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Post by ewellholmes on Nov 6, 2025 2:52:00 GMT

Near Earth Orbit 66 Millions Years Ago

In the void of space, death is lurking among the stars. Hurtling towards Earth is an an asteroid the size of Mount Everest, bringing with it the promise of calamity for the inhabitants below. For most life on the planet, these are their final hours for today is the end of the Mesozoic. The uncontested reign of the Dinosaurs, arguably the greatest reign in all of natural history, has reached its twilight. With the coming apocalypse, an entirely new and almost alien Earth will be ushered in by the violent death of the old. Mammals and other assorted creatures will no longer live in the shadows as they have before. However, today will not be the end of the Dinosaurs themselves.

As the Chicxulub asteroid begins its final descent animals of all sorts around the world look up in incomprehension. Among them, along the Western coast of South America, are several colonies of Thescelosauridae. In our own world, these basal Ornithopods were limited in range to North America and East Asia but here things have taken a different turn; via rafting dispersal, they have successfully established a presence on this new continent. Descendants of Orodromeus, they have retained the small stature of their ancestor as well as the burrowing capability which is innate among their kind. Competition with the local Elasmarians has, however, has pushed them into expanding their diet to adapt compared to their ancestors. As a result, they now have a more generalist diet that has added tubers and seeds to their existing low browsing of plant matter. All of these traits will serve them well in what is to come, for though they do not know it the future of the Non-Avian Dinosaurs now rests upon their shoulders.

{Orodromeus, the ancestor to our survivors}

Briefly, the asteroid appears as bright as the sun before slamming into the Earth and beginning one of the greatest mass extinctions in Earth’s history. Driven by well honed instincts and fear, the Thescelosaurs immediately flee to their burrows underground. This serves them well, as the protection of the ground allows them ample safety from the armageddon outside. Although distance alone is sufficient to protect them from the initial blast, it is their own burrows that enable them to endure the mega earthquakes which come soon after. This security persists as the main element of the K-Pg extinction, the Infrared Thermal Event triggered by the return of ejecta falling back to Earth, begins. As tektites rain down, they trigger a global firestorm that will burn 70% of the Global bio-sphere, but our survivors are able to endure this too thanks to the safety of the ground between them and the chaos outside. Once things finally calm down sufficiently to re-emerge, these survivors enter emerge into a shattered world. Although they cannot know it, this act signifies the start of something new for their kind: This is the dawn of the Cenozoic.

The landscape that greets them is desolate, charred almost beyond recognition. Still, confusion at these monumental changes gives way as it must to basic instincts; after a day underground, it is time to eat. While little foliage has survived to feast upon, plenty of seeds and tubers have been protected by the topsoil to sustain them. Their smaller body size and thus reduced caloric needs compared to other Dinosaurs help them here, but they are still the largest surviving herbivore on the entire continent of South America in an ironic twist of fate. Thus, they are easily able to muscle out their competition in the form of mammals and their Avian cousins in the Birds. This first contest between the differing lineages will intensify in the eons to come and will have a huge impact on the evolution of all involved as they fill the now cleared ecological niche but for now the focus is on immediate survival and recovery. The future can wait in that context, of course.

As they forage among the broken terrain, the changing climate is becoming obvious as temperatures noticeably cool. The combination of soot from the fires and aerosols generated directly by the impact itself are working together in the atmosphere to reduce sunlight as the impact winter begins to set in. Their innate feathering helps to dull the worst of the chill and the ability to retreat to their warm burrows will prevent them from the worst of exposure but they are being aided by forces beyond their control in the form of local geography. In a stroke of luck, Northwestern South America is one of the few places where average temperatures will stay above freezing consistently and precipitation is sufficient to avoid aridification as occurs elsewhere. This lets the fern spike-and thus a steady supply of food-to emerge far sooner here than elsewhere while also keeping the ground soft enough that our plucky survivors can continue to dig their meals up from the ground until then.

{Climate Modeling for the Impact Winter}

While the days ahead of them will still be full of hardships, they have already survived the worst part thanks to their innate characteristics and a little luck. As the plucky survivors spread out across the land and eat their fill before the onset of night and associated chill forces them to retreat back to their lairs, the genesis of not just their own survival but the revival of the Non-Avian Dinosaur line is being set into place. While the Dinosaurs will never regain their uncontested reign globally, they can still make their own way here in this new world. Amid the death and destruction of their former world, a new and beautiful chapter is just getting written and will shape the world to come in amazing ways.

Last Edit: Nov 7, 2025 2:39:00 GMT by ewellholmes

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Post by ewellholmes on Nov 26, 2025 3:00:05 GMT

A map of Earth as it appeared 60 million years ago during the Paleocene Epoch, Selandian Age​

** The Paleocene Epoch**

66-56 Million Years Ago

Gradually, the world has begun to recover from the effects of the Impact. With this new found stability in the climate, the biotic recovery begins to accelerate as new species rush to radiate into the gaps left open by those that failed to survive. For most of the planet, things are proceeding as they did historically; South America’s isolation shields them for now. However, on the island continent itself, things are rapidly diverging and will never be the same again.

