Sunday, November 8, 2015

Paleo Files: Stegosaurus

  • Stegosaurus: “Roofed lizard”
  • Height: 4 meters
  • Length: 9 meters
  • Weight: 3 tons
  • Time Period: Jurassic Period (155-145 MYA)

The state dinosaur of Colorado and one of the most iconic members of Dinosauria, Stegosaurus, has a lot of history and theories surrounding it including; where it was found, its behavior, and how it lived. As with most fossilized organisms, there is only so much information that can be gleaned from the material and, because of this, some aspects of its anatomy and behavior can easily be inferred due to the connections that can be made between the animal and its relatives. Stegosaurus has earned its spot as one of the most iconic dinosaurs due to the profound amounts of remains that have been uncovered that allows Paleontologists to understand more about Stegosaurus than other, more fragmentary species.
(Copyright belongs to the Artist; Robert Nicholls) 
The Bone Wars was a period of extreme increases in the collecting and discovery of as many fossils as possible due to a rivalry between two Paleontologists Edward Drinker Cope and Othniel Charles Marsh. During the Bone Wars, many new species of previously unknown organisms were discovered. Othniel Charles Marsh originally discovered Stegosaurus in 1877 in Morrison, Colorado. When Othniel Charles Marsh found the remains, he initially thought they belonged to a turtle-like animal thinking that the large plates overlapped one another along the back, but he later retracted this theory for the one accepted today which is to say, the plates lined the back standing vertically. In 1886, Marshal Felch discovered remains of a separate species of Stegosaurus, dubbed sulcatus, in Cañon City, Colorado. The largest Stegosaurus could stand four meters high at the tallest back plate and could reach lengths of up to nine meters that, in contrast, is the size of a modern bus. But the size alone is not what sets Stegosaurus apart from the other animals it shared its ecosystem with.
(Copyright= Public Domain)
The Plates that line the spine of Stegosaurus are still very much an enigma. Paleontologists have put forth many theories over the years regarding the use of the plates. When Othniel Charles Marsh first found the remains, he thought the plates lay flat against the body like the armor of a Pangolin. Marsh later revoked this theory for the much more familiar one that is in use today, in which the plates stand up vertically along the back. Through the years, Paleontologists have refined the theory regarding the exact configuration of the dorsal osteoderms which went from two lines of identical plates on the back (this theory has been found to be incorrect due to a second, identical, row of plates mirroring the other having never been found), to one row of plates that alternate. What these plates were used for is still up for debate and has been ever since the animal’s discovery. Many scientists have posed theories as to the purpose of the plates including Robert Bakker, World renowned Paleontologist and curator of the Houston Museum of Nature and Science, speculated that the plates of Stegosaurus were the inside, or core, of a bigger plate made of a keratinous material. Bakker also suggested that these horny outgrowths could be semi-movable and the animal might have been able to use them as a sort of defense, splaying them out to the sides to deter predators from coming too close. The other theories include, temperature regulation like the large flat area of an elephant’s ear, to appear larger to predators, and as a sexual display to the opposite sex. Many Paleontologists presently accept that the plates would have had multiple uses and could have been used for any one of these theories as well.
(Copyright of skeletal belongs to; Scott Hartman, Copyright for plate belongs to TaylorMadeFossils)
Even if the plates of Stegosaurus were not used for defense, Stegosaurus also carries with it four spike-like osteoderms on the end of its tail. These spines bent outward to the sides and backwards and could have been used an incredible defense against many of the large predators of the Morrison Formation. In 2014, Robert Bakker found evidence in the pelvis of the Mid-Jurassic Carnivore, Allosaurus, at the Glenrock Paleontological Museum in Glenrock, Wyoming. Bakker found that there was a large open hole in the pubic bone of one of the mounted Allosaurus skeletons and later revealed that the wound fit the tail spike of a Stegosaurus (informally known as a Thagomizer) almost perfectly. Evidence suggests that bacteria, fragmentary bone, and other debris stayed in the wound and infected the area until it eventually killed the animal. According to Robert Bakker, “A massive infection ate away a baseball-sized sector of the bone, Probably this infection spread upwards into the soft tissue attached here, the thigh muscles and adjacent intestines and reproductive organs.” This theory was also reinforced by the fact that the vertebrae in the tail of Stegosaurus had no locking mechanisms and could be swung in many directions as Bakker points out, “Most dinosaur tails get stiffer towards the end, The joints of a stegosaur tail look like a monkey’s tail; They have no locking joints and were built for 3-dimensional combat.” This heavily points to the Thagomizer of a Stegosaurus being used a powerful defense mechanism towards anything it perceived as a threat. Although Stegosaurus was unique in the family Dinosauria, it wasn’t the first or last of its kind.

