Troodon Leidy 1856

Troodon was a relatively small, bird-like dinosaur from the Late Cretaceous (75-65 million years ago). It was discovered in 1855 and ranged from Alaska south to Wyoming and possibly Texas and New Mexico. Troodon is known from the Judith River Formation and the upper Two Medicine Formation of Montana, the Judith River Group and Horseshoe Canyon Formation of Alberta, the Prince Creek Formation of Alaska, and in the Lance and Hell Creek Formations of the USA. There is some evidence that Troodon favoured cooler climates, as it seems to have been most abundant in northern and even arctic areas and during cooler intervals, such as the early Maastrichtian.[1] Possible Troodon teeth have been found in the lower Javelina Formation of Texas and the Naashoibito Member of the Kirtland Formation in New Mexico.[2][3]

It seems unlikely that all these fossils, from localities hundreds or thousands of miles apart, separated by millions of years of time, represent one species of Troodon.

'Troodon' is Greek for "wounding tooth". The teeth bear apically oriented serrations, which resemble those of herbivorous reptiles and suggest a possibly omnivorous diet.[4] A partial Troodon skeleton had preserved puncture marks.[5] Troodon was around 2.4 m (7.9 ft) long and weighed about 50 kg (110 lb). It was the largest troodontid. It had very long, slender hind limbs and could probably run quickly. It had large, retractable sickle-shaped claws on its second toes, which were raised off the ground when running. Its large, slightly forward-facing eyes suggest a partially nocturnal lifestyle with some degree of depth perception. In Troodon "the transverse sinus probably drained into a middle cerebral vein that exited the brain in the ridge on the dorsal edge of the trigeminal foramen."[6] The crista supporting the tympanic membrane were ossified at least in their dorsal and ventral regions and their remaining portions were cartilaginous or too delicate to be preserved.[6] The metotic strut of Troodon was "laterally hypertrophied".[6] This condition resembles that of Dromaeosaurus and primitive birds like Archaeopteryx and Hesperornis.[6] The type specimen of Troodon is based only on a tooth from the Judith River Formation. The tooth was classified as a "lacertilian" (lizard) by Leidy, but re-assigned as a megalosaurid dinosaur by Nopcsa in 1901; the Megalosauridae was a wastebin taxon for most carnivorous dinosaurs. In 1924, Gilmore suggested that the tooth belonged to the herbivorous pachycephalosaur Stegoceras and that Stegoceras was a junior synonym of Troodon. Troodon was clasified as a pachycephalosaur for many years and the family Pachycephalosauridae was named Troodontidae. In 1945, Charles Mortram Sternberg rejected the possibility that Troodon was a pachycephalosaur due to its teeth being similar to those teeth of other carnivorous dinosaurs. As Troodon was classified as a theropod, the family Troodontidae could not be used for dome-headed dinosaurs, so Sternberg called the family Pachycephalosauridae.[7] The first specimen assigned to Troodon that was not a tooth was named Stenonychosaurus by Sternberg in 1932, based on a foot, hand fragments and some tail vertebrae from Alberta. The enlarged claw on the second toe is recognized as characteristic of Deinonychosauria. Sternberg y classified Stenonychosaurus as a member of the family Coeluridae. In 1951, Sternberg speculated that as Stenonychosaurus had a "very peculiar pes" and Troodon "equally unusual teeth", they may be closely related.

In 1969, Dale Russell described a skeleton of Stenonychosaurus from the Dinosaur Park Formation; this formed the scientific foundation for a life-sized sculpture of Stenonychosaurus accompanied by its fictional, human-like descendant, the "dinosauroid".[8] Stenonychosaurus and Saurornithoides formed the family Saurornithoididae. Based on differences in tooth structure and the very fragmentary nature of the original Troodon specimens, Saurornithoidids were thought to be close relatives while Troodon was considered a dubious possible relative of the family. In 1987, Phil Currie showed that supposed differences in tooth and jaw structure among troodontids and saurornithoidids were based on age and position of the tooth in the jaw, rather than a difference in species. He reclassified Stenonychosaurus inequalis, Polyodontosaurus grandis and Pectinodon bakkeri as junior synonyms of Troodon formosus and made Saurornithoididae a junior synonym of Troodontidae.[9].

In Currie and colleagues thought the Judith River troodontids were all T. formosus, but thought troodontid fossils from other formations, such as the Hell Creek and Lance Formations, may belong to different species. In 1991, George Olshevsky assigned the Lance formation fossils to Troodon bakkeri (formerly Pectinodon) and several other researchers (including Currie) reverted to keep the Dinosaur Park Formation fossils separate as Troodon inequalis (formerly Stenonychosaurus).[10]

In his 1988 book Predatory Dinosaurs of the World, Gregory S. Paul included the Mongolian Saurornithoides mongoliensis as Troodon mongoliensis.[11].

Troodon is thought to have been a predator like other theropods. This is supported by its sickle claw on the foot and apparently good binocular vision. Troodon teeth differ from most other theropods. One study suggests that Troodon could have been an omnivore.[4] The jaws met in a broad, U-shaped symphysis similar to that of an iguana. The teeth of Troodon bore large serrations, each being a denticle. There are pits at the intersections of the denticles, the points of which point towards the apex of each tooth. The teeth show wear facets on their sides. Holtz (4) noted that characteristics used to support a predatory habit for Troodon, the grasping hands, large brain and stereoscopic vision, are characteristics shared with the herbivorous/omnivorous primates and omnivorous raccoon.

