Troodon
Encyclopedia
Troodon is a genus
of relatively small, bird
-like dinosaur
from the Late Cretaceous
period (75-65 mya). Discovered in 1855, it was among the first dinosaurs found in North America
. Its species ranged widely, with fossil remains recovered from as far north as Alaska
and as far south as Wyoming
and even possibly Texas
and New Mexico
.
Its name is Greek
for "wounding tooth", referring to the dinosaur's teeth, which are different from those of most other theropods. The teeth bear prominent, apically oriented serrations. These "wounding" serrations, however, are morphometrically more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet. A partial Troodon skeleton has been discovered with preserved puncture marks.
and Unenlagia
. Troodon is the largest known troodontid. It had very long, slender hind limbs, suggesting that the animal was able to run quickly. It had large, retractable sickle-shaped claws on its second toes, which were raised off of the ground, when running.
Its eyes were large (perhaps suggesting a partially nocturnal lifestyle), and slightly forward facing, giving Troodon some degree of depth perception
. Their light skulls contained a capsule similar to those found in ostrich dinosaurs.
' s cerebrum-to-brain-volume ratio was 31.5% to 63% of the way from a nonavian reptile proportion to a truly avian one. In Troodon "the transverse sinus probably drained into a middle cerebral vein
that exited the brain in the ridge present on the dorsal edge of the trigeminal foramen." Troodon had crista supporting its tympanic membrane that were ossified at least in their dorsal and ventral regions and their remaining portions either cartilaginous or too delicate to be preserved. The metotic strut of Troodon was "laterally hypertrophied". This condition resembles that of Dromaeosaurus and primitive birds like Archaeopteryx
and Hesperornis
.
in 1856, which was officially amended to its current status by Sauvage in 1876. The type specimen of Troodon has caused problems with classification, as the entire genus is based only on a single tooth from the Judith River Formation
. Troodon has historically been a highly unstable classification and has been the subject of numerous conflicting synonymies with similar theropod specimens.
The Troodon tooth was originally classified as a "lacertilian" (lizard
) by Leidy, but re-assigned as a megalosaurid
dinosaur by Nopcsa in 1901 (Megalosauridae having historically been a wastebin taxon for most carnivorous dinosaurs). In 1924, Gilmore suggested that the tooth belonged to the herbivorous pachycephalosaur Stegoceras
, and that Stegoceras was in fact a junior synonym of Troodon (the similarity of troodontid teeth to those of herbivorous dinosaurs continues to lead many paleontologists to believe that these animals were omnivores). The classification of Troodon as a pachycephalosaur was followed for many years, during which the family Pachycephalosauridae was known as Troodontidae
. In 1945, Charles Mortram Sternberg
rejected the possibility that Troodon was a pachycephalosaur due to its stronger similarity to the teeth of other carnivorous dinosaurs. With Troodon now classified as a theropod, the family Troodontidae could no longer be used for the dome-headed dinosaurs, so Sternberg named a new family for them, Pachycephalosauridae.
The first specimen currently assigned to Troodon that was not a tooth was named Stenonychosaurus by Sternberg in 1932, based on a foot, fragments of a hand, and some tail vertebrae from Alberta
. A remarkable feature of these remains was the enlarged claw on the second toe, which is now recognized as characteristic of Deinonychosauria
. Sternberg initially classified Stenonychosaurus as a member of the family Coeluridae
. Later, in 1951, Sternberg speculated that since Stenonychosaurus had a "very peculiar pes
" and Troodon "equally unusual teeth", they may be closely related. Unfortunately, no comparable specimens were available at that time to test the idea.
A more complete skeleton of Stenonychosaurus was described by Dale Russell
in 1969 from the Dinosaur Park Formation
, which eventually formed the scientific foundation for a famous life-sized sculpture of Stenonychosaurus accompanied by its fictional, humanoid
descendant, the "dinosauroid". Stenonychosaurus became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Along with Saurornithoides
, it formed the family Saurornithoididae. Based on differences in tooth structure, and the extremely 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. Phil Currie
, reviewing the pertinant specimens in 1987, 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 as well as Polyodontosaurus grandis and Pectinodon bakkeri as junior synonyms of Troodon formosus. Currie also made Saurornithoididae a junior synonym of Troodontidae. In 1988, Gregory S. Paul
went farther and included Saurornithoides mongoliensis in the genus Troodon as T. mongoliensis, but this reclassification, along with many other unilateral synonymizations of well known genera, was not adopted by other researchers. Currie's classification of all North American troodontid material in the single species Troodon formosus became widely adopted by other paleontologists
, and all of the specimens once called Stenonychosaurus were referred to as Troodon in the scientific literature through the early 21st century.
