Retro vs Modern #13: Stegosaurus stenops

The first known stegosaur fossils were found in England and South Africa between the 1840s and 1870s, but these dinosaurs weren’t properly recognized as a highly distinctive group until the discovery of Stegosaurus itself in North America during the late 1870s.


1880s

The first Stegosaurus reconstructions were based on fragmentary and disarticulated fossil material, and its life appearance was very poorly understood. Initially it was depicted as a bipedal long-necked animal, with its plates laying flat against its back like a turtle shell, numerous spikes across its back, and more plates running along its tail.


1890s-1970s

Better skeletons of the species Stegosaurus stenops were discovered in the late 1880s, and by the 1890s stegosaur anatomy was becoming clearer. Reconstructions quickly adopted an arch-backed body shape with a tiny head and drooping tail, short semi-sprawling forelimbs and long hindlimbs, and with the plates now properly upright on the back and the spikes at the end of the tail.

Stegosaurus‘ unique appearance rapidly made it one of the most famous and recognizable dinosaurs to the general public. Its comical-seeming tiny head and even tinier brain also unfortunately ended up contributing to the prevailing early 20th century attitude that dinosaurs were sluggish and unintelligent, with the myth that it needed a “second brain” in its hips to control its huge body becoming a popular notion for quite some time.

The exact arrangement of the iconic back plates and tail spikes was uncertain for several decades, with early versions in the 1890s having up to eight tail spikes and a single row of plates. This was then updated in the 1900s to a double row of symmetrical plate pairs, and by the 1920s the standard arrangment had soon become an alternating two-row pattern with the tail spikes reduced to four – a layout that’s still considered correct today.


2020s

In the second half of the 20th century a combination of numerous new stegosaur species from China and the Dinosaur Renaissance began to revise the way Stegosaurus was understood, bringing it into a fully upright posture with its head and tail held high, and recognizing the convergently sauropod-like anatomy of its hands and feet.

But something still wasn’t right.

Compared to other known stegosaurs, Stegosaurus itself was starting to seem… rather weird. Its short neck, short forelegs, giant plates, sloping back and high rump were much more exaggeratedly proportioned than any of its relatives.

This was finally resolved in the 2010s when a near-complete specimen nicknamed “Sophie” was thoroughly described – and revealed that Stegosaurus’ proportions had been wrong the whole time. All previous skeletal reconstructions had been composites, put together from remains of multiple individuals that had all been different ages and sizes, and in the process had heavily distorted our idea of what this animal actually looked like.

Our modern view of Stegosaurus is now a much more typical stegosaur than before. It lived during the Late Jurassic, about 155-145 million years ago, across the Western and South Central United States (with a possible additional occurence in Portugal), alongside several other iconic dinosaurs of the “Jurassic savanna” like Brontosaurus, Diplodocus, and Allosaurus.

It grew up to around 9m long (~30ft), and had a small head with a long narrow snout, with a toothless beak at the front of its jaws and small peg-shaped teeth further back. Bony ossicles lined the underside of its neck, possibly providing chainmail-like protection to its throat, and its skin was covered in tiny pebbly scales interspersed with “rosettes” formed around slightly larger oval scales. Its neck was longer than previously thought, more in line with other stegosaurs, and its torso and hind legs were a bit shorter, making its posture more horizontal and its back less arched.

The actual function of the large back plates is still uncertain. Ideas about them being defensive armor (and speculation about them even being moveable!) have mostly been discounted at this point, since they were actually relatively fragile – although their keratinous covering may have had a fairly sharp edge. Thermoregulation has been a popular explanation for many decades, with blood vessel impressions in the plates being proposed as evidence they were used as “radiators” to prevent overheating like the ears of modern African elephants.

But currently the most likely primary plate function is thought to be visual display, with the large plates increasing the perceived size of Stegosaurus either to intimidate predators and rivals or to impress potential mates. If this was the case then they may have also been strikingly colored and patterned in life.

Meanwhile the “thagomizer” on its tail actually does seem to have been a weapon, with injuries to that area of the body being fairly common, and several Allosaurus fossils have been found with puncture wounds the exact size and shape of Stegosaurus spikes. Articulated specimens have also shown that the tail curved downwards at the tip, holding the thagomizer with the spikes pointing horizontally outwards and backwards.

