While most of the text in this file is “in character”, the sections that say “real life inspiration” provide commentary on, well, the real life inspiration for each clade of these fictional prehistoric monsters.  While the specific kaiju characters in A.T.O.M., like Tyrantis and Ahuul, are owned by me, I have offered up the concept of retrosaurs as public domain, so if you want to include the monster clades listed here in your own stories you are more than welcome to.

RETROSAURS: RULERS OF THE REPTILIAN AGE

EVOLUTIONARY HISTORY
One of several archosaur groups competing for dominance of the planet at the end of the Permian and beginning of the Triassic, retrosaurs are somewhat hard to place on the tree of life.  They are definitely part of the Pseudosuchia clade, though how close they are to modern crocodilians is a matter of dispute.  Scientists are likewise uncertain of what specifically gave retrosaurs an advantage over competing archosaur clades, such as the prehistoric birds that were also rising in diversity at the start of the Triassic.  The two clades were similar in a lot of respects, with both producing bipedal hunters that evolved from quadrupedal ancestors.  Nothing about the bone structure of these two competitors seems to settle the issue, and as such many paleontologists hypothesize that the retrosaur’s advantage must not have been something that could fossilize; the most popular theory is that their ectothermic metabolism allowed them to conserve resources better than their avian competitors during times of drought and famine.   Regardless, birds and mammals stayed in a few specialized niches while retrosaurs took over the environment and became extremely diversified, ruling the world for millions of years before their sudden and cataclysmic extinction.

COMMON RETROSAUR BIOLOGY
All retrosaurs have digitigrade hind limbs which stand mostly erect, rather than sprawling out like their fore limbs tend to do.  Retrosaurs are also believed to have had an ectothermic metabolism, meaning they relied on their environment to provide them with heat rather than producing it themselves.  With rare exception, they laid soft-shelled eggs and are believed to have abandoned their young like other reptiles.  Retrosaurs are believed to have been slow of mind and body, which is why their name means “backwards lizards” – they are believed to have been a mistake in the grand design of creation.

PRIMITIVE RETROSAURS

The earliest retrosaurs were fairly plain compared to their more notable descendants, looking little different than the modern reptiles we share the earth with today.  However, by the end of the Triassic they obtained a massive size, and began developing many of the strange features that would make the clade so bizarre and interesting during its long domination of the earth.

Primitive Carnivorous Retrosaurs
Early on in its history, the retrosaur clade diverged in form because of function; specifically, the function of eating.  Descended from carnivorous pseudosuchian reptiles, the first retrosaurs were obligate carnivores, and many of the most ancient species in the clade looked very similar to crocodiles we have today, albeit with a few more frills, spikes, and horns.  Slowly they developed longer, larger, and altogether more powerful hind legs, and their descendants would eventually become fully bipedal.

Primitive Herbivorous Retrosaurs
Herbivores arose from the primitive retrosaur clade as well, looking coincidentally similar to many modern herbivorous and omnivorous lizards.  Members of these early herbivorous retrosaurs are sometimes called “hunchbacked retrosaurs” because of their distinctly large, curved spines, which gave their bodies a hump-like shape.  Slow moving yet heavily armored, these primitive reptiles carried the first drafts of traits their descendants would specialize in to great success.

Real Inspiration: Primitive Retrosaurs are based on the earliest reconstructions of dinosaurs, such as the very Iguana-like Crystal Palace sculpture of Iguanodon from the 1800’s.  They’re also inspired by Slurpasaurs, i.e. the old film technique of putting latex makeup on real lizards and using film trickery to make them look much larger so they can be “dinosaurs.”

CARNIVOROUS RETROSAURS

Paleo Tyrants
The largest predators ever to walk the earth, Paleo Tyrants were fierce killers that hunted the largest and deadliest game imaginable.  Bipedal creatures with digitigrade legs, they would have been remarkably fast despite their reptilian metabolisms, and were armed with saber fangs and razorsharp claws on their hands and feet.  Many Paleo Tyrants had short, thin arms, which bordered on being vestigial in a few species.  The Paleo Tyrant clade is split into three subclades:

True Tyrants – the most famous of the Paleo Tyrants, True Tyrants were massive predators with large heads, comparatively small arms, and enormous strength matched only by their ferocity.  Despite their great size, they operated in a number of niches and had a notable variety of species right up to the end of the Mesozoic era.

Tiny Tyrants – while the general perception of retrosaurs is that they were large, Tiny Tyrants were exceptions to this rule.  Few were larger than a dog, with some species being little larger than a chicken.  They were not nearly as successful as their larger cousins, as they were in direct competition with prehistoric birds like Archaopteryx and Velociraptor.  These two separate groups of creatures were even prone to parallel evolution, as many species of Tiny Tyrants and prehistoric birds sport large, curved inner toe claws.