However, the concept that all Late Cretaceous North American troodontids belong to one species began to be questioned soon after Currie's 1987 paper was published, including by Currie himself. Currie and colleagues (1990) noted that, while they believed the Judith River troodontids were all T. formosus, troodontid fossils from other formations, such as the Hell Creek Formation
and Lance Formation
, might belong to different species. In 1991, George Olshevsky assigned the Lance formation fossils, which had first been named Pectinodon bakkeri but later synonymized with Troodon formosus to the species Troodon bakkeri, and several other researchers (including Currie) have reverted to keeping the Dinosaur Park Formation fossils separate as Troodon inequalis (formerly Stenonychosaurus).
In 2011, Zanno and colleagues reviewed the convoluted history of troodontid classification in Late Cretaceous North America. They followed Longrich (2008) in treating Pectinodon bakkeri as a valid genus, and noted that it is likely the numerous Late Cretaceous specimens currently assigned to Troodon formosus almost certainly represent numerous new species, but that a more thorough review of the specimens is required. Because the holotype of T. formosus is a single tooth, this may render Troodon a nomen dubium
.
and the upper Two Medicine Formation
of Montana
, the Judith River Group
of Alberta
, the Horseshoe Canyon Formation
of Alberta, the Prince Creek Formation
of Alaska
, and in the famous Lance
and Hell Creek Formation
s of the USA. There is some evidence that Troodon favored cooler climates, as it seems to have been particularly abundant in northern and even arctic
areas and during cooler intervals, such as the early Maastrichtian
. Possible Troodon teeth have been found in the lower Javelina Formation
of Texas
and the Naashoibito Member of the Kirtland Formation
in New Mexico
.
It seems unlikely that all of these fossils, which come from localities hundreds or thousands of miles apart, separated by millions of years of time, represent a single species of Troodon. However, further study and more fossils are needed to determine how many species of Troodon existed.
.
Troodon teeth, however, are different from most other theropods. One comparative study of the feeding apparatus suggests that Troodon could have been an omnivore
. The jaws met in a broad, U-shaped symphysis
similar to that of an iguana
, a lizard species adapted to a plant-eating lifestyle. Additionally, the teeth of Troodon bore large serrations each of which is called a denticle. There are pits at the intersections of the denticles, and the points of the denticles point towards the tip, or apex, of each tooth. The teeth show wear facets on their sides. Holtz (1998) also noted that characteristics used to support a predatory habit for Troodon - the grasping hands, large brain and stereoscopic vision, are all characteristics shared with the herbivorous/omnivorous primate
s and omnivorous Procyon
(raccoon).
One study was based on the many Troodon teeth that have been collected from Late Cretaceous deposits from northern Alaska. These teeth are much larger than those collected from more southern sites, providing evidence that northern Alaskan populations of Troodon grew to larger average body size. The study suggests that the Alaskan Troodons may have had access to large animals as prey because there were no tyrannosaurids in their habitat to provide competition for those resources. This study also provides an analysis of the proportions and wear patterns of a large sample of Troodon teeth. It proposes that the wear patterns of all Troodon teeth suggest a diet of soft foods - inconsistent with bone chewing, invertebrate exoskeletons, or tough plant items. This study hypothesizes a diet primarily consisting of meat
Age determination
studies performed on the fossilized remains of Troodon using growth ring counts suggest that this dinosaur reached its adult size probably in 3–5 years.
, but none were found.
and their accession numbers are MOR 246, 299, 393, 675, 676, 750, 963, 1139. Horner (1984) 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
. Horner and Weishampel (1996) reexamined the embryos preserved in the eggs and determined that they were those of Troodon, not Orodromeus. Varricchio et al. (1997) made this determination with even more certainty when they described a partial skeleton of an adult Troodon (MOR 748) in contact with a clutch of at least five eggs (MOR 750), probably in a brooding position.
Varricchio et al. (1997) described the exact structure of Troodon nests. They were built from sediments, they were dish shaped, about 100 cm in internal diameter, and with a pronounced raised rim encircling the eggs. The more complete nests had between 16 (minimum number in MOR 246) and 24 (MOR 963) eggs. The eggs are 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 that, on average, the upper half is closer to the center of the nest. There is no evidence that plant matter was present in the nest.
