Tenontosaurus

Tenontosaurus Temporal range: Ma PreꞒ Ꞓ O S D C P T J K Pg N
T. dossi on exhibit in the Perot Museum of Nature and Science
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Ornithopoda
Clade:	†Iguanodontia
Genus:	†Tenontosaurus Ostrom, 1970
Species
†T. tilletti Ostrom, 1970 (type) †T. dossi Winkler, Murry & Jacobs, 1997

Tenontosaurus (

The genus is known from the late Aptian to Albian ages of the Early Cretaceous period sediments of western North America, dating between 115 and 108 million years ago. It contains two species, Tenontosaurus tilletti (described by John Ostrom in 1970^[2]) and Tenontosaurus dossi (described by Winkler, Murry, and Jacobs in 1997).^[3] Many specimens of T. tilletti have been collected from several geological formations throughout western North America. T. dossi is known from only a handful of specimens collected from the Twin Mountains Formation of Parker County, Texas.

History of discovery

The first Tenontosaurus fossil was found in Big Horn County, Montana by an American Museum of Natural History (AMNH) expedition in 1903. Subsequent digs in the same area during the 1930s unearthed 18 more specimens, and four specimens were found during the 1940s. Despite the large number of fossil specimens, the animal was not named or scientifically described during this time, though Barnum Brown of the AMNH gave it the informal name "Tenantosaurus", "sinew lizard", in reference to the extensive system of stiffening tendons in its back and tail.^[4]

During the 1960s, Yale University began an extensive, long-term dig in the Big Horn Basin area (Cloverly Formation) of Montana and Wyoming. The expedition was led by John Ostrom, whose team discovered more than 40 new specimens. Following his expedition, Ostrom became the first to describe and name the animal, calling it Tenontosaurus,^[5] a slight variation in spelling of Brown's informal name.

Since 1970, many more Tenontosaurus specimens have been reported, both from the Cloverly and other geological formations, including the Antlers Formation in Oklahoma, Paluxy Formation of Texas, Wayan Formation of Idaho, Cedar Mountain Formation of Utah, and Arundel Formation of Maryland.^[4]

Description

Tenontosaurus was a facultative quadruped, capable of assuming either a bipedal or quadrupedal stance. It may have used a quadrupedal stance while feeding,^[5][6] but was probably incapable of rapid quadrupedal movement.^[5] Tenontosaurus resembles quadrupedal ornithischians in having a tibia that is shorter than the femur and an anterolateral process on the ulna, but it resembles bipedal ornithischians in having a relatively narrow pelvis and a pendant fourth trochanter.^[6] The manus (the hand/front foot) shows a mixture of traits associated with bipeds and quadrupeds. It retains narrow claws, unlike the hoof-like unguals of quadrupedal ornithischians,^[6][7] has a short metacarpus, as in the bipedal Hypsilophodon, and retains grasping adaptations. However, the phalanges are also shortened as an adaptation to weight bearing.^[7] An analysis of the overall proportions and center of mass of Tenontosaurus found it to be a quadruped, although the analysis only distinguished bipeds from quadrupeds without including a facultatively bipedal category.^[8]

Classification

The cladogram below follows an analysis by Butler et al, 2011.^[9]

Paleobiology

Diet

Plant life in the Tenontosaurus ecosystem was likely dominated by ferns and tree ferns, cycads, and possibly primitive flowering plants. Larger plants and trees were represented by gymnosperms, such as conifer and ginkgo trees. Tenontosaurus was a low browser, and an adult would have had a maximum browsing height of about 3 meters (10 ft) if it adopted a bipedal stance. This restricted Tenontosaurus, especially juveniles, to eating low-growing ferns and shrubs. Its powerful, U-shaped beak and the angled cutting surfaces of its teeth, however, meant it was not limited to which part of the plant it consumed. Leaves, wood, and even fruit may have formed part of its diet.^[4]

Predators

Teeth and a number of skeletons belonging to the carnivorous theropod Deinonychus have often been discovered associated with Tenontosaurus tilletti remains. Tenontosaurus specimens have been found at over 50 sites, and 14 of those also contain Deinonychus remains. According to one 1995 study, only six sites containing Deinonychus fossils contain no trace of Tenontosaurus, and Deinonychus remains are only rarely found associated with other potential prey, like Sauropelta.^[10] In all, 20% of Tenontosaurus fossils are found in close proximity to Deinonychus, and several scientists have suggested that this implies Deinonychus was the major predator of Tenontosaurus. Adult Deinonychus, however, were much smaller than adult Tenontosaurus, and it is unlikely a single Deinonychus would have been capable of attacking a fully grown Tenontosaurus. While some scientists have suggested that Deinonychus must therefore have been a pack hunter, this view has been challenged based on both a supposed lack of evidence for coordinated hunting (rather than mobbing behavior as in most modern birds and reptiles, though crocodilians have been documented to hunt cooperatively on occasion^[11]) as well as evidence that Deinonychus may have been cannibalizing each other, as well as the Tenontosaurus, in a feeding frenzy.^[12] It is likely that Deinonychus favored juvenile Tenontosaurus, and that when Tenontosaurus reached a certain size, it passed out of range as a food source for the small theropods, though they may have scavenged larger individuals, or preyed on adults that were sick or injured. The fact that most Tenontosaurus remains found with Deinonychus are half-grown individuals supports this view.^[4][13] It also lived in the same area as the large carnivorous dinosaur Acrocanthosaurus.^[14]

