Retro vs Modern #19: Quetzalcoatlus northropi

Named after an Aztec deity and often called “the largest animal to ever fly”, Quetzalcoatlus northropi is probably the most famous large pterosaur after Pteranodon – but despite its popularity for a long time we actually knew very little about it.

1970s

Discovered in Texas in the United States during the early 1970s, the first known fossils of Quetzalcoatlus were just a few giant wing bones, along with several partial skeletons of smaller individuals which at the time were thought to be juveniles.

But although it was given its charismatic name in 1975, the fragmentary nature of the find and it only being given a brief non-formal description meant it was very poorly understood at the time. Worse, the known fossil material was notoriously difficult for other paleontologists to study for several decades afterwards, with the museum housing it often refusing access requests entirely or demanding promises of total secrecy from anyone who was actually allowed to see it.

Most pterosaurs at the time were thought to be soaring seabird-like fish-eaters, but this huge species had been discovered in an inland environment. So Quetzalcoatlus was interpreted as being a huge vulture-like scavenger, with early reconstructions based on this idea ending up highly speculative due to the lack of good anatomical information. A common paleoart meme in the 1970s and 1980s depicted it with a long snake-like neck, a bizarrely tiny head, snaggly teeth, and a small nub-like crest.

1990s

In the mid-1980s Quetzalcoatlus was recognized as belonging to the newly-discovered azhdarchid lineage, a group with extremely long necks, toothless beaks, and long legs – although these pterosaurs were also rather poorly-known until more complete specimens were found in the mid-to-late 1990s.

Reconstructions during this period were considerably less demonic, reflecting the changing ideas about pterosaurs at the time. Quetzalcoatlus became sleeker and more bird-like, depicted with long narrow wing membranes, its neck held in an S-curve, its legs sticking out straight behind it in flight, and sometimes pycnofiber fuzz on parts of its body.

(…And sometimes there was excessive shrinkwrapping.)

During the early 1990s Quetzalcoatlus was also often shown with a blunt chunky beak based on fossil jaws found in west Texas, but in 1996 these remains were determined to belong to a different pterosaur. At the same time more material of the “juveniles” suggested they actually represented an entire second species of Quetzalcoatlus, about half the size of Quetzalcoatlus northropi, with partial skulls showing evidence of a bony crest on the head.

Quetzalcoatlus was also interpreted as a skim-feeder during this time, thought to fly along just above bodies of water with its lower jaw trawling over the surface, snapping up any fish it contacted.

2020s

The skimming hypothesis was widely accepted until 2007, when a detailed study showed that it was physically impossible for any pterosaurs to have actually fed that way. Based on their anatomy and known habitats azhdarchids like Quetzalcoatlus were subsequently reinterpreted as highly terrestrial predators, spending a lot of their time stalking around on all fours snagging prey in their huge beaks like a stork with the proportions of a giraffe.

A full technical description of the known fossil material had been promised since the early 1980s, but decades had passed and by the 2010s it still hadn’t been published. The specimens continued to be inaccessible, information was still under heavy embargo, and what little had been published in the 1970s and 1990s was argued to be sparse enough and undiagnostic enough that it was starting to be genuinely unclear if Quetzalcoatlus northropi was even a valid species name at all.

Finally, finally, after over 40 years, a whole collection of papers about this pterosaur were released in 2021 – and in a refreshing contrast to the many years of secrecy and hoarding they were all free and open access. The second species finally got a name, Quetzalcoatlus lawsoni, and the genus as a whole is now properly and officially defined, instantly going from dubious and almost unknown to one of the most complete azhdarchids so far.

The study isn’t without its issues or controversy, particularly in regards to some of its retro-seeming interpretations of Quetzalcoatlus’ posture, proportions, and launch mechanics. But since the publication means that the fossils are finally unrestricted to other researchers, there’s probably going to be plenty more studies and arguments and new discoveries about it in the future.

While Quetzalcoatlus northropi is still only known from fragments, the new knowledge about its smaller relative means we now have a much better idea of what it was probably like. It lived at the very end of the Cretaceous period, about 68-66 million years ago, and is currently known just from Texas – but it probably ranged much further than that, since azhdarchids are thought to have been able to fly for potentially thousands of kilometers at a time using energy-efficient thermal soaring.

