The Big Dinosaur Podcast

Episode 41 is all about Stegoceras, a pachycephalosaurid with a smooth domed head.

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In this episode, we discuss:

• The dinosaur of the day: Stegoceras, whose name means “horned roof”
• Pachycephalosaurid dinosaur that lived in North America during the Late Cretaceous
• First named in 1902, by Lawrence Lambe
• Bones first found in 1889
• When Stegoceras was first found, scientists thought the bones were belly ribs, which are not found in other ornithischians (but now thought to be ossified tendons
• Type species is Stegoceras validum (based on 40 specimens found in the Belly River Group of Alberta, Canada)
• Between the 1920s to 1945, Stegoceras was thought to be Troodon, because they had similar teeth (but then better specimens were found)
• Used to be more Stegoceras species, such as Stegoceras lambei, Stegoceras sternbergi, and Stegoceras breve, but were later assigned to other genera
• In 1983 Stegoceras browni was renamed Ornatotholus, but is now considered to be a juvenile of S. validum
• One genus, Ornatotholus, is thought to be a juvenile Stegoceras validum, based on an analysis of the cranial dome ontogeny
• 2011 PLOS ONE Cranial Ontogeny in Stegoceras validum (Dinosauria: Pachycephalosauria): A Quantitative Model of Pachycephalosaur Dome Growth and Variation”: study that shows the skull changed with age and Ornatotholus browni is a juvenile of S. validium
• In 1990, Mark Goodwin described the skull of an adult Stegoceras, but the skull was large for a Stegoceras. In 2003, Robert Sullivan wrote a review of the fossils found, and thought it was distinct enough to be named Hanssuesia sternbergi. But a more recent study by Ryan Schott and David Evans argues the skull is an adult Stegoceras, even though it lacked nodes in the back of the skull which is seen on younger Stegoceras (not sure why it doesn’t have the nodes, possible they just changed with age)
• In 2011, a new valid species was named by Steven E. Jasinski and Robert M. Sullivan, called Stegoceras novomexicanum, based on two partial skulls
• Stegoceras novomexicanum was only about 4 feet long, compared to S. valdium which was over 6 feet long
• Stegoceras is a more common, better known pachycephalosaur
• Part of the group Marginocephalia and Pachycephalosauria
• Probably evolved from Hypsilophodon (from episode 28)
• Bones have been found in Alberta Canada and New Mexico
• About 6.6 ft (2 m) long and weighed 22-88 lb (10-40 kg)
• About 4 ft or 1.2 m tall
• Bipedal
• May have gone on all four feet to look for plants to eat
• Had small teeth that were curved, with serrated edges (again, similar to Troodon)
• Had legs three times longer than arms
• Had an S- or U-shaped neck
• Stegoceras had round forward facing eye sockets, so probably had good vision and binocular vision
• Probably a herding dinosaur
• Other dinosaurs that lived by Stegoceras were Albertosaurus, Maiasaura, T-rex, Ankylosaurus, Parasaurolophus, Corythosaurus and Dryptosaurus
• Had a large brain that was in a thick dome about 3 in or 7.5 cm thick (and divided into two parts)
• The dome was smooth
• Males had thicker domes than females (so did older Stegoceras)
• In 1981 the Journal of Paleontology published “A Morphometric Study of the Carnium of the Pachycephalosaurid Dinosaur Stegoceras” which measured the braincases of 29 specimens and found that what was once thought to be two types of Stegoceras was just male and female (males had thicker domes than females)
• Scientists originally thought male Stegoceras rammed their heads together (like bighorn sheep or musk oxen), but in 1997 some paleontologists said the dome was not large enough for that kind of impact, and would not have worked unless the heads hit at just the right spot; also, their head, neck and body would have had to be in a horizontal line to transmit stress, but scientists think their necks were S- or U-shaped; an alternative is flank-butting, which involves moving the neck and rotating the head and not seriously injure the opponent (bone rim above the eye may have protected its eyes)
• Mark Goodwin from University of California Berkeley analyzed pachycephalosaur skulls and found no evidence of healed scars, and found hat the skull bone is porous and fragile under pressure, so they would have killed each other in fights
• In 2011 Eric Snively and Theodor analyzed CT scans of Stegoceras validum skulls, and found they could have head-butted, based on an extra layer of dense bone in the middle of the dome, which would have been extra protection
• Eric Snively and researched published a study in 2011 in PLOS One  called”Common Functional Correlates of Head-Strike Behavior in the Pachycephalosaur Stegoceras validum (Ornithischia, Dinosauria) and Combative Artiodactyls” that showed Stegoceras could have head butted
• They did CT scans on modern animal skulls and Stegoceras, then made a virtual simulation showing beasts going head to head
• Found its brain was more protected than bighorn sheep and musk ox
• Stegoceras had an extra layer of dense bone in the middle of its dome, in addition to a stiff rind outside with spongy material that can absorb energy (and keep them conscious when butting heads)
• Eric Snively said there are “alternating layers of stiff and compliant bone in the domes…almost as if they are wearing a double motorcycle helmet”
• May have butted heads to attract mates
• The study of Stegoceras that found the domes could dissipate impact forces doesn’t prove they rammed heads, they may have flanked each other by swinging their heads into each others sides instead
• Stegoceras was a heavy breather
• In 2014 Anatomical Record published Jason Bourke and his team’s study that showed Stegoceras cooled its brain by breathing
• It breathed like a bird or reptile and took long, deep breaths, based on a CT scan by Jason Bourke and colleagues (breathing helped cool its brain by cooling blood vessels in the brain; also may have not had nose hairs like modern reptiles so would have had lots of mucous to avoid inhaling small, airborne objects
• Dinosaurs did not have nose hairs (needed mucous)
• Bourke and colleagues scanned the Stegoceras skull and found the turbinates
• Jason Bourke from the University of Ohio found that Stegoceras had turbinates, intricate structures in their nose, to help cool blood and prevent water loss
• They ran virtual air through a 3D model of the dinosaur’s nose to see how the turbinates altered airflow
• That’s how they found it acted as a cooling system, where the air breathed in cooled the warm blood inside before flowing to the brain (helped them keep cool when running away from predators)
• Because dinosaurs were so large, overheating was a major issue
• Stegoceras is a relatively small dinosaur, but still cooled its brain
• Had a good sense of smell, to sniff out predators, mates, and food
• Marginocephalia (“fringed heads”) is a clade of ornithiscians that were herbivores (both bipedal and quadrupedal), with bony ridges of frills at the back of the skull; lived in Jurassic and Cretaceous
• Pachycephalosauria (“thick headed lizards”) is also a clade of ornithiscians; lived in the late Cretaceous in North America and Asia
• Pachycephalosaurs were herbivores with thick skulls
• They were all bipedal
• Had thick skulls
• Some had domed skulls, others flat or wedge shaped
• Fun Fact: The phaeomelanosomes that potentially stored pigments in dinosaurs’ red feathers is the same as the pigment that makes Garret’s hair red!