- Main article: Triassic Divergence
The world at the time of the Last Ice Age
The climate of the Paleogene began as one even warmer and wetter than the preceeding Cretaceous. All continents were isolated from one another, and similar jungles dominated the landscape of each unil the Paleocene-Eocene Thermal Maximum. After this time, the world followed a general trend of becoming colder and drier, culminating in the Last Ice Age, which ended about 10,000 to 15,000 years ago. The continents again moved together in a formation one could almost describe as a supercontinent until they reached the point at which we find them today. Frequent land bridges connected North America to Eurasia, South America recently collided with North America, and Africa collided with Eurasia. Australia separated from Antarctica, pushing up the largely volcanic islands of Indonesia, while India collided with Eurasia to create the Himalayas. Madagascar drifted even farther away from Africa, and carbonate island chains came to dominate the South Pacific and Caribbean.
Embryophyta[]
In the warm, wet conditions of the Paleocene and Eocene, the neomeiales were among the first plants to rebound after the extinction. Though originally tree-like forms, they began to fill a variety of niches due to their unique reproductive strategy. However, they were never able to conquer constantly dry environments, as their spores still require water for the final transfer.
The conifers and ginkgoes were increasingly marginalized during the Paleogene, but the cold, dry conditions of the Neogene allowed them to re-establish themselves in the high latitudes. Today, ginkgoales are the dominant temperate climate trees, and boreal forests are made up of familiar conifer trees.
Smaller continued to inhabit the understory in every environment. In everwet conditions, this means club mosses, while ferns make up the "soft" understory of the temperate latitudes. Cycads are somewhat marginalized today, but they are present in some of the drier forests (especially in the southern continents).
Gnetophyta were also limited somewhat during the wet conditions of the Paleogene, but, with the return of arid and semi-arid conditions in the Neogene, they once again became the dominant desert plants. Most of the "grasslands" of this timeline are made up of pseudograminoids: gnetophytes that adopted a rapid lifecycle that suited them very well in semi-arid conditions.
Cordaitales almost became extinct during the Paleogene, but they have persisted as isolated genera in Australia and New Caledonia.
Invertebrates[]
Strophomenid beds rebounded after the extinction, allowing them to occupy an oyster-like niche in today's world. In fact, true colonialism evolved (independently) again in the aggreostreae. This group steadily became the most common of the strophomenids, although other groups persist into the present day. The rhynchonellids also recovered quite quickly, and they are the dominant reclining shellfish in the world today. One lineage of these -- the chlororhynchonellids -- adopted a symbiotic relationship with photosynthetic algae not unlike the giant clams of our world. Terebratulid brachiopods are common as deep water cemented suspension feeders, but they are seldom present in nearshore environments. Lingula still persists into the present (as one of the oldest animal genera on the planet), but it is the only representative of its group left.
Clams weathered the extinction better than most groups, and they only increased in diversity. The bivalves today match the brachiopods in species number, but the bivalves are generally limited to an infaunal niche. However, they have diversified into every concieable form underneath the sediment: from suspension feeders with two meter siphons and rapidly-burrowing razor clams to deposit-feeders and even a few predators. Some even returned to a life above the sediment or adopted a lifestyle burrowing into hard substrates.
Gastropod diversity in this world is perhaps the most similar to that of our world. "Archaegastropods" and "mesogastropods" remain the prevalent aquatic deposit feeders and grazers, while the neogastropods are predatory (almost without exception). However, a number of forms are present, including a few suspension feeders and some parasitic genera. Pulmonate land snails and slugs are about as diverse as ours, and, though they lack a fossil record, nudibranchs are common in marine environments.
Ceriatitids recovered from the extinction perhaps more quickly (and certainly to a much more impressive extent) than any other group. They went from one genus to dozens of genera by the end of the Paleogene. Today, they have a marine diversity similar to that of the teleost fish in our timeline, but several other groups invaded some of their niches as they recovered. In addition, they are almost entirely limited to open, salt water environments. Belemnites and octopuses filled many of their roles in the recovering reefs of the Cenozoic, as their lack of an external shell allowed them to more easily manuver in narrow areas.
Echinoderms are another very familar group of invertebrates in this timeline. Their diversity is comparable to ours, with infaunal echinoids accounting for most of the species number. The one exception is that stalked crinoid beds are not an unknown sight in the world's oceans, though these animals are by no means common.
Many other invertebrate groups (flatworms, nematodes, rotifers, hemichordates, etc.) are present in the world today, but they have left such a spotty fossil record that their evolutionary histories are not known. However, round worms are exceptionally common in all manner of habitats, parasitic and free living flatworms are found almost everywhere, and annelids turn the soil and feed in the oceans, much like in our timeline.
"Fishes"[]
Conodont animals became the oceans' primary scavengers after the Cretaceous, pushing the surviving hagfish into deeper waters. Some have actually attained respectable sizes of multiple meters, and a variety of forms are known. Parasitism and predation evolved again in some of the smaller lineages, and some very unusual forms are known in these roles.
