- See also: Triassic Divergence
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 Pangea; 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 difinitive 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, espeically in the shallow inland seas of the Late Cretaceous. Truely 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.
The scallops persisted until the end of the Cretaceous but died out at the boundary due to their specialized nature. The clams, however, went through the Cretaceous and subsequent extinction with most major groups intact, even if the genus number was almost halved. Their infaunal habit gave them access to a niche not exploited by any other group and thus proved to be the key to their success.
Ammonite ammonids became the dominant group of ammonids during the Cretaceous by genus number. However, this group was made entirely extinct at the boundary, leaving the ceriatitids as the only ammonids to make it to the Cenozoic, if only just (only one genus is found on both sides of the boundary). The pseudorthocerids also died out at the boundary, but the nautilids somehow made it through the Cretaceous and the extinction. These remained deep-water forms, however, and were mostly untouched by the changing surface world. Coleoids almost died out at the boundary, as well, due to their dependence on reefs. The only groups to survive were the belemnites and the octopuses.
Acipenseriformes were represented by three major groups by the end of the Cretaceous: acipenseridae (freshwater bottom-feeders common in Laurasia), polyodontidae (smaller, freshwater to marine filter-feeders), and chondrosteidae (typically large marine filter-feeders). Of these, the chondrosteidae became entirely extinct, while polydontidae and acipenseridae became extinct in Gondwana and marine environments. Thus, the remaining forms -- the paddlefishes and sturgeons -- were present only in the freshwater environments of Laurasia after the extinction.
The holostei were relatively untouched by the extinction. The gar and bowfin in Laurasia were among the least impacted groups at the boundary, allowing them to bounce back relatively soon into the Paleogene.
Gondwana's unique freshwater chondrichthyan composition surprisingly came through the boundary more-or-less intact. Freshwater chimaerae remained the dominant bottom-feeders, and the smaller xenacanths survived (though, naturally, the largest genera did not). In the seas, the hybodont sharks declined through the Cretaceous, becoming entirely extinct at the boundary, while the ctenacanths survived with a handfull of genera.
The parasitic and predatory conodonts diversified through the Cretaceous but were hit fairly hard by the extinction. The parasitic conodonts were represented only by one genus on both sides of the boundary, and the predatory forms entirely died out. However, the scavenging species weathered the extinction better than any of their competetors.
The limnarchia persisted until the end of the Cretaceous in all the southern continents, but only in Australia-Antarctica did they manage to weather the extinction.
The impact of the Cretaceous-Paleogene Mass Extinction on the lissamphibia was about the same as in our timeline. Because of their poor fossil record, it is impossible to determine the full impact. However, all four major groups of the lissamphibia persist until the present day, suggesting that they at least weathered the Cenozoic better than on our Earth.
Any Laurasian representatives the cynognathia may have had during the Jurassic seem not to have persisted into the Cretaceous. However, in Gondwana they were enormously successful until the boundary. Some forms even achieved sizes of up to a meter in length, and herbivorous, omniviorous, and carnivorous genera were present. At the boundary, they became extinct in Africa but persisted in South America and Australia-Antarctica.
Probainognathia continued to diversify (especially in Laurasia) through the Cretaceous. However, the non-mammalian forms were too specialized to survive the extinction, meaning that they disappeared even in their areas of Laurasian dominance. The mammals never grew much larger than a shrew through the Cretaceous and became extinct in Australia-Antarctica, South America, and North America at the end of the period.
Fluminitherian dicynodonts were the only representatives of the group to survive until the end of the Cretaceous. These gradually became less diverse until the only continents on which they were well-represented were Eurasia and Africa, and they became entirely extinct at the Cretaceous-Paleogene boundary.
The generalist procolophonoids did quite well during the Cretaceous, filling most of the roles traditionally associated with mammals during the Mesozoic. However, for unknown reasons, they became almost entirely extinct at the boundary.
Unlike in our timeline, the ichthyosaurids did quite well during the Cretaceous, mostly filling the niches of our modern sharks during the Late Cretaceous (and gradually outcompeting the hybodonts). Thus they were able to survive the extinction, though their diveristy was much reduced. The shastasaurs that had dominated the Jurassic persisted until the end of the Mesozoic, but their history during the Cretaceous was that of gradual decline. Other groups filled their niches as the apex predators in the sea, but not before they produced a few truly spectacular forms.
Placodonts continued to diversify in both of their typical roles unitl the end of the Cretaceous, but none were able to survive the extinction. The semi-aquatic nothosaurs, however, retained dominance of the coastlines through the Cretaceous and beyond. The plesiosaurs persisted in a limited capacity unitl the boundary but not beyond.
Sphenodontids were quite successful in a variety of roles (mostly lizard-like) during the Cretaceous. However, the Laurasian sphenodontids became extinct at the end of the Cretacous, leaving their South American, African, and Australian-Antarctican relatives as the surviving members of this clade.