It was one of the largest animals known to have been capable of powered flight, but not necessarily the largest ever. Some past Quetzalcoatlus wingspan estimates got ridiculously over-enthusiastically huge, in some cases up to to 21m (69′), but modern estimates based on better knowledge of azhdarchid proportions suggest something shorter-winged and much closer to 10m (33′) – and some other azhdarchids are now thought to have had similar or possibly even slightly larger wingspans.

Still, Quetzalcoatlus was very big, and when standing on all fours it was probably similar in size and shape to a modern giraffe, about 6m tall (~20′) with at least half of its height just being its neck. It had limb proportions more like an ungulate mammal than most other pterosaurs, suggesting it was highly adapted for walking and running around on the ground – but it could also catapult itself up into the air using its powerful forelimbs to take flight.

It would have been a ground-stalking predator similar to some modern storks and ground hornbills, using its long sharply pointed beak to snatch up any smaller animals it could fit into its mouth. Since its Texan habitat was a semi-arid “fern prairie” dominated by the titanosaur Alamosaurus, hatchlings and small juveniles of this sauropod may have made up a major part of Quetzalcoatlus’ diet. 

Retro vs Modern #18: Pterodactylus antiquus

Pterodactylus antiquus was the first pterosaur ever discovered, and in popular culture the name “pterodactyl” has become commonly associated with the group as a whole.

1800s

The first known Pterodactlyus specimen came from southeast Germany, and was described (although not yet named) in the 1780s. The modern concept of extinction hadn’t yet been established, so at the time unknown fossil species were generally assumed to still exist alive somewhere in remote regions of the world. Initially it was unclear what type of animal this specimen represented, and it was interpreted as being aquatic because the oceans seemed like the best place for such a strange creature to hide undiscovered.

In 1800 it was recognized as instead being a flying animal, with naturalist Johann Hermann creating both the first known life restoration of a pterosaur and one of the first known examples of scientific paleoart in general. He depicted it as a bat-like mammal with extensive wing membranes, external ears, and a covering of fur, and made two different sketches of this interpretation. The first shows an odd rounded wing shape with the wing finger seeming to form a stiff “hoop” around to the ankles, but the second version has some interesting additions – showing an understanding of the wing finger being straightened and stretching out the membrane, and adding a very large colugo-like propatagium between the neck and the wrist.

In light of our modern understanding of pterosaurs this was an incredibly good attempt at a reconstruction, despite the total lack of soft tissue impressions and the mistaken mammal classification.

The name Pterodactylus was established for this animal by the late 1810s, and while it was correctly identified as a flying reptile by some early paleontologists, others also saw it as being more mammal-like or bird-like.

In this pre-Darwinian time there was no modern concept of evolutionary relationships, and pterosaurs were instead thought to be a type of bat positioned inbetween mammals and birds in the “chain of being“. This “bat model” became influential on the early study of pterosaurs, and some paleontologists depicted highly mammalian versions even as late as the 1840s.

(The aquatic interpretation also stuck around as a competing idea until at least 1830, with  Pterodactylus‘ wings restored as huge penguin-like flippers.)

1850s-1970s

By the mid-19th century the reptile interpretation had become standard but the bat influence remained, with pterosaurs commonly assumed to have been furry, warm-blooded, and quadrupedal and clumsy on the ground. Fossil evidence of hair-like fuzz had even been found on a specimen of Scaphognathus in the 1830s, but this was later disputed and was only confirmed as being real almost two centuries later.

British paleontologist Richard Owen disagreed with the bat model for pterosaurs, considering them to be scaly sluggish cold-blooded gliders, and in the 1850s oversaw the creation of the heavily-scaled and oddly goose-like Crystal Palace Pterodactylus statues – one of their first major portrayals to the general public, and influential in the popular perception of these animals at the time.

But even into the start of the 20th century some paleontologists were still arguing for active warm-blooded pterosaurs, with the first popular book on the group in 1901 suggesting they were closely related to birds. German paleontologists continued to interpret pterosaurs this way into the 1930s, but in contrast English and American scientists largely lost interest in these animals over subsequent decades – and depictions of pterosaurs went the same way as non-avian dinosaurs during this period, descending into awkward evolutionary failures that could barely even fly, shown as scaly-skinned or naked, and hanging upside-down from trees and cliffsides like giant wrinkly bats.

