The Big Dinosaur Podcast

Episode 38 is all about Stegosaurus, one of the most famous dinosaurs, known for its plates and spiky tail.

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In this episode, we discuss:

• The dinosaur of the day: Stegosaurus, whose name means “roof lizard”
• Not to be confused with Stegoceras (pachycephalosaurid, with a dome head)
• Lived in the Late Jurassic, in western North America (though one found in Portugal in 2006)
• At least 3 species found in Morrison Formation (from 80 individuals)
• Stegosaurus was the first dinosaur named in the family Stegosauridae (it’s the type genus)
• In the family Stegosauridae, Stegosaurus is the largest
• Stegosaurus was found during the Bone Wars. Charles Marsh named Stegosaurus armatus in 1877, based on fossils found near Morrison, Colorado. At first Marsh thought it was an aquatic animal, similar to a turtle. The name “roofed lizard” comes from Marsh thinking the plates were flat on Stegosaurus’ back, like shingles on a roof. Lots of Stegosaurus fossils were found and Marsh wrote many papers. Like with many dinosaurs, at first multiple species were named, but now there are only a few valid ones
• Stegosaurus armatus (armored roof lizard) was named based on two partial skeletons, two partial skulls, and 30 fragments of individuals. It had four tail spikes (Marsh originally thought it had 8) and somewhat small plates. It was about 30 feet or 9 m long (longest Stegosaurus species); found in Colorado, Wyoming and Utah in the Morrison Formation
• Stegosaurus ungulatus (hoofed roof lizard); Marsh named in 1879 based on fossils found in Wyoming. It is possible these fossils are actually Stegosaurus armatus, but the fossils found in 2006 in Portgual are considered Stegosaurus ungulatus
• Stegosaurus sulcatus (furrowed roof lizard), named by Marsh in 1887 from a partial skeleton. Many have thought it was the same as Stegosaurus armatus, but recent studies show it may be its own species. The type specimen had a spike, originally thought to be part of the tail, that some scientists now think was part of the shoulder
• Stegosaurus stenops (narrow-faced roof lizard), named by Marsh in 1887, holotype found by Marshal Felch in Colorado in 1886. Species is known from at least one complete skeleton, so it’s the best known. Had four tail spikes and broad plates. Known from 50 partial skeletons of adults and juveniles, one complete skull, and four partial skulls. Only 23 ft or 7 m long, compared to Stegosaurus armatus, and has been found in Colorado, Wyoming and Utah
• In 2008 Susannah Maidment and a team pushed to synomymize Stegosaurus stenops and Stegosaurus ungulatus with Stegosaurus armatus, as well as change Hesperosaurus and Wuehosaurus into Stegosaurus and renaming them Stegosaurus mjosi and Stegosaurus homheni. So there would be 3 Stegosaurus species (armatus, homheni, and mjosi), with Stegosaurus ranging from Late Jurassic in North America and Europe to Early Cretaceous in Asia). But most researchers do not agree.
• Some nomina dubia (dubious names) and junior synonyms include Stegosaurus affinis (Marsh described in 1881, but based on only a pubis); Stegosaurus laticeps (Marsh described in 1881 from jawbone fragments); Stegosaurus duplex (name means two plexus roof lizard, Marsh named in 1887 based on the large area near its tail that Marsh called the “posterior brain case”) but it’s probaly just Stegosaurus armatus
• Former Stegosaurus‘ include Stegosaurus longispinus (Charles Gilmore named) but is now the type species of the genus Natronasaurus; also Stegosaurus madagascariensis (described in 1926 based on teeth found in Madagascar, but is now considered to be something else, like a hadrosaur or ankylosaur); Stegosaurus marshi (described in 1901 and renamed Hoplitosaurus in 1902); and Stegosaurus priscus, described in 1911 and now the type species of Loricatosaurus
• Kenneth Carpenter and Peter Galton published a couple papers in the 2010 that Stegosaurus stenops may be a better type species than Stegosaurus armatus (since it is the best known, most well studied and has the most fossils and a near complete skeleton)
• Kenneth Carpenter said there’s debate on the number of valid species, and if you’re a “taxonomic clumper” you may only see one Stegosaurus species as valid, since there can be so much variation in one species (like how dogs all belong to Canis lupus familiaris)
• Most Stegosaurus fossils were found in the Morrison Formation
• Morrison Formation was semiarid with wet and dry seasons and flat floodplains. Vegetation included conifers, ferns, green algae, fungi, mosses, horsetails, cycads, and ginkgoes
• Other dinosaurs included Allosaurus, Torvosaurus, Brachiosaurus, Apatosaurus, Diplodocus, Camarasaurus, Camptosaurus, Dryosaurus (Stegosaurus often found near Allosaurus, Apatosaurus, Camarasaurus and Diplodocus)
• Other animals include snails, frogs, ray-finned fish, turtles, salamanders, pterosaurus, and early mammals
• Matthew Mossbrucker found tracks that show Stegosaurus may have lived in herds among a number of different aged Stegosauruses. One set of tracks had 4-5 baby Stegosaurus moving together and another had a juvenile track with an adult track over it
• Stegosaurus had pebbly throat armor (lumps under its neck to help shield it from predators)
• Predators included Allosaurus and Ceratosaurus
• Marsh at first thought Stegosaurus was bipedal, because its fore limbs were so short, but then in 1891 he decided it was two heavy to walk on just two legs. Some scientists, however, think Stegosaurus may have been able to rear up on its hind legs, using its tail to help support its weight, so it could eat higher up plants
• Forelimbs were shorter than hindlimbs, giving it an interesting posture
• Short forelimbs, kept head low to the ground (so ate low lying plants), with a stiff tail high in the air
