Product Code Database
Troodon ( ; Troödon in older sources) is a controversial genus of relatively small, bird-like Theropoda definitively known from the Campanian age of the Late Cretaceous period (about 77 million years ago). It includes at least one species, Troodon formosus, known from Montana. Discovered in October 1855, T. formosus was among the first dinosaurs found in North America, although it was thought to be a lizard until 1877.
Several well-known Troodontidae specimens from the Dinosaur Park Formation in Alberta have been historically considered members of this genus. In a 2017 analysis, the genus was considered undiagnostic, and some of its specimens were referred to the genus Stenonychosaurus (long believed to be synonymous with Troodon), some to the genus Latenivenatrix, and some to the genus Pectinodon. However, this has been disputed since, and a 2025 paper recommended the designation of a neotype to preserve Troodon's validity, with Stenonychosaurus as a junior synonym. The genus name is Ancient Greek for "wounding tooth", referring to the teeth, which were different from those of most other known at the time of their discovery. The teeth bear prominent, apically oriented serrations. These "wounding" serrations, however, are more similar to those of herbivorous reptiles, and suggest a possibly omnivorous diet.Holtz, Thomas R., Brinkman, Daniel L., Chandler, Christine L. (1998) Denticle Morphometrics and a Possibly Omnivorous Feeding Habit for the Theropod Dinosaur Troodon. Gaia number 15. December 1998. pp. 159-166.
The Troodon tooth was originally classified as a "lacertilian" (lizard) by Leidy, but reassigned as a megalosaurid dinosaur by Franz Nopcsa von Felső-Szilvás 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 time the family Pachycephalosauridae was known as Troodontidae. In 1945, Charles Mortram Sternberg rejected the possibility that Troodon was a pachycephalosaur thanks 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.
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 formosus specimens, saurornithoidids were thought to be close relatives, while Troodon was considered a dubious possible relative of the family. Phil Currie, reviewing the pertinent 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, 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 paleontology and all of the specimens once called Stenonychosaurus were referred to as Troodon in scientific literature through the early 21st century.
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, but that a more thorough review of the specimens is required. Because the holotype of T. formosus is a single tooth, this renders Troodon a nomen dubium.
In 2017, Evans and colleagues further discussed the undiagnostic nature of the holotype of Troodon formosus and suggested that Stenonychosaurus be used for troodontid skeletal material from the Dinosaur Park Formation. Later in the same year, Aaron J. van der Reest and Currie came to a similar conclusion as Evans and colleagues and also split much of the material assigned to Stenonychosaurus into a new genus: Latenivenatrix. In 2018, Varricchio and colleagues disagreed with Evans and colleagues, citing that Stenonychosaurus had not been used in the thirty years since Currie and colleagues synonymized it with Troodon and they indicated that " Troodon formosus remains the proper name for this taxon". This conclusion by Varricchio was agreed upon by Sellés and colleagues in their 2021 description of Tamarro. Varricchio's comments were later addressed by Cullen and colleagues in their 2021 review of Dinosaur Park Formation biodiversity, where they noted that, while Stenonychosaurus has indeed not been used for 30 years, Currie's original hypothesis of subjective synonymy (based on tooth and jaw morphology) was never directly tested and, given that later research found that teeth were not diagnostic below the family level in troodontids, Currie's original hypothesis is therefore not supported by the available data, regardless of the amount of time since it was originally proposed. They suggested that the description of more complete skeletal material (i.e. containing dental, frontal, and postcranial elements) that can be tied to the holotype could allow the direct testing of the synonymy hypothesis, but re-affirmed that, for now, given the lack of supporting evidence, the synonymy of Troodon and Stenonychosaurus cannot be maintained and that merely remaining untested for 30 years is insufficient justification to accept a proposed lumping of taxa lacking overlapping diagnostic materials.
In 2025, Varricchio and colleagues proposed the troodontid material (MOR 553, "a collection of elements representing multiple individuals of differing ontogenetic stages") from the Jack's Birthday Site of the Two Medicine Formation as the neotype of T. formosus and considered Stenonychosaurus as a possible junior synonym of the former. While the specimen is not from the Judith River Formation, the authors considered the neotype proposal to be appropriate, since both the Judith River and Two Medicine Formation are "contemporary and confluent". However, because the holotype is not lost or destroyed, in accordance with the International Code of Zoological Nomenclature (ICZN) article 75.5, the authors are preparing a petition to the ICZN for a formal neotype designation. Text was copied from this source, which is available under a Creative Commons Attribution 4.0 International License.
Below is a cladogram of Troodontidae by Zanno et al. in 2011.