Varricchio et al.(1997) were able to extract enough evidence from the nests to infer several characteristics of Troodon reproductive biology. The results are that Troodon appears to have a type of reproduction that is intermediate between crocodiles and birds, as phylogeny would predict. The eggs are statistically grouped in pairs, which suggests that the animal 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. also found evidence for iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. MOR 363 was found with 22 empty (hatched) eggs, and the embryos found in the eggs of MOR 246 were in very similar states of development, implying that all of the young hatched simultaneously. The embryos had an advanced degree of skeletal development, implying that they were precocial
or even superprecocial
. The authors estimated 45 to 65 total days of adult nest attendance for laying, brooding, and hatching. The authors found no evidence that the young remained in the nest after hatching and suggested that they dispersed like hatchling crocodiles or megapode birds instead.
Varricchio et al. (2008) examined the bone histology of Troodon specimen MOR 748 and found that it lacked the bone resorption patterns that would indicate it was an egg-laying female. They also measured the ratio of the total volume of eggs in Troodon clutches to the body mass of the adult. They graphed correlations between this ratio and the type of parenting strategies used by extant birds and crocodiles and found 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 likely did not brood eggs, that the males did, and this may be a character shared between maniraptoran dinosaurs and basal birds.
In 1982, Dale Russell, then curator of vertebrate fossils at the National Museum of Canada
in Ottawa
, conjectured a possible evolutionary path that might have been taken by Troodon had it not perished in the K/T extinction event 65 million years ago, suggesting that it could have evolved into intelligent beings similar in body plan to humans. Over geologic time, Russell noted that there had been a steady increase in the encephalization quotient
or EQ (the relative brain weight when compared to other species with the same body weight) among the dinosaurs. Russell had discovered the first Troodontid skull, and noted that, while its EQ was low compared to humans, it was six times higher than that of other dinosaurs. If the trend in Troodon evolution had continued to the present, its brain case could by now measure 1,100 cm3; comparable to that of a human. Troodontids had semi-manipulative fingers, able to grasp and hold objects to a certain degree, and binocular vision.
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 have been 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. However, it would not have possessed mammary glands, and would have fed its young, as some birds do, on regurgitated food. He speculated that its language would have sounded somewhat like bird song
.
Russell's thought experiment has been met with criticism from other paleontologists since the 1980s, many of whom point out that his Dinosauroid is overly anthropomorphic. Gregory S. Paul
(1988) and Thomas R. Holtz, Jr., consider it "suspiciously human" (Paul, 1988) and Darren Naish
has 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 in the manner of a bird, rather than with human-like "hands".
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
of relatively small, bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
-like dinosaur
Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
from the Late Cretaceous
Late Cretaceous
The Late Cretaceous is the younger of two epochs into which the Cretaceous period is divided in the geologic timescale. Rock strata from this epoch form the Upper Cretaceous series...
period (75-65 mya). Discovered in 1855, it was among the first dinosaurs found in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. Its species ranged widely, with fossil remains recovered from as far north as Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
and as far south as Wyoming
Wyoming
Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...
and even possibly Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
and New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
.
Its name is Greek
Ancient Greek
Ancient Greek is the stage of the Greek language in the periods spanning the times c. 9th–6th centuries BC, , c. 5th–4th centuries BC , and the c. 3rd century BC – 6th century AD of ancient Greece and the ancient world; being predated in the 2nd millennium BC by Mycenaean Greek...
for "wounding tooth", referring to the dinosaur's teeth, which are different from those of most other theropods. The teeth bear prominent, apically oriented serrations. These "wounding" serrations, however, are morphometrically more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet. A partial Troodon skeleton has been discovered with preserved puncture marks.
Description
Troodon was a small dinosaur, around 2.4 metres (7.9 ft) in length, and weighing on the order of 50 kilograms (110.2 lb). It was comparable in size to DeinonychusDeinonychus
Deinonychus was a genus of carnivorous dromaeosaurid dinosaur. There is one described species, Deinonychus antirrhopus. This 3.4 meter long dinosaur lived during the early Cretaceous Period, about 115–108 million years ago . Fossils have been recovered from the U.S...
and Unenlagia
Unenlagia
Unenlagia is a genus of, possibly dromaeosaurid, theropod dinosaur from the Late Cretaceous of Argentina....