(Copyright of art belongs to; Robert Bakker), Copyright of Fossil segment is Public Domain)
Since Stegosaurus was the first of its kind to be discovered, its phylogenic suborder and family were named after it. After Stegosaurus’ discovery many other members of Stegosauridae were discovered including; Kentrosaurus, Dacentrurus, Lexovisaurus, Miragaia, and Hesperosaurus from different parts of the world like Africa and Europe. These relatives generally all shared a similar body plan as Stegosaurus only variations being the plates, spines, and skull shapes. Stegosaurus existed from the early Jurassic to the early Cretaceous and then went extinct, most likely due to other more successful herbivores taking over the Stegosaur niche. Stegosaurus lived during the Late Jurassic period from around 155-150 million years ago. It was the largest member of Stegosauridae yet to be discovered and would have to eat constantly. However, the exact feeding strategy of Stegosaurus and Stegosaurs in general is still not yet fully understood. The teeth of Stegosaurus are peg shaped and coupled with the wear on the teeth and the restrictions regarding the jaws, suggests that Stegosaurus could only move its jaws up and down. Since the head of Stegosaurus was small, pointed, and stood at most three feet off the ground at all times the animal was most likely a low level browser of soft material like shoots and fruits. According to an analysis done on the jaw strength of Stegosaurus done in 2010 the bite force of the animal was about half that of a common dog, or about 140 Newton for the anterior, 183 Newton for the middle, and 270 for the posterior teeth. This proves problematic to the understanding of how an animal the size of Stegosaurus could have a big enough food intake to survive and more research need to and is being done on the subject. The one thing fossils have a hard time explaining is how the animal that the fossils belonged to lived.
(Copyright for art belongs to; Geocities)
The brain of Stegosaurus, although not quite walnut-sized, was unusually small compared to the body mass and has the smallest brain size to body mass of any other dinosaur species. This presented a problem to the animal; how could it survive without a higher intelligence, and giant bony billboards and four large sword-like spikes were the answer. Paleontologist Matthew Mossbrucker discovered footprints of Adult, juvenile, and Hatchling specimens in 2007 in the Morrison Formation, which suggests that Stegosaurs stayed together in small groups, most likely for protection against predators. Another problem that Stegosaurs faced but fixed with armor was its speed. Stegosaurus has extraordinarily short front legs compared with the back legs and the femur was longer than the tibia and fibula of the leg which would have made walking any faster than six to seven kilometers an hour rather difficult. The other animals that Stegosaurus shared its environment with include, Allosaurus, Ceratosaurus, Apatosaurus, Camarasaurus, and many others. Stegosaurus most likely fit into a niche that none of the other organisms around it did, however that niche is not well understood as stated previously. Although there is evidence that suggests whenever an Allosaurus did attack a Stegosaurus it met its end, there is also fossil evidence for the opposite suggesting that Allosaurus did, in fact, prey on Stegosaurus and were able to bring them down. There is a specimen of Stegosaurus in which a U-shaped bite mark is visible on one of the plates on the neck. This suggests a hunter/prey relationship that went both ways in fatalities.
(Copyright for art belongs to; Geocities)
Stegosaurus can be considered the Rhinoceros or Hippopotamus of the Late Jurassic as it was both an herbivore and highly dangerous to anything it perceived as a threat. Due to the excessive deposits of this animal’s fossils, it has become the state fossil of Colorado and a universal icon of the small group of famous dinosaurs of pop culture. Stegosaurus ruled whatever specifically was its niche in its time period and then it and its relatives died out near the end of the Jurassic leaving only fossils and footprints as a reminder of its existence. However, Paleontologists have been able to, using the fossils and a little bit of guesswork, roughly understand how this animal behaved, how it lived, and how it died.
(Copyright for Stegosaurus belongs to; Gregory S. Paul)

(Copyright belongs to; PrehistoricWildlife)
Works Cited
Castro, Joseph. "Stegosaurus: Bony Plates & Tiny Brain." Livescience. Purch, 08 Dec. 2014. Web. 18 June 2015.

"Stegosaurus - Dinosaur - Enchanted Learning Software." Enchanted Learning. Enchanted Learning, n.d. Web. 19 June 2015.

Holtz, Thomas R. Jr. (2012) Dinosaurs: The Most Complete, Up-to-Date Encyclopedia for Dinosaur Lovers of All Ages, Winter 2011 Appendix.

Lambert D (1993). The Ultimate Dinosaur Book. Dorling Kindersley, New York. pp. 110–29. ISBN 1-56458-304-X.

Carpenter K (1998). "Armor of Stegosaurus stenops, and the taphonomic history of a new specimen from Garden Park Colorado". The Upper Jurassic Morrison Formation: An Interdisciplinary Study. Part 1. Modern Geol. 22. pp. 127–44.

Carpenter K, Galton PM (2001). "Othniel Charles Marsh and the Eight-Spiked Stegosaurus". In Carpenter, Kenneth. The Armored Dinosaurs. Indiana University Press. pp. 76–102. ISBN 0-253-33964-

Pastino, Blake De. "Allosaurus Died from Stegosaur Spike to the Crotch, Wyoming Fossil Shows." Western Digs. Western Digs, 23 Oct. 2014. Web. 20 June 2015.
"Stegosaurus; Colorado State Fossil." State Symbols USA. STATE SYMBOLS USA, n.d. Web. 21 June 2015.

Jacobson, Rebecca. "First Steps of a Baby Stegosaurus, Captured in 3-D." PBS. PBS, 16 July 2014. Web. 22 June 2015.

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