Pre-Cenozoic aquatic mammals
The notion that mammals were small, shrew like critters in the Mesozoic is a sad notion that is long discreditted. While admitely still mostly no larger than a wolverine, Mesozoic mammals had a massive diversity of forms, from the rodent like multituberculates to the tasmanian devil like repenomamids to the bat like volaticotheres. Even early “ungulates”, like Kharmerungulatum, are known.
If there is one niche mammals seemed to have been historically lacking, however, was aquatic niches. The appearence of large marine mammals would only have occured in the Eocene; in the Cretaceous, there were no whales or pinnipede like forms around. Even otter like forms were notoriously scarce. Reasons for this were largely the overwhelming abundance of aquatic sauropsids: sphenodontians, choristoderes, squamates and crocodiles all occupied piscivorous niches, maybe discouraging otter like mammals from evolving. Even temnospondyls posed a threat; after all, they were the ones turning non-mammal synapsids away from the water.
However, as it turns out, aquatic mammals did still evolve back then, and they appearently weren’t as rare as previously thought.
Non-mammal synapsids: the first ones to get wet again
Very early in their evolution, synapsids returned to the water, even when temnospondyls were enjoying their golden age. In retrospect, it’s not hard to see why: early ancestors of the mammals were very weird critters that seemed suited to life in the water from the get-go. We now know “pelycosaurs” had fish scales on their bellies, and their skin was quite similar to that of modern amphibians, wet and secreting (where do you think sweat came from?). Indeed, it wouldn’t surprise me the least if it turned out non-therapsid synapsids were on average semi-aquatic, and that Amniota is actually polyphyletic, given how amphibian-like these animals were.
Within the menagerie of non-therapsid pelycosaurs, Ophiacodon and kin were the earliest to be suspected of aquatic behaviours. With teeth similar to those of piscivorous dinosaurs and long jaws, these were the first form of aquatic synapsid, stalking the shallows for fish and amphibians. The animal’s limbs appear to have had few adaptations for swimming, but the long tail appears to have been suited for aquatic locomotion. The more derived Secodontosaurus, more closely related to the iconic Dimetrodon, was also suspected of semi-aquatic habits for quite a while, largely due to it’s jaws, remarkably similar to those of a spinosaurid. The nostrils even appear slightly dorsal in position, implying that, like choristoderes and turtles, this animal used them as a snorkel.
Within non-mammalian therapsids, the number of defenitely aquatic forms appears rarer, but they are still known. The dicynodont Lystrosaurus has historically been suggested as having been semi-aquatic, which could explain why an animal it’s size managed to survive the Permian extinction, though no extensive bio-mechanical studies have been made.
The cynodont Procynosuchus was pretty much the first known otter like animal to have existed, though, unlike them, this animal swam laterally like a crocodile. It’s tail is longer than that of contemporary cynodont taxa, and it’s limbs are paddle like. Strong flexor muscles suggest that, like modern crocodiles, it relied on the hindlimbs and tail to propell itself through the water.
Procynosuchus and kin appear to have been a short evolutionary experiment, having disappeared in the Permian extinction if not earlier. Unambiguously aquatic synapsids are not known from the Triassic, which makes sense as it was the time of the most massive aquatic sauropsid radiation ever known.
And then came the docodonts.
Castorocauda: the (not so unusual, after all) otter of the Jurassic
Docodonta is a lineage of mammal-like synapsids from the Jurassic and Cretaceous periods, ostensibly one of the last non-mammalian therapsid clades to exist. Sometimes they are classified within Mammalia proper, but most often they are outside of it. Still, we may never know, as docodont DNA isn’t preserved.
Docodonts, as the name implies, are mostly known from teeth and jaw fragments. For the most part, they seem to have been omnivorous critters, as most Mesozoic Mammaliformes were. So, logically, nothing remarkable was thought about them.
This was until the discovery of an unique Jurassic genus known as Castorocauda. Known from the Daohugou beds of Mongolia, dating to the Middle-Late Jurassic, also known to have provided the gliding mammal Volaticotherium as well as crucial fossils to the understanding of avian and pterosaur evolution, Castorocauda is among the most complete docodont fossils known, most of the animal’s skeleton being preserved. And, while it did fill previous expectations of a borrowing lifestyle, it shows many specialisations to an aquatic lifestyle, raising docodonts from a mere clade of “living fossils” when they were alive, to one of the Mammaliforme clades that depict a much higher diversity of taxa than small, rodent/shrew like beasts.
Castorocauda, as the name implies, had a massive, beaver like tail. Given the structure of the vertebrae, it seemed that, like a beaver, it moved it’s tail in vertical undulations rather than lateral ones, making it one of the first synapsids to have swam like a whale or an otter rather than like a reptile (a trait modern tenrecs still possess). The feet also have preserved membranes, showing that Castorocauda also had webbed feet, and the teeth are quite remarkably similar to those of seals, pantolestids and hapalodectid mesonychians, lineages of aquatic placentals. Suggesting that it predated exclusively on aquatic prey.
More impressive, however, is that it’s closest relatives, Krusatodon and Simpsonodon, both european animals (Asia and Europe were back then seperated by the Turgai Strait, connecting the Tethys to the Arctic Ocean), might had also been similar, having similar dentitions. This both implies there were more aquatic docodonts and that these animals might actually have swam their way across the Turgai Strait, suggesting that otter like docodonts were the first marine synapsids. Other docodonts from North America might have similar teeth, suggesting that the waterbodies of Laurasia were populated by therapsids like these.