Retro vs Modern #12: Edmontosaurus annectens

Hadrosaurs were first discovered during the 1850s in North America, with the eponymous Hadrosaurus being both one of the most complete dinosaurs known at the time and also the first dinosaur skeleton to ever be mounted and displayed.

Like many other dinosaurs of the time hadrosaurs were initially reconstructed as bipedal with an upright kangaroo-like pose. Early in the history of their study their wide flat “duckbill” snouts were thought to indicate they were semi-aquatic, and they were frequently portrayed swimming and wading while feeding on soft water plants.

While elaborately bony-crested hadrosaurs like Parasaurolophus have become some of the most famous and recognizable members of the group, the species that’s gone through the most radical changes in our understanding in recent years is probably Edmontosaurus annectens.


1890s-1960s

Edmontosaurus has had an especially messy taxonomic history with various specimens spending decades under many different names, commonly being labelled as Anatosaurus and Trachodon for much of the 20th century. For the sake of avoiding a lot of confusion I’m just going to keep referring to it here as “Edmontosaurus”, even though the naming issues weren’t properly sorted out until the 1990s.

The earliest specimen of what we know call Edmontosaurus was discovered in the 1890s, and the first to actually bear the genus name was the closely related species Edmontosaurus regalis discovered in the 1910s. For many decades it was mostly reconstructed in the then-typical “tripod” posture and seen as being highly aquatic, with an exceptionally well-preserved “dinosaur mummy” specimen being used to support that view – skin impressions around its hands were interpreted as paddle-like webbing used to swim.

The mummy also showed fairly thin and delicate skin, with a pattern of many tiny scales dotted with clusters of larger scales, and what appeared to be a fleshy skin frill running along Edmontosaurus’ neck and back.


2020s

The idea of amphibious hadrosaurs was finally challenged in the mid-1960s, at the start of the Dinosaur Renaissance, with details of their anatomy, possible stomach contents, and the environments that their fossils had been preserved in all being used to help reinterpret them as fully terrestrial herbivores that walked on four legs and ran on two. The discovery of Maiasaura nesting colonies in the late 1970s also revealed a lot of new information about the life history of these dinosaurs, and helped to popularize the image of them as social animals living in herds and caring for their young.

From the 1990s onwards new discoveries of additional “mummies” of both Edmontosaurus and other hadrosaurs have given us even more insights into the soft parts of their anatomy. Their necks and tails were much more thickly muscled and chunky than their skeletons alone suggest, the frill may have had a sort of rectangular segmented appearance, and the webbing on their forelimbs was actually more of a “mitten” that bound their hands into fleshy weight-bearing pads. And instead of a broad “duckbill” they actually had large hooked beaks covering their snouts, giving then more of a horse-like head shape.

We now know Edmontosaurus lived during the very end of the Cretaceous, about 73-66 million years ago, with the older part of that time range represented by Edmontosaurus regalis in Western Canada and the younger part represented by Edmontosaurus annectens in Western Canada and the Western and West North Central United States. It was one of the largest known hadrosaurs with most adult specimens around 9-12m long (~30-39′), but some of the very largest known partial remains suggest the existence of rare enormous “super-adults” that were about 15m long (49′).

Edmontosaurus was probably a grazing animal primarily eating tough low-growing foliage like horsetails, cropping off mouthfuls with its beak and then grinding them up with batteries of hundreds of teeth in the back of its jaws using a unique complex chewing motion.

Its skin had a complex texture of varying scale shapes and sizes across its body, and one mummified specimen of Edmontosaurus regalis shows a raised bumpy pattern of large scale clusters on its neck and a fleshy crest on the top of its head. It’s currently unclear if these were sexually dimorphic features and we don’t know if Edmontosaurus annectens actually had them too, but I’ve speculatively included them in this reconstruction anyway.

And despite being one of the most intensely-studied and completely known non-avian dinosaurs in the world, Edmontosaurus is somehow still continuing to surprise us. Parts of the mummy specimen nicknamed “Dakota” are still being carefully prepared, and in late 2019 the North Dakota Geological Survey teased an unexpected discovery – a large single hoof-like nail on the front of its hand, unlike anything ever seen before on a dinosaur, and suggesting that Edmontosaurus may have been much more specialized for purely quadrupedal movement than previously thought.

