Adalatherium

Even for a fossil species from an isolated island, Adalatherium hui is very weird.

This mammal was part of an enigmatic group known as gondwanatheres, which were probably early members of the theriiform lineage – slightly closer related to modern marsupials and placentals than to monotremes. Found in the southern continents of Gondwana between the Late Cretaceous and the Miocene, these animals were adapted for herbivory with convergently rodent-like ever-growing front teeth that helped them chew through tough plant matter.

They were previously known mainly from isolated teeth and jaw fragments, with some rare full skull material, but Adalatherium is remarkable for being represented by a complete skeleton.

And it’s turned out to be far stranger than anyone expected.

Living in northwestern Madagascar during the Late Cretaceous, about 70-66 million years ago, Adalatherium was one of the larger known Mesozoic mammals at around 60cm long (2′) – although the one known specimen seems to have been a juvenile, so mature individuals were probably slightly larger.

(And based on its body proportions, its close relative Vintana may actually have been even bigger than previously thought. Whether this sort of large size was common in Cretaceous gondwanatheres or if this was just island gigantism is still unknown, though.)

It was probably a marmot-like digging animal, excavating burrows with its large claws and powerful limbs, and since it likely evolved from ancestors that had become isolated on Madagascar over 20 million years earlier it had developed a very unusual mixture of both “primitive” and highly specialized anatomical features. It had more back vertebrae than any other known Mesozoic mammal, upright forelimbs, sprawling hind legs with bowed-out tibias, strong back and leg musculature, and a therian-like pelvis with epipubic bones.

And then there’s the snoot.

The snout region of Adalatherium‘s skull was pockmarked with a large number of foramina, holes that allow the passage of nerves and blood vessels through the bone. It had more of these than any other known mammal, and their presence suggests that it probably had a very sensitive upper lip and whiskery snout. Most mammals with a lot of whiskers just have one very big foramina, but Adalatherium seems to have evolved a different solution to the same problem.

It also had one other bizarre feature – a hole in the top of its nose. A large “internasal vacuity” between its nasal bones is a unique feature not known in any other mammal, and its function is a total mystery.

Since this hole was also surrounded by many foramina it may have supported some sort of soft-tissue sensory structure on top of its nose. So I’ve speculatively depicted it here with a leathery horn-like “shield”.

Adalatherium skull
From fig 2 in Krause, D. W. et al (2020). Skeleton of a Cretaceous mammal from Madagascar reflects long-term insularity. Nature 581, 421–427. https://doi.org/10.1038/s41586-020-2234-8

Island Weirdness #10 – The Kogaionids

Multituberculates were a group of rodent-like mammals that originated back in the Early Jurassic, at a point on the mammalian family tree between the origin of monotremes and the earliest therians (represented today by marsupials and placentals).

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.

Skull of Barbatodon, from fig 2 in Smith T, Codrea V (2015) Red iron-pigmented tooth enamel in a multituberculate mammal from the Late Cretaceous Transylvanian “Haţeg Island.” PLoS ONE 10(7): e0132550. doi: 10.1371/journal.pone.0132550  | CC-BY-4.0

Some of them also had oddly domed skulls and proportionally tiny brains, along with highly acute senses of smell, eyesight, balance, and motor control.

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.

Litovoi

Litovoi tholocephalos, a multituberculate mammal from the Late Cretaceous of Romania (~70-66 mya). Living on what was at the time the large offshore Hațeg Island, this rat-sized animal (about 25cm /10″ long) was part of a lineage of insectivorous multis called the kogaionids, with the same sort of red-colored enamel on its teeth as other species like Barbatodon.

Its brain was surprisingly tiny proportional to its size – one of the smallest known brain-to-body ratios of any mammal, and more similar to those of non-mammalian cynodonts – but it also seems have been highly specialized for processing sensory input, with relatively enormous regions associated with smell, eyesight, balance, and motor control. The olfactory bulbs of its brain were so enlarged, in fact, that they caused its skull to bulge out into an unusually dome-shaped forehead.

Its reduced brain size may have been due to limited food availability on its isolated island home. Brains are very metabolically expensive organs, and some other extinct island mammals like hippos, hominids, and goats are also known to have evolved smaller brain sizes. Modern shrews even seasonally shrink their own brains during winter for similar energy-saving reasons.