A study was based on the many Troodon teeth collected from Late Cretaceous deposits from northern Alaska. These teeth are much larger than those collected from more southern sites, indicating that northern Alaskan populations of Troodon grew larger and may have had access to large animals as prey as there were no tyrannosaurids to compete for those resources. The study analysed the proportions and wear patterns of Troodon teeth. It proposes that the wear patterns of Troodon teeth suggest a diet of soft foods, inconsistent with bone chewing, invertebrate exoskeletons or tough plant items. This study hypothesizes a diet mainly of meat[12]

Troodon probably reached its adult size in 3–5 years.[12]

John R. Horner discovered dinosaur eggs and nests in the Two Medicine Formation of Montana in 1983. Varriccho et al. (13) described eight of these nests. Horner (14) found isolated bones and partial skeletons of the hypsilophodont Orodromeus very near the nests in the same horizon and described the eggs as those of Orodromeus.[15] Horner and Weishampel (16) reexamined the embryos in the eggs and determined they were those of Troodon, not Orodromeus.[18] Varricchio et al. (17) described a partial skeleton of an adult Troodon in contact with a clutch of at least five eggs, probably in a brooding position. They described nests that were built from sediments, dish-shaped, about 100 cm in internal diameter with a raised rim encircling the eggs. The more complete nests had 16-24 eggs, shaped like elongated teardrops, with the more tapered ends pointed downwards and imbedded about halfway in the sediment. The eggs are pitched at an angle so the upper half is usually nearer the centre of the nest. There is no evidence of plant matter in the nest. Varricchio et al.(17) inferred that Troodon may have had an intermediate type of reproduction between crocodiles and birds. The eggs are statistically grouped in pairs, so the animal probably had two functional oviducts, like crocodiles, rather than one, as in birds. Crocodiles lay many eggs that are small proportional to adult body size. Birds lay fewer, larger, eggs. Troodon was intermediate, laying an egg of about 0.5 kg for a 50 kg adult. This is 10 times larger than reptiles of the same mass, but two Troodon eggs are roughly equivalent to the 1.1 kg egg predicted for a 50 kg bird. Varricchio et al. found evidence for iterative laying, where an adult may have laid a pair of eggs every one or two days and ensured simultaneous hatching by delaying brooding until all eggs were laid. One nest had 22 empty (hatched) eggs and the embryos in the eggs of another nest were in very similar states of development, implying that all the young hatched simultaneously. The embryos had an advanced degree of skeletal development, implying they were precocial or even superprecocial. The authors estimated 45-65 days of adult nest attendance for laying, brooding, and hatching. The authors found no evidence that the young stayed in the nest after hatching and suggested that they dispersed like hatchling crocodiles or megapode birds.[13] Varricchio et al. (15) found that the bone histology of Troodon specimen lacked the bone resorption patterns that would indicate it was an egg-laying female. They measured the ratio of the total volume of eggs in Troodon clutches to the body mass of the adult and graphed correlations between this ratio and the type of parenting strategies used by birds and crocodiles. That the ratio in Troodon was consistent with that in birds where only the adult male broods the eggs. From this they concluded that Troodon females probably did not brood eggs, but males did. This may be a character shared between maniraptoran dinosaurs and basal birds.[15]

The light skull contained a capsule similar to that of ostrich dinosaurs.[9]Troodon had one of the largest known brains of any dinosaur, relative to its body mass (comparable to modern birds).[6] The cerebrum-to-brain-volume ratio was 31.5% to 63% of the way from a nonavian reptile proportion to a truly avian one.[7] In 1982, Dale Russell (8) conjectured a possible evolutionary path that Troodon might have taken if it had survived the K/T extinction event 65 million years ago. Russell suggested it could have evolved into intelligent beings similar in body plan to humans. Over geologic time, Russell noted a steady increase in the encephalization quotient or EQ (the relative brain weight compared to other species with the same body weight) among the dinosaurs. Russell noted that the EQ of the first Troodontid skull was low compared to humans, but six times higher than that of other dinosaurs. If the trend in Troodon evolution had continued to the present, its brain case could now measure 1,100 cm3; comparable to that of a human. Troodontids had semi-manipulative fingers, able to grasp and hold objects to some certain degree, and binocular vision.[8] Russell proposed that this "Dinosauroid", like most dinosaurs of the troodontid family, would have had large eyes and three fingers on each hand, one of which would be partially opposed. As with most modern reptiles and birds, he conceived of its genitalia as internal. Russell speculated that it would have required a navel, as a placenta aids the development of a large brain case. It would not have had mammary glands, but would have fed its young, as birds do, on regurgitated food. He speculated that its language would have sounded like bird song.[8][18]. In his 1977 book, The Dragons of Eden: Speculations on the Evolution of Human Intelligence, Carl Sagan speculated about the related genus Saurornithoides evolving into into more intelligent forms in the absence of an extinction event. In a world dominated by Saurornithoides, Sagan mused, arithmetic would be Base 8 rather than Base 10. Russell's thought experiment has been met with criticism from other paleontologists since the 1980s, many of whom say his Dinosauroid is overly anthropomorphic. Gregory S. Paul (11) and Thomas R. Holtz, Jr. consider it "suspiciously human" (11) and Darren Naish argued that a large-brained, highly intelligent troodontid would retain a more standard theropod body plan, with a horizontal posture and long tail, and would probably manipulate objects with the snout and feet like a bird, rather than with human-like "hands".[18]