Official details on the “hoof” still haven’t been published yet, but whenever it happens it’ll be exciting to find out just what’s actually going on there.

Retro vs Modern #03: Hylaeosaurus armatus

Despite being the third-ever scientifically named dinosaur genus, and being used in the first official definition of dinosaurs as a group, Hylaeosaurus armatus has ended up as a much less well-known name than Iguanodon or even Megalosaurus.

It was also the very first ankylosaur to be discovered, found as a partial skeleton in Southeast England in the early 1830s. Its large bony spikes were quickly recognized as being some sort of defensive armor, initially thought to be arranged in a vertical row along the animal’s back and tail.


1850s

The Victorian Crystal Palace statue of Hylaeosaurus is surprisingly decent for such an early attempt at reconstructing something as weird as an ankylosaur. It gives the impression of a slower and much more lizard-like animal than Iguanodon or Megalosaurus, showing it as a large squat quadruped with hoof-like claws and heavily armored scaly skin, with long spines running along its back and numerous smaller bony bumps over its head and sides.


1860s-1920s

Discoveries of other more complete armored dinosaurs began to give a better picture of what ankylosaurs actually looked like. But although Hylaeosaurus was soon recognized as having had multiple rows of spikes rather than just one, actual reconstructions of it seem to have been scarce during this period – mostly all derivative of a single 1869 image that depicted it as a sort of fat sprawling pinecone-lizard bristling with spikes.


2020s

Still only known from fragmentary material, Hylaeosaurus has remained rather obscure for a long time. In the 21st century it’s started to get a bit more attention, however, with the original specimen being further prepared and examined – and 2020 was Hylaeosaurus’ big year, with both a redescription of the genus being published and it also being featured on a special-edition British 50p coin.

Hylaeosaurus was probably around 4m long (~13′), and lived in southeast England about 140-136 million years ago. It may have also ranged further across Europe, with possible remains known from Germany and some more dubious records from France, Spain, and Romania. Generally classified as an early nodosaurid, most of our modern knowledge of what it would have looked like comes from other discoveries of much better-known ankylosaurian relatives, including some exquisitely well-preserved examples in the last few years like Borealopelta and Zuul.

It would have had a low triangular head, with a toothless beak at the front of its jaws and leaf-shaped teeth further back, and a pair of short horns on the back of its skull behind its eyes. Rows of spiky osteoderm armor ran along is body, with longer curving spines over its shoulders, all covered in thick keratin sheaths that would have made them look even larger in life. Numerous smaller bony nodules in its skin filled in the gaps between the armor, forming a tough protective shield over its entire back. Its short powerful limbs had hoof-like claws, and if it was indeed a nodosaurid its tail would have lacked the famous club of its ankylosaurid cousins.

Based on Borealopelta we even now know a little bit about the potential coloration and patterning of these animals – some of them were reddish-colored, with a countershaded camouflage pattern, darker on top and lighter on the underside.

Retro vs Modern #02: Iguanodon bernissartensis

Named just a year after Megalosaurus, in 1825, Iguanodon has remained a fairly iconic dinosaur ever since.

Discovered in a different region of Southeast England, its fossilized teeth were soon recognized as being similar to those of modern iguanas – but much much larger. Partial skeletal remains were initially reconstructed as belonging to a gigantic herbivorous lizard, with what was thought to be a horn placed on the tip of its nose.


1850s

The Victorian Crystal Palace statues of Iguanodon depicts a more bulky reptile with a nose horn, a toothless beak at at the front of its jaws, scaly skin, thick upright legs and hoof-like claws. Much like the Megalosaurus of the time it’s really not nearly so bad of a reconstruction as it’s often accused of being, showing a surprisingly naturalistic and almost mammal-like interpretation of these animals compared to later portrayals.

Technically the particular “Iguanodon” species at Crystal Palace has more recently been renamed Mantellisaurus atherfieldensis, but it was considered to be Iguanodon at the time so it’s included here anyway.