2020s

The discovery of definite hair-like structures (known as pycnofibers) on Sordes brought pterosaurs into their own renaissance in the 1970s, and among a flood of new discoveries they were reinterpreted as active warm-blooded bird-like animals. Reconstructions sometimes went a bit too bird-like, though, attempting to distance themselves from the older saggy-repto-bat portrayals, with forced bipedal postures and much more slender wing membranes attaching to the waist.

But early 21st century studies into biomechanics, soft-tissue remains, and trackways confirmed that some elements of the bat model had actually been right the whole time. Pterosaurs had flight membranes attached to their hind limbs and were quadrupedal when on the ground – but instead of being awkward bat-like sprawlers they were actually competent walkers and runners with an energy-efficient upright posture.

We now know Pterodactylus lived during the Late Jurassic, about 150-148 million, at a time when the region of southern Germany was part of an island archipelago in a shallow tropical sea. Fragmentary remains are also known from elsewhere in Europe and in Africa, suggesting this genus had a fairly wide range.

It was a fairly small pterosaur, with the largest adults having am estimated wingspan of around 1m (3’3″), and had long straight jaws lined with numerous pointed teeth. Most known specimens are juveniles, but fossils of larger adults preserve evidence of a soft tissue crest with a backwards-pointing “lappet”, and long mane-like pycnofibers on the back of the neck.

Like other pterosaurs it was fuzzy and warm-blooded, and it had hollow bird-like bones and air sacs lightening its body. Its wings were highly complex with layers of strengthening fibers and muscles that allowed the flight surface shape to be precisely controlled, and when walking on the ground it could fold up its wing fingers and stow the membranes well out of the way of its limbs.

It was probably a generalist carnivore, feeding mostly on small prey like invertebrates, and the shape of the sclerotic rings in its eye sockets suggest it was mainly active during the daytime.

Retro vs Modern #17: Ammonites

Ammonites (or ammonoids) are highly distinctive and instantly recognizable fossils that have been found all around the world for thousands of years, and have been associated with a wide range of folkloric and mythologic interpretations – including snakestones, buffalo stones, shaligrams, and the horns of Ammon, with the latter eventually inspiring the scientific name for this group of ancient molluscs.

(Unlike the other entries in this series the reconstructions shown here are somewhat generalized ammonites. They’re not intended to depict a specific species, but the shell shape is mostly based on Asteroceras obtusum.)

1830s

It was only in the 1700s that ammonites began to be recognized as the remains of cephalopod shells, but the lack of soft part impressions made the rest of their anatomy a mystery. The very first known life reconstruction was part of the Duria Antiquior scene painted in 1830, but to modern eyes it probably isn’t immediately obvious as even being an ammonite, depicted as a strange little boat-like thing to the right of the battling ichthyosaur and plesiosaur.

The argonaut octopus, or “paper nautilus”, was considered to be the closest living model for ammonites at the time due to superficial similarities in its “shell” shape, but these modern animals were also rather poorly understood. They were commonly inaccurately illustrated as floating around on the ocean surface using the expanded surfaces on two of their tentacles as “sails” – and so ammonites were initially reconstructed in the same way.

1860s

While increasing scientific knowledge of the chambered nautilus led to it being proposed as a better model for ammonites in the mid-1830s, the argonaut-style depictions continued for several decades.

Interestingly the earliest known non-argonaut reconstruction of an ammonite, in the first edition of La Terre Avant Le Déluge in 1863, actually showed a very squid-like animal inside an ammonite shell, with eight arms and two longer tentacles. But this was quickly “corrected” in later editions to a much more nautilus-like version with numerous cirri-like tentacles and a large hood.

The nautilus model for ammonites eventually became the standard by the end of the 19th century, although they continued to be reconstructed as surface-floaters. Bottom-dwelling ammonite interpretations were also popular for a while in the early 20th century, being shown as creeping animals with nautilus-like anatomy and numerous octopus-like tentacles, before open water active swimmers eventually became the standard representation.

