In this month-long "Retrosaur Challenge" from TyrantisTerror, we will revert back to the olden days of Mesozoic reptile restorations - featuring tail-dragging carnosaurs, featherless raptors, swamp-dwelling sauropods, naked flying pterosaurs, kaiju-like mosasaurs, and many more!

Day 6: Tiny Tyrant

    Nanotherus ("dwarf beast") was a genus of indeterminate Tiny Tyrant that resided in what is now southeast Wyoming during the Late Cretaceous, approximately 86 million years ago. It appears to have been moderately-large for a Tiny Tyrant, with the only fossil specimen recovered as of yet reaching an estimated length of around 17 feet (5.2 meters) in life. Due to their incomplete nature, however, those holotype remains have aroused a great deal of contention regarding the validity of Nanotherus over the years. Many paleontologists assert that it is not a distinct genus and instead represents a juvenile specimen of the True Tyrant Gigantotherus - citing evidence such as the unossified neural arches in the holotype’s vertebrae, similar cranial features, and the fact that Gigantotherus remains are known from the same formation. Others have challenged this notion, arguing that the lack of vertebral ossification could indicate that the holotype was not necessarily young at the time of its death, but rather a subadult individual. A meager few have even gone as far as to theorize that Nanotherus could actually represent a fairly-small True Tyrant rather than a Tiny Tyrant - namely a laelapid or a dwarf carnosaurid related to Gigantotherus. Whatever the case may be, this retrosaur will continue to baffle paleontologists in terms of its taxonomic placement until more complete fossils are uncovered.   

    The only-known Nanotherus specimen was excavated from the Steer Springs Formation in southeast Wyoming, whose sediments date back to the early Santonian stage of the Late Cretaceous. Sandstone and mudstone account for much of the formation’s geologic composition, indicating that the region was covered by a sprawling floodplain or river delta near the young Western Interior Seaway during the time in which this retrosaur existed. Similar to the present-day Mississippi River Delta, the region would have had a humid and subtropical climate that received steady rainfall throughout the year - along with a natural labyrinth of cypress forests, marshes, and swamps occurring in close proximity to the seaway. Turtles, brackish-water bony fish, and anadromous ammonites thrived in this environment, as well as the primitive crocodilians and freshwater-tolerant sharks that likely sought them out as prey. Though not as abundant as those of the aforesaid animals, retrosaur fossils belonging to at least a few genera have also been unearthed from the Steer Springs; among them are the aforementioned Gigantotherus, the early agathaumid Clypeocephalus, and the Duck-Billed Goliath Telmatites selwyni. 

    While the true identity of Nanotherus is still an enigma, it can be said with certainty that this (supposed) Tiny Tyrant occupied the niche of a medium-sized predator in its habitat. Taking into account the size, robust build, and proportionately-large skull of the holotype specimen, it can be assumed that the retrosaur targeted bigger quarry than most contemporary Tiny Tyrants; small Bipedal Goliaths, juvenile Long-Necked and Horned Goliaths, and even lesser Tiny Tyrants have all been theorized to have constituted this reptilian flesh-eater’s diet. It has been proposed that adult Nanotherus might have also appropriated kills from larger predators whenever the opportunity presented itself, in addition to utilizing their large size to intimidate smaller carnivores into relinquishing their kills. Despite its impressive size compared to many other Tiny Tyrants, it is believed that Nanotherus still would have been a fairly-nimble and maneuverable animal in life, its slender frame allowing it to weave through tangled patches of coastal forest in pursuit of prey or to evade the wrath of larger retrosaurs. Whereas most smaller animals would struggle with wading through waterways and trudging across mired patches of ground, the stature and somewhat lanky hindlegs of adult Nanotherus likely enabled them to traverse such terrain with surprising ease. It is also possible that Nanotherus utilized its labyrinthine habitat to its advantage when hunting - forcing prey into constricting chokepoints formed by thickets of vegetation or cypress roots, and perhaps even driving them into swampy areas to trap them for easier takedowns. Nevertheless, due to only a single Nanotherus specimen being known, this is as far as paleontologists can speculate about its potential lifestyle and habits. Until more complete fossils are discovered, any other aspects of this dubious retrosaur’s paleobiology shall remain mere suppositions. 

    Retrosaurs are typically perceived as being enormous, powerful, and intimidating creatures - but the Tiny Tyrants were among the few exceptions to that standard. They were a diverse clade of generally-small carnivores that occupied a wide array of niches throughout their evolution, from agile insectivores barely larger than a chicken to prodigious specialists of aquatic prey that reached sizes comparable to some mid-sized True Tyrants. It has also been theorized that competition with these retrosaurs was the catalyst for the evolution of birds as we know them, with primitive birds being forced to adopt arboreal lifestyles and rapidly develop the anatomical features necessary for flight in order to evade their oppressors. The syntarsids were the most basal group and became highly successful despite their primitivity, eventually spreading to nearly every continent and diversifying into a myriad of forms - including those that developed elaborate crests for display, cursorial herbivores, and even a few venomous species. The bizarre allolestids were prevalent in Cretaceous Gondwanaland, with many venomous genera alongside tree-dwelling and semiaquatic forms emerging from this branch. The most well-known group, the dryptonychosaurians, evolved in Laurasia during the Middle Cretaceous and went on to terrorize the region’s herbivorous retrosaurs for the remainder of the Mesozoic with their ferocious speed, lethal weaponry, and alarmingly-high intelligence. Yet another branch of Tiny Tyrants took to the trees in pursuit of protobirds in the Late Jurassic and would themselves split into two distinct lineages. One would become the Flying Tyrants as they developed the power of flight in order to conquer the skies of the Cretaceous, while the other retained their arboreal lifestyle and became the pithecusaurians - whose evolution eerily mirrored that of primates later on in the Cenozoic. While the prosperity of the Tiny Tyrants was cut short by the disastrous events at the end of the Mesozoic that terminated the retrosaurs’ dominion as a whole, this retrosaur clade stands as a stark reminder that size doesn’t always matter in the grand scheme of things. 

Species:

- Nanotherus laramiensis ("small beast from Laramie County")

Most people would probably create a dromaeosaur analog or something along those lines for this entry, but I thought fabricating an analog to the infamous invalid tyrannosaur Nanotyrannus instead would be a much more unique idea. That said, I intended for it to be among the more mysterious retrosaurs I’ve created for the “RetroVerse”. Whether Nanotherus is its own distinct taxon or simply a juvenile Gigantotherus is completely up to individual interpretation. Thanks once again to my friend OviraptorFan for making a canonified redesign of my original Nanotherus, which I referenced for the updated version you see here!