kogaionidae – Nix Illustration

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.

Month of Mesozoic Mammals #19: Red In Tooth

Barbatodon

Living during the Late Cretaceous of Romania (70-66 mya), Barbatodon was part of a group of European multituberculates known as the kogaionids. These multis originated on the then-isolated Hațeg Island alongside dwarf dinosaurs and giant pterosaurs, and adapted to a highly insectivorous diet. They even managed to survive through the end-Cretaceous mass extinction, and spread across Europe for a further 10 million years before going extinct in the early Eocene about 56 million years ago.

Barbatodon is mainly known from teeth and partial skull material, so its full size is uncertain, but it was likely rat-sized at around 25-30cm long (10-12″). In one specimen its teeth were also preserved with their original coloration – a distinctive “blood red”. This feature is seen in some modern rodents and shrews, and is caused by iron minerals in the enamel that are thought to add extra strength. Since multis didn’t have ever-growing teeth like rodents, this added durability would have been especially important to them.

Another group of multis, the taeniolabidoids, also had red teeth, but since fossil enamel is rarely so well-preserved and unaltered we don’t know whether this was a shared ancestral feature or due to convergent evolution.