Islands are natural sites for evolutionary experiments. Their isolation and limited resources put a lot of selective pressure on their native species, often resulting in spectacular and unique adaptations. Big animals become small, small animals become big, and ecological niches can end up being filled in unexpected ways.
From the dodo becoming the first well-known example of human-caused extinction, to Darwin’s Galápagos finches being influential in the development of the theory of natural selection, to famous-but-endangered living examples like the kiwi and marine iguana, island species are fascinating and often fragile examples of how diverse life can get even in restricted conditions.
In fact, this theme ended up containing so many species I wanted to feature that I can’t possibly fit them all into just a single month. So, for the first time, a theme is going to need two months – with part 1 happening right now, and part 2 coming later this summer.
For much of the Mesozoic Europe was an archipelago of islands in a shallow tropical sea. During the Late Triassic, about 205-201 million years ago, some of the paleo-islands in this region existed around southern Wales and South West England, near the city of Bristol.
Thecodontosaurus was actually one of the first non-avian dinosaurs ever named by modern science, discovered in the mid 1830s – several years before the term “dinosaur” was even created to classify the “great ancient lizards”.
It was an early member of the herbivorous sauropodomorphs, the group that would eventually include the largest ever land animals. But unlike its enormous later cousins it was short-necked and bipedal, and was particularly small compared to other contemporary “prosauropods”, measuring only about 2m long (6′6″). This would make it one of the oldest known examples of insular dwarfism.
A layer of rock just above the deposit of Europasaurus fossils also gives us a clue about their eventual fate. Footprints of large carnivorous theropods – bigger than the mini-sauropods themselves – suggest that at some point the sea level dropped and predators from the mainland were able to reach the island.
Since there were no large predators on the island before then,the small Europasaurus had no defenses against these new giant invaders. They very likely were literally eaten into extinction.
And towards the very end of the Cretaceous, about 72-66 million years ago, this Adriatic-Dinaric island was home to the hadrosauroid dinosaur Tethyshadros.
Surprisingly it wasn’t very closely related to earlier European hadrosauroids, and its ancestors seem to have actually originated in Asia, island-hopping their way westward over to the Adriatic-Dinaric.
At around 4m long (~13′) it was much smaller than most of its close relatives and was another example of insular dwarfism. But it had some odd body proportions: its head was relatively large, its neck and tail were fairly short, its limbs were long and gracile, and it had a reduced number of fingers in its hands. It appears to have be specialized for running, sort of like a dinosaur mimicking a horse.
It also had a weird highly serrated edge to its beak, which in life would have been even more pronounced and spiky-looking. The purpose of this is unknown for certain, but it may have been an adaptation for a specific food source – and since some hadrosaurs seem to have occasionally snacked on shellfish for extra protein, it’s possible Tethyshadros was also doing something more omnivorous along the shores of its island home.
By far the biggest island in the Late Cretaceous European archipelago, the Ibero-Armorican island (sometimes also known as the Ibero-Occitan island) was made up of most of the Iberian Peninsula and France and was larger than modern-day Madagascar.
Around 73-71 million years ago one of the residents of this island was the aptly-named Gargantuavis – the largest known Mesozoic bird, and probably an example of island gigantism.
Although only known from a few isolated bones, it’s estimated to have been slightly larger than a modern cassowary, somewhere in the region of 2m tall (6′6″). At that size it would have also been secondarily flightless, which is surprising for a bird that was living alongside larger fast-moving theropods like abelisaurs.
Not much else is known about it due to the scarce remains, but it seems to have had a long slender neck and probably had a small head. Its hips were fairly broad, suggesting it wasn’t capable of running very fast, and it was likely a slow-moving herbivore that was a fairly rare member of its ecosystem.
Exactly where it belongs in the bird evolutionary tree is also unclear, with the best current guess being “some sort of euornithean”.
At the very end of the Cretaceous period, between about 72 and 66 million years ago, tectonic uplift from the start of the formation of the Alps created an island in the area corresponding to modern-day Romania.
Known as Hațeg Island, after the region where fossils of the native species were found, it was similar in size to Hispaniola and was surrounded by deeper waters than most of the other European archipelago islands.
