The Sobral Formation: Part 2
The First Tarrasques
For one hundred and sixty million years, the dinosaurs ruled the planet. From their humble progenitors in the Triassic to the Tyrant Lizard King of the Late Cretaceous, Dinosauria was a clade unmatched in its domination. And then 66 million years ago, that came to an abrupt and near total end. With the sole exception of Aves, every clade of dinosaur went extinct… But what if they hadn't?
Dragons of the Cenozoic follows the development of a world where the K-Pg event took place, but where a little bit of luck allowed a handful of species of Non-Avian dinosaurs to survive in a refugium in Eastern Antarctica. This is the story of how the greatest dynasty the world has ever known recovered in a world it no longer ruled.
The low grumble of an angry boar Eokastros rolls over the water lilies as the pig-sized cornoderm flares its bright red nostrils at a rival. The water dances along its back as the old boar sings an infrasound intimidation song. A complex mixture of audio and visual displays is used by both the older resident male and his young rival before the two commit to their ritualized combat. But when at last they had finished their dueling songs, they pushed forward through the water lilies.
Their momentum carries them past each other, and for a moment, it seems as if they will pass by one another altogether. But these two have not overshot their target—they are exactly where they want to be. Raising the osteoderm macuahuitl at the end of their tails, they strike with a thunderous crack, battering each other.
As an armored tail slams against an armored hide, a nearby Marambiotherium waddles out of the river. Like the larger cornoderms, the beaver-like gondwanathere spends much of its time in the water, feeding on lilies and Azolla to bulk up for the coming winter. But an accidental swipe of an armored tail would be enough to shatter flesh and bone, so for now, it wants to be as far away from the battling males as possible.
On the shore of the Antarctic lake, a family of very different thyreophorans nervously paces as they cautiously watch the battle in the water. This Hoplophosphorus buck already went through his own ritualized mating combat at the dawn of spring; the four-month-old chicks surrounding him are a testament to his success. He has no interest in the fighting of these ferocious boars, and so he turns to lead his family to a quieter watering hole.
Beneath the churned-up surface waters, another strange thyreophoran bites into the root of a Nelumbo water lily as she bulks up to lay her own clutch of eggs. She has no time for the conflicts of her larger cousins. Before too long, summer will end, and she and her future brood will have to be buried deep in their burrow, emerging only occasionally to hunt fish when the darkness of the winter night puts the plants to sleep.
And so, life moves on for the armored tarrasques of Paleocene Antarctica as they all do their best to prepare for the dark to come.
Phylogenetic tree for Pachyaspidia (Thyreophora) (subject to revision)
In the 740,000 years since the K-Pg mass extinction, the ecological dominance of fungal and fern landscapes—characteristic of the immediate post-impact period—has long since given way to more complex and stable ecosystems. Within this more robust and diverse environment, the surviving Thyreophorans have followed suit, diversifying into a variety of forms.
Over the 20th and 21st centuries, the classification of extant thyreophorans has undergone significant revisions as new fossil evidence and molecular analyses have reshaped their taxonomic framework. Early naturalists traditionally classified all surviving armored taxa within Thyreophora as a single monophyletic group, Aquilohippoidae. This clade included the groups Titanogryphia, Notohippogriffia, Pachyhippogriffia, Testudoloxodonta, Aspidochelonia, and Chelosoma. This taxonomic approach mirrored historical misclassifications in mammalian phylogeny in our own world, such as the now-defunct order Pachydermata, which erroneously grouped elephants, rhinoceroses, and hippopotamuses together based on shared morphological traits rather than evolutionary relationships.
However, subsequent phylogenetic analyses have demonstrated that Chelosoma, Testudoloxodonta, and Aspidochelonia are far outside of Aquilohippoidae in their own grouping dubbed Cornodermata. With Cornodermata and Aquilohippoidae having last shared a common ancestor during the Early Jurassic, approximately 190 million years ago, paleontologists have established that the three surviving families belong to a single lineage—remnants of a once more diverse group of robust Cornoderms.
In the world of Dragons of the Cenozoic, Cornodermata is most analogous to what we would call Eurypoda, a clade that includes Stegosauria and Ankylosauria. However, due to the persistence of Parankylosaurs into the Cenozoic, contemporary zoologists have classified Parankylosauria within the clade Eucornodermia, while the extinct Euankylosaurs are referred to as Pseudocornodermia.
And yes this is a retcon.
