This page details the history after the point of divergence in the project Triassic Divergence.
Extended Triassic (201 to 182 million years ago)Edit
The climate of the Extended Triassic became increasingly wet as Pangaea began to split up. During the Extended Triassic, the divide between Laurasia (modern Europe, Asia, and North America) and Gondwana (modern South America, Africa, Australia, Antarctica, and India) became fully apparent as North America and South America/Africa split to form the early North Atlantic. Central deserts were present on most continents in the middle latitudes, but most areas were either tropical jungles, seasonal climates, or temperate forests. The seas were warm, and the plankton had fully recovered from the Permian-Triassic Mass Extinction providing the basis for an intricate and stable food chain.
The stable, wet forests along the coast were dominated by the pleuromeiales, the primitive relatives of the club mosses. Because they reproduced via spores, they were limited to at least seasonally wet areas, and some grew to enormous sizes rivaling most modern home Earth trees. In tropical environments, sphenophyllales dominated the understory, where their broad leaves could absorb more sunlight and enough moisture was available to reproduce. Cordaitales formed the dominant large flora in the semi-arid regions, while conifers dominated the temperate forests. Primitive ginkoales were also evolving, and the gnetophyta were the dominant shrub-like plants in the deserts.
The parallel world had its own version of the Mesozoic Marine revolution, beginning with the forms that diversified in the Extended Triassic. However, without the blow to invertebrate diversity that was the Triassic-Jurassic Mass Extinction, the players were slightly different. Perhaps the closest analogue to our timeline is the evolution of the gastropods, with genera that seem almost identical to those of our own Earth. The mesogastropod deposit feeders and grazers continued to diversify, and fossils deemed to be generally ancestral to the neogastropods date to this time. There was even a lineage of archegastropods that evolved as slightly motile suspension feeders.
While in our timeline the brachiopods were gradually replaced by the bivalves, this was not the case on this Earth. Strophomenid brachiopods continued to diversify, and some even formed extensive beds on the sediment similar to modern-day oysters. Spiriferid brachiopods generally lived attached via their pedacles to hard sediments, though some forms lived reclining on the sediment. Rhynchonellid brachiopods also formed a significant portion of the marine fauna, living either attached or reclining.
The bivalves were still a driving force, however. Primitive clams adopted an infaunal habit that allowed them to colonize an essentially untapped environment, while scallops' motility gave them a certain edge over the strictly sessile brachiopods. The first coral reefs since the Paleozoic began to form with primitive scleractinian corals providing the reef-building component, and the stalked crinoids managed to make a slight resurgence along with bryozoans and octocorals towards the end of the period, even forming limited mounds in areas of high nutrient content and strong flow. This was likely due to the restored planktonic flora and the lack of modern suspension-feeders in these environments.
Ceriatitid ammonids formed the bulk of the pelagic suspension feeders and micropredators, with ammonite amonids being represented only by a few genera towards the end of the period. Pseudorthocerid nautiloids grew to become active predators, and their nautilid relatives managed to cling to life in deeper waters. Squid, cuttlefish, octopuses, and belemnites began to become a prominent force in reef and mound environments as well.
In our timeline, the teleost fishes rose to dominance after the Triassic-Jurassic Mass Extinction due to their unique feeding mechanism, rapid capacity for breeding, and an unusual whole genome duplication. However, there was no void in the Extended Triassic for them to fill in this timeline. They were about as common as the non-teleost ray-finned fishes are today in our timeline. The non-teleost actinopterygii underwent an impressive radiation, though mostly in fresh water. Chondrostians in particular are particularly common from the Extended Triassic.
Sarcopterygians were somewhat more diverse than in our home timeline, but not by much. They radiated into a variety of freshwater and saltwater habitats during the Extended Triassic, and the lungfish had long since attained a mode of life similar to that of our home timeline (a mode which they retain to this day in both timelines).
An unusual feature of this timeline is the presence of the conodont animals beyond 201 million years ago. Though they stayed small, they filled several ecological niches. Most were scavengers, but a few were active predators. One genus is even hypothesized to have been parasitic towards the end of the period, living like modern-day lampreys. Because of the presence of conodonts, the living jawless vertebrates -- the lampreys and hagfish -- were not quite as common. These still were present, of course, but mostly restricted to freshwater (lampreys) and deep ocean (hagfish) environments.