Having already started from the advantageous position as the largest extant herbivores, the Thescelosaurid survivors strengthen their competitive lead thanks to their innate Dinosaurian traits. As R-selected breeders, they are able to recover their numbers faster than their mammalian competition which in turn helps to muscle out their competition by enabling them fill new niches faster on an evolutionary time scale. This is further compounded by their efficient respiratory system and lighter bone structure, which make it both easier and faster for them to achieve larger body sizes. While they lack the air sacks of their Saurchian cousins, placing a bio-mechanical constraint upon their maximum sizes, they will still be able to produce giants in the long run in this altered world.

Thanks to these inherent abilities, the Thescelosaurids rapidly radiate into new species as they seek to claim available ecological roles. By the mid-Danian, it is clear the Dinosaur survivors of South America are well positioned to secure the medium to large herbivore roles left open by their extinct relatives. This is made definitive with the onset of the Paleocene-Eocene Thermal Maximum (PETM) which eliminates any further serious competition until the Great American Biotic Interchange (GABI) almost 50 million years later. At the smaller trophic levels, however, the competition is more fierce and Mammals are able to retain more niches in a way they were not able to before in the Mesozoic. These differing conditions result in two main evolutionary groupings emerging within the Non-Avian survivors.

The Traditional Thescelosaurids are the first, defined by the retention of the bipedal body plan of their ancestors and occupying roles extending from small fossorialists like their Orodromeus ancestor to that of fast-running browsers/generalists. Their intense competition with the Mammals results in them having the greatest biological diversity in numbers of species as well as the largest population numbers. Most of their kind remain small but some get as large as* Parkosaurus* within a million years of the K-Pg Impact. Ultimately, their size range tops out as large as their namesake Thescelosaurus down to members the size of the Jurassic ornithischian Fruitdens by the end of the Paleocene.

The second group are the Neo-Hadrosaurs, which emerge from new successor species leaning into their heritage as basal Ornithopods to assume a quadrupedal stance. This enables them to quickly begin increasing in size as they move into medium and then large body plans with evolutionary time, laying the foundation for the great giants of the Cenozoic to come. By 64 million years ago, they have produced species as large as Ajnabia; by the onset of the PETM, their largest example taxa will rival Maiasaura. The previously mentioned lack of air sacs will prevent them from reaching truly massive Sauropod sizes in the long run but they will still develop adaptions that enable them to be worthy successors nevertheless.

Outside of the Dinosaurs themselves, the combined competitive pressures exerted by the diversifying Non-Avian survivors produces a mixed bag result for the Mammals of South America. The Gondwanatherians, the resident Multituberculates of the continent, are clear losers; shut out of most Herbivore niches and with the Sprassadonts fulfilling the mammalian carnivore roles already, they begin to enter terminal decline by the Mid-Danian. The last few relic taxa terminate in the PETM, far ahead of the historical extinction of their last few members in the Miocene. This loss, however, has an ironic and arguably positive side effect:the lack of their own competitive downward pressure on their fellow Mammals enables other lines a chance to diversify and expand themselves far earlier than they historically enjoyed. This enables the* Dryolestoids* to undergo a rapid diversification as they speciate into the niches now opening up to them as the resident Multituberculates die out. They are able to occupy everything from arboreal roles to aquatic specialists, producing their own unique takes on squirrels and otters for example via convergent evolution.

However, the clearest winner out of all Mammalians are the Sprassadonts. Having already begun to move into a carnivorous role before the Impact, they are unaffected by the intense competition over herbivorous niches and in fact massively benefit from the presence of the Non-Avians, as their fast recovery means there is a larger megafaunal prey population for themselves to hunt. Thus, they actually have a far accelerated diversification than in our world and see quickly increasing body sizes in line with the growth of their new prey. While biomechanical constraints on Mammlian size-along with the warm climate of the PETM itself limiting their growth-will still leave the Sebecids as the dominant predatory of the Continent, the Sprassadonts are in a strong second place for now.

Finally, we must turn our attention to the aforementioned Sebecids. Just as in our world they have established themselves as the dominant land predator in South America but here likewise benefit from the earlier arrival of large and abundant prey the surviving Dinosaurs provide. This too, just like with the Sprassadonts, causes an earlier development in larger body plans that will eventually produce giants for Crocodylomorphs unseen since the Triassic. The combined effects of a more rapid development by the Sebecids and Sprassadonts does produce a clear loser however: the South American line of Madtsoiidae snakes. The early arising of competitors in the apex predator role serve to limit their size and will eventually culminate in an earlier extinction for their lineage during the late Eocene cooling event.

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Post by ewellholmes on Jan 16, 2026 2:48:39 GMT

Earth during the middle Eocene, 45 million years ago

**The Early-Middle Eocene Epoch 56-41 Million Years Ago **The epoch dawns with the birth of the Eocene Climatic Optimum, ushered in by the PETM. Warm temperatures prevail globally, with palm trees and crocodilians even being found near the modern Arctic Circle. In this hothouse world, tropical environments are prevalent, radiating out far out from the equator. In many ways this world is a throwback to the Mesozoic, which is something not lost on the evolutionary biology of the Non-Avian survivors of South America. The extremely favorable conditions for life serve as a catalyst for a further explosion in diversity and adaptation as the survivors continue to evolve in their slice of the planet which suddenly seems very familiar.