Reproduction

The presence of medullary bone tissue in the

Throughout the Cloverly Formation, Tenontosaurus is by far the most common vertebrate, five times more abundant than the next most common, the ankylosaur

The ecological community was similar in other regions, with dinosaurs like Tenontosaurus and Deinonychus as the most common large vertebrates. The

crurotarsan reptile Bernissartia, the cartilaginous fish Hybodus buderi and Lissodus anitae, the ray-finned fish Gyronchus dumblei, the crocodilian Goniopholis, and the turtles Glyptops and Naomichelys.^[20][21] Possible indeterminate bird remains are also known from the Antlers Formation. The fossil evidence suggests that the gar Lepisosteus was the most common vertebrate in this region. The early mammals known from this region include Atokatherium boreni and Paracimexomys crossi.^[22]

Climate

In the Cloverly Formation of Montana and Wyoming, Tenontosaurus remains are common in two distinct rock units: the more ancient Little Sheep Mudstone Member (Cloverly Formation unit V) and the more recent Himes Member (units VI and VII). The oldest part of the formation, the Pryor Conglomerate, contains no Tenontosaurus fossils, and they only appear in the uppermost, most recent part of the Little Sheep Mudstone Member. Catherine Forster, in a 1984 paper on the ecology of Tenontosaurus, used this as evidence to suggest that Tenontosaurus populations did not arrive in the Bighorn Basin area until the time of the late Little Sheep Mudstone Member.^[4]

At the time Tenontosaurus first appeared in Wyoming and Montana (the early Albian age), the regions climate was

tropical, with river deltas, floodplains, and forests with swampy inlets reminiscent of modern Louisiana, though marked dry seasons persisted to create savannah-like environments as well.^[18] The change in rainfall levels is likely due to the advancing shoreline of the Skull Creek Seaway, a cycle of the Western Interior Seaway which, later in the Cretaceous period, would completely divide North America.^[4]

This dramatic shift in climate coincided with an apparent increase, rather than decrease, in the abundance of Tenontosaurus. This shows Tenontosaurus to have been a remarkably adaptable animal, which persisted for a long span of time in one area despite changes to its environment.[4]

References

1. OCLC 985402380
   .
2. ISSN 0079-032X
   .
3. JSTOR 4523810
   .
4. ^ ^{a b c d e f g h i} Forster, C. A. (1984). "The paleoecology of the ornithopod dinosaur Tenontosaurus tilletti from the Cloverly Formation, Big Horn Basin of Wyoming and Montana". The Mosasaur. 2: 151–163.
5. ^
   JSTOR 4523326
   .
6. ^
   ISSN 0567-7920
   .
7. ^
   ISSN 0272-4634
   .
8. ISSN 0094-8373
   .
9. doi:10.1111/j.1475-4983.2011.01046.x.{{cite journal}}: CS1 maint: multiple names: authors list (link
   )
10. doi:10.1080/02724634.1995.10011256. (abstract Archived 2007-09-27 at the Wayback Machine
    )
11. S2CID 84672219
    .
12. S2CID 84175628
    .
13. ^ ^{a b} Brinkman, Daniel L.; Cifelli, Richard L.; Czaplewski, Nicholas J. (1998). "First Occurrence of Deinonychus antirrhopus (Dinosauria: Theropoda) from the Antlers Formation (Lower Cretaceous: Aptain-Albian) of Oklahoma". Oklahoma Geological Survey (164): 1–27.
14. doi:10.1016/j.palaeo.2012.03.003
    .
15. PMID 18195356
    .
16. PMID 22470454
    .
17. S2CID 83674985
    .
18. ^ ^{a b c} Wedel, M. J.; Cifelli, R. L. (2005). "Sauroposeidon: Oklahoma's Native Giant" (PDF). Oklahoma Geology Notes. 65 (2): 40–57. Archived from the original (PDF) on 2008-07-05. Retrieved 2007-07-07.
19. ^ Weishampel, David B.; Barrett, Paul M.; Coria, Rodolfo A.; Le Loeuff, Jean; Xu Xing; Zhao Xijin; Sahni, Ashok; Gomani, Elizabeth, M.P.; and Noto, Christopher R. (2004). "Dinosaur Distribution", in The Dinosauria (2nd), p. 264.
20. ISSN 0272-4634
    .
21. ^ Cifelli, Richard L.; Gardner, James D.; Nydam, Randall L.; Brinkman, Daniel L. (August 1997). "Additions to the vertebrate fauna of the Antlers Formation (Lower Cretaceous), southeastern Oklahoma". Oklahoma Geology Notes. 57 (4): 124–131.
22. ISSN 0567-7920
    .

External links

Wikimedia Commons has media related to Tenontosaurus.