2020s

During the 20th century opinions on the closest living relatives of ammonites began to shift away from nautiluses and towards the coleoids (squid, cuttlefish, and octopuses). The consensus by the 1990s was that both ammonites and coleoids had a common ancestry within the bactridids, and ammonites were considered to have likely had ten arms (at least ancestrally) and were probably much more squid-like after all.

Little was still actually known about these cephalopods’ soft parts, but some internal anatomy had at least been figured out by the early 21st century. Enigmatic fossils known as aptychi had been found preserved in position within ammonite shell cavities, and were initially thought to be an operculum closing off the shell against predators – but are currently considered to instead be part of the jaw apparatus along with a radula. 

Tentative ink sac traces were also found in some specimens (although these are now disputed), and what were thought to be poorly-preserved digestive organs, but the actual external life appearance of ammonites was still basically unknown. By the mid-2010s the best guess reconstructions were based on muscle attachment sites that suggested the presence of a large squid-like siphon.

Possible evidence of banded color patterns were also sometimes found preserved on shells, while others showed iridescent patterns that might have been visible on the surface in life.

In the late 2010s the continued scarcity of ammonite soft tissue was potentially explained as being the same reason true squid fossils are so incredibly rare – their biochemistry may have simply been incompatible with the vast majority of preservation conditions.

But then something amazing happened.

In early 2021 a “naked” ammonite missing its shell was described, preserving most of the body in exceptional detail – although frustratingly the arms were missing, giving no clarification to their possible number or arrangement. But then just a few months later another study focusing on mysterious hook-like structures in some ammonite fossils concluded that they came from the clubbed tips of a pair of long squid-like tentacles – the first direct evidence of any ammonite appendages!

A third soft-tissue study at the end of the year added in some further confirmation that ammonites were much more coleoid-like than nautilus-like, with more evidence of a squid-style siphon, along with evidece of powerful muscles that retracted the ammonite’s body deep inside its shell cavity for protection.

Since ammonites existed for over 340 million years in a wide range of habitats and ecological roles, and came in a massive variety of shapes and sizes, it’s extremely likely that their soft anatomy was just as diverse as their shells – so there’s no single “one reconstruction fits all” for their life appearances. Still, at least we now have something less speculative to work with for restorations, even if it’s a bit generalized and composite, and now that we’re finally starting to find that elusive soft tissue there’s the potential for us to discover so much more about these iconic fossil animals.

Retro vs Modern #16: Uintatherium anceps

Discovered in the Western United States during the early 1870s, Uintatherium anceps was part of one of the earlier major conflicts in the the Bone Wars. Nearly 30 different scientific names were applied to various fossil specimens of this mammal in under two decades, and the taxonomic tangle wasn’t properly sorted out until nearly a century later in the 1960s when they were recognized as actually all being the same species.

1870s

Paleontologist Edward Cope considered Uintatherium (under the name “Loxolophodon”) and its close relatives to be proboscideans – part of the elephant lineage – due to some of the similarities in their anatomy. The first reconstruction of these animals showed this version, depicting elephant-like animals with downward-pointing tusks, short tapir-like trunks, and the multiple bony projections on their skulls speculatively shown as attachment points for large antler-like horns.

Cope’s rival Othniel Marsh heavily criticised that interpretation of Uintatherium, arguing that these huge mammals were instead a separate group within the ungulates named dinoceratans – although this wasn’t really as huge of a classification difference as it seems today, since at the time proboscideans were also considered to be ungulates!

The dinoceratan ungulate interpretation quickly won out, and for a while in the 20th century Uintatherium actually became a fairly popular and well-known prehistoric mega-mammal, commonly included in collections of cheap plastic “dinosaurs” and usually depicted as more of a knobbly-headed sabertoothed rhino.

2020s

In recent years the dinoceratans seem to have fallen into obscurity and some degree of paleontological neglect, with little modern work on the group and no major studies for the last couple of decades – although this might be starting to change.

Despite the early ideas about them being ungulates, the evolutionary relationships of dinoceratans have become much more murky over the last century or so. Due to different elements of their anatomy being highly convergent with various other mammals it’s easy to find “false positives” in morphological comparisons, and they’ve been proposed as being connected to a wide variety of groups including “condylarths“, “insectivores“, rodents, and cimolestans. But some mid-2010s research suggests they were in fact ungulates after all, closely related to early South American forms like Carodnia – a lineage whose own evolutionary relationships are murky, but may have close affinities with modern horses, rhinos, and tapirs.