The relative abundance of new forest environments proves particularly favorable for the Neo-Hadrosaur lineage. The already steady growth in size they were experiencing in the Paleocene is amplified with the increased availability in food to sustain such a drive. As a result, by the end of the Early Eocene Climatic Optimum (EEOC) in 49 MYA the largest species has come to rival the long extinct Shantungosaurus in weight. The lack of specialized traits like air sacs found in their Saurichian relatives, however, means this 20 ton size proves their biological limit. While they will thus never challenge the Titanosaurs, this still comfortably allows them to rival many Jurassic Sauropods. Other similarities to their extinct relatives further abound among several species outside of just weight, as they begin to occupy the high browsing niche of their predecessors.

Through convergent evolution, many of the same morphological traits emerge, most obviously in the form of elongated necks. This in of itself is nothing new; Ornithischia had experimented in this previously with Miragaia but the diversity of species doing so as well as their increasing adaptations to their niche is something entirely novel. Most prominent among these is the gradual development of dental batteries, creating an advanced chewing mechanism that opens up more and more food sources to help them sustain their bulk in the face of the floral turnovers occurring in the wake of the decline of gymnosperms. It is this combination of derived traits that leads to the formal arising of the Pseudo-Sauropods, as a branching family of the Neo-Hadrosaurs.

Outside of the quadrupedal lineages, the more basal Thescelosaurids have a more mixed early Eocene experience. The expansion of forest environs combined with stiff competition from the Neo-Hadrosaurs for the middle tier of herbivorous niches (As they themselves are mostly excluded from high browsing now) causes a decline in diversity of their medium sizes species. The smaller examples of their line, however, do well; the thick jungles that pervade the continent suit them well and the competition with mammals for them is not as fierce. Burrowers are most common, a direct continuation of their ancestors lifestyles, but many "speedsters" remain as their line fills the abandoned roles left by Elasmaria at the time of the K-Pg Extinction. Even with the decline of their larger members, they still enjoy the status of more species diversity than their sibling families, setting them up well for future success.

Of course, this boom time for Dinosauria has effects outside their own members. Just as abundant food is an advantage to the Non-Avians, so too are they in turn to their natural predators. While no Theropods yet exist that can credibly threaten most of them, the "land crocs" of* Sebecidae* is another matter entirely and unfortunately for the Non-Avians, they are the dominant terrestrial carnivores of Cenozoic South America. Under the prevailing conditions of the humid hothouse climate their ectothermic biology is not a hindrance and the thick undergrowth of their environment is well suited to a large ambush predator role anyway. As a result, as the Dinosaurs increase in size so too do the Sebecids as the largest among them soon rivals the Triassic era Fasolasuchus in size by the end of the EECO. Cemented as the largest predators on their island continent with this, they will retain their position for millions of years to come.

Rounding out the predators are the Sprassadonts, Methatherian mammalian predators and indeed the only large mammals that currently exist on the continent of South America in general. Firmly boxed out for the top by both climatic conditions and competitive pressures from the larger Sebecids, they have optimized for the medium and small predatory niches but with a focus on pursuit predation where their biology is more suited than that of the Sebecids. Their smaller size also makes them better equipped for open spaces, enabling them to get closer to their Dinosaurian (or otherwise) prey before beginning the chase. Gregariousness also begins to emerge as an adaptation to the increasing size of their prey sources, as group hunting enables them to overcome larger prey with numbers rather than just pure size alone. They do well in this roles and in some ways exert a measure of "revenge" for the Mesozoic, despite their overall environment being very "Triassic" in nature with them as the Synapsid analogue of that earlier epoch.

As with all good things, the bountiful days of the first half of the Eocene gradually comes to a close as forces on a global scale force change. Far to the east, India smashes into Asia and begins to gradually uplift the Himalayan Mountains. This greatly increases chemical weathering as more bedrock becomes exposed, and it is further worsened with the Azola Event sequestering carbon on a mass scale in the modern Arctic. Together, this serves to end the climate optimum gradually over the course of millions of years as colder temperatures set in for the remainder of the Eocene. This leads to a contraction of tropical environments and the expansion of a relatively new biome: grasslands. These immense changes serve to make the latter half of the epoch a fundamentally different time than the first, bringing with it new challenges and opportunities for the unique inhabitants of South America.

Last Edit: Jan 16, 2026 2:51:28 GMT by ewellholmes

Post by Max Sinister on Jan 18, 2026 0:02:07 GMT

Nice to have such a TL! The maps alone make it readworthy.

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Post by ewellholmes on Jan 19, 2026 4:38:29 GMT

Nice to have such a TL! The maps alone make it readworthy.

Much appreciated, glad you’re enjoying.