Stegosaurus: A Jurassic Icon

Sophie the Stegosaurus. On display at the Natural History Museum in London, England
Image Credit: Aya Reyad, https://commons.m.wikimedia.org/wiki/File:%D9%85%D8%AA%D8%AD%D9%81_%D8%A7%D9%84%D8%AA%D8%A7%D8%B1%D9%8A%D8%AE_%D8%A7%D9%84%D8%B7%D8%A8%D9%8A%D8%B9%D9%8A_8.jpg

In late 2014 the Natural History Museum revealed a new dinosaur display. It is situated almost immediately after you go through the east entrance, and is different from most fossils in that she has her own identity. Her name is Sophie, and she stands in front of the giant globe that marks the entrance to the Earth Hall as if guarding it. While she is small for her genus she is nevertheless an imposing sight; larger than any living elephant, two rows of plates lining her back and a spiked tail that she raises in defence. Her original home was the coniferous forests and floodplains that would later become the Western United States, and she walked the land 150 million years before humans. She is a special dinosaur, one that is instantly recognisable the world over by dinosaur lovers and casual observers alike. She is power, she is serenity, she is a Stegosaurus.

The first fossils of Stegosaurus (“roofed lizard”) were discovered in the state of Colorado, USA by Marshall Parker Felch in 1876. Felch was a veteran of the American Civil War and was a fossil hunter working with the palaeontologist Othneil Charles Marsh, who subsequently described and named this as a new dinosaur in 1877. The fossils were unearthed from what is known as the Morrison Formation; a 1 million square kilometre Late Jurassic rock sequence that stretches across a large area of the western USA including Arizona, Utah and Wyoming. The very first reconstructions of Stegosaurus was based on the initial assumption that it was an extinct turtle! This resulted in a wildly different animal to what we think today. In this initial reconstruction it reared on two legs, its spines were positioned on its back, its plates were laid flat on its back like roof tiles (hence “roofed lizard”) and it had two brains. That’s right those Victorian palaeontologists though Stegosaurus had TWO brains! A small, walnut sized brain in its head, and a “second brain” located in its hips. The thinking was that its main brain was so small compared to its body size that it needed a second one to help control its hind legs and tail. We know of course that it didn’t really have two brains. The “second brain” is thought to be a body cavity containing glycogen stores to help provide energy, in the form of glucose, to its muscles.

Since those early days many Stegosaurus fossils have been unearthed with Stegosaurus stenops, the species that Sophie belongs to, being the most common (in fact Sophie is the most complete Stegosaurus skeleton ever found). This has allowed palaeontologists to construct a more detailed picture of what this animal was like. It was a relatively slow moving, four legged animal belonging to its own group of ornithischian dinosaurs known as the Stegosaurs. Stegosaurus is the largest and most iconic member of the group, but Stegosaurs have been discovered across the world; such as Kentrosaurus from Africa, Tuojiangosaurus from Asia and Dacentrurus from Europe. Stegosaurus and its relatives were herbivores, using peg-like teeth to strip leaves off low growing ferns, mosses and shrubs. To help digest this, Stegosaurus would intentionally swallowed small stones (called gastroliths) that sat in its stomach and helped to grind up incoming plant matter. This diet may not seem very exciting, but consuming vast quantities of these simple shrubs allowed Stegosaurus to grow to a huge size; up to a maximum of 9 metres long and weighing 7 tons in the largest species (Stegosaurus armatus).

Size comparison between a human and two Stegosaurus species: S.ungulatus and S.stenops (the species Sophie belongs to)
Image Credit: KoprX, https://commons.m.wikimedia.org/wiki/File:Stegosaurus_size_comparison.svg

Stegosaurus is characterised by two eye-catching features. The first is the two rows of alternating pentagon shaped plates that stretch along its back from its neck to near the end of its tail. These plates were covered in a sheath of a horn like material called keratin, the same material that makes up fingernails, horns and antlers. Currently the best explanation for what these huge, lavish structures were used for is that they were multi-functional. The main use is in display and signalling to others. Whether males trying to attract a mate, Stegosaurus’ sizing each other up, ward off predators or to simply recognise each other these plates position on the body and their size would have made them very effective “billboards”. As a bit of fun speculation I wonder if these plates could have been brightly coloured, especially since it’s thought that the plates might have had an outer covering of thin skin. Unique variations in plate colours between different individuals would have acted like a fingerprint, allowing for identification and signalling. Another idea is that Stegosaurus used its plates to help regulate its body temperature. To do this Stegosaurus would have pumped blood into the plates, allowing the heat to be radiated outwards or to be taken in. This was a popular theory for a long time and while the size and positioning of the plates would have allowed them to passively radiate heat, and there are some channels within the plates that could have aided with this, the latest thinking is that the plates weren’t especially adapted for heat radiation or absorption.