We now know Uintatherium anceps lived across the Western and South Central USA during the mid-Eocene, about 46-40 million years ago, at a time when warm wet climates extended up into the Arctic and lush tropical-style rainforests covered much of the continent.

It was similar in size and build to a modern white rhino, about 4m long (13′) and stood around 1.7m tall at the shoulder (5’7″). It had three distinctive pairs of “horns” on its forehead, snout, and nose, that were similar in structure to the ossicones of giraffids, probably covered in skin and hair rather than keratin. Its elongated canine teeth were protected by bony flanges on its lower jaw, and seem to have been a sexually dimorphic feature that was much more prominent in males.

It also had an oddly concave skull, with its forehead dipping inwards, and an unusually tiny braincase for its size. It probably wasn’t a particularly intelligent animal, but it didn’t really need to be – as one of the first types of herbivorous mammal to get truly huge in the early Cenozoic, a fully-grown Uintatherium probably had no natural predators at all.

Retro vs Modern #15: Dimetrodon limbatus

With its prominent sailback Dimetrodon is one of the most iconic prehistoric animals – and one that still frequently gets mistaken for a dinosaur, despite being closer related to modern mammals.

1870s-1980s

The first known Dimetrodon fossil was an upper jaw fragment found in Canada in the 1840s, but at the time this specimen was thought to represent a dinosaur. It wasn’t until the late 1870s that species like Dimetrodon limbatus (initially called Clepsydrops limbatus) from the Midwestern and Southern United States were recognized as belonging to a much older and different group of animals given the name “pelycosaurs“.

While some paleontologists did propose pelycosaurs as being ancestral to mammals quite early on, for several decades the prevailing view was actually that they were an ancient branch of rhynchocephalian reptiles closer related to modern tuataras. From the 1910s onwards pelycosaurs were finally linked back to mammals, with their similarities to the therapsids placing them as early members of the synapsid lineage – although all these early mammal-relatives were still considered to be derived from reptiles, and “mammal-like reptile” became a commonly-used term for them. 

As a result reconstructions of Dimetrodon during this time period usually depicted a highly reptilian and heavily scaled lizard-like animal, with a sprawling belly-dragging pose, protruding crocodilian-like teeth, and a highly shrink-wrapped sail on its back modeled on those of some modern lizards. Some earlier images also showed a short stumpy tail, since Dimetrodon‘s longer tail proportions weren’t confirmed until the 1920s.

2020s

During the late 20th century new classification techniques led to the messy concept of “reptiles” being properly redefined as sauropsids, and synapsids being recognized as an entirely separate non-reptilian lineage of amniotes. Along with new studies and discoveries this has resulted in our understanding of Dimetrodon changing a lot in the last few decades, moving away from a heavily reptilian interpretation and instead letting it be its own weird “protomammal” thing. 

We now know there were at least a dozen different species of Dimetrodon living during the early-to-mid Permian, about 295-272 million years ago. Most of them are known from North America, but an additional species discovered in Germany suggests this genus ranged further across Pangaea than previously thought.

Dimetrodon limbatus was one of the larger species, about 3m long (10′), and like other members of the genus it had a tall narrow skull with high-set eyes and two distinct types of teeth in its jaws. The structure of its nasal cavities suggest it had a good sense of smell, and like the related synapsid Ophiacodon it may have had a closer to “warm-blooded” metabolism than previously thought.

It would have had a very poor sense of hearing, however, and probably didn’t even have any visible ears on its head. It may have been functionally deaf to air-borne sounds entirely, only able to detect vibrations by pressing its lower jaw to the ground.

No skin impressions are known for Dimetrodon. Scaly reptile-like skin has been found on varanopids, a group traditionally classified as very early synapsids – but some recent studies have suggested they were actually part of the true reptile lineage, so their extensive scaliness probably doesn’t apply to synapsids like Dimetrodon after all. There is some possible evidence of rows of square or rectangular scale-like scutes on the underside of the belly and tail in pelycosaur-grade synapsids, but otherwise the next-closest known synapsid skin comes from the distantly-related therapsid Estemmenosuchus, which seems to have had smooth glandular skin similar to a hairless mammal.