Sharks remained the dominant non-tetrapod predators in the oceans into the Extended Triassic. Ctenacanth and hybodont sharks formed the bulk of these groups (taking on many forms similar to our sharks), and chimerae also had an impressive radiation mostly as bottom-feeders. There is even one freshwater genus of holocephalan known from the Extended Triassic of South America. Having survived into the wet conditions of the Extended Triassic, the xenacanth sharks rebounded in the abundance of freshwater habitats. Some became real monsters of the swamps and rivers, attaining lengths of up to 5 m (16 ft).
In the increasingly wet climate of the Extended Triassic, amphibians began to diversify once again. The modern groups of lissamphibians (frogs, salamanders, caecilians, and albanerpetontids) had begun to emerge in the tropical forests of the low latitudes, and the limnarchian monsters (relics from the Paleozoic) filled roles similar to the crocodiles in swampy habitats, mostly in Gondwana.
Basal mammals evolved in the Extended Triassic of this Earth, but their diversity was rather limited, even more so than in our Mesozoic. Competition from basal cynodonts, as well as a number of "reptile" groups that filled rodent-type niches, severely limited their capacity for diversification. Forms remained small and mostly lived in alpine and far northern regions. The cynodonts were much more common, living as generally small scavenging or predatory animals through many environments.
Unusual compared to our timeline is the persistence of dicynodonts into the Extended Triassic. They primarily existed as browsing and rooting animals in open terrain. They were relatively uncommon, however, especially in the tropical and temperate forests. A few adopted a lifestyle similar to modern-day hippos, however, allowing them to graze on aquatic vegetation and escape predation.
Procolophonoids achieved greater diversity in the Extended Triassic due to their varied diets. They occupied niches similar to modern-day rodents or lizards, and began a new radiation with the newly lush environments.
Ichthyosaurs were, without question, the dominant predators (and tetrapods of any kind) of the Extended Triassic seas. Primitive snakelike toretocnemids were found in shallow habitats, but they were gradually being outcompeted by the ichthyosaurids. The largest predators of the time, however, remained the "primitive" shastosaur ichthyosaurs. These would prove to be the most successful of the group in the long term.
Turtles also began their diversification during this time, though the evolution of turtles from the Triassic through present more closely mirrors that of the home timeline than that of almost any other tetrapod group.
Nothosaurs and placodonts continued to diversify into the Extended Triassic. Some nothosaurs evolved into the plesiosaur sauropterygians during this time, while others remained coastal forms similar to present-day seals in habit. Most placodonts remained shellfish browsers, but at least two genera (independently) became algal grazers.
Thalattosaurs gradually became less and less diverse through the Extended Triassic. Only one genus is known from the very latest Triassic; the others were likely outcompeted by pseudosuchians, sauropterygians, and ichthyosaurs.
The sphenodontids filled most of the lizard-like roles of the time, with a large amount of diversity in small, carnivorous forms. There were even a few semi-aquatic varieties, but these were by no means common. The true squamates (our lizards and snakes) also evolved at this time, but the diversity was fairly low. Presumed arboreal forms are most common from this time, with a few small terrestrial carnivores.
The champosaurs -- the group that never seems to die -- also persisted as semi-aquatic hunters in swampy environments.
The phytosaurs are the most prominent basal archosauromorphs from the Extended Triassic. As in the earlier Triassic, they dominated semi-aquatic (mostly freshwater) environments as the top predators of their range. Some forms grew to enormous sizes, while others were no more than a meter long. All seemed to remain carnivorous or piscivorous through the end of the period.
Proterochampsids continued to live as semi-aquatic predators, though their numbers slowly dwindled for the remainder of the Mesozoic. The trilophosaurs, despite diversity in size and feeding strategies, were represented by only one genus in North America by the end of the Extended Triassic.
The pterosaurs (especially the eudimorphodontids which went extinct in our Triassic-Jurassic Mass Extinction) continued to diversify into the Extended Triassic. In particular small, forest-dwelling eudimorphodontids underwent a prominent radiation in the tropics.
The line between "dinosaur" and the rest is not as clear in the Extended Triassic. Two groups of dinosauromorphs -- lagerpetids and silosaurids -- survived into this time as small carnivores similar to the early theropod dinosaurs. Ornithischian dinosaurs seem never to have diversified, with heterodontosaurs being the sole representatives into the Extended Triassic. The primary groups of dinosaurs in this time were the sauropodomorphs -- the largest herbivores at the end of the period -- and the theropods -- which generally remained small carnivores though a few genera attained respectable sizes.
The pseudosuchia were the most diverse archosaurs of the Extended Triassic. Large aetosaurs lived as grazers and browsers in a variety of habitats and served as the largest herbivores on Earth aside from the sauropodomorphs. Through the end of the Triassic, they remained heavily armored, and a few used their adaptations for rooting to produce unusual forms.