Magyarosaurus here was was a titanosaur living on this island, and was one of the smallest known of all sauropod dinosaurs at just 6m long (19’8″). Much of that length would have been in its neck and tail, and its body was only actually about the size of a horse.
Like some other titanosaurs it had bony osteoderm armor along its back, although since only one isolated piece has been found the exact arrangement isn’t known.
It had a chunkier build than its closest relatives, with a deep skull, a large beak, and a rotund body. Like other rhabdodontids it would have had powerful jaw muscles and ridged cheek teeth specialized for scissoring, adaptations for cutting up particularly tough plant matter.
It was also quite small, about 2.4m long (7’10”), although since the largest known fossils represent subadults this may not have been its full size. A second species in the same genus (Zalmoxes shqiperorum) lived on the same island and was actually slightly bigger, suggesting that Z. robustus represented a minor case of insular dwarfism.
It was also the first dinosaur fossil found with a specific type of non-cancerous tumor known as an ameloblastoma on its lower jaw – a surprising discovery, since ameloblastomas were previously only known to occur in mammals and a single snake species. Various other types of abnormal tissue growth have been identified in other hadrosauroids and hadrosaurs, however, suggesting that this particular lineage of dinosaurs may have been unusually susceptible to developing tumors.
When Balaur was described in 2010 it was initially thought to be a dromaesaurid closely related to Asian forms like Velociraptor. With its particularly stocky legs built for strength rather than speed, two-fingered hands, and two large sickle claws on each foot, it was interpreted as a weird highly specialized predator terrorizing the other Hațeg Island species at the end of the Cretaceous. Although only 1.8m long (5’10”), it was hypothesized to have taken down prey much larger than itself with powerful slashing kicks.
But later analyses cast doubt on this interpretation.
A lot of the anatomical features of Balaur’s skeleton were odd for a dromaeosaurid, but matched those of avialans – a group of close evolutionary “cousins” to the dromaeosaurids, containing Archaeopteryx and the common ancestors of all modern birds. And, by 2015, multiple studies had confirmed Balaur wasn’t really a “raptor” but instead a little further along on the bird lineage.
So now our picture of this dinosaur is very different: a chunky-bodied island bird, grown large and secondarily flightless sort of like a Cretaceous equivalent to the dodo. Its double sickle claws were probably adaptations for climbing and perching in trees, and based on similar avialans it was likely a herbivore rather than a hypercarnivore.
The apex predator niche here instead seems to have been occupied by Hatzegopteryx, an enormous azhdarchidpterosaur. Standing up to 4.5m tall (14’9″) when on the ground, and with an estimated wingspan rivaling Quetzalcoatlus (~11m / 36′), it was one of the largest animals to ever fly – although like other azhdarchids it probably actually spent most of its time stalking around quadrupedally on the ground eating whatever it could fit into its mouth.
Its neck was shorter and chunkier than most other azhdarchids, and its skull was wider and more massively built. The walls of its hollow bones were also unusually thick and reinforced for a pterosaur, so much so that they were initially mistaken for those of a theropod instead.
Fossils of Hatzegopteryx are very fragmentary, however, so its full appearance and the specifics of its diet are still uncertain. But it would have probably been able to tackle much larger prey than other azhdarchids, possibly capable of using its sturdy beak to bludgeon or stab anything too big to pick up and swallow whole in a similar manner to modern marabou storks.
While they were an incredibly successful group, found around the world in large numbers, in Late Cretaceous Europe multis had become incredibly rare and restricted to just a single place: Hațeg Island.
Isolated there, they evolved into a unique family known as the kogaionids, diverging from their ancestral mostly-herbivorous diet to instead become specialized insectivores with distinctly red iron-pigmented teeth and huge blade-like lower premolars.
Kogaionon ungureanui was one of the first kogaionids to be discovered, and gives its name to the group as a whole. Although known only from a skull, it was probably rat-sized, around 30cm long (~12″).
Unusually for island species, which are often ecologically fragile and vulnerable, the kogaionids’ insectivorous habits allowed them to successfully survive through the end-Cretaceous mass extinction 66 million years ago while the Hațeg dinosaurs and pterosaurs perished. And when conditions changed and their island home became reconnected to the rest of Europe they rapidly spread out and became common across the entire region for a further 10 million years, only finally disappearing in the early Eocene about 56 million years ago.