Example Site: Santa Lucía Formation

A sample of mammals (and a sphenodont) from this timeline’s Santa Lucia Formation. Curiously, only three taxa are multituberculates (seven if you count the gondwanatheres), the remainder being dryolestoids and two metatherians. Artist is pale.relics

The Santa Lucía Formation is a fossil site in Bolivia dating to the Paleocene, usually dated to the Danian (the first five million years). It is part of the larger Potosí Basin and overlaps several Late Cretaceous formations, leading to dinosaur footprints and bones being erroneously associated with it. It seems to represent a tropical lake environment, seemingly adjacent to a tropical rainforest ecosystem. In our timeline, pollen and other floral fossils indicate a fundamentally modern flora dominated by legumes, but in this timeline the flora more closely matches that of Cretaceous South America, with around 50% of the plants being conifers, ferns, horsetails and ginkgos.

In our timeline, this formation is relevant because it is the start of the first South American Land Mammal Age (SALMA), the Tiupampan. Here, there are no signs of the earlier Gondwannan mammal lineages, but a variety of northern invaders: ungulates, basal eutherians like pantodonts and cimolestids and of course metatherians, leading both to marsupials and related groups like sparassodonts. Though many of these mammals would leave no descendents, the metatherians would go on to dominate the southern continents, invading Australia via the Antarctic land bridge.

In this timeline, none of those mammals are present, and this formation instead demonstrates A) a continuation of the Gondwannan Land Mammal Age and B) one of the earliest signs things are about to diverge immensely.

Like the flora, the mammalian fauna is primary a continuation of the Cretaceous situation. Gondwanatheres occupy a myriad of herbivorous niches, ranging from hamster to cow size, and are represented both by the generalistic ferugliotheriids and hypsodont sudamericids and greniodontids. But the true stars are not allotheres but rather dryolestoid mammals, which account for nearly 50% of all mammal remains. They are divided along two major lineages: the leonardids, which occupy all manner of insectivorous, omnivorous and carnivorous roles, and the mesungulatids, which are less speciose but compensate by being the megafaunal carnivores and largest herbivores, thus accounting for the most represented mammal fossils.

Just like our timeline, however, some groups have arrived from North America. These include cimolodont multituberculates (two ptilodontoideans, one microcosmodontid, one taeniolabidid) as well as therians (two metatherians and possible carpolestid remains). Also like in our timeline most of these will not leave a legacy, the difference being that metatherians are among them. Thus, marsupials and sparassodonts will never evolve in this timeline, with some dryolestoids seemingly even evolving rather convergently to some species in our Santa Lucía as the ultimate insult.

Curiously, there seems to be evidence of some animals from further south migrating northwards and possibly displacing some older Cretaceous clades. Santa Lucía thus is even more remarkable in this time, attesting to a massive shuffling of species across the Americas.

Besides mammals, the most common fossils are reptilian. Turtles, crocodylomorphs (including both aquatic alligatoroids and terrestrial notosuchians) and snakes are quite the abundant finds. Few lizards are present in both timelines, but here their niche was taken by sphenodonts, common in the Cretaceous and Paleocene in South America. Like therians, lizards will not get their chance to shine, instead replaced by relatives of the tuatara as well as allocaudate amphibians.

Sample Fauna

Hosca brusca is the only sudamericid genus found in Santa Lucía, and the only local gondwanathere genus that’s monotypic. This bear sized animal is characterised by a thinner snout than other gondwanatheres, which would have given it a somewhat bear-like profile in life. This is likely due to a more selective diet, a trend seen among several herbivorous mammals and dinosaurs. Isotope ratios suggest it fed primarily on cycads and conifers, represented in the formation by araucarias and podocarps. Biomechanical studies suggest its thin snout was still capable of a full palinal stroke and processing hard plant matter, so much like other “picky” herbivores it probably picked individual or few more nutrious leaves and branches while other herbivores bulldozed through. Phylogenetic studies indicate that it was closely related to Cretaceous genera like Gondwanatherium, likely representing a low-latitude radiation supplanted by migrants from Patagonia in the Paleocene. This might explain its speciation in comparison to the more generalistic greniodontid Glyptoguy, which represents an early example of greniodontids competitively excluding sudamericids.

There are 3-6 Glyptoguy species: a well understood G. sue, about the size of a sheep, an also fairly well understood G. giganteus, the largest of the taxa about as large as a cow, and poorly understood menagerie in between, which have been considered anything from different sexes to different morphs of a single species, as well as fragmentary remains not yet identified as recognised species. It is by far the second most common herbivore of the Santa Lucia Formation and likely took most large sized browser and grazer niches, its robust and short jaws apt to deal with a variety of plant matter from grass to conifers, though likely tending towards tougher food items in general. Glyptoguy is the most well represented greniodontid genus at the time (some contemporary Argentinian fossils have been attributed to the genus as well) and likely represents a then recent migration event from there, showcasing that northern South America underwent migrations not only from the north but the south as well.

