The Ancients

Stegosaurus: Titan of the Jurassic

December 14, 2025

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  • Stegosaurus lived during the Jurassic period, declining by the early Cretaceous, and was separated in time from *Tyrannosaurus Rex* by millions of years. 
  • Stegosaurs, including *Stegosaurus*, possess the smallest brain volume per body mass of any terrestrial animal known to have existed, with brains the size of a walnut. 
  • The iconic plates of *Stegosaurus* were likely used for multiple functions, including thermoregulation, display, and potentially defense, rather than solely as armor. 

Segments

Introduction and Guest Welcome
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(00:01:55)
  • Key Takeaway: Dr. Susannah Maidment from the Natural History Museum joins the podcast to discuss Stegosaurus.
  • Summary: The episode introduces Dr. Susannah Maidment, an expert on armored dinosaurs, who joins the host to discuss the iconic Stegosaurus. The discussion is also being filmed for the podcast’s YouTube channel. The host shares a listener shout-out before transitioning to the main topic.
Stegosaurus Anatomy and Iconography
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(00:03:31)
  • Key Takeaway: Stegosaurus possessed a walnut-sized brain, massive hind legs, and a tail armed with four razor-sharp spikes known as the Thagomizer.
  • Summary: The Stegosaurus is described as a tank-like giant with a tiny brain relative to its body size. Its most terrifying defensive feature is the four spikes on its tail, famously termed the Thagomizer. The episode promises to explore the secrets of its armor and bizarre anatomy.
Stegosaur Group vs. Species
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(00:05:35)
  • Key Takeaway: Stegosaurs are a diverse group of dinosaurs, of which Stegosaurus is just one specific genus.
  • Summary: The term ‘stegosaurs’ refers to the broader group of dinosaurs, while ‘Stegosaurus’ is a specific species within that group, similar to how ‘antelope’ describes a group while individual species exist within it. The UK is home to the first Stegosaur ever discovered, Dacentrus armatus (the Swindon Stegosaurus), which is often overlooked.
Timeline and Coexisting Dinosaurs
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(00:06:33)
  • Key Takeaway: Stegosaurs evolved in the Middle Jurassic, peaked in the Late Jurassic, and went extinct by the early Cretaceous, meaning they did not coexist with Tyrannosaurus Rex.
  • Summary: Stegosaurs first appeared around 167 million years ago and disappeared about 100 million years ago, long before T. Rex existed. During the Late Jurassic in North America, Stegosaurus lived alongside giant sauropods like Diplodocus and predators such as Allosaurus. The transition from Jurassic to Cretaceous involved a faunal turnover where ecosystem dominance shifted away from stegosaurs and sauropods.
Jurassic Ecosystems and Fossil Record
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(00:07:38)
  • Key Takeaway: Jurassic ecosystems where Stegosaurus lived were often seasonally arid in North America, while European finds suggest fossils were sometimes washed out to sea.
  • Summary: Stegosaurs were found across Laurasia and Gondwana, excluding Australia and Antarctica, though the fossil record for them is generally rare outside of North America. A unique specimen in the Natural History Museum consists of two tail spikes encrusted with bivalves, suggesting the carcass formed a small ecosystem on the seafloor after death.
Head Anatomy and Diet
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(00:13:33)
  • Key Takeaway: Stegosaurs had very small heads and teeth, suggesting they cropped vegetation using a strong, keratinous beak, similar to a turtle.
  • Summary: The small brain size of stegosaurs is noted as having the smallest brain volume per body mass of any terrestrial animal. Their teeth were tiny (about half a centimeter), leading to the hypothesis that they used a beak to crop vegetation, possessing a bite force comparable to a sheep. Unlike many later herbivores, stegosaurs did not process food by chewing in their mouths.
Neck Structure Variation
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(00:18:54)
  • Key Takeaway: Neck length varied significantly among stegosaurs, with Stegosaurus having an average neck, while Miragaia from Portugal possessed an unusually long neck with 17 vertebrae.
  • Summary: The neck structure was not uniform across all stegosaurs; Stegosaurus had a standard neck allowing it to reach the ground. Miragaia had significantly more neck vertebrae (17 vs. 11-13 in others), possibly evolving to reach higher foliage or mimicking sauropods. Evidence suggests some stegosaurs might have adopted a ’tripodal stance,’ rearing on hind limbs supported by the tail to reach higher branches.