The characteristic back sail, formed by highly elongated neural spines on the vertebrae, is now thought to have been covered in a different pattern of soft tissue than older reconstructions depicted. The texture of the bone along the spines’ length shows that at the base they were deeply embedded in the back musculature, then further up they were covered by skin webbing, but then at the tips they may actually have been unwebbed and free-standing, giving a much spinier profile.

While the sail was traditionally assumed to be used for temperature regulation, more recently this has started to seem less likely. The sail doesn’t seem to have been quite as well-supplied with blood vessels as previously thought, and there’s a lack of direct correlation between sail size and body size in different Dimetrodon species and age classes. Instead this structure may actually have been used for visual communication and display, and could therefore have been quite flashy and brightly-colored.

Fossilized trackways also suggest that Dimetrodon didn’t move with a low lizard-like sprawling gait but instead with something more like a crocodilian “high walk”, with its limbs much closer to upright. It was probably a fairly active terrestrial predator and would have eaten a wide variety of other smaller Permian animals, with its teeth having been found in association with the remains of the amphibians Eryops and Diplocaulus and the freshwater shark Xenacanthus.

Retro vs Modern #14: Therizinosaurus cheloniformis

Therizinosaurs were some of of the most unique theropod dinosaurs. It’s only in the last few decades that we’ve started to understand much about them, and they’re still somewhat enigmatic even today.

1950s

The first known therizinosaur fossil discovery was Therizinosaurus cheloniformis, found in Southern Mongolia during the late 1940s and described and named in the mid-1950s based on a few fragments that included some unusually large and elongated claws.

These remains were interpreted as belonging to a giant turtle-like reptile that used its scythe-like claws to harvest aquatic plants – inspiring both parts of its scientific name, with Therizinosaurus meaning “reaper lizard” and cheloniformis meaning “turtle-shaped”.

1990s

The turtle interpretation began to be questioned during the 1970s, and the discovery of some slightly better (but still fragmentary) specimens reclassified Therizinosaurus as an unusual theropod dinosaur.

In the late 1970s and early 1980s fossils of another group of dinosaurs known as “segnosaurs” were also starting to be discovered, with a confusing mixture of anatomical features that seemed to link them to multiple different dinosaur lineages. As a result opinions about their evolutionary relationships varied during the 1980s, sometimes considering them to be theropods, sometimes late-surviving “relic” prosauropods, and sometimes a whole new major lineage of rare and weird saurischians.

Similarities between segnosaurs and the known material of Therizinosaurus were soon noted, and the discovery of the fairly complete Alxasaurus in the early 1990s confirmed that they were all part of the same group of bizarre herbivorous theropods – and the name “segnosaurs” was dropped in favor of “therizinosaurs”, since older names usually get priority in taxonomy.

Reconstructions of Therizinosaurus during this time tended to look rather weird and awkward. As with the majority of dinosaurs during this period it was depicted as entirely scaly and reptilian, and was often shown with a stiff hunched downcurving neck and an oddly tiny-looking tail.

Further discoveries during the 1990s finally began to clarify therizinosaurs’ evolutionary affinities, eventually placing them as an early branch of bird-like maniraptoran theropods, closely related to both oviraptorosaurs and the alvarezsaurs – and in 1999 the discovery of the small early therizinosaur Beipiaosaurus helped to confirm this relationship, revealing impressions of an extensive coat of filamentous feathers and longer stiffer quill-like structures.

2020s

Over the next couple of decades more and more therizinosaur fossils were found in both Asia and North America, and details about these dinosaurs’ appearance, biology, and ecology gradually became better understood. While there’s still a lot we don’t know about them, we do now at least have some fairly complete examples like Nothronychus, fossilized therizinosaur footprints, more feathers, an idea of Beipiaosaurus’ coloration (it was brown!), and even eggs and potential colonial nesting sites.

And while Therizinosaurus itself is still only represented by fragmentary and incomplete material, we now have a much better idea of what it was probably like. It lived in what is now the Gobi Desert during the Late Cretaceous, about 70 million years ago, and would have been both the largest known therizinosaur and the largest known maniraptoran dinosaur, estimated to have been as much as 10m long (33′) based on the proportions of its relatives.

