Tag: gondwanatheria

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

Almost-Living Fossils Month #04 – The Last Multis

Known almost exclusively from the southern continents of Gondwana – hence their name – the gondwanatheres were part of a widespread and very long-lived group of mammals known as multituberculates.

Although multis resembled placental rodents, and gave birth to tiny undeveloped young in a similar manner to marsupials, they originated much further back in the mammal evolutionary tree. They existed by at least the Early Jurassic (~183 mya), and their ancestry may go even further back into the Late Triassic (~220 mya) if they were descended from haramiyidans.

Multis survived the end-Cretaceous extinction and became very diverse through the first half of the Cenozoic, until a combination of factors such as climate shifts, new types of vegetation, and the evolution of new mammalian predators (and possibly also competition from placental rodents) resulted in most of them going extinct by the Early Oligocene (~33 mya) – with only the gondwanatheres surviving past that point in the then-isolated continent of South America.

Patagonia peregrina was the very last known gondwanathere in the fossil record, living just 21-17.5 million years ago in the Early Miocene of Argentina. Although only teeth and jaw fragments have been found so far, it was probably about 15cm long (6″) and would have been a burrowing herbivore similar to modern gophers or tuco-tuco. Its ever-growing rodent-like teeth were adapted for grazing on tough grasses in its savanna-like habitat, and it would have lived alongside several other now-extinct types of mammal – but we’ll be getting to those ones later in the month.

Since it seems like these last gondwanatheres had survived by retreating into a rather specialized ecological niche, they sadly probably didn’t persist for very long beyond the time of Patagonia. A wave of extinctions associated with sudden climate cooling about 14 million years ago may well have been the final blow to the once-successful lineage of the multituberculates.

Month of Mesozoic Mammals #20: Hidden Herbivores

Vintana

For a long time very little was known about a group of mammals called gondwanatheres. Named for their occurrence in the southern continents that made up Gondwana, they were represented only by fossil teeth and jaw fragments, and it wasn’t even clear what type of mammal they actually were. But recent discoveries of more complete skulls (and a currently undescribed full skeleton*) are starting to reveal more information, and we now know they were actually part of the multituberculates, or at least very closely related to them.

* described and named as Adalatherium in 2020

They were also the latest-surviving of the multis, lasting well into the Cenozoic with the youngest known fossils dating to just 17.5 million years ago.

Vintana lived during the Late Cretaceous of Madagascar (70-66 mya), and is known from a single skull. It was a specialized herbivore with rodent-like teeth adapted for chewing tough plants, seeds, and roots, and huge powerful jaw muscles attached to downward-extended cheekbones – a feature convergently seen in a few other mammals.

It was one of the largest known mammals of its time, estimated to have been at least 60cm long (2′). It had relatively large eyes, well-developed inner ears, and an expanded area of its brain associated with processing scents, all features that indicate it had very keen senses and may have been quite a fast and agile animal.