- See also: Triassic Divergence
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 Pangea, 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 predation pressure. 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 dominatnt 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 aparent 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 thier 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 druing 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 quadropedal 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.