It would have had a tiny head with a toothless beak at the front of its jaws, a long neck, and a wide bulky “pot-bellied” body housing its huge plant-fermenting gut. With its especially large body size it probably wasn’t as extensively feathered as its smaller relatives, but it may have still been sparsely fuzzy across parts of its body.

Unlike most other theropods it walked on all four toes of its feet, with the dewclaw enlarged into an extra weight-bearing digit. Recent analysis of footprints has also suggested that the larger therizinosaurs like Therizinosaurus may actually have been plantigrade, walking with their feet completely flat on the ground. This might turn out to just be an artifact of how the tracks were preserved, but therizinosaurs are certainly already weird enough that it could be a plausible interpretation.

But Therizinosaurus’ most distinctive feature was its hands, with extremely long narrow claws each at least 50cm long (1’8″). Unlike the strongly curved claws seen in other therizinosaurs, these ones were fairly straight for most of their length, only curving more sharply towards their tips.

While in the past these claws have been proposed as being weapons or digging adaptations, they were actually relatively delicate and were probably mainly used for pulling clumps of vegetation closer in a convergently similar manner to the later mammalian chalicotheres and ground sloths.

Therizinosaurus would have been a heavy slow-moving animal, and probably spent a lot of time sitting on its haunches supported by its especially robust hip bones while it browsed on large amounts of vegetation. It likely relied on its pure bulk and intimidation to deter potential predators, possibly even making aggressive displays with its claws when threatened – essentially it may have been a giant goose-sloth.

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 #11: Brontosaurus excelsus

Discovered in the Western United States during the 1870s, in the early years of the Bone Wars, Brontosaurus excelsus was one of the most complete sauropod dinosaurs known at the time.

1900s-1970s

In the early 1900s the genus name Brontosaurus was declared invalid and it was reclassified as a species of the very-closely-related Apatosaurus, renaming it to Apatosaurus excelsus – but this change took decades to be recognized outside of scientific literature, and by that time the “Brontosaurus” name had already stuck in pop culture. With the prominence of the name’s use in early 20th century museum displays and its charismatic meaning of “thunder lizard”, it rapidly became one of the most famous and recognizable dinosaurs to the general public.

Like most sauropods of the time Brontosaurus was generally portrayed as a large bulky lizard-like creature with an arched back, thick elephant-like legs, and a long dragging tail. Opinions on its neck posture varied over time, ranging from low-slung and horizontal to highly vertical, and it was commonly depicted wallowing lazily half-submerged in swamps due to sometimes being considered too big to easily support its own weight on land.

And along with spending most of the century with the wrong name, Brontosaurus also spent most of it with the wrong head. While a slender Diplodocus-like skull had been found close to the rest of an Apatosaurus skeleton in the early 1900s, it was rejected by some paleontologists and both Brontosaurus and Apatosaurus were instead given boxy Camarasaurus-like skulls that were thought to be more fitting for such big beefy-necked sauropods.

2020s

The Dinosaur Renaissance in the late 20th century completely revolutionized the understanding of sauropods and their biology. They were dragged out of the swamps and put properly back onto dry land, reinterpreted as active animals with their long tails held up off the ground and bird-like air sacs lightening their bodies.

The correct skull shape for Brontosaurus and Apatosaurus was also finally recognized in the late 1970s, and during the the 1990s and early 2000s a very horizontal neck posture became the standard depiction for this type of diplodocid sauropod. But by the 2010s this was being argued as biomechanically wrong – animals usually hold their necks at a much higher angle than the bones alone would suggest, and sauropods almost certainly did the same.

And then in 2015 the name Brontosaurus was reinstated as valid after all, in a massively thorough analysis of the diplodocid family that found enough physical differences between Brontosaurus and Apatosaurus to justify them both being separate genera again.

So our modern view of Brontosaurus excelsus (formerly known as Apatosaurus excelsus, even-more-formerly known as Brontosaurus excelsus) is a large sauropod that grew to around 22m long (72′). It lived during the Late Jurassic, about 156-146 million years ago, at a time when the supercontinent of Pangaea was starting to break apart and much of Western North America was a warm and semi-arid “Jurassic savanna” environment.

