Stenonychosaurus
Stenonychosaurus | |
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Skeletal reconstruction, with missing parts based on related genera | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Family: | †Troodontidae |
Subfamily: | †Troodontinae |
Genus: | †Stenonychosaurus Sternberg , 1932
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Type species | |
†Stenonychosaurus inequalis Sternberg, 1932
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Synonyms | |
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Stenonychosaurus (meaning "narrow claw lizard") is a genus of
History of discovery
The first specimens currently assigned to Troodon that were not teeth were both found by Sternberg in 1928, in the
The holotype of Stenonychosaurus inequalis, CMN 8539, is a partial skeleton consisting of six caudal vertebrae, hand bones, the distal end of the left
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".[1]
Stenonychosaurus became a well-known theropod in the 1980s, when the feet and braincase were described in more detail. Along with
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) reverted to keeping the Dinosaur Park Formation fossils separate as Troodon inequalis.[4]
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.[5]
In 2017, Evans and colleagues, building on the work of Zanno and others, confirmed the currently undiagnostic nature of the holotype of Troodon formosus and suggested that Stenonychosaurus be used for troodontid skeletal material from the Dinosaur Park Formation.[6] Later in 2017, Van der Reest and Currie found Stenonychosaurus to be a valid genus, but reassigned much of the known material to the new genus Latenivenatrix.[7]
Many Dinosaur Park troodontid specimens once referred to Troodon have recently been referred to Stenonychosaurus and Latenivenatrix. The specimens now referred to Stenonychosaurus include several frontals (UALVP 5282, TMP 1986.078.0040, TMP 1988.050.0088, TMP 1991.036.0690), a partial dentary (TMP 1982.019.0151), and the distal portion of a left metatarsal III (TMP 1998.068.0090).
In 2021, a more comprehensive re-analysis of the morphology and stratigraphic positions of known skeletal material assigned to Stenonychosaurus and Latenivenatrix determined that several characters described as diagnostic of Latenivenatrix are in fact individually variable, that both taxa overlap stratigraphically, and that Latenivenatrix mcmasterae is a junior synonym of Stenonychosaurus inequalis.[8] This leaves S. inequalis as the only valid troodontid taxon currently identified from the Dinosaur Park Formation.
The "Dinosauroid"
In 1982,
However, 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.
Description
Stenonychosaurus was a small dinosaur, up to 2.5 metres (8.2 ft) in length and 35 kilograms (77 lb) in body mass.[10] The largest specimens are comparable in size to Deinonychus and Unenlagia.[11] They had very long, slender hind limbs, suggesting that these animals were able to run quickly. They had large, retractable, sickle-shaped claws on the second toes, which were raised off the ground when running.
Their eyes were very large (perhaps suggesting a partially
Brain and inner ear
Stenonychosaurus had one of the largest known brains of any dinosaur, relative to its body mass (comparable to modern birds).
Paleobiology
Stenonychosaurus are thought to have been
Stenonychosaurus teeth, however, are different from most other
(raccoon).A partial skeleton has been discovered with preserved puncture marks, possibly inflicted by a predator.[17]
Reproduction
Dinosaur eggs and nests were discovered by
Varricchio et al. (1997) described the exact structure of the nests. They were built from sediments, they were dish shaped, about 100 cm (39 in) 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 embedded 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 troodont reproductive biology. The results are that they appear to have had 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. The Two Medicine troodont was intermediate, laying an egg of about 0.5 kg (1.1 lb) for a 50 kg (110 lb) adult. This is 10 times larger than reptiles of the same mass, but two troodont eggs are roughly equivalent to the 1.1 kg (2.4 lb) egg predicted for a 50 kg (110 lb) 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 approximately simultaneously. The embryos had an advanced degree of skeletal development and empty eggs were relatively uncrushed, implying that hatchlings were
Varricchio et al. (2008) examined the bone histology of Two Medicine troodont 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 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 the troodont was consistent with that in birds where only the adult male broods the eggs. From this they concluded that troodont females likely did not brood eggs, that the males did, and this may be a character shared between maniraptoran dinosaurs and basal birds.[22] However, a later analysis of avian clutch mass found that the type of parental care cannot be determined using conventional allometric methods such as the one used by Varricchio et al.[23]
Paleoecology
Stenonychosaurus inequalis is known from the
References
- ^ a b c d Russell, D. A.; Séguin, R. (1982). "Reconstruction of the small Cretaceous theropod Stenonychosaurus inequalis and a hypothetical dinosauroid". Syllogeus. 37: 1–43.
- ^ Currie, P. (1987). "Theropods of the Judith River Formation". Occasional Paper of the Tyrrell Museum of Palaeontology. 3: 52–60.
- ISBN 978-0-671-61946-6.
- ^ 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.
- PMID 21949721.
- .
- ^ hdl:1807/78296.
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- ^ a b Naish, D. (2006). Dinosauroids Revisited Darren Naish: Tetrapod Zoology, April 23, 2011.
- ISBN 978-0-691-16766-4.
- PMID 17823350.
- .
- ISBN 978-1-84028-152-1.
- ^ a b Larsson, H.C.E. (2001). "Endocranial anatomy of Carcharodontosaurus saharicus (Theropoda: Allosauroidea) and its implications for theropod brain evolution". In Tanke, D.H.; Carpenter, K.; Skrepnick, M.W. (eds.). Mesozoic Vertebrate Life. Indiana University Press. pp. 19–33.
- ^ Holtz, Thomas R., Brinkman, Daniel L., Chandler, Chistine L. (1998) Denticle Morphometrics and a Possibly Omnivorous Feeding Habit for the Theropod Dinosaur Troodon. Gaia number 15. December 1998. pp. 159–166.
- JSTOR 4523488
- ^ Jacobsen, A. R. 2001. Tooth-marked small theropod bone: An extremely rare trace. p. 58-63. In: Mesozoic Vertebrate Life. Ed.s Tanke, D. H., Carpenter, K., Skrepnick, M. W. Indiana University Press.
- ^ Horner, John R. (1984) "The nesting behavior of dinosaurs". "Scientific American", 250:130–137.
- ^ Horner, John R., Weishampel, David B. (1996) "A comparative embryological study of two ornithischian dinosaurs – a correction." "Nature" 383:256–257.
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- PMID 23676654.