considered sufficiently distinct to merit its own genus. This is Zanabazar junior, originally described from a skull and partial skeleton by Rinchen Barsbold, and reclassified in a paper in the American Museum Noviates by a team lead by Mark Norell of the American Museum of Natural History.
The skull of Zanabazar junior.
Linhevenator tani comes from the Late Cretaceous of Inner Mongolia, China. It was described in a paper in PLoS ONE by a team lead by Xu Xing of the Chinese Academy of Sciences, from a single specimen, LVH0021 from the Mandahu Formation in the Linhe region. The specimen was fragmentary, but largely complete; certainly enough that Linhevenator can be considered to be a separate species with confidence. It is a large Troodontid, weighing about 23 kg, and had a raised claw on the second digit of each hind foot.
Reconstruction of Linhevenator tani by dinosaur artist Nobu Tamara.
Xixiasaurus henansis is a species of Late Cretaceous Troodontid described from a partial skull from the Majiacun Formation in Henan Province in eastern China. The skull is well preserved, but again the possibility that this species has been described elsewhere from other body parts cannot be ruled out. The description was published in a paper in the journal Acta Palaeontologica Polonica by a team led by Junchang Lü of the Institute of Geology at the Chinese Academy of Geological Sciences.
The skull of Xixiasuarus henansis.
Urbacodon itemirensis is a Troodontid dinosaur from the Late Cretaceous of Uzbekistan. It was described from a partial lower jaw, with teeth, and could well, therefore, have been described elsewhere as another species from a different part of its anatomy. The description of Urbacodon was published in a paper in the Journal of Vertebrate Paleontology by Alexander Averianov of the Zoological Institute of the Russian Academy of Sciences and Hans-Dieter Sues of the National Museum of Natural History at the Smithsonian Institution.
Two highly dubious Troodontids have been described from Europe. Elopteryx nopcsai was described from a partial femur and tibia from Romania, though the tibia is now considered to be from a separate animal. Euronychodon portucalensis was described from a single tooth from Portugal. A second species of Euronychodon, E. asiaticus was later described from a single tooth from Uzbekistan; this is considered an even more dubious classification.