In 2011, another derived troodontid, Linhevenator, was described from Inner Mongolia. It was noted by the authors as having relatively short and robust forelimbs, along with an enlarged second pedal ungual akin to that of the dromaeosaurids compared to more basal troodontids. It was proposed that derived troodontids had convergently evolved dromaeosaurid-style large second pedal unguals, likely as an adaptation relating to predation. The authors noted that it is plausible that this may be applicable to other derived troodontids, including Troodon, although this is currently uncertain due to a paucity of sufficient remains of the latter genus.Xu X, Tan Q, Sullivan C, Han F, Xiao D (2011) A Short-Armed Troodontid Dinosaur from the Upper Cretaceous of Inner Mongolia and Its Implications for Troodontid Evolution. PLoS ONE 6(9): e22916. doi:10.1371/journal.pone.0022916
In the past, remains have been attributed to the same genus as the Judith River Troodon from a wide variety of other geological formations. It is now recognized as 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 or even a single genus of troodontid. Further study and more fossils are needed to determine how many species of Troodon existed. It is questionable that, after further study, any additional species can be referred to Troodon, in which case the genus would be considered a nomen dubium.
Remains referred to Troodon are known from the Prince Creek Formation, a rock layer in Alaska that dates from the latest Campanian to Maastrichtian ages of the Late Cretaceous. Based on the presence of gypsum and pyrite in the rocks, it suggests that the formation was bordered by a large body of water. It seems that, based on the presence of pollen fossils, the dominant plants were , , , and . The temperature ranged from possibly 2-12°C, which roughly correlates to 36-54°F, and based on Alaska's position in the late Cretaceous, the area faced 120 or so days of winter darkness. This lived alongside many other reptiles, like the Centrosaurinae Pachyrhinosaurus, a species of the Saurolophinae Hadrosauridae Edmontosaurus, the pachycephalosaurin Alaskacephale, an unnamed Azhdarchidae pterosaur, and the Tyrannosaurinae Nanuqsaurus. It also lived alongside the Metatheria mammal Unnuakomys. Based on the amount of teeth found, this troodontid was the most common theropod of the formation, making up 2/3 of all specimens, which is a stark contrast to more southern deposits in Montana, where troodontids only comprise 6% of all theropod remains. This, along with evidence that Troodon was more abundant during cooler intervals, such as the early Maastrichtian, may indicate that Troodon favored cooler climates.
Additional specimens currently referred to Troodon come from the upper Two Medicine Formation of Montana. Troodon-like teeth have been found in the lower Javelina Formation of Texas and the Naashoibito Member of the Ojo Alamo Formation in New Mexico.Langston, Standhardt and Stevens, (1989). "Fossil vertebrate collecting in the Big Bend - History and retrospective." in Vertebrate Paleontology, Biostratigraphy and Depositional Environments, Latest Cretaceous and Tertiary, Big Bend Area, Texas. Guidebook Field Trip Numbers 1 a, B, and 49th Annual Meeting of the Society of Vertebrate Paleontology, Austin, Texas, 29 October - 1 November 1989. 11-21.Weil and Williamson, (2000). "Diverse Maastrichtian terrestrial vertebrate fauna of the Naashoibito Member, Kirtland Formation (San Juan Basin, New Mexico) confirms "Lancian" faunal heterogeneity in western North America." Geological Society of America Abstracts with Programs, 32: A-498.
Classification
Troodon is considered to be one of the most derived members of its family. Along with Zanabazar, Saurornithoides, and Talos, it forms a clade of specialized troodontids.
Paleobiology
One study was based on multiple Troodon teeth that have been collected from Late Cretaceous deposits in 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, hinting at Bergmann's rule. 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.Fiorillo, Anthony R. (2008) "On the Occurrence of Exceptionally Large Teeth of Troodon (Dinosauria: Saurischia) from the Late Cretaceous of Northern Alaska" Palaios volume 23 pp.322-328 A pellet possibly belonging to Troodon suggests it hunted early mammals such as Alphadon.
Communal nesting
A 2023 study using presumed Troodon eggshells from the Oldman Formation used Clumped isotopes thermometry to determine their formation and development. The study found that in contrast to the accelerated mineralization of eggs in modern birds, Troodon and likely other non-avian Maniraptora had slowed egg calcification akin to other reptiles. This would indicate that, unlike birds, Troodon and other maniraptorans had two functional Ovary that would limit the number of eggs produced. Thus, the study concluded that the large clutches of fossilized eggs present in the formation, despite the limited egg production each individual had, would indicate that Troodon had communal nesting behavior, where eggs would be laid together at a single nest by multiple females, forming large clutches. This is a strategy also used by some modern birds, such as Ostrich.
Paleoecology
The type specimen of Troodon formosus was found in the Judith River Formation of Montana. The rocks of the Judith River Formation are equivalent in age with the Oldman Formation of Alberta, which has been dated to between 77.5 and 76.5 million years ago.
See also
External links
|
|