. Troodon is the largest known troodontid. It had very long, slender hind limbs, suggesting that the animal was able to run quickly. It had large, retractable sickle-shaped claws on its second toes, which were raised off of the ground, when running.
Its eyes were large (perhaps suggesting a partially nocturnal lifestyle), and slightly forward facing, giving Troodon some degree of depth perception
Depth perception
Depth perception is the visual ability to perceive the world in three dimensions and the distance of an object. Depth sensation is the ability to move accurately, or to respond consistently, based on the distances of objects in an environment....
. Their light skulls contained a capsule similar to those found in ostrich dinosaurs.
Brain and inner ear
Troodon had one of the largest known brains of any dinosaur, relative to its body mass (comparable to modern birds). TroodonMiddle cerebral vein
The superficial middle cerebral vein begins on the lateral surface of the hemisphere, and, running along the lateral cerebral sulcus, ends in the cavernous or the sphenoparietal sinus.-Relations:It is connected:...
that exited the brain in the ridge present on the dorsal edge of the trigeminal foramen." Troodon had crista supporting its tympanic membrane that were ossified at least in their dorsal and ventral regions and their remaining portions either cartilaginous or too delicate to be preserved. The metotic strut of Troodon was "laterally hypertrophied". This condition resembles that of Dromaeosaurus and primitive birds like Archaeopteryx
Archaeopteryx
Archaeopteryx , sometimes referred to by its German name Urvogel , is a genus of theropod dinosaur that is closely related to birds. The name derives from the Ancient Greek meaning "ancient", and , meaning "feather" or "wing"...
and Hesperornis
Hesperornis
Hesperornis is a genus of flightless aquatic birds that spanned the first half of the Campanian age of the Late Cretaceous period . One of the lesser-known discoveries of the paleontologist O. C. Marsh in the late 19th century Bone Wars, it was an important early find in the history of avian...
.
History and classification
The name was originally spelled Troödon (with a diaeresis) by Joseph LeidyJoseph Leidy
Joseph Leidy was an American paleontologist.Leidy was professor of anatomy at the University of Pennsylvania, and later was a professor of natural history at Swarthmore College. His book Extinct Fauna of Dakota and Nebraska contained many species not previously described and many previously...
in 1856, which was officially amended to its current status by Sauvage in 1876. The type specimen of Troodon has caused problems with classification, as the entire genus is based only on a single tooth from the Judith River Formation
Judith River Formation
The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the upper Cretaceous, between 80 and 75 million years ago, corresponding to the "Judithian" land vertebrate age...
. Troodon has historically been a highly unstable classification and has been the subject of numerous conflicting synonymies with similar theropod specimens.
The Troodon tooth was originally classified as a "lacertilian" (lizard
Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
) by Leidy, but re-assigned as a megalosaurid
Megalosaurid
Megalosauridae was a family of relatively primitive tetanuran theropod dinosaurs, order Saurischia. They were small-to-large carnivores with sharp teeth and three claws on each hand. Some members of this group were Megalosaurus, Eustreptospondylus, Streptospondylus and Torvosaurus...
dinosaur by Nopcsa in 1901 (Megalosauridae having historically been a wastebin taxon for most carnivorous dinosaurs). In 1924, Gilmore suggested that the tooth belonged to the herbivorous pachycephalosaur Stegoceras
Stegoceras
Stegoceras is a genus of plant-eating pachycephalosaurid dinosaur that lived in what is now North America during the Late Cretaceous period....
, and that Stegoceras was in fact a junior synonym of Troodon (the similarity of troodontid teeth to those of herbivorous dinosaurs continues to lead many paleontologists to believe that these animals were omnivores). The classification of Troodon as a pachycephalosaur was followed for many years, during which the family Pachycephalosauridae was known as Troodontidae
Troodontidae
Troodontidae is a family of bird-like theropod dinosaurs. During most of the 20th century, troodontid fossils were few and scrappy and they have therefore been allied, at various times, with many dinosaurian lineages...
. In 1945, Charles Mortram Sternberg
Charles Mortram Sternberg
Charles M. Sternberg was an American-Canadian fossil collector and paleontologist, son of Charles Hazelius Sternberg.Late in his career, he collected and described Pachyrhinosaurus, Brachylophosaurus, Parksosaurus and Edmontonia...
rejected the possibility that Troodon was a pachycephalosaur due to its stronger similarity to the teeth of other carnivorous dinosaurs. With Troodon now classified as a theropod, the family Troodontidae could no longer be used for the dome-headed dinosaurs, so Sternberg named a new family for them, Pachycephalosauridae.