The ornithosuchids were the dominant midsize carnivores of the time. They were all highly predatory facultative bipeds (to varying degrees). With the split between Laurasia and Gondwana, a split in the ornithosuchids followed with the Gondwanan genera becoming larger to rival the rauisuchids.
Poposauroids are about as close to the dinosaurs as the pseudosuchians ever got. In particular, the shuvosaurids attained a form very convergent to the ornithomimosaurs. These were the omnivorous poposauroids, and they continued to diversify for the rest of the period. Other poposaurs inhabited a variety of niches from large herbivores and carnivores to semi-aquatic forms. By the end of the Triassic, all forms had lost osteoderms, and an unusual lineage of sailbacked arizonasaurs had risen to prominence in Laurasia.
The rauisuchidae were the largest carnivores the Extended Triassic ever saw. They were, without exception, medium to very large carnivores. The rauisuchidae were the terrestrial super-predators of the day. They were generally quadropedal (though some genera were capable of limited bipedalism), and they had large, curved teeth similar to the carnosaurs of our Mesozoic.
The crocodylomorphs of the Extended Triassic would appear truly alien to us today. While our popular conception of crocodylomorphs is the slow, semi-aquatic reptiles we see today, these were quite different. The dominant crocodylomorphs of this time were small, graceful, galloping forms. They were small predators capable of running down relatively swift prey. Some semi-aquatic forms were present towards the end of the period, but the bulk of the diversity was terrestrial.
Jurassic (182 to 145 million years ago)Edit
The climate of the Jurassic became gradually wetter and colder until the end of the period. This was spurred on by the continued breakup of Pangaea, with shallow seas spreading over much of the low-lying lands. The Tethys Ocean fully separated the continents of Laurasia and Gondwana, and shallow seas separated Laurasia into a series of several landmasses. Inland seas were common, with the Sundance Sea covering large tracts of Western North America and a large sea dividing Europe into a series of islands. Deserts shrank, jungles spread, and seasonal climates dominated the temperate areas. Towards the end of the period, some alpine glaciers were even able to form in the northern latitudes.
Everwet environments continued to be dominated by the pleuromeiales, with their spore-based reproduction, and the understory saw the continued diversification of the sphenophyllales. In the seasonal environments, however, some pleuromeiales -- the neomeiales -- adapted to the changing conditions with an entirely independent development of the seed. Spores remained the method of genetic transfer, but the gametophyte stage of the plant developed a hard exterior that could survive harsh environmental conditions. Temperate forests were made up of conifers and ginkgoes, and semi-arid regions retained their primarily cordaitales composition. Gnetophyta persisted in arid regions, with flower-like reproductive structures being seen in some genera.
The Mesozoic Marine Revolution continued into the Jurassic, with the increased predation pressure leading the way. Among these new predators were the neogastropods. These predatory "snails" developed complex shell structures complete with siphons to dispose of waste and spikes to ward off other predators.
Strophomenid brachiopod beds continued to spread, especially in shallow environments. A few forms seemed to have evolved true colonialism during this time. Spiriferids became the dominant solitary suspension-feeders, with their intricate lophophore supports. They even gradually outcompeted the crinoids and bryozoans in the mounds of the deeper waters. Rhynchonelid brachiopods began to show intricate shell designs, with thickened shells and large spikes. These adaptations allowed them to continue to live in their reclining habit, despite the increased pressure of predators. In addition, some of these forms attained sizes upwards of a meter in diameter.
The bivalves underwent their own radiation, though it was almost entirely below the sediment. Clams evolved progressively deeper burrows and methods to either extend their siphons (to remain permanently deep below the sediment) or burrow quickly (to evade predators when sensed nearby). By species number, they began to overtake the brachiopods, though the brachiopods remained the dominant forms. Scallops diversified as well, though their competition with the rhynchonelid brachiopods meant that they did not become as diverse as in our timeline.
Ammonids underwent a radiation similar to the teleost fishes in our timeline during the Jurassic. They filled a variety of niches, mostly pelagic. Ceriatitid and ammonite ammonids coexisted in roughly even proportions during this time. Nautilids clung to life in the form of a few deep-water genera, as they would for the rest of the Mesozoic, while the pseudorthocerids continued to diversify into predatory forms in open environments. Coleoids dominated reef environments, where their lack of bulky external shells allowed them to safely navigate small crevices.