There are two species of Wawa, W. cho and W. nyia. the latter larger and bearing a longer premolar 2 (in most cimolodonts the plagiaulacoid is the last lower premolar, but some lineages retain a peg-like remnant of another one in front of it; see them skulls). These are the earliest known gondwannan ptilodontoideans, likely having recently arrived from North America, a move repeated in both timelines by various mammal lineages and what makes this formation special. Unlike other northern invaders though, Wawa would go on to diversify, leading to the diverse clade known as Notoptilodontoidea, which would spread across not only South America but Antarctica and Australia as well, in a move similar to that of our marsupials. Wawa itself seems to have been arboreal and likely lead a lifestyle similar to that of possums and New World monkeys.

Allqu uyam is one of three mesungulatid species in the Santa Lucía Formation. Mesungulatids were a highly successful lineage of meridiolestidans, some of the largest Cretaceous mammals and enduring in our timeline as the giant Paleocene Peligrotherium tropicalis. A. uyam is one of the more conservative species, a raccoon sized animal that likely also lived a similar omnivorous lifestyle, though likely not as arboreal. That is fine, the treetops were crowded anyways. It is thought that this animal might represent a local mesungulatid radiation alongside its larger cousin Kuzcotherium elegans, perhaps forced into more carnivorous niches with the arrival of southern and northern herbivores.

Arguably the saddest thing from this timeline’s Santa Lucía Formation, Potentioperadectes vastata is the closest thing this world has to a marsupial (phylogenetic studies do recover it as a marsupialiform, though sometimes it groups more closely with peradectids and herpetotheriids than true marsupials) and while this would have been the time and place for their debut in our world, here this poor bastard will leave no descendents. Hell, it isn’t even a common fossil find, with only one semi-complete skeleton. It likely lived a lifestyle akin to that of our opossums, and show adaptations to both an arboreal ecology (opposite thumbs) and terrestriality (stronger pelvis). In a place already filled to the brim with opossum-wannabees, this animal is the definition of stunted potential.

The apex mammalian carnivore of this ecosystem is a mesungulatid meridiolestidan; mostly specialised towards herbivory, several lineages turned to more carnivorous lifestyles in the Palaeogene, with virtually no other large mammalian carnivores in Gondwanna at the time. Kuzcotherium elegans in particular seems to form a clade with Allqu uyam, likely representing an old low latitude mesungulatid clade that turned towards more carnivorous niches as herbivores from north and south invaded the region. Turning competition into food was a brilliant move, and thus this cougar-sized animal was capable of living like our big cats and fossa, using its powerful jaws and clawed forelimbs to bring down most of the megafaunal herbivores it co-existed with (aside from adult Baroauchenia canifacis, far too large), though it likely having a versatile diet that also included smaller prey. It differs from our timeline’s sparassodont metatherians by adopting a more cat-like build, thus relying more on the forelimbs than the jaws. This offered it a degree of versality, but time will tell if it will pay off, as it was still a hypercarnivore after all, now incapable of turning to plants like its ancestors did.

One of the two ferugliotheriid genera, Funzaiomys (a pun on Chibcha funzaiomy, “black potato”, with Greek mys, “mouse”) is the smallest of the Santa Lucia herbivores, ranging from hamster to guinea pig size. It is represented by three species, F. tribbitherium (hamster size), F. gilberti (guinea pig size) and F. troti (somewhere in between, and by far the most common though these are fairly rare finds overall). All are fairly rare finds, likely due to not only their small size but also fossorial habits. Like most ferugliotheriids it had brachydont molars instead of the hypsodont molars found in more derived gondwanatheres, so it likely fed primarily on soft plant matter like ferns, though a fairly large plagiaulacoid and robust jaws suggest it might have been more of a seed and tuber specialist. Phylogenetic studies place it in a clade with northern ferugliotheriids; whereas it descended from an endemic clade that would go on to populate the northern hemisphere in the Cretaceous/Paleocene or if it is a northern form that returned south is anyone’s guess, though its durophagous speciations seem to point to the former, since it likely represents yet another local forced to speciate in the light of more competition.

The first gondwannan taeniolabidid (unrelated lineages would later arrive by sea), Alicantotherium rigobelli is part of the laurasian radiation of these large herbivores in the Paleocene, the rare example that strayed southwards. Although it is seldomly bigger than Taeniolabis taoensis itself (already a fairly large animal), it seems to be more closely related to Late Paleocene titans from North America and Europe, likely shrinking in size both to to its tropical residence as well as competition from established herbivore groups; this mirrors in some ways South American pantodonts from our own timeline. It was semi-aquatic, a lifestyle already common among taeniolabidids and probably reinforced here due to a lack of competition in the water and too much competition on land. Still, it wasn’t quite as aquatic as latter taeniolabidids would become and seems to have been pretty gracile, likely foraging mostly along the shore on herbs, shrubs and horsetails and retreating to the water during heat or danger. Of the northern invaders, it is the most comon fossil find due to its large size and semi-aquatic habits, though not quite as common as giant gondwanatheres and Baroauchenia canifacis.

