The Leptictida were a group of Cretaceous-Paleogene eutherians usually thought to be outside crown-group placentals. Although not very diverse, at least some members, particularly the most well known member of the group, Leptictidium, were fully or partially bipedal. This puts them among a restricted category of bipedal mammals, which also includes macropods, pangolins, and humans. Why this evolved is unknown, but with their long legs and tails, as well as bipedality, they were surely unique among mammals in locomotion, a reminder of the theropod body plan that used to be common across the planet.
Generally small, insectivore-like omnivores/insectivores, they certainly had evolutionary potential. Given the chance, they had the design to become among the most successful of mammals. But that time wouldn't come in our timeline. The Eocene-Oligocene extinction almost wiped them out, with only a few lines surviving. The drying climate prevented them from ever rebounding. But here they didn't ever decline. They would become very successful across 5 of the 7 continents, from carnivores, to browsers, to weird hybirds of wallabies and anteaters, they would diversify in multiple radiations, that would lead to a group perhaps equivalent to the rank of primate, artiodactyl, or carnivoran. For in this world, placentals aren't the only eutherians.
The Leptictidae are one of two major groups of leptictidans, and the only one known from HE Oligocene fossil layers. However, as here the extinction marking the demise of the Leptictidium-line leptictidans never happened, they are not alone. Throughout the fossil record and to the modern day, the Leptictidae have been more successful then their cousins, the pseudorhyncocyonids, which lived in relative oblivion until the MTM, when two radiations boosted diversity. However, the MTM also greatly boosted the diversity of Leptictidae, certainly more so. But before that, they had stayed relatively unchanged, being generally insectivore like and seperated into three of small forest dwellers. More "advanced" forms are grouped together in a monophyletic group of derived leptictids, making four internal clades total.
Despite being an actual group, being crown group Leptictidae, the term here generally refers to a grade of primitive leptictids that don't fit into any of the other three pre-MTM groups. As such, it doesn't actually count towards the number of clades. The "Leptictinae" were the most common group of leptictids in the Oligocene, and indeed leptictidans period. The only large clade of leptictids to evolve in the Oligocene (from the parapyletic "Leptictinae") were the Acanthomysinae. As the pseudorhyncocyonids were still on the brink of extinction in many areas, this made basal forms, including Leptictis proper, among the most common forms.
Three notable Oligocene genera started towards a lifestyle in more open areas, as the minor drying was more gradual then HE, thought these went extinct when the world warmed again in the Miocene. This short lived, polyphyletic grouping included Striataleptictis, Erectomys, and the more derived Nothomys. The most basal form is Striataleptictis, being the third most basal non-HE leptictid. It's changed significantly from Leptictis, and can grow to the size of a porcupine, and is omnivorous, eating almost anything. They poses large leg muscles, and an overall robust body, walking around hunched bipedaly, balanced by a heavy tail that occasionally drags. Their hands have two large, robust claws, and two grasping fingers, and their dentition is very generalist. The most probable reason for extinction is competition from other generalists that instead adapted to the forests, restricting them to the plains. It's name means "Stripped Leptictis" in reference to the stripes of most of the seven taxa/populations. However, as S. australis and S. italiensis, the populations that spend the most time in dry, open areas apparently lack them, they aren't present across the entire taxon. These also happen to be the latest forms, living in the latest Oligocene, and into the earliest Miocene, thought only the former lasted any time into this epoch.
The second taxon, Erectomys, is more derived, and only a couple steps lower on the tree then Nothomys. Despite this, grouping the two still creates a polyphyletic grouping. Overall, it's the seventh most basal non-HE leptictid, making it close to the acanthomysine split. Classification aside, Erectomys is quite unique among leptictine-grade in being oddly thin and gracile, almost lizard like, with a long tail, and growing up to almost a meter long in some species. All this odd extremities come as a side effects of a feature, and indeed the one that gives it it's name. Despite being in an area of the Leptictidae that is mostly quadrupedal, they will often run on their back legs, and stand in an almost vertical pose to look above the ferns and shrubs for predators, or for water sources. Their gracile body and tail the supports them as a third leg while "peeking" are the best ways to achieve this in a basal leptictids case, giving it it's lizard like build. Built upon gracile limbs, it can be quite fast when need be. It has a small proboscis, similar to that of Leptictidium, varying in length between species. It uses these and its small but sharp claws to support a varied diet composed mostly of small vertebrates, like lizards, rodents, and indeed small leptictidans. Canibilistic acts aren't rare. This diet was a major shift from the mostly insectivorous one of it's forest ancestors, and because of its ancestry, it has a different way of killing and butchering its prey than most mammals of its niche. It catches prey using canines on the upper and lower jaw, then pushes them back to the large, hyper-sharp molar teeth on it's lower jaw, crushing prey between the smaller, flatter up molars, all near the middle of the mouth. The carcass is then left to the back molars, which butcher the kill before swallowing it mostly whole. This way of killing prey was probably derived from the fact that it feeds on mostly backboned prey, which they kill by crushing the spine. This generalized diet led them to being the most successful of the plains lectictids, and the last to go extinct. Despite this, it has the least amount of variation among populations. On a physical level at least. There's significant genetic variation between said populations, a side effect of living from the eastern stretches of the European islands, what we know as Moscow. They slowly declined, with the final populations of the Balkans going extinct in the dawn of the Miocene, with the re-expansion of forest. One of the smallest (non-insect or miscrobial) life forms in the ecosystem, brought down by the largest. Ironic, perhaps.