• Probably weight balanced towards its high hips and was carried by the hind legs, so Stegosaurus could make tight turns when defending itself
• Could not walk fast, otherwise the back legs would overtake the front legs (max speed of 4-5 mph or 6-7 kmh
• Hind feet had three toes, and fore feet had five toes (but the inner two toes had a blunt hoof)
• Stegosaurus and relatives were herbivores, but had different teeth and jaws compared to other herbivores, so may have had a unique feeding strategy. Stegosaurus had peg-shaped teeth (not grinding teeth) and jaws could only do up-down movements; also no evidence they swallowed gastroliths, so it’s not clear exactly how they ate their food
• No front teeth, instead had a horny beak (easier to eat low growing vegetation
• Teeth were small, flat, and triangular; Stegosaurus ground up its food and possibly had cheeks to keep food in its mouth when it chewed
• In 2010, scientists did a detailed computer analysis of how Stegosaurus ate, using 2 3D models of Stegosaurus teeth. Also calculated bite force and found it was less than half the force of a Labrador retriever, so although it could bit through small young branches, could not bite through anything over 12 mm in diameter
• Fossilized teeth showed more wear on the sides that were sharpest, so Stegosaurus probably bit on a plant, pulled back its head, and then teeth cut through the vegetation (possibly swallowed gastroliths to help digest)
• Probably ate mosses, ferns, horsetails, cycads and conifers (would not have grazed on grasses, since grass was not around until the late Cretaceous, after Stegosaurus went extinct)
• Stegosaurus was up to 30 ft (9 m) long; about the size of a bus
• In 1994 a subadult Stegosaurus was found in Wyoming (15 ft or 4.6 m long, 7 ft or 2 m high, 2.6 tons or 2.3 metric tons). Can see it on display in the University of Wyoming Geological Museum
• A 90% specimen found in 2003 in Wyoming by Bob Simon, president of the dinosaur excavation and preservation corporation Virginia Dinosaur Company and Dinosaur Safaris
• Stegosaurus weighed more than 5 short tons (4.5 metric tons), but brain was about 80 g (made people think for so long that dinosaurs were not smart, until more recently (around Jurassic Park)
• Long, narrow skull (but small compared to the rest of its body)
• Braincase no larger than a dog’s, though it’s body was much bigger
• Brain was thought to be the size of a walnut, but according to Kenneth Carpenter, director of th USU Eastern Prehistoric Museum in Utah, “its brain had the size and shape of a bent hotdog”
• Had a low EQ (brain to body mass ratio), so not the smartest
• Charles Marsh got a case of a brain cavity (also called an endocast) in the 1880s, and showed it was the smallest proportionally of all dinosaurs, at least the ones known at the time
• Marsh described a “large canal in the hip region of the spinal cord” which could fit something more than 20 times bigger than the Stegosaurus’ brain. This led to the idea that Stegosaurus had a “second brain” in its tail to help control its body, especially when threatened. But this area has also been found in sauropods and “may have been the location of a glycogen body, a structure in living birds whose function is not definitely known” but it probably has something to do with energy storage
• Known from its plates and spikes on the tail (probably used for defense)
• Kite shaped plates on its rounded back, and two pairs of long spikes at the end of its tail
• Plates may have been used for defense, display, and/or thermoregulation
• Had 17 flat plates (dermal plates) that were osteoderms (bony-cored scales) similar to osteoderms in modern crocodiles and lizards. The plates came from the skin (not the skeleton), and the largest plates were 2 ft (60 cm) wide and 2 ft (60 cm) tall
• Lots of arguments over how the plates were arranged on Stegosaurus.
• Marsh at first thought the plates lay flat, but in 1891 said it had a single row of plates
• Another idea is there were pairs of plates in a row along the back (seen most often in images, especially early ones before the 1970s, seen in the 1933 film King Kong this way) but no two identical sized and shaped plates have been found for the same Stegosaurus
• Another idea was two rows of alternating plates (many accepted it by early 1960s, though some argue we don’t see this in other reptiles, so how could that evolve that way)
• Robert Bakker speculated the plates were somewhat mobile, and Stegosaurus could flip them from side to side to deter a predator from attacking
• In 1914 Gilmore said the spikes on the tail were for display only, but Robert Bakker later said the tail was probably pretty flexible (no ossified tendons) so probably used as a weapon (said they looked like a monkey tail, with no locking joints, so could fatally stab)
• But, plates seem to overlap with tail vertebrae so may have limited it somewhat
• Lots of debate over the purpose of the plates. Thought to be armor at first, but they’re too fragile and they leave the sides of Stegosaurus unprotected; however they may have made Stegosaurus look bigger and more menacing to predators or impressive to female Stegosaurus (though males and females had plates); may have helped control body temperature
• Could also be used for warning, blood would rush to plates, making them “blush” a red warning (could also use to attract mates)
• Stegosaurus plates were not made of solid osteoderms, but had lattice-like structures and blood vessels
• 2005 analysis in Paleobiology found the “microstructure of the plates suggest they weren’t used to radiate heat”; 2010 study published in the Swiss Journal of Geosciences found the plates may have passively helped control body temperature (because the plates were so large with so many blood vessels, like how a toucan’s bill naturally radiates body heat), but not the main purpose
• The size and shape of Stegosaurus plates help identify whether it was male or female.
