Earth's continental configuration during the extended Triassic
- See also: Triassic Divergence
The climate of the Extended Triassic became increasingly wet as Pangea 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.
Viridiplantae[]
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.
Invertebrates[]
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.
"Fishes"[]
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).
"Amphibia"[]
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.
Synapsida[]
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.
Basal "Reptiles"[]
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.
Lepidosauromorpha[]
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.
Archosauromorpha[]
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.
Avemetatarsalia[]
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.
Pseudosuchia[]
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.