The first specimen currently assigned to Troodon that was not a tooth was named Stenonychosaurus by Sternberg in 1932, based on a foot, fragments of a hand, and some tail vertebrae from Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
. A remarkable feature of these remains was the enlarged claw on the second toe, which is now recognized as characteristic of Deinonychosauria
Deinonychosauria
The Deinonychosauria were a clade of coelurosaurian theropod dinosaurs from the Late Jurassic and Cretaceous periods. These omnivores and carnivores are known for their switchblade-like second toe claws and for displaying numerous bird-like characteristics. The clade has been divided into two...
. Sternberg initially classified Stenonychosaurus as a member of the family Coeluridae
Coeluridae
Coeluridae is a historically polyphyletic family of generally small, carnivorous dinosaurs from the late Jurassic Period...
. Later, in 1951, Sternberg speculated that since Stenonychosaurus had a "very peculiar pes
Pes (zoology)
The pes is the zoological term for the distal portion of the hind limb of tetrapod animals. It is the part of the pentadactyl limb that includes the metatarsals and digits . During evolution, it has taken many forms and served a variety of functions...
" and Troodon "equally unusual teeth", they may be closely related. Unfortunately, no comparable specimens were available at that time to test the idea.
Dale Russell
Dale A. Russell is a Canadian geologist/palaeontologist, currently Research Professor at The Department of Marine Earth and Atmospheric Sciences of North Carolina State University...
in 1969 from the Dinosaur Park Formation
Dinosaur Park Formation
The Dinosaur Park Formation is the uppermost member of the Judith River Group, a major geologic unit in southern Alberta. It was laid down over a period of time between about 76.5 and 75 million years ago. The formation is made up of deposits of a high-sinuosity fluvial system, and is capped...
, which eventually formed the scientific foundation for a famous life-sized sculpture of Stenonychosaurus accompanied by its fictional, humanoid
Humanoid
A humanoid is something that has an appearance resembling a human being. The term first appeared in 1912 to refer to fossils which were morphologically similar to, but not identical with, those of the human skeleton. Although this usage was common in the sciences for much of the 20th century, it...
descendant, the "dinosauroid". Stenonychosaurus became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Along with Saurornithoides
Saurornithoides
Saurornithoides is a genus of troodontid maniraptoran dinosaur, living during the Late Cretaceous period. These creatures were predators, which could run fast on their hind legs and had excellent sight and hearing...
, it formed the family Saurornithoididae. Based on differences in tooth structure, and the extremely 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. Phil Currie
Phil Currie
Philip John Currie, AOE is a Canadian palaeontologist and museum curator who helped found the Royal Tyrrell Museum of Palaeontology in Drumheller, Alberta and is now a professor at the University of Alberta in Edmonton...
, reviewing the pertinant specimens in 1987, 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 as well as Polyodontosaurus grandis and Pectinodon bakkeri as junior synonyms of Troodon formosus. Currie also made Saurornithoididae a junior synonym of Troodontidae. In 1988, Gregory S. Paul
Gregory S. Paul
Gregory Scott Paul is a freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dinosaurs and his detailed illustrations, both live and skeletal...
went farther and included Saurornithoides mongoliensis in the genus Troodon as T. mongoliensis, but this reclassification, along with many other unilateral synonymizations of well known genera, was not adopted by other researchers. Currie's classification of all North American troodontid material in the single species Troodon formosus became widely adopted by other paleontologists
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
, and all of the specimens once called Stenonychosaurus were referred to as Troodon in the scientific literature through the early 21st century.
However, the concept that all Late Cretaceous North American troodontids belong to one species began to be questioned soon after Currie's 1987 paper was published, including by Currie himself. Currie and colleagues (1990) noted that, while they believed the Judith River troodontids were all T. formosus, troodontid fossils from other formations, such as the Hell Creek Formation
Hell Creek Formation
The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...
and Lance Formation
Lance Formation
The Lance Formation is a division of Late Cretaceous rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the latest Mesozoic vertebrate faunas...
, might belong to different species. In 1991, George Olshevsky assigned the Lance formation fossils, which had first been named Pectinodon bakkeri but later synonymized with Troodon formosus to the species Troodon bakkeri, and several other researchers (including Currie) have reverted to keeping the Dinosaur Park Formation fossils separate as Troodon inequalis (formerly Stenonychosaurus).