Eotestudoides and the Persistence of Eucornodermia
reconstruction of Eotestudodes eleutherospondylus (Free-spined dawn turtle mimic) while swimming
As the basalmost identified member of Chelosoma, Eotestudoides eleutherospondylus provides valuable insight into how Cornoderms managed to survive the K-Pg extinction. Eotestudoides was not a large animal—measuring between 45–50 cm in body length, it was significantly smaller than even the Maastrichtian Stegouros. In contrast to its predominantly herbivorous and heavily armored Mesozoic relatives, Eotestudoides exhibits adaptations indicative of a semi-aquatic lifestyle, paralleling speculative reconstructions of Liaoningosaurus paradoxus. The absence of fusion between its spinal and hip bones represents a key adaptation to this semi-aquatic niche, enhancing flexibility and facilitating movement through aquatic environments. This ecological shift provided access to a broad spectrum of nutritional resources, securing an adaptive niche that helped the group endure the environmental upheavals of the K-Pg event.
Central to Eotestudoides’ survival was its generalist dietary strategy. Unlike contemporaneous freshwater mosasaurs and plesiosaurs, whose piscivorous and macropredatory specializations made them vulnerable to ecosystem collapse, Eotestudoides exploited a diverse array of food sources, including small fish, crustaceans, insects, mollusks, and plant matter such as roots and tubers. This dietary flexibility gave it a competitive edge amid the erratic resource availability of the post-impact world, while exclusively carnivorous endothermic marine reptiles like mosasaurs and plesiosaurs were unable to survive the trophic collapse.
Additionally, Eotestudoides—like all dinosaurs—was endothermic, setting it apart from ectothermic archosaurs such as crocodilians. While crocodilians were able to persist in most of their ranges due to their lower metabolic demands, they still experienced localized extirpations throughout much of Southern Gondwana during the impact winter. Endothermy, coupled with burrowing behavior (or at least the use of natural shelters), likely provided Eotestudoides with thermal buffering and protection from climatic extremes during this difficult period, positioning it well for survival when the worst of the extinction had passed.
Beyond the absence of fusion between its spine and hip bones, Eotestudoides possessed cranial adaptations that further supported its semi-aquatic lifestyle. These included elevated nostrils and orbits, analogous to those of modern crocodilians, allowing it to remain partially submerged while maintaining sensory awareness above the water’s surface. This adaptation enabled it to detect both predators and prey while resting at the bottom of bodies of water or when surfacing.
From Chelosoma and Eotestudoides, two divergent clades would arise: Testudoloxodonta and Aspidochelonia. Testudoloxodonta transitioned to a terrestrial existence, developing reinforced limbs and adopting a predominantly herbivorous diet, filling the now-vacant niches left by the extinction of Pseudocornoderms. In contrast, Aspidochelonia retained and refined its semi-aquatic heritage, evolving into robust benthic feeders and seagrass grazers.
The discovery of Eokastros notios (Southern Dawn Fortress) within the Sobral Formation further exemplifies the rapid diversification of Eucornoderms in the aftermath of the K-Pg extinction. Unlike its smaller, more generalist relative Eotestudoides, Eokastros attained a size comparable to Stegouros, demonstrating how quickly this lineage adapted to exploit newly available ecological niches. Retaining several anatomical features of its predecessor, Eokastros still had elevated eyes and nostrils, a feature analogous to modern hippopotamuses, allowing it to remain largely submerged while still breathing and monitoring its surroundings—an advantageous trait in its lacustrine and riverine habitats. However, despite its larger size, Eokastros had not yet fully abandoned burrowing behavior. Fossil evidence suggests that individuals may have still excavated or utilized existing burrows, particularly during the harsh Antarctic winters of the Paleocene environment in the Sobral Formation. This behavior likely provided insulation against temperature fluctuations and refuge from predators, ensuring the species’ survival in a climate that, while warmer than present-day Antarctica, still imposed seasonal hardships.
Dietary analyses of Eokastros indicate a shift toward greater herbivory compared to its omnivorous ancestor, though it retained significant dietary flexibility. Fossilized gut contents and isotope analysis suggest that while it fed extensively on floating and emergent aquatic vegetation—such as Notonuphar and Nelumbo water lilies and the ubiquitous Azolla water ferns—it also incorporated terrestrial flora into its diet, including Antarctic Nothofagus trees and other angiosperms. This allowed Eokastros to exploit multiple feeding grounds within its environment.
The emergence of Eokastros in less than a million years after the K-Pg extinction underscores the evolutionary plasticity of Eucornoderms, particularly in response to ecological upheaval. While Eotestudoides occupied a generalized semi-aquatic niche, virtually unchanged since the K-Pg, Eokastros represents an early step toward increased size and specialization, paving the way for future evolutionary radiations.