Lampreys and hagfish are similar to those of our timeline but are somewhat less diverse due to competition with conodonts. However, conodonts are strictly marine animals, allowing the lampreys to diversify in fresh water. Hagfish are restricted to the deep oceans, where the conodonts (with their huge eyes) are not usually found.
Xenacanths diversified in the warm, wet conditions of the Paleogene, but the drying during the Neogene hit them particularly hard. They entirely died out in Australia, and there are only two genera present in Africa (and another in Madagascar). In South America they are now most diverse, especially in the Amazon River and its tributaries. Some genera even moved north and can be found in parts of (what is to us) the southern United States. Ctenacanths filled much of the gap left by the hybodonts at the end of the Cretaceous. Most forms are one sort of bottom-feeder or another, but a lineage of large filter-feeders is present in the southern oceans.
Marine chimaerae are somewhat more diverse than in our timeline, mostly due to lack of competition in the Cenozoic. However, they are not common and are mostly restricted to cold or deep waters. The freshwater chimaerae survived in all three southern continents with a good deal of diversity. They even colonized rivers and lakes into Laurasia and are now found on every inhabited continent.
Polypteriformes diversified in Africa as the most common midsize freshwater predators, though they became extinct everywhere else during the extinction. They have extended their range during the Neogene, however, and can now be found throughout the Old World. Some "walking bichirs" are known from Southeast Asia.
Polydontidae and acipenseridae diversified into a number of freshwater niches in Laurasia, although the polydontidae tended to remain filter-feeders while the acipenseridae tended to stay bottom-feeders. A lineage of marine paddlefish evolved during the Paleogene, and these are now the dominant large filter-feeders in the northern oceans. The acipenseridae were hit particularly hard by competition with fluminochimaerae during the Neogene, retaining most of their diversity in Europe.
Holostei diversified as the most common Laurasian midsize freshwater predators during the Paleogene. Though both groups are found across the northern continents (and beyond), the gar are most common in North America while the bowfins are most common in Eurasia. In the gap left by the hybodonts, marine gar evolved during the Paleogene. These fish rival our sharks in size and predatory ability.
Teleosts were particularly hard-hit by the extinction. In fact, they were believed to have gone extinct, but three species are known from deep waters of the Indian and Pacific Oceans. These are considered "living fossils," as the group has changed relatively little from the forms known in the Late Cretaceous.
Coelocanths are common midsize marine predators. They are excellent ambush hunters, as their large surface area in the tail gives them capability for quick bursts of speed, while thier lobed fins give them excellent manuverability. Lungfish are known from all six inhabited continents, and both lineages are fairly well-represented.
"Amphibians"[]
Limnarchia[]
The limnarchia managed to survive to the present day solely in Australia, where they fill most of the crocodile-type niches across the continent. Although they are not exceptionally well adapted to terrestrial life, they are found deep into the outback because of their recently-evolved ability to enter a dormant state similar to a lungfish. Thus, amphibians of multiple meters in length can remain hidden under dried up riverbeds until the next wet season, allowing them to be the apex predators of these waterways.
Lissamphibia[]
The lissamphibians of this world seem mostly familiar to an observer from ours. The three groups of our timeline (frogs and toads, salamanders and newts, and apodans) are all well-represented. They are even somewhat more diverse, as they have adapted to many of the typical "lizard" roles with which we are familiar. In addition, a fourth group -- the allocaudata -- has survived into the present day in Eurasia. They are quite diverse on this continent, in fact, as they took over many of the lizards' roles first after the Cretaceous-Paleogene Extinction and then again during the Last Ice Age.
Synapsida[]
In South America, the cynognathia underwent a significant radiation during the Paleogene but died out at the end of the Oligocene, except for one small omnivorous lineage. These forms persist into the present day and have moved into North America. There are some unusual Central and South American forms that are more specialized, however. In Australia, though, the cynognathia became the dominant terrestrial animals in almost every niche. A variety of forms can still be found on the continent.
Mammals spread back into North America during the Neogene, where they diversified into a number of forms filling the lizard-type niche. At the end of the Oligocene, however, mammals became extinct everywhere except there. Now, they have recolonized every continent except Austrialia and Antarctica, but all forms alive today are small, sprawling insectivores.
Procolophonoidia[]
The procolophonoids recovered fairly soon after the extinction, moving back into North America during the warm conditions of the Paleogene. However, they then began a steady decline culminating in their extinction during the Last Ice Age.
Ichthyosauridae[]
The ichthyosaurids survived the extinction in a number of typically "shark" niches. They did extremely well during the Paleogene, but falling ocean temperatures of the Neogene limited their range. Today, all ichthyosaurids are predatory inhabitants of warm (typically tropical) shallow seas.