1880s-1960s

A massive discovery of the remains of nearly 40 Iguanodon individuals in a coal mine in Bernissart, Belgium, revealed the full anatomy of these dinosaurs for the first time. Much more well-preserved and complete than the patchy English material, these larger Iguanodon bernissartensis eventually became the official type species for the whole genus – a standard used to help determine whether similar-looking fossils are Iguanodon or not.

The Bernissart specimens were restored as bipedal animals in an upright kangaroo-like pose, with their tails dragging behind them acting like a tripod to prop them up. What had previously been the single “horn” was finally realized to instead be a thumb spike on each hand, interpreted as a defensive weapon against predators.

This image of Iguanodon persisted for decades, with a giraffe-like long prehensile tongue sometimes also depicted (including a particularly bizarre interpretation of it sticking out through a hole in the lower jaw!).


2020s

The Dinosaur Renaissance in the late 20th century corrected Iguanodon‘s posture to hold its body horizontally, and it was eventually recognized as being capable of both bipedal and quadrupedal movement. Juveniles were found to have walked more on their hindlimbs, while adults spent more time on all fours but were still capable of running bipedally when they needed to.

We now have fossils of Iguanodon from across much of Europe during the Early Cretaceous, about 126-122 million years ago. Our modern view of this animal is a heavily built ornithopod that grew to around 9m long (~30′), with a horse-like head, a large keratinous beak at the front of its jaws, chewing teeth further back, and cheeks covering the sides of its mouth. Its chunky forelimbs each had a large thumb spike, hoof-like claws, and a prehensile grasping pinky finger, while its powerful hindlimbs ended in three-toed vaguely bird-like feet.

Soft tissue preservation discovered in related hadrosaurs suggests it probably also had a very bulky body with a thick heavily muscled neck and tail, and possibly an ornamental “frill” running along its back. Skin impressions show a covering of numerous tiny pebbly scales, generally too small to have been visible from a distance.

Stegouros

While some ankylosaurs are famous for their specialized tail clubs, Stegouros elengassen here had something else entirely going on with its rear end.

Known from the late Cretaceous of southern Chile, about 75-72 million years ago, this small ankylosaur was around 1.5m long (~5′), roughly the size of a large dog. It had a proportionally larger head and more slender limbs than most other ankylosaurs, and a pelvis more resembling a stegosaur, but its most distinctive feature was its tail – it had a completely unique never-before-seen type of tail weapon, with a flat “frond-like” structure formed from several pairs of large fused osteoderms making a shape resembling a macuahuitl.

It seems to have been part of a previously unrecognized very early-branching lineage of Gondwanan ankylosaurs – the parankylosaurians – with its closest relatives Antarctopelta and Kunbarrasaurus also included in this new group. And since the tail regions of both of those other species are poorly known, this means they may also have possessed macuahuitls.

Aquilarhinus

Aquilarhinus palimentus here was an early hadrosaurid dinosaur known from the Late Cretaceous of Texas, USA, living about 80 million years ago. Around 5m long (16″5″), it had a prominent humped nose that seems to have been an evolutionary prelude to the larger and much more elaborate crests found in later hadrosaurs.

It also had an unusually wide and shovel-like beak, unlike any other known hadrosaur, which was probably a specialization for a different diet than its relatives. Since it lived along coastal marshlands it may have used its broad jaws to scoop up large mouthfuls of soft vegetation – or, much like the “shovel-tusker” proboscideans that were once thought to have a similar lifestyle, it may actually have been doing something else entirely with that beak.

Eons Roundup 9

New year, new PBS Eons commission roundup day!

The ancient walruses Neotherium and Valenictus, from “How the Walrus Got Its Tusks”
https://www.youtube.com/watch?v=BKDGYGV2LK8


The nodosaurid ankylosaur Borealopelta, in both alive and “bloat-and-float” carcass states, from “The Dinosaur Who Was Buried at Sea”
https://www.youtube.com/watch?v=a-UZXBF63z4


The ankylosaurid ankylosaurs Gobisaurus and Dyoplosaurus, from “How Ankylosaurs Got Their Clubs”
https://www.youtube.com/watch?v=lRt-4SdzWrk

Prenoceratops

Although much less famous than their larger horned and frilled relatives, the leptoceratopsids were a widespread and successful group of ceratopsian dinosaurs during the Late Cretaceous, with fossils known from North America, Asia, and Europe (and, dubiously, Australia).