Lizards were hit very hard by the extinction due to their arboreal lifestyle. However, during the Cretaceous, a few terrestrial forms originated (independently) in North America and Africa. These were the groups that survived past the boundary.
The tethyosaurs underwent an impressive radiation during the Cretaceous, with some forms becoming huge and even replacing the shastosaurs as the apex predators of their environments. However, due to their specialization, they became extinct at the end of the Cretaceous.
The islasaurs were efficient predators in the European islands during the Cretaceous. Some forms even colonized areas as far away as North America and Africa. However, only the island-dwelling European forms made the Cretaceous-Paleogene transition. Both the redondasaurs (in North America) and the nicrosaurs (in Euraisa) survived the extinction as crocodile-like forms.
The pterosaurs continued to diversify into the Cretaceous, with both the rhamphorhynchidae and the caelidracones producing exceptionally large forms in the Late Cretaceous. The caelidracones inhabited mostly open terrestrial habitats, while the rhamphorhynchidae diversified in marine roles. However, both groups became extinct in the Cretaceous extinction, leaving the neodimorphodontidae as the only representatives of the pterosauria (and the only flying vertebrates) at the start of the Cenozoic.
The lagerpetids expanded on their specialized niche during the Cretaceous, living as small, fast carnivores in the Laurasian continents. They were very successful in this capacity for the remainder of the Mesozoic. However, this niche proved to be too specialized, and they became extinct at the end of the Cretaceous.
The last Laurasian sauropods became extinct in the Early Cretaceous, and their Gondwanan relatives became extinct at the boundary between the Early and Late Cretaceous. This was due to competition from other groups: heterodontosaurs and aetosaurs in Laurasia and their relatives the prosauropods in Gondwana. One lineage of prosauropods re-evolved a sauropod-type lifestyle in Gondwana, rivaling their extinct relatives in size during the middle of the Late Cretaceous. However, the prosauropods became extinct in every continent except South America during the extinciton, and the lineage that did survive was small (about 1 meter) by comparison to its relatives.
Heterodontosaurs diversified across the world over the Cretaceous, but mainly in Laurasia. In the gap left by the sauropods, one lineage adopted a pseudo-hadrosaur method of life. This group attained respectable sizes (mainly in Eurasia), primarily because of its diverse tooth forms. However, this specialized lineage (along with all heterodontosaurs) became extinct at the end of the Cretaceous.
Dilophosaurs became extinct druing the Cretaceous along with the sauropods, leaving the coelophysoidea as the only theropods by the Late Cretaceous. However, one lineage -- the neosauria -- achieved great success in Laurasia due to a smaller, more generalistic body plan. This was the only lineage of theropods to survive into the Cenozoic.
Desmatosuchines persisted as large grazers -- primarily in Laurasia -- until the end of the Mesozoic, but they declined over the course of the Cretaceous. They became extinct at the end of the period.
The thypothorascisines maintained the "rooter" lifestyle in the majority of their lineages, but a few moved into large grazer niches as the desmatosuchines and sauropods declined over the course of the Cretaceous. Only a few "rooter-browser" aetosaurs survived the extinction in Eurasia.
The gondwanasuchia remained the apex predators of the southern continents until their total extinction at the end of the Cretaceous; some groups attained massive sizes of up to 13 meters (43 feet) druing the Late Cretacous.
The laurasiasuchia declined steadily over the course of the Cretaceous due to competition from both the arizonasaurs and the lagerpetids. One genus is reported from North America on both sides of the boundary, but it is considered a dead lineage as it totally disappears from the fossil record by 64.5 million years ago.
Shuvosaurs continued to diversify throughout the Cretaceous. One lineage -- called the pseudotheropoda -- even attained sizes and lifestyles similar to that of our tyrannosaurs in Laurasia. In Gondwana, the shuvosaurs adopted a lifestyle simlar to the neosauria of Laurasia, ensuring that this lineage survived the extinction.
Arizonasaurs achieved great success during the Cretaceous, though much of the diversity returned to a small-bodied plan (due to competition from the pseudotheropoda). The sail-back substitute for endothermy is considered a stepping stone that allowed the smaller neopoposaruia to survive the extinciton on the Laurasian continents as small carnivores.
The rauisuchidae saw a minor increase in diversity during the Late Cretaceous due to a piscivorous lineage in Africa, Asia, and Europe. However, this group -- along with the other rauisuchidae -- were extinct before the end of the period.
The velocisuchia, though arguably the most successful carnivores of the Cretaceous, did not survive into the Paleogene in most of the world. In fact, the crocodylomorphs became extinct in every continent except for South America, where a small 0.5 to 1 meter lineage survived into the Cenozoic.