Hoplophosphorus pugnax (Fighting Armored morning star)
The second major thyreophoran clade represented in the Sobral Formation is Aquilohippoidae, which includes an indeterminate species of Eoaspisaurus alongside the significantly larger Hoplophosphorus pugnax. As with its Late Cretaceous ancestor Jakapil, Hoplophosphorus exhibited the highly derived dental and mandibular adaptations that distinguish aquilohippoids from more basal thyreophorans such as Scelidosaurus. While these more primitive thyreophorans employed simple shearing teeth for folivory, the advanced mastication mechanics of Hoplophosphorus and other aquilohippoids suggest an ability to process more fibrous and abrasive vegetation. This has fueled ongoing debate regarding the presence and ecological significance of grass in Maastrichtian and Paleocene Gondwana.
The co-occurrence of Hoplophosphorus with hypsodont Gondwanatherian mammals further complicates this discussion, as both clades exhibit morphological adaptations typically associated with grazing. Some researchers propose that these traits evolved in response to grass consumption, indicating an earlier proliferation of grasses in Gondwanan ecosystems than traditionally assumed. Alternatively, it has been suggested that the development of hypsodonty in Gondwanatheres and the mastication adaptations of aquilohippoids may have arisen as an adaptation to processing ash-laden vegetation, a consequence of increased volcanic activity associated with the rising Andes. However, the presence of hypsodont Gondwanatheres in Madagascar and India, where they coexisted with grasses, suggests that at least some early Paleocene ecosystems supported true grazing niches.
The diet of Hoplophosphorus also provides insight into the evolutionary drivers behind neck elongation in aquilohippoids. While some later, highly derived families, such as Titanogryphia, were undoubtedly adapted for high browsing, many long-necked aquilohippoids appear to have primarily consumed grasses, despite possessing cervical proportions that rivaled those of modern giraffes. This paradox has given rise to competing hypotheses regarding the selective pressures responsible for neck elongation in the clade. As with giraffids, both sexual selection and dietary specialization have been proposed as primary drivers of cervical elongation in Hoplophosphorus and its relatives.
Fossil evidence from the Sobral Formation indicates that Hoplophosphorus was already undergoing significant modifications to its cervical vertebrae, including increased length and reinforced musculature. These structural changes suggest that, beyond potential browsing advantages, neck elongation may have been primarily driven by intraspecific combat, with individuals using their lengthened necks and reinforced musculature to batter competitors with their armored cervical regions—a behavior akin to that observed in extant giraffes and some testudinids.
As new discoveries continue to refine our understanding of these enigmatic taxa, their complex histories remind us that survival is not merely a matter of endurance, but of adaptation. Whether through ecological flexibility, shifts in feeding strategies, or novel anatomical modifications, these once-overlooked survivors of the Mesozoic carved out new niches in a radically altered world. Their evolutionary paths serve as a testament to the unpredictable yet opportunistic nature of these Dragons of the Cenozoic.
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Thanks for reading, everyone! This post was supposed to be out last week but unfortunately, life got in the way. The Q&A video will be out next Thursday with Gondwanathere video coming out a couple days later on Saturday, so if you have any more questions that you didn’t ask in the comment section of the previous video, be sure to ask them here or in the comments when the video version of this post goes live on Youtube.
Now, time to discuss the big change. Like I mentioned earlier in the video, the survival of Parankylosaurs is a significant retcon to the established list of survivors. I had initially intended for the Aquilohippoids to diverge into the more gracile Aquilohippid lineages while another lineage converged on the body plan of ankylosaurs, which would be called Cornoderms. However, after looking at the diminutive Stegouros, I started thinking more and more about how these more derived thyreophorans could have survived. The postulated semi-aquatic nature of Liaoningosaurus really struck me as a novel way to make that happen. I tried to ensure that the circumstances and forces of survival were just as plausible as those of the other surviving dinosaurs, so I hope everyone is on board with the retcon.
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Sources:
Diverse evidence for grasslands since the Eocene in Patagonia https://www.sciencedirect.com/science/article/abs/pii/S0895981121002042
Fish Hunting Ankylosaurs( Dinosauria,Ornithischia) from the Cretaceous of China
http://web.archive.org/web/20171108222649/http://en.cnki.com.cn/Article_en/CJFDTotal-JSDZ201602028.htm
Unfortunately, the English language abstract is very difficult to find and only exists if you use the Wayback machine and I can’t find the full paper anywhere. Making the Everything Dinosaur blog one of the best places to see Liaoningosaurus as it was initially speculated to be like
https://blog.everythingdinosaur.com/blog/_archives/2016/08/20/fishing-ankylosaurs.html
potential for Gondwanatheres to have eaten abrasive foods like grass
https://www.researchgate.net/publication/268228495_First_cranial_remains_of_a_gondwanatherian_mammal_reveal_remarkable_mosaicism