The other defining feature of Stegosaurus was the two pairs of long spines that jutted out sideways from the end of its tail. This formed a structure that is referred to as a “Thagomizer”. This term is unique as it originated from a cartoon in the comic “The Far Side”, which had a caveman name the spiny tail after the fictional “late Thag Simmons”. The name has stuck since with palaeontologists accepting it as a valid scientific term. The thagomizer was a deadly weapon that Stegosaurus used by positioning itself sideways or with its tail facing its attacker, giving it room to swing and forming an impenetrable barrier to protect the more vulnerable front end. A thagomizer would have certainly been required as Stegosaurus was prey for the large predators stalking the area at the time. Large theropod dinosaurs such as Torvosaurus and Saurophagonax would have been among these, but by far the most notorious was Allosaurus. Fossil evidence of conflicts between Stegosaurus and Allosaurus have been found frequently, ranging from a damaged piece of Stegosaurus plate to large holes in Allosaurus bones caused by a Stegosaurus thagomizer (including one fossil where a thagomizer went straight through an Allosaurus pelvis, hitting a place where the sun doesn’t shine!). It seems that the two dinosaurs clashed frequently, sharing a rivalry similar to the one between T-Rex and Triceratops 90 million years later. Thagomizers may not have been Stegosaurus’ only method of defence. One theory is that Stegosaurus might have lived in small groups, with the numbers giving mutual protection. Furthermore it’s been speculated that Stegosaurus might also have formed mixed herds (like those seen between Wildebeest, Zebra and Ostrich’s on the African Savannah) with other plant eating dinosaurs such as the smaller Camptosaurus. This arrangement would have provided mutual benefits; Camptosaurus’ keen eye sight would have allowed it to act as a scout, while the Stegosaurus would have been the heavily armoured knights.

A life like model of Stegosaurus
Image Credit: DinoTeam, https://commons.m.wikimedia.org/wiki/File:Stegosaurus_02_DinoPark_Ko%C5%A1ice.jpg

For me personally Stegosaurus has held a fond place in my heart for a long time. It was my favourite dinosaur when I was young, so much so that I still have a toy one amongst my collection. I think I was fond of it it so much because not only did it have an eye catching appearance, quite unlike any animal alive today, but also because I saw it almost like a superhero. Normally it would peacefully munch on ferns and go about its business. But if it or its herd were threatened it wouldn’t hesitate to swing into action, lashing its spiked tail at any predator brave enough to take it on. While many prehistoric animals have left a mark on me over the years, none have done so quite like Stegosaurus, and for that I will always love it.

References/Further Reading

A paper (Carpenter et.al. 2005) on the predator-prey relationship between Allosaurus and Stegosaurus

Carpenter, Kenneth & Sanders, Frank & Mewhinney, Lorrie & Wood, Lowell. (2005). Evidence for Predator- Prey Relationships Examples for Allosaurus and Stegosaurus, The Carnivorous Dinosaurs, Chapter: 17, 325-350

A paper (Farlow, Hayashi & Tattersall 2010) reviewing the possible heat regulatory properties of Stegosaurus plates, using comparisons with Alligator osteoderms

Farlow, J.O., Hayashi, S. & Tattersall, G.J. Internal vascularity of the dermal plates of Stegosaurus (Ornithischia, Thyreophora). Swiss J Geosci 103, 173–185 (2010). https://doi.org/10.1007/s00015-010-0021-5

A blog article, published on Scientific American (originally on Tetrapod Zoology), by renowned palaeontologist Darren Naish on Stegosaurus plates and their research history

Naish, Darren, “The Stegosaurus Plate Controversy”, Scientific American, Jul. 11, 2016, blogs.scientificamerican.com/tetrapod-zoology/the-stegosaurus-plate-controversy/

An article by The Evolution Institute on the geology and fossils of the Morrison Formation

Marano, Michael, “Utah’s Morrison Formation: A Fossil Treasure Chest”, The Evolution Institute, Dec. 13, 2012, evolution-institute.org/utahs-morrison-formation-a-fossil-treasure-chest/

A New Scientist article on the origin of the word “thagomizer”

“The word: Thagomizer”, New Scientist, Jul. 5, 2006, newscientist.com/article/mg19125592-200-the-word-thagomizer/?ignored=irrelevant

A National Geographic article on the history of Stegosaurus reconstructions

Laelaps, “Out With the Old Stegosaurus”. National Geographic, Apr 29, 2013, nationalgeographic.com/science/phenomena/2013/04/29/out-with-the-old-stegosaurus/

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