Limb Proportions and Posture
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(00:20:31)
  • Key Takeaway: As descendants of bipedal ancestors, stegosaurs had significantly shorter forelimbs than hind limbs, resulting in a naturally downward-sloping back posture.
  • Summary: All four-legged dinosaurs evolved from two-legged ancestors, resulting in uneven limb lengths in many bird-hipped dinosaurs like Stegosaurus. Their forelimbs were much shorter than their hind limbs, suggesting a crouched posture where the back angled downwards. Furthermore, their forelimb movement was restricted, unlike modern mammals.
Function of Dorsal Plates
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(00:22:02)
  • Key Takeaway: The two rows of bony plates on Stegosaurus were embedded in the skin, covered in keratin, and likely served multiple roles including display and thermoregulation.
  • Summary: Stegosaurus had 19 plates and spikes down its back, arranged in two offset rows, unlike the paired plates seen in most other stegosaurs. The plates had a bony core with blood vessel channels, suggesting a keratinous covering made them larger than preserved. While defense is possible, the plates’ structure suggests functions like display (for mating or species recognition) or radiating excess body heat due to a slow metabolism.
Tail Spikes and Defense
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(00:31:41)
  • Key Takeaway: The four tail spikes on Stegosaurus (Thagomizers) were powerful enough to cause bone-crushing damage when swung side-to-side.
  • Summary: Stegosaurus possessed four spikes on its tail, which the host prefers to call ‘spikes’ rather than the cartoon-derived term ’thagomizer.’ Computational modeling indicates that swinging the tail generated forces capable of impacting and potentially breaking bone. Other stegosaurs, like Kentrosaurus, had spikes running further up the tail rather than just four terminal spikes.
Juveniles and Social Behavior
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(00:36:42)
  • Key Takeaway: Fossil evidence for juvenile stegosaurs, nests, or eggs is virtually non-existent, making conclusions about their social structure or parental care speculative.
  • Summary: The lack of juvenile fossils suggests stegosaurs may have laid soft-shelled, leathery eggs that do not preserve well in the fossil record, unlike calcite shells. It is unknown if they lived in family groups or herds, though herding is considered a sensible defense strategy given their inability to outrun predators. Their defense relied on elaborate structures rather than speed.
Predators and Extinction
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(00:40:02)
  • Key Takeaway: Allosaurus was the most abundant predator in the Morrison Formation alongside Stegosaurus, and stegosaurs ultimately went extinct without evolving into later Cretaceous armored dinosaurs.
  • Summary: In North America, Allosaurus was the dominant predator coexisting with Stegosaurus, far outnumbering other contemporary carnivores like Ceratosaurus. Evolutionary analysis strongly suggests that stegosaurs died out completely and did not evolve into the later Cretaceous armored dinosaurs like ankylosaurs.
New Discoveries in Morocco
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(00:40:32)
  • Key Takeaway: The discovery of Adraticlet Bulafa in Morocco confirmed the presence of stegosaurs in North Africa during the Middle Jurassic, initiating new paleontological collaborations.
  • Summary: The specimen Adraticlet Bulafa was identified from bones initially sold commercially and represents one of the oldest known stegosaurs, dating to the Middle Jurassic of Morocco. Tracing the specimen’s origin led to the establishment of Morocco’s first vertebrate paleontology labs. This discovery highlights that Africa remains largely unsampled for Jurassic fossils.
Media Depictions of Stegosaurus
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(00:47:29)
  • Key Takeaway: Media depictions of Stegosaurus, such as in Walking with Dinosaurs, are generally accurate, though the tendency to show them galloping is scientifically inaccurate.
  • Summary: The host notes that the classic depiction of Stegosaurus with large plates and a small head is largely plausible. The primary inaccuracy seen in media is depicting them running or galloping, which contradicts the understanding of their limb structure and locomotion. The limited fossil record for stegosaurs allows for some latitude in artistic interpretation.