During the Jurassic, the chondrostei essentially split into two dominant groups: the filter- and suction-feeding acipenseriformes and the predatory polypteriformes. The Jurassic, though, was dominated by the acipenseriformes. Some genera grew to large sizes to feed on the plankton in the seasonal seas, being the "whales":of the Mesozoic. Others became the freshwater bottom-feeders of Laurasia, similar to our sturgeons. The holostei (our gar and bowfin) became efficient midsize freshwater predators, especially in the Laurasian rivers and lakes.
While Laurasian fresh waters were dominated by the bony fishes, Gondwana became increasingly populated by chondrichthyes. Freshwater chimaerae diversified as bottom-feeders, while the xenacanth sharks filled a variety of predatory niches in the southern continents. The sharks of the oceans remained dominated by the hybodonts and ctenacanths, with the hybodonts living primarily like the "typical" sharks (great whites, tiger sharks, etc.) and the ctenacanths assuming niches similar to our dogfish and cat sharks.
Parasitic conodonts continued to diversify as their scavenging relatives faced competition from the jawed fishes. Predatory genera continue to be reported, but these forms were by no means common.
During the gradual cooling during the Jurassic, cold-intolerant reptiles were pushed out of the colder waterways at the high latitudes. This allowed the limnarchia to fill crocodile-like roles in the extreme south of Gondwana. At the southern pole, these were the dominant ambush predators of shallow waterways.
The cynodonts' split into the cynognathia and the probainognathia became very apparent in the Jurassic. The probainognathia (specifically the mammals) were present on both continents, while the cynognathia seem to have only been represented in Gondwana. Thus, the larger cynodonts in Laurasia were of probainognathian origin. The tritheledontidae functioned as small carnivores, while their relatives, the tritylodontidae, were specialized herbivores. The mammals remained small, shrew-like forms but were moderately successful in these roles, especially at the high latitudes.
The dicynodonts' diversity continued to be depleted throughout the Jurassic. Only a few genera outside the fluminitheria (large semi-aquatic herbivores in freshwater environments globally) survived to the end of the period, and these died out at the boundary.
Procolophonoids retained many of the small generalist roles in the low latitudes, where there was little pressure from endothermic mammals and dinosaurs. Their ability to tackle varied diets suited them well, and this led to a good deal of success in their environments. In Laurasia, they were also the dominant lizard-like small insectivores.
Toretocnemids finally became extinct at the Triassic-Jurassic boundary, the ichthyosaurids moving into most of their niches. The Jurassic was the time of peak diversity for the ichthyosaurids, with their familiar dolphin-like body shape making them swift midsize predators. The shastasaurs grew to be the largest oceanic predators in Earth's history towards the end of the Jurassic, with upper size estimates of up to 25 m (82 ft). These were likely very slow creatures, but their chondrostean prey was almost certainly even slower.
The grazing placodonts gradually increased in diversity over the course of the Jurassic, while the shellfish-eating placodonts' diversity was gradually depleted due to increased defenses of their prey. They were still doing quite well by the end of the period, however, and many were able to adapt to sift sand to feed on bivalves or else expand their jaw musculature and tooth ornamentation to feed on the reclining brachiopods.
Nothosaurs continued to dominate the water's edge across the world. Some smaller, freshwater forms developed in larger waterways, but the bulk of their diversity was in the sea coasts. Of course, their relatives the plesiosaurs diversified as well, but these fully aquatic forms were largely outcompeted by the ichthyosaurs and placodonts in these roles.
Sphenodontids retained many of the small carnivore roles traditionally associated with lizards, especially in Gondwana. True lizards diversified on both continents but were mainly restricted to marginal niches due to competition with procolophonoids and sphenodontids. However, they became the most common arboreal animals by the end of the period.
The phytosaurs remained the dominant "crocodile" type predators in waterways across the low latitudes, especially in Laurasia. The two main groups adopting this lifestyle were the redondasaurs (most common in North America at this time) and the nicrosaurs (most common in Eurasia), which were characterized by unusual bony structures on their snouts. On isolated islands in Europe, some more terrestrial forms evolved in the form of the islasaurs, and a lineage of marine phytosaurs -- the tethyosaurs -- became quite successful in the Tethys Ocean.
The eudimorphodontidae proved extremely successful in forested environments of the Jurassic due to their small size and primitive (more terrestrially adapted) features. One clade -- the neodimorphodontidae -- evolved strengthened wing membranes and more developed hands and feet for a "perching" lifestyle in the forests.