In 2011, Zanno and colleagues reviewed the convoluted history of troodontid classification in Late Cretaceous North America. They followed Longrich (2008) in treating Pectinodon bakkeri as a valid genus, and noted that it is likely the numerous Late Cretaceous specimens currently assigned to Troodon formosus almost certainly represent numerous new species, but that a more thorough review of the specimens is required. Because the holotype of T. formosus is a single tooth, this may render Troodon a nomen dubium
Nomen dubium
In zoological nomenclature, a nomen dubium is a scientific name that is of unknown or doubtful application...
.
Distribution
Troodon is known from the Judith River FormationJudith River Formation
The Judith River Formation is a fossil-bearing geologic formation in Montana, and is part of the Judith River Group. It dates to the upper Cretaceous, between 80 and 75 million years ago, corresponding to the "Judithian" land vertebrate age...
and the upper Two Medicine Formation
Two Medicine Formation
The Two Medicine Formation is a geologic formation, or rock body, that was deposited between 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma , during Campanian time, and is located in northwestern Montana...
of Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
, the Judith River Group
Judith River Group
The Judith River Group is a group of geologic formations in western North America dating from the late Cretaceous and noted as a site for the extensive excavation of dinosaur fossils. The formation is named after the Judith River in Montana. The group is also called the Judith River Wedge...
of Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
, the Horseshoe Canyon Formation
Horseshoe Canyon Formation
The Horseshoe Canyon Formation is part of the Edmonton Group and is up to 230m in thickness. It is Late Campanian to Early Maastrichtian in age and is composed of mudstone, sandstone, and carbonaceous shales...
of Alberta, the Prince Creek Formation
Prince Creek Formation
The Prince Creek Formation is a geological formation in Alaska with strata from the early Maastrichtian stage of the Upper Cretaceous, dating to between 70 and 69 million years ago...
of Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
, and in the famous Lance
Lance Formation
The Lance Formation is a division of Late Cretaceous rocks in the western United States. Named after Lance Creek, Wyoming, the microvertebrate fossils and dinosaurs represent important components of the latest Mesozoic vertebrate faunas...
and Hell Creek Formation
Hell Creek Formation
The Hell Creek Formation is an intensely-studied division of Upper Cretaceous to lower Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana...
s of the USA. There is some evidence that Troodon favored cooler climates, as it seems to have been particularly abundant in northern and even arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
areas and during cooler intervals, such as the early Maastrichtian
Maastrichtian
The Maastrichtian is, in the ICS' geologic timescale, the latest age or upper stage of the Late Cretaceous epoch or Upper Cretaceous series, the Cretaceous period or system, and of the Mesozoic era or erathem. It spanned from 70.6 ± 0.6 Ma to 65.5 ± 0.3 Ma...
. Possible Troodon teeth have been found in the lower Javelina Formation
Javelina Formation
The Javelina Formation is a geological formation in Texas. Dating has shown that the strata date to the mid-late Maastrichtian stage of the Late Cretaceous, with the middle part of the formation dated to about 69 million years ago plus or minus 1 Ma and the top situated near the K-Pg boundary,...
of Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
and the Naashoibito Member of the Kirtland Formation
Kirtland Formation
The Kirtland Formation is a sedimentary geological formation. It is the product of alluvial muds and overbank sand deposits from the many channels draining the coastal plain that existed on the inland seashore of North America, in the late Cretaceous period. It overlies the Fruitland Formation...
in New Mexico
New Mexico
New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...
.
It seems unlikely that all of these fossils, which come from localities hundreds or thousands of miles apart, separated by millions of years of time, represent a single species of Troodon. However, further study and more fossils are needed to determine how many species of Troodon existed.
Paleobiology
Troodon is thought to have been a predator like other theropods. This view is supported by its sickle claw on the foot and apparently good binocular visionDepth perception
Depth perception is the visual ability to perceive the world in three dimensions and the distance of an object. Depth sensation is the ability to move accurately, or to respond consistently, based on the distances of objects in an environment....
.
Troodon teeth, however, are different from most other theropods. One comparative study of the feeding apparatus suggests that Troodon could have been an omnivore
Omnivore
Omnivores are species that eat both plants and animals as their primary food source...