Docodonts appear to have become extinct during the Mid-Cretaceous, somewhat earlier than the Turonian extinctions, likely due to competition from therian mammals. How long these otter like forms lasted is unknown; given that the european otter-like forms did co-exist with crocodyllians, it is possible that the warmer temperatures didn’t cause replacement by aquatic sauropsids, and they might have lasted longer than other docodont taxa.
The most basal mammals known, monotremates seem to have branched off from the lineage that gave rise to the other mammalian clades quite early, having evolved in the then polar conditions of Australia. And early on, they became aquatic.
Two lineages of basal monotremates are known from the Cretaceous, Steropodontidae and Kollikodontidae. Both appear to have been aquatic, just like modern platypi, even having developed beaks as their jaw bones seem to suggest. However, they retained teeth, and their teeth depict clearly their radiation in aquatic niches back then; steropodontids appearently were piscivorous like otters, while kollikodontids specialised in eating hard shelled aquatic animals like crustaceans and shellfish. These mammals patrolled the rivers and lakes of southernmost Gondwanna, and possibly also their coastlines.
It is easy to see monotremates as the product of the polar conditions of Australia-Antarctica, evolving in the absence of aquatic sauropsids, alongside the relic temnospondyls like Koolasuchus. However, the South Pole back then might had been warmer than previously expected, and in fact aquatic crocodyllians are known from the same deposits as these animals, suggesting that the evolution of these mammals wasn’t likely influenced by colder-than-“normal” conditions.
Monotremates are only known outside of Oceania via the Palaeocene fossils of Monotrematum/Obdurodon sudamericum, endemic to Argentina. However, given how close they are to the KT event, it is likely that they already became that widespread across Gondwanna at least in the Maastrichtian.
Gondwanatheria is one of the most poorly known clades of Cretaceous mammals. Known from from teeth, jaw fragments and a single complete skull, gondwanatheres have been placed everywhere in the mammalian phylogenetic tree, from xenarthrans to metetherians, though consensus is that they are allotheres, related closely or within Multituberculata.
Gondwanatheres, therefore, have little to offer to shed light on their ecological habits. However, what they did offer seems to imply that they were yet another clade of semi-aquatic Mesozoic mammals.
The teeth of gondwanatheres are quite distinctive and unusual. They have well formed cusps and groves, similar to those of modern grazing mammals. This, therefore, would naturally indicate a specialisation towards grass-eating. However, how widespread grasses were in the Cretaceous is unknown. It is certain they present in the Maastrichtian of India, but the earliest gondwanathere fossils come from the early Campanian of Soth America, and they already display the iconic structures. While this could imply an early origin and dispersion of grass, only Bharattherium is certain to have been capable of gazing.
Another possibility is that these mammals were mollusc eaters, as this type of teeth also crops up in molluscivore mammals. The groves and cusps therefore would have evolved to deal with the shells of bivalves, not grass. The wide distribution of these mammals in Gondwanna, as well as their disappearence during the climatic changes of the Eocene – most of it’s tetrapod victims being aquatic – seems indicative of a semi-aquatic lifestyle for the gondwanatheres.
Within Theria, aquatic mammals were most well represented by Stagodontidae, a lineage of metatherians that lasted across the post-Turonian Cretaceous in North America and Asia. Stagodontids are best known for the most common north american genus, Didelphodon, which co-existed with the giant tyrannosaurs of the Maastrichtian, as BBC most famously depicted.
However, rather than badger like opportunists, stagodontids actually took to the water. Since at least the 90’s, the tail morphoogy of these animals has been compared to that of sea otters, and their robust skeletons seem to support this comparation. Their long toes also seem indicative towards an at least semi-aquatic lifestyle, indicating that these mammals swam like modern otters, using their tails and hindlimbs. Didelphodon itself also seems to have teeth adapted to crush molluscs, and was found in the more aquatic zones of Hell Creek, which ranged from inland areas to offshore islands. The fact that possible stagodont teeth also occur in Appalachia seems to seal the deal.
The main historical opposition to this idea was the notion that metatherians are poorly suited for an aquatic lifestyle, what with their pouches. However, the south american Yapok most defenitely shows that pouches are not enough to keep these mammals out of aquatic niches and indeed fossils dating at least as back in time as the Miocene shows that south american marsupials have produced aquatic forms. The only reason australian marsupials appearently didn’t go that far was due to competition with monotremates, and even then some forms from Riversleigh seem to imply otherwise. Furthermore, not only have some metatherians developed placentas independently from placentals – though stagodonts, with their long prepubic bones, were likely not among them -, but most non-placental eutherians appearent bred just like marsupials do, and true placentals are only known in Laurasia during the Cenozoic, thus etatherians likely had very little competition for aquatic niches.
Stagodonts are mostly classified as ameridelphians, but most modern studies place them as sister taxa to the weasel like deltatheroids, which are metatherians well outside of Marsupialia. A radiation of mustelid like metaatherians was appearently well underway in the late Cretaceous, and the aquatic, otter like stagodonts were the finishing touch.