Island Weirdness #21 – The Malagasy Mini-Hippos

Much like elephants, hippos have frequently made their way onto islands and developed dwarfed forms. These mini-hippos are mostly known from the Mediterranean, but further south they also occurred on Madagascar.

Hippopotamus madagascariensis (also sometimes called Hexaprotodon madagascariensis or Choeropsis madagascariensis; its exact classification is uncertain) was similar in size and appearance to the modern West African pygmy hippo – which it might have been closely related to, or may have just ended up resembling through parallel evolution.

Standing under 1m tall at the shoulder (3′3″) and measuring about 1.8m long (5′11″) it lived in the forested highlands and was much more terrestrial than its larger cousins. Its eyes were further down on the sides of its head, and it was better adapted for walking and running around on land, with proportionally longer legs and a more digitigrade posture.

It also had an unusually small brain for its size – about 30% smaller than expected – which may have been an energy-saving adaptation.

Two other species of dwarf hippo have been identified on Madagascar – the similarly-sized but more aquatic Hippopotamus lemerlei in the west of the island, and the larger and poorly-known Hippopotamus laloumena in the east. It’s not clear when exactly the ancestors of these various hippos first arrived on the island, and they may even represent three independent colonization events.

The last known subfossils date to about 1000 years ago, but surprisingly accurate accounts of similar creatures in Malagasy folklore raise the possibility that small isolated populations of these hippos may have survived into more recent times. One of the most intriguing examples is the kilopilopitsofy, described as having large floppy ears and supposedly being sighted as recently as the 1970s.

Island Weirdness #20 – Megaladapis

While some of the lemurs of Madagascar were surprisingly sloth-like, another lineage of these primates evolved in a different direction entirely.

Megaladapis was built similarly to a koala, with a rather squat body and hands and feet adapted for clinging onto branches. Three different species have been identified, with the largest measuring around 1.5m long (4′11″).

Its skull resembled that of a cow, with eyes on the sides of its head, a long snout, and powerful chewing jaw muscles for processing its diet of tough leaves. It also had a very unusual nose for a primate, with nasal bones similar to rhinos – suggesting it may have had an enlarged prehensile upper lip used for grasping foliage.

Much like some of the sloth lemurs, carbon dating of subfossil remains indicates that these “koala lemurs” may have survived until surprisingly recently – possibly only going extinct about 500-600 years ago.

Island Weirdness #19 – Archaeoindris fontoynontii

The small and medium-sized sloth lemurs of Madagascar were incredibly convergent with modern tree sloths, and the biggest member of the group likewise seems to have been the closest they came to evolving a ground sloth equivalent.

Archaeoindris was the largest known lemur – and one of the largest primates – similar in size to a modern gorilla at about 1.5m tall (4′11″). It would have been a slow-moving animal which fed mostly on leaves, and while it was still capable of climbing around in larger trees it was probably much too bulky for upside-down suspension like its smaller relatives, and would likely have had to regularly traverse the ground to reach new feeding sites.

It seems to have been a fairly rare member of the ecosystem, living only in the Central Highlands, and the last known remains date to just over 2000 years ago – around the same time that humans first reached that area of the island. Sadly a combination of factors such as the giant lemurs’ slow reproductive rate, habitat loss, and hunting pressure was too much for their population to recover from all at once, and they probably went extinct very soon after that date.

Island Weirdness #18 – Palaeopropithecus

The island of Madagascar has been isolated from other landmasses for almost 90 million years, and as a result there are many lineages present there found nowhere else on Earth.

Lemurs are one of the island’s most famous residents, having arrived from Africa via a rafting event sometime early in the Cenozoic and evolving to fill the ecological niches occupied elsewhere by monkeys and apes. But while there are around 100 lemur species alive today, there used to be more before the arrival of humanssubfossil remains from the last 25,000 years hint at an ecology with even greater diversity, and types of lemurs much larger than any still living today.

The sloth lemurs, as their name suggests, resembled modern sloths in a remarkable case of convergent evolution. With long limbs and long hooked fingers and toes they were adapted for swinging through trees and hanging from branches, feeding on a wide range of plant material such as leaves, fruit, and seeds.

Palaeopropithecus was one of the larger members of this group, and the most specialized for sloth-like upside-down suspension. Three different species have been identified, with the biggest (Palaeopropithecus maximus) possibly measuring around 1m long (3′3″).

It probably spent almost its entire life in the trees, and would have been slow and awkward on the ground. Malagasy folklore about a creature known as the tretretretre or tratratratra, which couldn’t navigate on smooth flat surfaces, may even represent a cultural memory of Palaeopropithecus from before its extinction – which may have happened as recently as within the last 500-1000 years.

Island Weirdness #17 – Tyto gigantea

The largest terrestrial predators on the Late Miocene Gargano-Scontrone island(s) were unusually big hedgehog-relatives, and likewise the local aerial predators also increased in size compared to most of their cousins on the mainland.

Tyto gigantea was a massive barn owl, estimated to have been at least as large as the modern eagle-owl – probably measuring somewhere around 80cm in length (2′7″). It likely grew so big thanks to the lack of competition and in order to keep up with the larger sizes of its prey, since the rodents and pikas of Gargano-Scontrone were also comparative giants.

Alongside the slightly smaller species Tyto robusta it would have been the dominant nocturnal predator on the island(s), while during the daytime the golden-eagle-sized buzzard Garganoaetus occupied the same large-carnivorous-bird niche.

Island Weirdness #16 – Garganornis ballmanni

While some of the main big herbivores on the Late Miocene Gargano-Scontrone island(s) were the larger species of Hoplitomeryx, they weren’t the only animals filling that ecological niche.