Egg Thieves – closely related to Tiny Tyrants, Egg Thieves were exceptionally odd retrosaurs that coincidentally bore a slight resemblance to large, modern, flightless birds, such as ostriches and emus.  With slender bodies, long legs, and slender necks, these retrosaurs were built for speed.  Most species preyed on small animals, while a significant number were specialized at poaching eggs from the nests of other, larger retrosaurs (hence the clade’s nickname).  This behavior may also be why they were often a common prey item of True Tyrants, with many Egg Thief skeletons being found in the stomach of their larger, more gluttonous kin.

Real Inspiration:  Paleo Tyrants are inspired by theropods.  They’re split up according to size like scientists used to do with theropods before we realized things were a bit more complex than that – True Tyrants are a rough parallel to the outdated clade of “carnosaurs,” for example.  Tiny Tyrants are probably the closest retrosaurs get to modern, accurate dinosaur  depictions, being based on 1980’s illustrations of dromaeosaurs.

Flying Tyrants

Descended from the Tiny Tyrant clade, Flying Tyrants sported a pair of leathery wings similar to a bat’s.  However, these wings were supported by only one long, muscular digit, allowing the Flying Tyrant’s other fingers to be used for grasping prey.  Flying Tyrants filled a variety of niches, almost all of which were predatory in nature.  Their hind limbs, which they could stand on without too much trouble, were equipped with strong grasping talons.  Most species sport long, thin tails for balance, as well as an impressive cranial crest.  While Flying Tyrants appeared to have beaks, their jaws retained their teeth.

Real Inspiration: Flying Tyrants are based on pterosaurs, which in real life were only distantly related to dinosaurs.  Since a lot of people mistakenly call pterosaurs “flying dinosaurs,” I thought it’d be funny if the retrosaur equivalent of pterosaurs actually were flying members of the retrosaur clade.  This is all about making the misconceptions into a mythology, after all.

Transitional Tyrants

One of the more nebulous clades in the retrosaur family, some paleontologists argue that Transitional Tyrants should be folded into their ancestor clade, the Paleo Tyrants.  Originally thought to be just a transitional form between Paleo Tyrants and their aquatic descendants, the Sea Tyrants, recent discoveries have shown that Transitional Tyrants were a diverse lot that occupied many diverse niches on their own.  All Transitional Tyrants are plantigrade, walking with their heels firmly on the ground, which is a significant difference between them and their ancestors, the digitigrade Paleo Tyrants.  Many Transitional Tyrants were also either amphibious or burrowing creatures, lurking beneath the waves or deep within the earth when not hunting prey.  Notably, this clade of retrosaurs managed to survive for millions of years alongside their terrestrial ancestors and aquatic descendants.

Real Inspiration: Transitional Tyrants are based on the man-in-a-rubber suit special effect, i.e. characters like Godzilla, Gomora, Baragon, et cetera.  They’re plantigrade and weird – monsters like Baragon often stretch our idea of what dinosaurs could be while still feeling very prehistoric.  This clade is where you can get as far from the reality of what dinosaurs were as you can.

Sea Tyrants
Evolved for an almost completely aquatic existence, Sea Tyrants dominated the Mesozoic seas, driving sharks and other oceanic predators into smaller niches.  Some species were even ovoviviparous, incubating their eggs inside their body until they hatched rather than laying their eggs on land.

Long-Necked Sea Tyrants – superficially similar to Long Necked Goliaths, these carnivorous retrosaurs have serpentine necks, round, hump-backed bodies, and sinuous tails tipped with a broad and powerful fluke.  Generally more peaceful than the other members of their clade, long necked sea tyrants tended to feed on schools of fish instead of on other retrosaurs.  Their front limbs were closer to legs than flippers, with fully articulate elbows and short paws tipped with long talons.  Their hind limbs, however, were fully adapted for moving in the water.  Most long necked sea tyrants spent a good chunk of time on land as well as in the water, both to lay their eggs and to bask in the sunlight.

Long-Tailed Sea Tyrants – the most successful and varied of the Sea Tyrants, these retrosaurs had flexible, serpentine bodies, long crocodilian jaws, and a series of dorsal fins stretching from their necks down to their tails.  Their limbs were adapted for an entirely aquatic existence, being near-useless on land but working as powerful flippers in the sea.  Long Tailed Sea Tyrants were prolific, with many species in a variety of sizes and niches.

Short-Tailed Sea Tyrants – largest of the Sea Tyrants, this clade was also the least common, often inhabiting only the deepest and farthest parts of the ocean.  With stocky builds and enormous jaws, these massive leviathans were the largest predators ever to stalk the sea, with the greatest of their clade being twice the size of the largest terrestrial carnivorous retrosaur.  A sub-group of this clade were smaller and more fish or dolphin-like in shape, being designed for speed instead of strength.  Despite their size, they were not a very successful clade, and Short Tailed Sea Tyrants died out early in the Cretaceous Period.

Real Inspiration: The different sea tyrants are based on all the different marine reptiles from the Mesozoic era – plesiosaurs (long-necked sea tyrants), mosasaurs (long-tailed sea tyrants), and pliosaurs and ichthyosaurs (short-tailed sea tyrants).  Like pterosaurs, these creatures often get mistaken for being dinosaurs themselves, and so their retrosaur counterparts literally are just a subclade of retrosaur.  Sea Tyrants also mix in a good deal of medieval sea monster in their designs, just as early reconstructions of their real life counterparts often did.

