The rodent-like multituberculates were a major lineage of mammals that were only distantly related to modern marsupials and placentals. They originated around the time of the mid-Jurassic (~168 million years ago), survived through the end-Cretaceous mass extinction, and went on to become one of the most diverse and successful types of mammal in the Paleocene. After that point they began to decline, and after anw over-130-million-year-long run they went extinct* in the early Oligocene (~33 million years ago).
(* Except, possibly, in South America, where an enigmatic fossil known as Patagonia peregrina may represent a multi surviving as recently as about 18 million years ago in the early Miocene.)
First discovered in North America in the 1880s, Catopsalis foliatus was part of a group of multituberculates called taeniolabidoids. These multis got significantly larger than the rest of their kind – averaging beaver-sized but with some species getting up to at least capybara-sized – and were some of the first mammals to evolve into relatively big herbivores after the extinction of the non-avian dinosaurs.
Catopsalis was named based on a partial jawbone and a few teeth, and over the next century or so various other similar-looking fossils from both North America and Asia were added into the genus as additional species. Eventually Catopsalis contained eight different species, ranging over about 10 million years from the late Cretaceous to the early Eocene – not especially big compared to some other wastebaskets we’ve looked at this month, but it was still a problem, muddying up attempts to understand the actual evolutionary relationships and biogeography of the taeniolabidoids.
Cladistic studies in the 1980s showed that Catopsalis was paraphyletic, made up of at least five separate lineages, and a few of them were subsequently renamed and reclassified. The Cretaceous Asian forms became Djadochtatherium and Catopsbaatar, and are now considered to be part of a different lineage of multis known as djadochtatherioids, while one of the remaining North American species then became Valenopsalis.
…But a couple of other new Catopsalis species have also been named in the meantime (one as recently as 2018), so there are still seven different species that need sorting out in this particular wastebasket.
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.
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.
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.