Spotlight: Stegosaurus

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Stegosaurus stenops — Narrow-faced covered reptile

Specimen Name & Sex: Jean (female)
Specimen Height: 2.8m
Specimen Length: 6.4m
Specimen Weight: 2.7 tonnes

Stegosaurs are among the most charismatic of all dinosaurs. Where tyrannosaurs have tiny limbs, ceratopsids have spiky head ornamentation, sauropods have long necks, and hadrosaurs have “duck bills,” stegosaurs have some of the most defining “dinosaurian” traits; the dorsal plates and the thagomizer. The arrangement and function of the plates is a debate dating back to Stegosaurus‘s discovery in the late 1800s by Othniel Charles Marsh. Marsh hypothesized the plates would be arranged in a manner similar to that of a pangolin, where they covered the entire animal. This is where the name Stegosaurus comes from — “covered reptile.” Of course this restoration is outdated. S. stenops and its sister species S. ungulatus have somewhere in the neighborhood of 18-24 plates (depending on who’s counting) that run at least semi-bilaterally along the animal’s back. I have elected to keep the plates as display structures, disregarding hypotheses of temperature regulation due to the probable existence of a layer of keratin covering the vascular structures. Here, Jean is shown with the plate arrangement based on the subadult Sophie specimen currently housed in London, which has an astounding >90% completeness!

She’s also not much larger than Sophie. S. stenops was the smaller of the two recognized species of Stegosaurus, with mass estimates close to that of a rather porky rhinoceros. Despite a comparatively smaller size to some contemporaries (BrachiosaurusBrontosaurusDiplodocus, etc.), Jean and the rest of her kind were rather successful, low-browsing ornithischians. The recent find of further neck vertebrae extends the stationary feeding range of a steg, allowing it to stand in one place and clear a larger volume of plant matter than it would with the typically-renditioned stubby neck.

Skeletal reference by Scott Hartman.

Artist Notes:

What strikes me the most about the new Stegosaurus is the strange plate arrangement along with the proportionality of said plates. To an extent, they appear asymmetrical as opposed to the more traditional reconstructions with a very clear, very concise shift in size from the neck to the hip and then tapering towards the end of the tail. It’s come to my attention that I also might’ve placed the plates too low, so Jean might get a refurbishment in the near future (nothing major, just adjusting the plates and maybe her head). Maybe a pattern update, too?

The duality of the blue and pinkish tones on Jean is something I consider to be my trademark for Stegosaurus. I interpret stegosaurs to be docile, rather skittish animals. The big eyespots on the plates were meant to deter attacks from predators, with the thagomizer acting as a last resort. The thagomizer was no laughing joke, however. The strength of the tail was more than capable of lodging a spike deep into any opposing party. I complemented the tail spikes with some sharp keratin barbs, just in case the animal lost or broke a spike in combat.

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Spotlight: Pachycephalosaurus

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Pachycephalosaurus wyomingensis — Wyoming’s thick-headed reptile

Specimen Name: Gracy (female)
Specimen Height: 1.67m
Specimen Weight: 450kg
Specimen Length: 4.98m

One of the most striking things about Pachycephalosaurus is the character that this species is named for — its skull. At the thickest point, the bone was close to 22cm (~9 in) thick. This initial observation sent scientists running to the quickest conclusion they could make: the head was a weapon. Though some doubt has been cast in the time after the initial hypotheses were formed, a well-constructed analysis by Peterson et al. suggests that pachycephalosaurids with dome heads (as opposed to the flatheaded varieties/morphs) frequently confronted one another in antagonistic behaviors. In this study, the same kind of skull lesions/fractures were found between many different pachycephalosaurs across different geographic areas. In addition, Horner et al. found that the skull dome was made of bone known to have fibroblasts (cells that aid in rapid bone regeneration). Plenty of head injuries + a means to heal them => something ahead of its time.

Pachycephalosaurus is also one of the last non-avian dinosaurs to have ever existed. Specimens are known to be found in America’s Hell Creek Formation — a fossil bed known for specimens such as Tyrannosaurus rexTriceratops prorsus, and so on. All of these taxa existed right until the very end of the Cretaceous, to the event known as the K-T boundary extinction. It and its famous contemporaries struggled and failed to survive the aftermath.

Artist Notes:

Pachycephalosaurus is an animal that I’m not unfamiliar with. This is actually my…sixth(?) or seventh time restoring one. For reference, this was my most recent pachy otherwise:

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Some of the things I’ve retained from this to the new pachy are as follows:

  • The color palette (primarily yellow and tan)
  • The cluster of filaments at the end of the tail, accented with a bit of blue and white
  • Filaments on the forearms

The quills/filaments/feathers on this animal are largely speculative. As far as I know, we’ve no direct skin impressions from a pachycephalosaur (I’m writing this 3 March, 2017. If any developments/discoveries are made, I’ll update this.), so integument is largely a task left to phylogenetic bracketing. Ceratopsids and pachycephalosaurs are placed in their own monophyletic group, dubbed “Marginocephalia.” Basal ceratopsids such as Protoceratops and Psittacosaurus are known to have long bristle-like filaments on the tail. Assuming the common ancestor of both pachycephalosaurs and ceratopsids had similar traits, it’s not so strange to think a pachycephalosaur could retain at least some bristle ornamentation.

I made the clumps of bone at the back of the skull pointier in the March ’17 pachy. This is to reflect more recent findings of pachycephalosaur development, particularly in the skull shape. In life, I assume these sharp pseudo-horns(?) would be used to cause superficial injuries to an opponent. Assuming the animal swung its head (as opposed to the straightforward impact as seen in modern rutting animals) like a club, the additional movement of pulling the skull back to a neutral pose had the potential to scrape/cut as well. The head dome is also very brightly pigmented, which is simply something of my own design.

Gracy is depicted in a near-tripodal pose, crouching on her legs to rear her upper torso high. She’s bleating, sending a call to fellow pachycephalosaurs. I don’t imagine these to be exactly social animals, which might’ve formed loose family-based clusters, at best. She would only be vocalizing to ward off intruders to her territory, to find mates, or simply to check if there are others of her kind in the area. The snarl/bleating of the Jurassic Park Pachycephalosaurus is fairly close to what I envision for my renditions