The rhamphorhynchidae (specifically the rhamphorhynchines) were the other of the two most common groups of pterosaurs in the Jurassic. However, these pterosaurs (with typically long tails and comparatively thin membranes) were restricted to more open environments such as waterways and open lands.
Finally, the first caelidracones are known from the Jurassic. However, their true radiation took place primarily in the Cretaceous, with these forms remaining small.
Although the silesaurids became extinct at the end of the Triassic, the lagerpetids survived into the Jurassic. These animals were primarily found in Laurasia, though one genus is known from Gondwana. They were small carnivores specialized for speed, likely the fastest animals in their environments.
Some basal ornithischians were known from the Jurassic, almost all heterodontosaurids. They were generally small, fast herbivores, allowing them to fill a niche slightly different from that of the other dominant grazers and browsers.
In addition to these smaller herbivores, the dinosaurs produced the largest herbivores in history during the Jurassic. Like in our Earth, the sauropodomorphs (including both the sauropods and the prosauropods) grew to very large sizes on both continents. The smallest prosauropods were still meters in length, the smaller lineages having gone extinct at the end of the Triassic.
The theropods were generally small during the Jurassic. Coelophysoidea and dilophosauria were the most common representatives, but the coelophysoidea were most common by far. This group remained small, with a maximum size of around 2 meters (6.5 feet) in one Gondwanan genus. Dilophosaurs actually attained respectable sizes of up to 4-5 meters (13-16 feet) during this time.
Aetosaurs were, aside from the sauropodomorphs, the dominant herbivores of the Jurassic on both continents. The two primary groups were the typothoracisinae and the desmatosuchinae. While individual genera of each group had different specializations, thypothorascisines generally continued to evolve as rooters and browsers, and desmatosuchines grew to be larger grazers in open ground (because of their more extensive defenses).
With the extinction at the end of the Triassic, the ornithosuchians were split fairly well between the gondwanasuchia (larger, more heavily-built, and more quadrupedal ornithosuchians that became the largest predators of the continent) and the laurasiasuchia (smaller, more bipedal carnivores).
Shuvosaurs were very successful omnivores on both continents during the Jurassic, with their speed, agility, and diverse diet allowing them to exploit a variety of niches. The other major group of poposaurs -- the arizonasaurs -- diversified with some genera becoming the largest terrestrial predators in Laurasia. Their ancestral trait of having a "sail back" for thermoregulation meant that they were able to live in a wider range of temperatures than other suchians.
The rauisuchidae were still prominent predators well into the Jurassic, but they were not nearly as large as they were during the Triassic. They gradually declined over the course of the period, especially due to their intolerance of colder environments.
Crocodylomorphs of the Jurassic were built for speed. They grew longer legs and extensions of two carpals allowed them to extend their stride length. In particular, the velocisuchia of Gondwana were among the fastest predators on the planet at the time, detaching the pectoral girdle to a certain extent to allow for a greater stride length.
Cretaceous (145 to 66 million years ago)Edit
The Cretaceous period began with the peak of the cool, wet trend that had dominated the Jurassic. However, the Cretaceous is marked by continual warming and drying associated with the greenhouse emissions from volcanic activity. This was associated with the continued breakup of Pangaea; by the end of the period, Africa and South America had finally split, Australia and Antarctica had separated from them, and the Atlantic Ocean marked a definitive separation between Europe and North America. Around 66 million years ago, the combined Deccan Traps eruptions and a meteor impact in the Yucatan marked the end of the Mesozoic Era, with 65% of all animal life dying out.
The neomeiales flourished in the drying conditions of the Cretaceous. Co-evolution with spore transporters (not unlike the relationship between insects and angiosperms in our timeline) allowed them to spread and dominate seasonal environments everywhere. Basal pleuromeiales persisted in everwet environments, but these plants were gradually outcompeted by gymnosperm trees, especially ginkgoes and went extinct at the end of the period. "Understory" plants such as the sphenophyllales (which also died out at the boundary) and club mosses in the tropics and cycads, club mosses, and ferns in temperate or seasonal climates continued to persist. Cordaitales and gnetophyta also did rather well in the drying conditions of the Cretaceous.
Strophomenid beds reached their peak in the Cretaceous, especially in the shallow inland seas of the Late Cretaceous. Truly colonial forms continued to be found, but these would die out during the extinction. Stalked spiriferids also reached their peak in the Cretaceous, only to be entirely wiped out in the Cretaceous-Paleogene Mass Extinction due to their specialization. The reclining rhynchonelids survived the extinction, but only just. Only two genera are reported from both sides of the boundary.