Troodon

''Troodon'' ( ; ''Troödon'' in older sources) is a wastebasket taxon and a dubious genus of relatively small, bird-like dinosaurs known definitively from the Campanian age of the Late Cretaceous period (about 77  mya). 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 troodontid specimens from the Dinosaur Park Formation in Alberta were once believed to be members of this genus. However, recent analyses in 2017 have found the genus to be undiagnostic and referred some of these specimens to the genus '' Stenonychosaurus'' (long believed to be synonymous with ''Troodon'') and others to the genus '' Latenivenatrix''.
The genus name is Ancient Greek for "wounding tooth", referring to the teeth, which were different from those of most other theropods known at the time of their discovery. 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.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.

History of discovery

Early research

The name was originally spelled ''Troödon'' (with a diaeresis) by Joseph Leidy 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 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 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''.

Naming of related species

The first specimens assigned to ''Troodon'' that were not teeth were both found by Sternberg in the early 1930s, in the Dinosaur Park Formation of Alberta. The first was named '' Stenonychosaurus inequalis'' by Sternberg in 1932, based on a foot, fragments of a hand, and some tail vertebrae. A remarkable feature of these remains was the enlarged claw on the second toe, which is now recognized as characteristic of early paravians. Sternberg initially classified ''Stenonychosaurus'' as a member of the family Coeluridae. The second, a partial lower jaw bone, was described by Gilmore (1932) as a new species of lizard which he named '' Polyodontosaurus grandis''. Later, in 1951, Sternberg recognized ''P. grandis'' as a possible synonym of ''Troodon'', and 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. In a recent revision of the material by van der Reest & Currie, ''Polyodontosaurus'' was determined to be a ''nomen dubium'', not fit for synonymy with other taxa.

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'' 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.

Dissolution of the one species model

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'' (now ''Stenonychosaurus inequalis'').Currie, P. (2005). "Theropods, including birds." in Currie and Koppelhus (eds). ''Dinosaur Provincial Park, a spectacular ecosystem revealed, Part Two, Flora and Fauna from the park.'' Indiana University Press, Bloomington. Pp 367–397.

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 genera, 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 not sufficient justification to accept a proposed lumping of taxa lacking overlapping diagnostic materials. However, Varricchio and other still insist on their naming method.

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.

Below is a cladogram of Troodontidae by Zanno ''et al.'' in 2011.

Paleobiology

One study was based on multiple ''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, 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.

In 2011, another derived troodontid, '' Linhevenator'', was described from Inner Mongolia, China. 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, in comparison 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.

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.

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 genus of troodontid dinosaurs. 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''.

A species of ''Troodon'' is known from the Prince Creek Formation, a fossil site from Alaska that dates from the latest Campanian to Maastrichtian 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, based on the presence of pollen fossils, that the dominant plants were trees, shrubs, herbs, and flowering plants. 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 Maniraptoran lived alongside many other reptiles like the centrosaurine '' Pachyrhinosaurus perotorum'', a species of the saurolophine hadrosaur ''Edmontosaurus'', the pachycephalosaurin '' Alaskacephale gangloffi'', an azhdarchid pterosaur, and the tyrannosaurine ''Nanuqsaurus hoglundi''. It also lived alongside the metatherian mammal '' Unnuakomys hutchisoni.'' Based on the amount of teeth found, this troodontid was the most common theropod of the formation, making up 2/3 of all specimens, 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.

See also

* Timeline of troodontid research

References

* Russell, D. A. (1987). "Models and paintings of North American dinosaurs." In: Czerkas, S. J. & Olson, E. C. (eds) ''Dinosaurs Past and Present, Volume I.'' Natural History Museum of Los Angeles County/University of Washington Press (Seattle and Washington), pp. 114–131.

External links

Dinosauroid

{{Taxonbar, from=Q131043

Late Cretaceous dinosaurs of North America
Troodontids
Campanian genera
Fossil taxa described in 1856
Taxa named by Joseph Leidy
Paleontology in Montana
Nomina dubia