. The jaws met in a broad, U-shaped symphysis
Symphysis
A symphysis is a fibrocartilaginous fusion between two bones. It is a type of cartilaginous joint, specifically a secondary cartilaginous joint.1.A symphysis is an amphiarthrosis, a slightly movable joint.2.A growing together of parts or structures...
similar to that of an iguana
Iguana
Iguana is a herbivorous genus of lizard native to tropical areas of Central America and the Caribbean. The genus was first described in 1768 by Austrian naturalist Josephus Nicolaus Laurenti in his book Specimen Medicum, Exhibens Synopsin Reptilium Emendatam cum Experimentis circa Venena...
, a lizard species adapted to a plant-eating lifestyle. Additionally, the teeth of Troodon bore large serrations each of which is called a denticle. There are pits at the intersections of the denticles, and the points of the denticles point towards the tip, or apex, of each tooth. The teeth show wear facets on their sides. Holtz (1998) also noted that characteristics used to support a predatory habit for Troodon - the grasping hands, large brain and stereoscopic vision, are all characteristics shared with the herbivorous/omnivorous primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...
s and omnivorous Procyon
Procyon (genus)
Procyon is a genus of nocturnal mammals, comprising three species commonly known as raccoons, in the family Procyonidae. The most familiar species, the common raccoon , is often known simply as "the" raccoon, as the two other raccoon species in the genus are native only to the tropics and are...
(raccoon).
One study was based on the many Troodon teeth that have been collected from Late Cretaceous deposits from northern Alaska. These teeth are much larger than those collected from more southern sites, providing evidence that northern Alaskan populations of Troodon grew to larger average body size. The study suggests that the Alaskan Troodons may have had access to large animals as prey because there were no tyrannosaurids in their habitat to provide competition for those resources. This study also provides an analysis of the proportions and wear patterns of a large sample of Troodon teeth. It proposes that the wear patterns of all Troodon teeth suggest a diet of soft foods - inconsistent with bone chewing, invertebrate exoskeletons, or tough plant items. This study hypothesizes a diet primarily consisting of meat
Age determination
Age determination of dinosaurs
Age determination in dinosaurs is mainly used to determine the approximate age of a dinosaur when the animal died.-History:Early attempts to estimate the longevity of dinosaurs used allometric scaling principles. Ages were determined by dividing individual mass estimates by rates of growth for...
studies performed on the fossilized remains of Troodon using growth ring counts suggest that this dinosaur reached its adult size probably in 3–5 years.
Pathology
A parietal bone catalogued as TMP 79.8.1 referred to Troodon formosus bears a "pathological aperture". In 1985 Phil Currie hypothesized that this aperture was caused by a cyst, but in 1999 Tanke and Rothschild interpreted it as a possible bite wound. One hatchling specimen may have suffered from a congenital defect resulting in the front part of its jaw being twisted. In a 2001 study conducted by Bruce Rothschild and other paleontologists, 21 foot bones referred to Troodon were examined for signs of stress fractureStress fracture
A stress fracture is one type of incomplete fracture in bones. It is caused by "unusual or repeated stress" and also heavy continuous weight on the ankle or leg...
, but none were found.
Reproduction
Dinosaur eggs and nests were discovered by John R. Horner in 1983 in the Two Medicine Formation of Montana. Varriccho et al. (2002) have described eight of these nests found to date. These are all in the collection of the Museum of the RockiesMuseum of the Rockies
The Museum of the Rockies, is located in Bozeman, Montana. The museum, originally affiliated with Montana State University in Bozeman, and now, also the Smithsonian Institution, is known for its paleontological collections, although these are not its sole focus...
and their accession numbers are MOR 246, 299, 393, 675, 676, 750, 963, 1139. Horner (1984) found isolated bones and partial skeletons of the hypsilophodont
Hypsilophodont
Hypsilophodonts were small ornithopod dinosaurs, regarded as fast, herbivorous bipeds on the order of 1–2 meters long . They are known from Asia, Australia, Europe, New Zealand, North America, and South America, from rocks of Middle Jurassic to late Cretaceous age...
Orodromeus very near the nests in the same horizon and described the eggs as those of Orodromeus
Orodromeus
Orodromeus is a genus of herbivorous ornithopod dinosaur from the Late Cretaceous of North America.-Discovery and naming:...
. Horner and Weishampel (1996) reexamined the embryos preserved in the eggs and determined that they were those of Troodon, not Orodromeus. Varricchio et al. (1997) made this determination with even more certainty when they described a partial skeleton of an adult Troodon (MOR 748) in contact with a clutch of at least five eggs (MOR 750), probably in a brooding position.