They were fairly small stocky quadrupedal dinosaurs, sort of pig-like, with short deep jaws and powerful beaks adapted for eating fibrous low-level plants like ferns and cycads – and to process such tough food they even evolved a chewing style similar to mammals like rodents.

Prenoceratops pieganensis here is known from the Two Medicine Formation bone beds in Montana, USA, dating to about 74 million years ago. Around 1.5-2m long (~5′-6’6″), it was very similar to its later relative Leptoceratops, but had a slightly lower, more sloping shape to its skull.

Weird Heads Month #21: Honking Hadrosaurs

The ceratopsians and pachycephalosaurs weren’t the only ornithischian dinosaurs to do weird things with their skulls.

The hadrosaurs are commonly referred to as “duck-bills” (despite how their beaks weren’t actually duck-like at all), and are famous for the elaborate crests seen on some of the group’s members, with shapes ranging from lobes to helmets to hatchets to spikes – and even some of the apparently crestless species are now known to have sported fleshy combs instead of the bony structures seen in their relatives.

But by far the most recognizable of the crested hadrosaurs is Parasaurolophus walkeri, with its long curved backwards-pointing tubular crest.

This particular species was mid-sized for the genus, growing up to around 10m long (32’10”) and is known from Western North America during the Late Cretaceous, about 76-73 million years ago.

Its crest was intermediate in size and shape between the other two known species. The larger Parasaurolophus tubicen had a longer and slightly straighter crest, while the smaller Parasaurolophus cyrtocristatus had a shorter more strongly curved one. Juveniles developed these crests as they matured, starting off with much smaller bumps on their snouts that gradually grew backwards and upwards.

Some hadrosaur crests were purely for visual display, but in the lambeosaurine lineage that Parasaurolophus belonged to they also incorporated complex looping nasal passages that were probably used as resonating chambers, allowing each species to make a unique-sounding loud bellowing call to communicate with each other.

There are also rumors of a currently-undescribed specimen of Parasaurolpphus that has preserved soft tissue around its crest, possibly a keratinous covering or skin flaps that made it appear even larger and more flamboyant in life than the underlying bone. So I’ve given this reconstruction a speculative structure like that, along with hoof-like claws on its hands similar to those recently revealed for Edmontosaurus.

Weird Heads Month #18: Boneheaded Dinosaurs

Pachycephalosaurs are highly recognizable dinosaurs with their thick spiky skulls, and it’s not hugely surprising that they were the evolutionary cousins of the equally weird-headed ceratopsians.

Much like their frilled relatives they had beaks at the tips of their snouts and large gut cavities for digesting plant matter, but they also had surprisingly sharp theropod-like teeth in front of their more standard herbivore teeth further back – suggesting they may also have been opportunistic omnivores, occasionally snacking on carrion or small animals similarly to modern pigs or bears.

Their striking-looking dome heads were probably used for combat, headbutting or flank-butting each other, and many fossil skulls show evidence of injuries that would have been caused by that sort of behavior.

The eponymous Pachycephalosaurus wyomingensis lived in North America right at the end of the Cretaceous, about 70-66 million years ago. It was one of the largest of its kind, reaching lengths of around 4.5m (14’9″), and was characterized by a large bony dome-head surrounded by small blunt spikes.

But it turns out that was probably only what it looked like as a fully mature adult.

Recent discoveries of juvenile Pachycephalosaurus skulls confirmed a hypothesis proposed a few years earlier: these dinosaurs changed appearance drastically as they grew up, and younger individuals had been mistaken for separate species. They started off with domeless flat heads, bristling with long spikes (a form previously named Dracorex hogwartsia) then as they matured their domes began to grow (previously Stygimoloch spinifer) and by full maturity they had big domes with the spikes shrunk down to smaller stubbier knobs (the classic Pachycephalosaurus look).

This particular reconstruction depicts a Stygimoloch-like subadult individual, not quite fully mature and still sporting some longer spikes.