HERBIVOROUS RETROSAURUS

Long-Necked Goliaths

Perhaps the most famous retrosaur clade, Long Necked Goliaths are what many first think of when they hear the word “retrosaur.”  There was an inherent majesty to these enormous creatures, some of which grew so large that they managed to dwarf the rest of their gargantuan kin.  With their long, serpentine necks, massive torsos, and tree trunk-like legs, they towered above all their neighbors.  Long Necked Goliaths were slow moving and placid creatures, eating foliage and avoiding conflict just by growing to a size where nothing could threaten them.  It’s no wonder that these long gone gentle giants remain a source of fascination to this very day.

Real Inspiration:  These guys are based on Sauropods, and there’s not much more we need to say.

Bipedal Goliaths
While most herbivorous retrosaurs were quadrupeds, one fairly successful clade imitated their carnivorous cousins and developed bipedal locomotion, albeit at a much later date (the earliest Bipedal Goliath appeared in the mid-Jurassic Period, while Paleo Tyrants, i.e. the first bipedal retrosaurs, evolved in the Late Triassic).  Most Bipedal Goliaths lived in herds, and fossilized tracks show their took care of their young to some extent.  This clade also has at least two sub-clades, as well as a number of species that don’t fit cleanly into either of them.

Duck-Billed Goliaths – far and away the most common clade of the Bipedal Goliaths, these creatures had bills that were superficially similar to a duck’s, although the many grinding teeth within these bills set them apart from their feathered imitators.  Duck Billed Goliaths were often very similar in shape, and most species would be practically identical save for their unique head ornamentation.  Many had fanciful head crests, and at least a few species also sported long, hollow horns that are theorized to have been used to produce a variety of calls.

Helmeted Goliaths – with thick, armored skulls, these Bipedal Goliaths were more equipped to fend off predators than their kin.  Helmeted Goliaths had muscular necks designed to withstand collisions, which in turn allowed them to use their dense skulls like battering rams against both their predators and rivals of their own species.  Despite this powerful weapon, Helmeted Goliaths are the rarest of the herbivorous retrosaur clade.  It is theorized that competition from their duck-billed cousins, as well as the even more aggressive Horned Goliaths, kept them from successfully spreading to a wider variety of niches.

Real Inspiration: These two clades are based on hadrosaurs and pachycephalosaurs respectively, though they can mix in traits from other bipedal herbivorous dinosaurs as well (I tend to draw them with Iguanodon thumb spikes, for example).

Armored Goliaths
Covered by a mix of scutes and osteoderms, these living tanks proved to be incredibly difficult meals for predators.  Armored Goliaths evolved in the mid-Jurassic and lasted till the end of the Mesozoic era, getting larger, heavier, and better protected with each successive species.  Nearly all species Armored Goliaths had a large cluster of bone spikes or plates on the tips of their tails.  This defensive structure is called a Thagomizer, and was so useful at deterring predators that some members of the other herbivorous retrosaur clades evolved their own thagomizers as well.

Spike-Tailed Goliaths – the earliest of the Armored Goliaths, species in this sub-clade were armed with a cluster of bony spikes on their tail tips.  Earlier species sometimes had the spikes stretch up farther than the tip, with one having them extend all the way to the base of the tail.  Members of this clade also tended to have particularly large and wide dorsal plates running on their back.  While in some species these plates were used for protection, in many they were incredibly brittle and frail, which has made paleontologists question their purpose.

Club-Tailed Goliaths – evolving in the late Cretaceous, Club-Tailed Goliaths quickly replaced their spike tailed cousins.  The number of spikes on their thagomizers dwindled while the size of those spikes grew, turning their tails into powerful hammers to shatter the bones of their foes.  Club-Tailed Goliaths tended to be squatter than their ancestors as well, with wide, stocky bodies that were bristling with short spikes and thick armored scutes.

Real Inspiration: These clades are based on stegosaurs and ankylosaurs respectively, with as much over-the-top armor as you can muster.

Horned Goliaths

With their reptilian beaks, shield-like crests, and massive goring horns, the Horned Goliaths were a formidable force to be reckoned with.  The first Horned Goliath appeared in the early Cretaceous, setting off an evolutionary arms race with their main predators, the Paleo Tyrants.  Horned Goliaths tended to live in herds, where they were aggressively protective of their young.  They proved so successful that the Paleo Tyrants that hunted them were forced to become more skilled at killing, leading to the evolution of several specialized killers that culminated in legendary carnivores like Dynamosaurus imperiosus and Manospondylus gigas.  Horned Goliaths evolved in kind, though, growing larger horns, thicker crests, and eventually dermal armor and spikes that imitated the defensive adaptations of their Armored Goliath cousins.  They also managed to outcompete their fellow herbivorous retrosaurs in the process, with both Long Necked Goliaths and Armored Goliaths nearing extinction towards the end of the Cretacerous while Horned Goliaths only grew more varied and prosperous.

Real Inspiration: This clade is based on ceratopsians, obviously.