Varricchio et al.(1997) were able to extract enough evidence from the nests to infer several characteristics of Troodon reproductive biology. The results are that Troodon appears to have a type of reproduction that is intermediate between crocodiles and birds, as phylogeny would predict. The eggs are statistically grouped in pairs, which suggests that the animal 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. also found evidence for iterative laying, where the adult might lay a pair of eggs every one or two days, and then ensured simultaneous hatching by delaying brooding until all eggs were laid. MOR 363 was found with 22 empty (hatched) eggs, and the embryos found in the eggs of MOR 246 were in very similar states of development, implying that all of the young hatched simultaneously. The embryos had an advanced degree of skeletal development, implying that they were precocial
Precocial
In biology, the term precocial refers to species in which the young are relatively mature and mobile from the moment of birth or hatching. The opposite developmental strategy is called "altricial," where the young are born or hatched helpless. Extremely precocial species may be called...
or even superprecocial
Superprecocial
The term superprecocial refers to a level of physical maturity and capability in young animals that is most extreme. Examples are the Megapode birds, which have full flight feathers and which, in some species, can fly on the same day they hatch from their eggs...
. The authors estimated 45 to 65 total days of adult nest attendance for laying, brooding, and hatching. The authors found no evidence that the young remained in the nest after hatching and suggested that they dispersed like hatchling crocodiles or megapode birds instead.
Varricchio et al. (2008) examined the bone histology of Troodon specimen MOR 748 and found that it lacked the bone resorption patterns that would indicate it was an egg-laying female. They also measured the ratio of the total volume of eggs in Troodon clutches to the body mass of the adult. They graphed correlations between this ratio and the type of parenting strategies used by extant birds and crocodiles and found 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 likely did not brood eggs, that the males did, and this may be a character shared between maniraptoran dinosaurs and basal birds.
The "Dinosauroid"
Canadian Museum of Nature
The Canadian Museum of Nature is a natural history museum in Ottawa, Ontario, Canada. Its collections, which were started by the Geological Survey of Canada in 1856, include all aspects of the intersection of human society and nature, from gardening to gene-splicing...
in Ottawa
Ottawa
Ottawa is the capital of Canada, the second largest city in the Province of Ontario, and the fourth largest city in the country. The city is located on the south bank of the Ottawa River in the eastern portion of Southern Ontario...
, conjectured a possible evolutionary path that might have been taken by Troodon had it not perished in the K/T extinction event 65 million years ago, suggesting that it could have evolved into intelligent beings similar in body plan to humans. Over geologic time, Russell noted that there had been a steady increase in the encephalization quotient
Encephalization quotient
Encephalization Quotient , or encephalization level is a measure of relative brain size defined as the ratio between actual brain mass and predicted brain mass for an animal of a given size, which is hypothesized to be a rough estimate of the intelligence of the animal.This is a more refined...
or EQ (the relative brain weight when compared to other species with the same body weight) among the dinosaurs. Russell had discovered the first Troodontid skull, and noted that, while its EQ was low compared to humans, it was six times higher than that of other dinosaurs. If the trend in Troodon evolution had continued to the present, its brain case could by now measure 1,100 cm3; comparable to that of a human. Troodontids had semi-manipulative fingers, able to grasp and hold objects to a certain degree, and binocular vision.
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 have been 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. However, it would not have possessed mammary glands, and would have fed its young, as some birds do, on regurgitated food. He speculated that its language would have sounded somewhat like bird song
Bird song
Bird vocalization includes both bird calls and bird songs. In non-technical use, bird songs are the bird sounds that are melodious to the human ear. In ornithology and birding, songs are distinguished by function from calls.-Definition:The distinction between songs and calls is based upon...
.
Russell's thought experiment has been met with criticism from other paleontologists since the 1980s, many of whom point out that his Dinosauroid is overly anthropomorphic. Gregory S. Paul
Gregory S. Paul
Gregory Scott Paul is a freelance researcher, author and illustrator who works in paleontology, and more recently has examined sociology and theology. He is best known for his work and research on theropod dinosaurs and his detailed illustrations, both live and skeletal...
(1988) and Thomas R. Holtz, Jr., consider it "suspiciously human" (Paul, 1988) and Darren Naish
Darren Naish
Darren Naish is a vertebrate palaeontologist and science writer. He obtained a geology degree at the University of Southampton and later studied vertebrate palaeontology under British palaeontologist David Martill at the University of Portsmouth, where he obtained both an M. Phil...
has 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 in the manner of a bird, rather than with human-like "hands".