• It’s hard to tell whether dinosaurs are male or female (reproductive organs and soft tissues rarely found), so scientists guess based on modern animals
• In 2015 (covered in the news in this podcast) in PLOS One a study said that Stegosaurus fossils (Stegosaurus mjosi) found in Montana with two types of plates (large and round v. tall and spiky) were not different species but different gender
• Evan Saitta, lead author said the large wide plates were probably from males (for display), and the tall spiky ones were from females (used as deterrents)
• They were found together, which shows they probably co-existed, and the plates had similar growth rings (so the dinosaurs were around the same age, and it’s not that the plates changed with age)
• Early on scientists thought the tail spikes were upright, but now they think they stuck out to the sides
• McWhinney and team published a study of tail spikes that showed that 9.8 % of Stegosaurus examined had tail spike injuries, helping support the idea that they fought with their tails (also an Allosaurus was found with a punctured tail wound)
• Informally the tail spikes are called thagomizers, after a Gary Larson “Far Side” cartoon was published in 1982 showing cavemen calling the spikes thagomizers (line was “Now this end is called the thagomizer…after the late Thag Simmons”)
• In 1977 paleontologists found a nearly complete juvenile Stegosaurus at Dinosaur National Monument (the most complete one found so far) with limb bones, shoulder blades, most of the hips, some ribs, and skull fragments (cast on display at Quarry Exhibit Hall)
• Can see an adult and juvenile Stegosaurus stenops at the Denver Museum of Nature and Science (look like they’re being attacked by an Allosaurus fragilis)
• Can see Stegosaurus ungulates at the Carnegie Museum of Natural History in Pittsburgh, PA or in the Nebraska Stage Museum in Lincoln, NE
• Can also see a Stegosaurus stenops (nickname “Sophie”) at the Natural History Museum in London
• Image of Stegosaurus in 1884 issue of Scientific American showed Marsh’s first thoughts on Stegosaurus, with tail spikes on its back, back plates on its tail, and up on its hind legs and tail in a tripodal pose
• In 1920 journalist W.H. Ballous wrote that Stegosaurus would flap its plates and glide through the air
• BBC’s 1999 Walking With Dinosaurs gave Stegosaurus some frontal swagger to show the shortness of its forelimbs compared to its hindlimbs
• Stegosaurus is the state dinosaur of Colorado (as of 1982)
• Stegosaurus became CO state dinosaur after a two-year write-in campaign by thousands of fourth graders
• Stegosaurus is also in Jurassic Park II and III
• Spike in Land Before Time
• Although Stegosaurus is famous, there are less than 2 dozen types in the Stegosaurid family, so it’s a rare type of dinosaur
• Stegosaurus was the first named genus in the Stegosauridae family (making it the type genus)
• Closest relatives to Stegosaurus were Wuerhosaurus from China and Kentrosaurus from East Africa)
• Stegosauridae is one of the two families in the infraorder Stegosauria (other famiy is Huayangosauridae)
• Stegosauridae skulls were shallower compared to Huayangosauridae and there was a bigger difference between its short forelimbs and long hinglimbs, and had larger plates and tail spikes
• Huayangosaurus is the only genus in Huayangosauridae, and lived 20 million years before Stegosaurus (Scelidosaurus from Jurassic England, lived 190 million years before and had features of stegosaurs and ankylosaurs
• Stegosauria is in the suborder Thyreophora (armored dinosaurs that includes ankylosaurs)
• Stegosauridae is further divided into subfamilies: Dacentrurinae and Stegosaurinae (Stegosaurinae are the larger ones)
• Earliest stegosaur is Lexovisaurus from England
• Other small, lightly armored dinosaurs related to stegosaurs direct ancestor include Emausaurus from Germany (small quardruped) and Scutellosaurus from Arizona, bipedal)
• A trackway of an early armored dinosaur, from 195 million years ago was found in France
• Stegosaurids lived in late Jurassic to early Cretaceous
• Usually large
• Their front legs were shorter than their back legs, so they were slow
• Could probably shear branches with teeth
• They have plates and tail spikes
• Fun Fact: Why do teeth fossilize so well? Teeth are made from Dentin which is harder and denser than bone, they are surrounded by a very hard enamel shell which protects them, in many cases they fall out and are replaced regularly, and they aren’t very tasty so other animals are unlikely to break them down.