Acanthoracosaurus

A fierce giant walks through the forest clearing. The anoplosaurs and even the gorgoraptors stay clear off its path. The reptile’s claws are sharp and its head huge, sporting powerful jaw muscles. It bears no teeth, yet its tortoise-like beak is sharp enough to bite a man’s head clean off. Its bony brows have frozen its demeanour into one of permanent anger. Despite its fierce appearance, Acanthroracosaurus jobini is a herbivore, but that does not mean that it is harmless.

On an island inhabited by predators as large as some of the most gruesome dinosaurs from the fossil record, a moving hunk of flesh this large cannot afford to be placid. As if the claws and beak were not already enough of a defensive weapon, the back of the animal is covered in a carapace, arranged in a unique stegomorph pattern, where huge bone plates covered in keratin overlap each other like shingles on a house roof. If a predator such as Basilosuchus were to attack from the top, it would break against the carapace, maybe even get impaled by the double-row of horn-spikes that run down the back. If it were to attack from the side, it might get stabbed by the sharp, pointy tips of the shingles. Many a predator has met their death at trying to crack this tough nut open.

Even with its armour, the saurian does not afford itself leisure. At even the faintest signs of threat it may show aggression, though thankfully rarely without warning. Sometimes it will even walk towards predators it sees in its vicinity to attack them, perhaps thinking that offense is the best defence. This is why even some of the most fearsome predators like Decarnodon, who could theoretically bite through its armour, stay clear off the plated reptile. It is simply not worth the risk of injury.

When not in a fighting mood, Acanthoracosaurus can show other sides of its personality. Most of its day is spent mindlessly eating away at tough vegetation, sometimes even tree stems and bark, which it can bite through like butter. It has no teeth, but its digestion is aided by a rough rasping tongue and gastroliths lining its stomach. It can also often be spotted taking a dive in the island’s many waterways, perhaps to wash itself off the many parasites on its hide, the one enemy even its armour cannot do much against. Its breeding behaviour is somewhat peculiar, in that it actually resembles that of some flightless birds. A male and a female will find each other and mate, the female will lay her eggs in a nest that the male has prepared and then leave. The male is then left to care for the chicks by himself until they are old enough to live alone, though it is difficult to imagine a father more capable than a living fortress.

The origins of Acanthoracosaurus have remained mysterious until recently. While it outwardly resembles the common image of a dinosaur, it bears little actual characteristic traits of one apart from the upright bipedal gait. With its toothless beak, anapsid skull and carapace, some early theories have actually suggested it may be the descendant of some kind of proto-turtle, such as Permian Eunotosaurus. However, the osteoderms are not fused to the ribcage, making this unlikely. Other suggestions have been put forward, such as it being a squamate, archosauriform, pareiasaur or even a cyamodontoid placodont re-adapted to land. The discovery of two vestigial toe bones of digits I and II in the padded foot of the beast have opened up a new and much more likely possibility, which is that Acanthoracosaurus is a giant member of the Eubolosauria, albeit in its own family, Cheloniopsidae. In this regard it is interesting that it is often seen gregariously living with its much smaller relative Pointilisaurus. The small herbivores likely seek the giant out for extra protection, while the beast itself seems to tolerate them, though without a clear benefit from the relationship. Except for the few times when it itself snaps up one of the hapless little critters. While it may be mostly herbivorous, even it will occasionally eat smaller animals as an extra source of protein. That includes humans.

Pointilisaurus

Ever since naturalists have first set foot on the island, they have made note of a variety of small, bipedal herbivorous reptiles running around. This being the early twentieth century with its limited understanding of paleontology, the reptiles in question were early on classified as genuine surviving relic dinosaurs, specifically of the small ornithischian types such as fabrosaurs or hypsilophodonts. But as their skeletons were more closely examined, and our understanding of fossil reptiles grew, it was recognized that the small runners on Ryl Madol were something else entire.

Click to enlarge

One of the first clues was the structure of their feet. Ornithischian dinosaurs only had four toes, whereas the Rylian lizards have five. Moreover, ornithischians walked on digits II – IV, with the inner toe digit I being reduced to a dewclaw. The island reptiles, however, walk on digits III – V, with digits I and II being reduced. The same pattern is also seen in their hands. While the elongation of the outer digits is common in some reptiles, it would be highly aberrant in dinosaurs.  The skull of these animals finally negates any association with the Dinosauria, for it lacks the antorbital fenestra typical of archosaurs and the back of the skull is even in a partial anapsid condition.

The final clue came, surprisingly, from Germany, when in the Permian aged Tambach Formation was found the fossil of Eudibamus cursoris. This was a little bolosaurid, a type of parareptile related to the larger pareiasaurs. Its hindlegs were highly elongated compared to the arms, leading to many reconstructing it as possibly the earliest biped. The condition of the skull, as well as the arrangement of the toes, is mostly identical to the bipedal mystery reptiles on Ryl Madol. Today, there is consensus that these creatures are descendants of this often-overlooked little parareptile, having adapted to obligate bipedalism and a fast-running lifestyle convergently to dinosaurs. The names that have been given to this new group in the technical literature are both Eubolosauria and Hylobolosauria and there is dispute over which name has preference.

One of the most common and well-known of the eubolosaurs is Pointilisaurus sibbicki. It is a small, fleet-footed herbivore, about the size of a green iguana. Gregarious, it can often be seen in small herds browsing on the lowland meadows or the forest edges. The groups are loosely organized and they quickly scatter when a predator like Gorgoraptor strikes. Social interactions are minimal, though some researchers have noted that the reptiles vocalize when in groups through a strange, cat-like purring sound. How exactly they produce this sound remains debatable. Reproduction is quite simple. After mating, the female lays her eggs in a hidden nest in the ground and leaves the eggs to their fates. The hatchlings usually survive through strength in numbers alone, with many of the little lizards falling prey to the voracious jungle shortly after emerging out of the ground.

Click to enlarge

While the most common eubolosaur on the island, it is also the most primitive, still sharing many traits with Permian Eudibamus, such as the iguana-like mouth with leaf-shaped teeth. Through anatomical and molecular studies, it has been revealed that many of the more enigmatic reptiles on the island can also be placed in the same group, however within a separate clade. These derived eubolosaurs have, in an even stronger parallel to some dinosaurs, evolved a predentary bone at the tip of their snout, giving them a sharp beak well-suited for chipping vegetation. These are the Aviopsia, which as of writing consist of three families. Saltopusauridae are small herbivores with parrot-like faces that move by saltation, similar to kangaroos or wallabies. The families Anatopsidae and Cheloniopsidae together form an even more derived clade, signified by the loss of the two inner digits of the foot, which for a long time obscured their relationship to the other eubolosaurs. Anatopsidae are huge, amphibious reptiles, with duck- or geese-like bills they use to feed on a wide variety of aquatic plants. Cheloniopsidae, such as the massive Acanthoracosaurus, are more terrestrial animals, which shield themselves against the island’s predators with strong, stegomorph armour and spikes.

While non-avian dinosaurs, as to our knowledge, still remain absent from Ryl Madol, these magnificent beasts are no less exotic nor less worthy of our awe.