Lepidosauromorpha[]
Nothosauria[]
Nothosaurs persisted past the boundary. Basal nothosaurs maintain their semi-aquatic habit to the present, but one lineage -- the pseudoplesiosauria -- evolved along the same lines as the plesiosaurs of the Mesozoic. Today, they have long necks, and they propel themselves using alternations of their four flippers. One lineage during the Paleogene became the apex predators of the oceans, but this lineage became extinct as the oceans began to cool. Finally, in New Zealand, the aotearoasaurs re-evolved a terrestrial lifestyle with a great deal of success.
Sphenodontia[]
In the southern continents, sphenodontids achieved great success in small insectivorious roles. They are now found in warm envoronments worldwide, and have diversified into a number of niches on the island of Madagascar.
Squamata[]
The lizards re-established themselves in arboreal niches in both North America and Africa. In fact, the North American group evolved a gliding membrane during the Paleogene. The African lineage of lizards became extinct at the end of the Oligocene, but arboreal and gliding forms are still present across Laurasia.
Phytosauria[]
Redondasaurs are the only phytosaurs in the Americas, mostly inhabiting crocodile-like modes of life. One genus (represented by a few species) is found in the temperate latitudes of Asia. They tend to hunt terrestrial organisms more than fish, but the South American redondasaurs underwent an impressive radiation in the vacuum left by the end of the Mesozoic.
The nicrosaurs are essentially the Old World analogues to the redondasaurs, but they are represented by many more species overall than the redondasaurs. These phytosaurs are now found in Africa, Asia, and Australia, the European lineages having gone extinct at the end of the Oligocene.
Islasaurs diversified in the Paleogene of Europe, and they were the apex terrestrial predators of the Paleogene. Like much of the reptilian diversity, however, they did not fare well when the world began to cool. They became extinct in Europe, but they remain the largest predators of Africa and South Asia.
Avemetatarsalia[]
Pterosauria[]
The neodimorphodontia are the only pterosaurs to survive into the Cenozoic. However, they have undergone an impressive radiation. During the Paleogene, most forms were small, perching forms. However, when the world began to cool and more open terrain was available, larger forms evolved. Their endothermic habit and insulating fur allowed them to colonize every land mass on Earth, even the fringes of Antarctica, and the largest forms today can have wingspans of a few centimeters to up to 7 meters (23 feet).
Dinosauria[]
The prosauropods diversified in South America after the extinction, becoming the dominant herbivores of the continent. Their initial browsing lifestyle suited them well in the forests of the Paleogene, and many forms adopted a grazing lifestyle during the Neogene. During the Oligocene, a few forms attained sizes that rival the sauropods (and later the pseudosauropods) of the Mesozoic. Today, they are found well into North America. However, they have lost much of their former diversity.
The Neosauria are one of the most diverse groups of fully terrestrial animals on Earth today. Their endothermy allowed them to exploit environmental conditions closed to most other groups, and they are today found on every continent except Australia and Antarctica. Sizes range from 0.1 meters to large 6 meter carnivores in the low latitudes. Most are either carnivorous or omnivorous, with just a few herbivorous lineages.
Pseudosuchia[]
Aetosauria[]
Neothypothorascisines diversified into a number of different forms and became the dominant herbivores in the northern continents. When the world began to dry, some adopted a grazing lifestyle allowing them to take over the prominent herbivore roles anywhere warm enough for them to survive. Thus, they are the dominant grazers in warmer parts of the world today. However, they can not be found very far north or south -- due to their intolerance of cold -- and the Americas have a lineage entirely separate from that of the Old World.
Suchia[]
Shuvosaurs became extinct in Australia at the end of the Cretaceous, and the South American forms died out due to competition from North America when the Isthmus of Panama closed. However, they evolved as the dominant land animals of Africa, filling roles from large (almost pseudotheropod) predators to small rooters and everything in between. Shuvosaurs can now be found across the Old World in a variety of niches.
Neopoposaruia of this world are perhaps the most bizare clade of organisms alive today. Their ability for thermoregulation translated into a third independent development of endothermy. In addition, scales evolved into a coat of downy, fur-like feathers during the Eocene. The basal (unfeathered) arizonasaurs became extinct at the end of the Oligocene, but the neopoposauria exploited every niche available to them in the cooling world. Today, they are present in every continent except Australia and Antarctica, they have come to dominate the colder parts of the world, and one lineage even returned to the oceans as bizare predators of the northern and southern oceans.
The velocisuchia became the dominant carnivores of South America due to their capacity for speed. They continued to radiate into a variety of niches, with one even becoming semi-aquatic like the crocodilians of our timeline (though this lineage went extinct when redondasaurs reached South America in the Oligocene). They were largely outcompeted by the neosaurs and neopoposaurs when the land bridge opened between North and South America, but they are still present as far north as Florida.
Fossil Sites[]
Green River Formation (54 - 49 million years ago) - A limestone deposit from a series of freshwater lakes details the fish composition of Eocene North America. Fossils of early avesaurs, feathered neopoposaurs, and piscivorous pterosaurs are also present, along with numerous phytosaurs, turtles, and insects.