The most derived and therefore the last to appear of the three was Nothomys. Arguably the least successful of the three, it had the smallest distribution or Iberia, the Alps, and the Italian islands, and was the first to go extinct, before the end of Oligocene. It's name means "false mouse" because of it's overall mouse like appearance, on the outside. Most species are larger then the average modern HE mouse, and have a more noticeable neck, and larger limbs and head, but the external differences to the naked eye end here. However, being no rodent, there is a major difference in the teeth. They lack the large, jutting, ever-growing incisors of rodents. Instead sporing a more general dentition. They live in burrows, and because of small size feed almost exclusively on insects. This specialization is probably what led them to extinction when the trees came back and the prey changed. Note all populations, however, had this general size. Some from the larger islands, like N. insularis and N. gigas grow up to 20kg, and the small N. gracilis and it's relatives reach less then a pound, and jumps around on the shrubs and ferns. In fact, this was probably the most diverse of three, in terms of anatomy. However, this couldn't save it from the ever shifting climate, and it joined the giant category of animals that time left behind.
Aside from these divergent taxa, there was many more familiar taxa, which were all quite similar in terms of anatomy. Of the pre-Acanthomysines, these are, in ascending order of derividity, Mys, Borealis, Borealidon, Artemis, Australeptictis, Neomys, and Rodentodon. Mys was the most succesful Leptictid that doesn't fit into any of the four large clades, and even among them it is notable. Evolving 2 milllion years before thee end of the Eocene epoch, it had over 20 notable popultions across it's range, and lasted until the Miocene. Despite this being similar to the human lumping concepts, the lack of much genetic diversity indicates they do indeed all fall under the same taxon. Spanning almost the entire old world, it originated in Chinease forests, after a quick migration of middle eastern leptictids eastward. The first populations were M. paleae, M. mys, and M. apis. M. mys gave rise to M. major and M. minor, the former of which giving rise to M. borealis, as well as various other populations. From M. borealis sprang up all of the over 17 populations of the Late Oligocene and onwards. The most successful of these was M. occidentalis, M. insularis, and M. sinensis. Most of these were fairly conservative, being small hunters of insects and other arthropods with a generalized body, not overly robust, but not enlogate like Erectomys either.
M. apis made up over 60% with bees and honey, where the species name comes from. M. minor fed almost exclusively on small insects, M. major and M. borealis were large and were about 50/50 between invertebrates and vertebrates for their diets. M. major lived in China and Mongolian, whereas the more successful M. borealis lived across northern Asia, China, and some of central Asia. M. occidentalis lived on the eastern floodplains and coastlines of Asia, feeding on small vertebrates, crustaceans, and some larger insects. It lived from the late Oligocene to the MTM. M. sinensis lived in chinease forests, and was the largest species, at a meter and a half. It again, had a mostly vertebrate diet. Appearing in the early Miocene, it lived to the MTM, and was the last species to go extinct, as the other extant species at that time depended on coasts and floodplains that became flooded in the rising sea levels. M insularis evolved in the early Miocene, and as was among the first of the African radiation, where the last new populations of this taxon occured. Most of these went extinct soon, but some Southern populations and M. insularis were fairly successful, and the latter longlived. M. insularis lived in some of the more isolated islands of the Saharan sea, and island magic is evident. As far as size goes it's moderate, but it's notable in that it's the only species which has plants as over half of it's diet. It also swims better then most, as it has webbed feet, evolved because of the habitat of small islands. It to lived to the MTM, thought these three successful populations were the only to make it this far. Mys went into decline almost immediately after the African radiation, partly because of competiton from Artemis. These three were relicts by the time of their extinction, and even this title was not earned after the climate chaos commenced.
The next few taxa were all of European origin. The first one of these to appear was Borealis, which came around 2 million years after Mys. It enjoyed success across the European islands as a taxon that lived very similar to the ancestral Mys populations, feeding mostly on insects. In the late Oligocene, when Mys started to radiate in Europe, Borealis went into decline, with the last individuals living in the north western chains, before even these were replaced. Of this short lived success, there were ten important populations. B. mus and B. germanicus were the first to appear, in Iberia, France, and Germany, mostly, with the latter giving rise to four more taxa, and two of these leaving one and two descendants respectively. B. mus gave rise to B. ukrainius, which was the westernmost species, living alongside some of the smaller Mys populations for a brief period. It was the shortest lived species indicating the smaller and more generalist Mys had been the better of the two since the start. From there, B. orientalis, B. aquas, B. borealis, B. australis, B. raptor, B. occidentalis, and finally the final individuals of B. gigadon were wiped out. Among these, B. aquas, B. raptor, and B. gigadon were the most derivative. The preferred habitat of B. aquas was riverbanks, as opposed to the drier forests most of the others lived in. Because of this, fish and freshwater crustaceans and mollusks were a major part of it's diet. B. raptor was the only cannibalistic species, and would often bully other small insectivores out of it's area. This may have actually evolved as a adaptation to the Mys competition, as it evolved around the time they invaded it's range. B. gigadon was more omnivorous, a foreshadow of future taxa, and had two large teeth for crushing nuts. Going extinct at the end of the Oligocene, warming may have also played a role in its extinction. Whether or not this taxon shows the adaptability of a Chinese taxon, or the incapability of German one, is up to interpretation.
|Mammalia||Eutheria||Laurasiatheria||Artiodactyla||Cetancodontamorpha • Dichobunoidea • Ruminantia • Tylopoda|
|Ferae||Carnivora • Creodonta • Pholidota|
|Perissodactyla||Chalicotherioidea • Hippomorpha • Rhinocerotoidea • Tapiridae|
|Euarchonta||Dermoptera • Haplorhini • Scandentia • Strepsirrhini|
|Afrotheria • Leptictida • Xenarthra|
|Sauropsida||Aves||Neoaves||Passeriformes • Phorusrhacidae|
|Choristodera • Crocodylomorpha • Lepidosauria • Testudines|
|Habitats||India • South America|