The token non-mammal in this presentation, Zysquy zoia is a small sphenodont, rather similar to our tuatara. Sphenodonts were common in South America during the Cretaceous and even endured into the Paleocene in our timeline, but quickly became extinct, perhaps due to climatic changes at the PETM. Here though, they’re only walking the first steps; in both timelines, squamates were severely decimated by the KT event, in some cases taking up to ten million years to recover. In our timeline lizards bounced back, but not here; by mere chance, sphenodonts and allocaudate amphibians beat them to the punch. If Potentioperadectes vastata is a pathetic little waste of potential, Zysquy zoia is an era about to begin.

The third mesungulatid genera is the most charismatic of megafauna. Closely related to Patagonian taxa like Peligrotherium tropicalis, Baroauchenia is the present culmination of their trend towards larger body sizes, becoming some of the largest land animals of the Paleocene at weights of over 3 tonnes and heights of four meters. Several species are known across South America, Antarctica and Australia, which B. canifacis representing the northernmost extreme of the genus’ range. These giant herbivores are worthy successors to the sauropods of old, having developed long necks to browse above their gondwanathere competition. Nonetheless, they possess more robust jaws than their dinosaur counterparts, hence a considerably broad diet of pretty much any vegetation above the reach of their competitors, as well as lower browse, aquatic plants and tubbers dug up by powerful claws. Unlike allotheres and their palinal strokes, meridiolestidans use a side-to-side chewing mechanism much like our ungulates, and unlike sudamericids and greniodontids they have lophodont rather than hypsodont teeth; both of these suggest a prefference for softer plant matter than the contemporary Glyptoguy and Hosca. B. canifascis is at any rate the most common mammal fossil, owning both to its large size and gregarious habits, with dozens of specimens often grouped together. The presence of such a large sized herbivore likely altered the biome of this timeline’s Santa Lucía Formation, resulting in open canopies and clearings; this perhaps explains a local flora more similar to that of the Cretaceous than our Paleocene. Males seem to have been slightly larger than females and are overall much more rare, suggesting that like other larger herbivores females lived in herds alongside their young while adult males were either solitary or formed bands at the fringes. While this depiction is conservative and depicts the animal with a full pelage, the tropical latitudes could mean that B. canifacis was naked; we may never know. What we do know that that its long neck, tail and ears likely played a role in cooling the body, and while not specialised to an aquatic ecology it is likely that herds spend daylight hours bathing and playing in the water, further explaining why these animals are such common finds.

The other ferugliotheriid genus, Fusuamys (a pun on Chibcha fusuamuy, “maize”, and Greek mys, “mouse”) is represented by two hyrax-sized species, F. gameri and F. kosemeni, distinguished by aspects of the ankle articulations and more lophodont teeth in the former’s case. Larger than Funzaiomys, Fusuamys was more cursorial (being the only local herbivore to be specialised for running) and had more lophodont dentition, suggestion a speciation towards browsing. It likely occupied a mid-sized herbivore niche alongside the smaller Glyptoguy species, with F. gameri in particular showing adaptations for climbing; overall, a comparison to our hyraxes or goats is not to far off, if less capable of eating grass. Fusuamys shows several similarities to Australian ferugliotheriids, suggesting that it might have been yet another migration from the south; however, it is less similar to Patagonian and Antarctic taxa that existed in between, offering a puzzling mystery. Are members of this clade yet to be discovered in southern South America and Penguinland? Or does it represent an older ferugliotheriid lineage largely displaced to the fringes of their former range? Another possibility is that these similarities are just convergent evolution. Who knows.

Sacadelphys is the most locally common genus of leonardid, a lineage of meridiolestidans related to the Late Cretaceous Leonardus and Cronopio (as well as necrolestids, which are highly derived members of this group). Several remnants attributed to therian mammals from the Late Cretaceous of South America seem to actually belong to this group (I can only recommend the book Mesozoic Mammals from South America and Their Forerunners for a complete and in-depth discussion), so it seems likely most if not all Santa Lucía leonardids represent old endemic lineages dating back to the Cretaceous. Sacadelphys seems to form a clade with the smaller Sorilestes andina and other contemporary taxa from Brazil, Pseudodidelphidae, which seems to have taken (or perhaps continued in, since Cretaceous leonardid remains are fragmentary at best so we know little of their post-cranial anatomy) niches occupied in our world by opossums and other metatherians, just to rub it in Potentioperadectes‘ face even more. Sacadelphys in particular has a muscular, prehensile tail, suggesting arboreal habits for its ten species. Ranging from the Monito del Monte sized S. minor to the spider-monkey sized S. chadi, they all seem to omnivorous, though larger species like S. tridentis show speciations towards carnivory like more carnassial-like molars, effectively acting as the mesopredators to Kuzcotherium elegans‘ apex predator. Some like S. planodens seem to have been pretty social, occuring in large numbers, something mirrored by some of our timeline metatherians of the same formation. Some even appear to have pneumatized hyoids similar to those of howler monkeys, suggesting that their troupes filled the canopies with noise.