Garganornis was an enormous anatid bird, closely related to modern ducks, geese, and swans. Although only known from fragments of its skeleton it’s estimated to have stood up to 1.5m tall (4′11″), making it the largest known waterfowl to have ever lived.

It probably reached such a size thanks to the lack of large terrestrial predators, and possibly also as protection against the island eagles and owls – literally growing too big for them to be able to eat.

It was flightless, with small wings, and had reduced webbing between its toes, suggesting it spent most of its time walking around on land. It also had bony knobs on its wrists that would have been used to give some extra force to wing-slaps when fighting with each other over territory or mates.

Island Weirdness #15 – Deinogalerix

Thanks to the absence of large terrestrial carnivores on the Gargano-Scontrone island(s) during the Late Miocene, animals that were usually small had the opportunity to become larger, moving into the vacant ecological niches and evolving into predators unlike anything existing on the mainland.

Deinogalerix was a giant member of the gymnures – close relatives of hedgehogs without the quills – with a proportionally big head and a long snout full of large fangs at the front and bone-crushing molars at the back.

Several different species have been found, with the largest Deinogalerix koenigswaldi having a head-and-body length of around 60cm (2′). Along with its tail that would have made it at least 90cm long (2′11″), making it the biggest eulipotyphlan ever discovered.

It would probably have hunted smaller mammals, birds, and reptiles, filling a niche on the island similar to dogs or cats.

Island Weirdness #14 – Hoplitomeryx

During the mid-Miocene, about 15 million years ago, a region of central and southeast Italy around Gargano and Scontrone was cut off from the mainland by rising sea levels.

For the next 7-10 million years this island (or perhaps a cluster of islands) was left isolated, and an unusual ecosystem developed known as the “Mikrotia fauna”. With the island starting off lacking large predators, small herbivorous animals like rodents, pikas, and waterfowl became huge – and then small predators like gymnures and carnivorous birds also grew to keep up with the increasing size of their prey.

One of the strangest residents of the island(s) was Hoplitomeryx, an early type of ruminant that resembled a deer or pronghorn. Nicknamed the “prongdeer”, it had a total of five horns on its head and large protruding fangs similar to some modern deer.

Multiple species of Hoplitomeryx have been identified, representing four different size classes ranging from huge down to tiny insular dwarfs. The largest is estimated to have been similarly sized to modern moose, standing around 2m tall at the shoulder (6′6″), while the smallest would have been under 50cm (1′8″).

Each of these size classes was specialized for slightly different ecological niches, eating different types of vegetation to avoid directly competing with each other for the limited amount of food on the island.

Island Weirdness #13 – Tiny Elephants On Parade Part 1: Japan

Part of the “island rule” is that large animals often become smaller – and no group seems to exemplify this more than the elephants.

Although they’re the largest living land animals today, and in the past included some of the largest known land mammals ever, ancient elephants also frequently ended up on islands thanks to their ability to swim long distances. They produced many different dwarfed forms around much of the world, and a few of them will be featured intermittently throughout both months of this theme.

The earliest known examples were the stegodontids of Japan in the early Miocene. These animals weren’t quite true elephants, instead being close evolutionary cousins to them, and had two small additional tusks in their lower jaws similar to the related gomphotheres.

A stylized illustration of an extinct dwarf elephant. It has four tusks – two short straight ones in its upper jaw, and another two smaller ones in its lower jaw.
Stegolophodon pseudolatidens

Stegolophodon pseudolatidens first arrived in Japan about 18 million years ago, and within just 2 million years they’d developed into insular dwarfs that were probably around 2m tall at the shoulder (6′6″)– still reasonably large, but only about 60% the size of their mainland relatives.

A stylized illustration of an extinct dwarf elephant. It has long curving tusks and small ears.
Stegodon aurorae

Much later in the Early Pleistocene another small almost-elephant appeared in Japan. Living between about 2 million years ago and 700,000 years ago, Stegodon aurorae was about the same size as the then-extinct Stegolophodon but probably wasn’t descended from them. Instead it was probably the result of a separate arrival and dwarfing of a larger Stegodon species from mainland Asia.

Island Weirdness #12 – Anatoliadelphys maasae

The Pontide island 43 million years ago had no placental mammalian carnivores, and in their absence a different type of mammal took up the main predatory niches – the metatherians.

Metatherians today are represented solely by marsupials, but there were once many other branches of this mammal lineage. One group present in Europe for much of the Cenozoic were the herpetotheriids, and these rat-sized animals seem to have also occupied the small predator and opportunistic omnivore niches on the Pontide island.

And then there were the bigger metatherians.

Anatoliadelphys was a specialized carnivore, with strong jaws and bone-crushing teeth similar to the modern Tasmanian devil. It had a body size similar to a domestic cat, giving it a total length of up to 1m long (3′3″), and had opossum-like grasping digits and a prehensile tail indicating it was capable of climbing trees.

It was initially thought to be descended from another type of metatherian found in Europe at the time, the peradectids, but a later study came up with a more surprising result for its evolutionary relationships. It instead may be closer related to the polydolopimorphs, a group known mainly from South America. Quite how its ancestors got as far as Turkey is a bit of a mystery, but it’s possible they’d previously dispersed into Africa at a time when the continents were still much closer to each other than they are today, and then spread northwards until they reached the Pontide island.

The main problem with this hypothesis is the current lack of any polydolopimorphans in the African fossil record – but the African Eocene is still poorly understood, so there may be some metatherian fossils yet to be discovered that could fill in this gap.