The design philosophy for World of Teeth is to focus on lesser known genera (no big names like T.rex or Triceratops), and to keep the skeletal anatomy accurate to contemporary paleoart on one hand, while going absolutely hog wild with speculative tissue, keratin sheaths, spikes, feathers and more on the other hand, to make something that isn't necessarily educational nor necessarily "awesomebro", but rather something that's unique and my own.
The Psittacosaurus' quill coverage far surpasses what we usually see in paleoart today. Instead of one row of quills on part of the tail, likely for display, this animal has quills cover its entire tail and a significant portion of its upper body. The quills on the tail are long and sharp, and, much like porcupine quills, are used for defense against predators. The rest of the body is covered in different types of scales. While pebble-shaped scales are the most common, the tan-colored underbelly is covered in long, rectangular scales more like those of squamates than dinosaurs.
The beak has an extensive keratin coverage which gives it the appearance of a parrot's beak, making it a useful tool for breaking hard plant matter as well as scavenging. The same keratin coverage also greatly extends the length of the horns.
For the Baryonyx , maybe it could behave and look like a smaller version of Spinosaurus, at least in regards to the tail? Since I imagine you'll have prehistoric crocodiles, maybe it would behave more like a grizzly bear and fish from the shore/in very shallow water. An additional alternative would be to give it waterproof feathers and behave like a certain species of egret (the exact species escapes me) that folds its wings around its head to create a shelter to lure fish within striking distance.
I was thinking of possibly including creatures from all eras, but not large Cenozoic mammals (maybe for a future sequel or expansion ).
How obscure? Whatever I think is cool, really, with preference to animals that aren't already well known from Jurassic Park or WWD (looking at you, Postosuchus!)
For a Pachycephalosaurus, that both sexes of these dinosaurs had sharp theropod-like teeth on their beaks, perfect for preying on small animals, young and/or weak dinosaurs, eating carrion, shredding tough vegetations, and especially being piscivore, scavenger, and kleptoparasite. Larger animal prey can be killed by a bite to the head or neck, or (in case of a young, or small adult) simply pinned down and mauled. Predatory behavior in a Pachycephalosaurus is typically taught to whelps by their mother. Otherwise, they used crown-like cheek teeth, for grinding on meat and, especially vegetations, makes a Pachycephalosaurus an omnivore. They can use their tongues to eat termites and honey.
The males have bright yellowish colored skull-heads based on the sun and fire, and the tips of their spikes are blood-red, but their eye patches and nostrils are dark blood red, while females however do had bright colors, like the males, but dark-indigo with their spike-tips being light pinkish-mauve, and their eye-paches and nostrils are bluish black, and the female's dome is marked like one front of a magenta human skull. Both sexes however had their skins being in the shade of dark-grey, while their proto-feathers can be based on a mix between striped and brown hyaenas in the markings and mane. Females are 12.5% bigger than males, and the sleuths can be ruled by the oldest, aggressive, and/or powerful matriarchs.
Pachycephalosaurus can be considered the biggest and the most deadly pachycephalosaurs, but most of them, like smaller pachycephalosaurs, can be gentle omnivores. Unlike most dinosaurs and birds, which they lay eggs, female pachycephalosaurs can give live birth, and as young and newborn pachycephalosaurs are born flat-domed, but they grow their domes by age. Even in winter, even Pachycephalosaurus can be solitary to head to their den for their hibernation, but sometimes they can be woken in search of food without any harm to one another. Their behavior is like a mix between the blackbear(Ursus americanum) and the common warthog(Phacochoerus africanus), the way that the minimum social is one individual, and the maximum is like 16 adults(e.g. 1-6 boars + 1-16 sows).
Thanks! Plus, a full-grown Pachycephalosaurus can stand 1.1 meter tall in the hips and measured 12 feet long, based on its skull alone being 2 feet long. This dinosaur's rival whose a pachycephalosaur-mimicking troodontid whose skull(from the tip of the beak to the edge of the spikey crown/frill) measured 3 feet long, whose a frilled and nocturnal Werrysaurus zachus.