One of the strangest leonardids is Xenotamandua allocaellus, a mymercophage similar to an anteater and indeed having convergently acquired many post-cranial adaptations like large clawed forelimbs, xenarthrous vertebrae and a heavy tail to balance while walking bipedally, but retained a very conservative skull, only slightly longer and with more spaced teeth than Leonardus itself. This has lead to differing interpretations that it is either part of a lineage dating deep into the Cretaceous or a more recent innovation. Not helping is that its phylogenetic placement is mangled by this mosaic of primitive and advanced features; sometimes it is the most basal Cenozoic leonardid known, sometimes it is part of a clade leading to the flying allochiropterids. Connecting it to other ant-eating meridiolestidans of the Cenozoic of Australia and South America has proven fruitless, so it seems it was one of a kind, either the last representative of a long-lasting lineage or a single bizarre offshoot. Specialised distally broad epipubic bones suggest that the tongue anchored here, thus spanning the torso like in pangolins.

The smallest of the Santa Lucia mammals, Sorilestes andina is known primarily from jaw fragments, with a single articulated spine demonstrating it had a long tail much like its larger relative, Sacadelphys. It appears to have been a terrestrial animal, likely occupying a similar niche to that of several Cretaceous leonardids and our timeline’s terrestrial opossums and shrew-opossums. Its long tail is muscular but not prehensile, suggesting that it was used for balance while running. It might also have been used in communication, though its unclear if it was a social animal.

Another tiny insectivore is the mole sized Tumi yumi, more robust and heavier than Sorilestes andina. This is South America’s only Palaeogene microcosmodontid, a lineage that diversified in the northern continents both as small insectivores and eventually as carnivores, some of the largest terrestrial mammalian predators of all time. None of that grandeur is seen in this tiny South American offshoot, which didn’t even leave descendents. It appears to be closely related to weasel-like forms in North America, which makes its subterranean habits quite perplexing, especially since necrolestids also occur in the Santa Lucia Formation. It indeed converged upon them in several ways, most notably its large clawed forelimbs. The skull is shorter, but bears massive lower incisors and shearing molars and plagiaulacoid, which might suggest a speciation towards larger prey like beetles and small vertebrates.

As equally perplexing as it is elegant, Fo gaxie is undeniably a meridiolestidan, but it is unclear if it is a leonardid or if it belongs to the poorly understood clade Brandoniidae (whose very existence is somewhat controversial in the first place, with alleged dental remains often considered synonims for leonardid or mesungulatid taxa). If a brandoniid, it fits the expected ecological niche this group occupied as a fast-running insectivore/omnivore since their teeth are noted as similar to those of sengi, being one of the only two cursorial mammals known from this formation alongside Fusuamys. Phylogenetic studies seem to place it as a leonardid closely related to allochiropterids; if this is true for all brandoniids remains to be seen. At any rate, Fo gaxie is one of the rarest of Santa Lucia’s mammals, known only from two semi-complete specimens and a lower jaw, owing perhaps to its delicate frame and perhaps solitary habits like those of our sengi.

The second metatherian is the otter-like Chibchalutra finalis. This is a stagodontid, a group of carnivorous metatherians that was common across the northern continents in the Late Cretaceous and that included the famous Didelphodon. In our timeline, the group might have endured as recently as the Eocene in South America in the form of the aptly named Eobrasilia from Brazil (gasp), representing an independent metatherian invasion from North America from the one that lead to marsupials. In this timeline, a stagodontid is definitely present in the Santa Lucía Formation when one wasn’t in ours… and that’s about it, as no younger stagodontids are known from this universe. Monkey’s Paw indeed. Anyways, Chibchalutra finalis was an aquatic predator with a particular speciation towards molluscs and crustaceans, much like its Cretaceous predecessors. It is indeed one of the few non-allothere mammals in this formation to have a durophagous diet, albeit relying on much more crude orthal (up-and-down) chewing instead of the more efficient palinal stroke. It might also have scavenged or even ambushed small prey at the water’s edge, though the abundance of crocodilians would limit this behaviour.