Its head was small and fairly delicate, similar to that of Diplodocus, atop a wide deep neck with chunky vertebrae. It had a deep chest and stout limbs, with its hands being semi-tubular pillars with a single “thumb” claw, and its feet having three large curved claws. Its tail made up over half its body length and was relatively slender, tapering into a long whip-like tip that may have been able to make loud cracking sounds like a bullwhip.

It was probably capable of briefly rearing up to reach higher vegetation, and small juveniles may even have been able to run on just their hind legs.

Soft tissue impressions from other diplodocids show keratinous spines running along the top of their tails, and complex variation in the sizes and shapes of scales across different parts of their bodies – so Brontosaurus may have been similarly ornamented.

Retro vs Modern #10: Plateosaurus trossingensis

First discovered in southeast Germany in the 1930s, Plateosaurus was only the fifth non-avian dinosaur known to science – but its fossils were fragmentary and poorly understood until the early 20th century, when large bonebeds full of much better specimens began to be excavated.

1910s-1970s

Between the 1910s and 1930s around 80 near-complete skeletons of Plateosaurus were found in two German quarries, quickly making it one of the most abundant and best known dinosaur species of the time. Although it had previously been classified as a theropod dinosaur, in the 1920s the more complete material allowed it to be properly identified as a “prosauropod“, an early herbivorous relative of the giant sauropods. 

Like many bipedal dinosaurs during this period Plateosaurus was generally interpreted as having an upright kangaroo-like posture with a dragging tail – although some paleontologists were arguing for it having a sprawling quadrupedal lizard-like stance as late as the mid-1930s.

1980s

Unfortunately much of the German fossil material was destroyed during World War II bombing raids, and interest in Plateosaurus didn’t pick up again until the time of the Dinosaur Renaissance when a third major fossil site was discovered in Switzerland during the 1970s.

Plateosaurus was reinterpreted with a horizontal body posture and fully upright limbs. As an early member of the sauropodomorph lineage it was often depicted as a transitional form between bipedal ancestral dinosaurs and the later quadrupedal sauropods, thought to primarily walk on all fours but also able to run on its hind legs like a hadrosaur – although some studies instead concluded it was fully quadrupedal with a downwards-curling tail that made bipedal movement impossible.

The large numbers of skeletons found together were considered to represent evidence for herding behavior, with groups of Plateosaurus being caught in catastrophic mudflows all at once.

2020s

Extensive biomechanical studies in the 2000s and early 2010s clarified what sort of posture Plateosaurus was really capable of. It was found to be completely unable to position its arms in a quadrupedal stance, and so was actually purely bipedal – and skeletons that had been mounted in the quadrupedal position had needed many of their joints to be completely dislocated to achieve the pose!

A huge number of different Plateosaurus species had been named over the genus’ nearly-200-years of history, too, creating a confusing mess of dubious and invalid names. These were all finally revised in 2019 leaving just three valid species, with Plateosaurus trossingensis as the best known and the new type species.

We now know Plateosaurus lived across central and northern Europe during the Late Triassic, around 214-204 million years ago, at a time when the region had a subtropical climate. It had a small head on a long flexible neck, with teeth convergently resembling those of modern iguanas suggesting it was probably primarily herbivorous (with possible opportunistic omnivory). Its arms were proportionally short for a prosauropod but were well-adapted for grasping, with large claws that may have been used to dig up roots and tear down branches

It had a rapid growth rate and bird-like lungs and air sacs that suggest it was warm-blooded, and different individuals showed an unusually high amount of variation in adult size and age of maturity. Some appear to have been fully grown at about 5m long (~16′) and as young as 12 years old, while others reached 10m long (~33′), and were still growing at 27 years old.

The bonebeds are no longer thought to represent mass mortalities of herds, but instead were probably a scenario more similar to the La Brea Tar Pits – mud-miring traps that smaller lighter animals could escape from but larger individuals became stuck and died.

No prosauropod skin impressions have been found yet, so it’s still unknown whether Plateosaurus was scaly like later sauropods or if it had some degree of protofeather hair-like fuzz.