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Around the boundary of the Jurassic and Cretaceous periods, about 145 mya, was a time during the age of dinosaurs when the world actually started to cool down due to a minor glaciation. However, what if there wasn't any volcanic activity after the glaciation? How much more severe would the world get? What groups would go extinct if this happened? What groups would evolve? Would the non-avian dinosaurs survive the K-T mass extinction in this alternate timeline?
Early Cretaceous (145-120 MYA)Edit
The split for this alternate timeline happens at about 140 MYA, after the Berriasian age ended and the Valanginian age began. However, in this timeline, there are none of these volcanic eruptions. This causes the world's temperature to quickly drop and causes dinosaur groups such as the stegosaurs and most other armored dinosaurs to go extinct as they couldn't adapt very well to the cooling climate. Sauropods also had a hard time with the cooling climate, and most groups such as the diplodocids and brachiosaurs went completely extinct, with only the titanosaurs surviving by the skin of their teeth. The theropods lost some groups as well: the colder climates caused the extinction of the spinosaurs, the majority of abelisaurs, and most carnosaurs; while the coelurosaurs easily survived, with most species having evolved primitive insulating feathers. The dinosaurs weren't the only group hit hard; the rhamphorhynchs and temnospondyls, already on the decline, also went extinct.
Some other groups fared better. The mammals, crocodilians, and birds easily survived; the mammals with endothermy and fur; the birds with endothermy and feathers; and the crocodiles with a conservative metabolism and low bodily needs. The ornithopods suffered some losses, but generally did well as flowers and fruits evolved around this time.
In the oceans, life was also hit hard. Ichthyosaurs and Thalattosuchia went extinct by this time. Long-necked plesiosaurs and pliosaurs went on the decline due to the cold oceans while the short-necked plesiosaurs fared better. However, on the land the colder, drier climates caused some ferns to evolve into grass-like forms with much smaller leaves. Ankylosaurs, therizinosaurs and ceratopsians nearly went extinct because they were designed for eating leaves and not these grass-like ferns. Titanosaurs and ornithopods fared better with this new food source since the could evolve teeth designed to chew this tough plant matter.
Mid-Late Cretaceous (120-66 MYA)Edit
By the mid Cretaceous, about 90 MYA, life was starting to recover to these colder climates. The dominant predators in the north were the carnosaurs and dromaeosaurs since the tyrannosaurs were less plentiful due to competition from the larger carnosaurs. In the south, the carnosaurs and abelisaurs ruled. Like in our home timeline, the titanosaurs are the dominant herbivores in the south while ornithopods are dominant in the north. However, the titanosaurs in this alternate timeline are smaller due to the colder climates.
By the end of the Cretaceous, around 66 MYA, the dinosaurs were thriving. While in our timeline they had gone extinct by this time due to a meteor strike, the cooling trend of the Early Cretaceous in this alternate timeline causes the K-Pg mass extinction to be much less severe and the majority of non-avian dinosaurs survived because of that. However, ankylosaurs nearly went extinct due to the spread of fern prairies, and were down to just several species by the end of this period. Pliosaurs also went extinct by this time due to competition from short-necked plesiosaurs.
Paleogene (66-23 MYA)Edit
Since the K-Pg mass extinction was much less severe in this timeline, the majority of dinosaurs were able to survive. However, the cooling trend of the Cretaceous was put to a halt by the Paleocene-Eocene thermal maximum. The titanosaurs and other large dinosaurs started to shrink in size due to the expansion of forests. The therizinosaurs, ankylosaurs and ceratopsians started to recover during this time since there were more leaves to eat.
At the end of the Eocene however, the fern prairies once again started to spread, allowing some dinosaurs to grow large again. The therizinosaurs, ankylosaurs and ceratopsians started to once again decline, but they mostly stayed diverse in the tropical zones. The carnosaurs started to decline during the Oligocene due to the decline of their sauropod prey.
Neogene (23-2 MYA)Edit
Ceratopsians and therizinosaurs migrated from Asia to Africa and brought serious competition for the African titanosaurs and they nearly went extinct. The Isthmus of Panama also allowed tyrannosaurs, therizinosaurs and dromaeosaurs to migrate to South America during the Pliocene. The tyrannosaurs also drove the abelisaurs to extinction in India and South America. By this time, the dromaeosaurs had spread to every continent except Antarctica. One group of dromaeosaurs in Africa grew to large sizes and became the tyrant raptors, they also outcompeted the carnosaurs into extinction in that continent. The tyrant raptors were part a new subgroup of dromaeosaurs called Gigaraptors. They get their name because of the large claws on their front hands.
Quaternary (2 MYA-Present Day)Edit
When the Pleistocene arrived, the ceratopsians and therizinosaurs went extinct in most of the northern hemisphere due to the decline of forests. The dromaeosaurs continued to diversify, the smaller ornithomimids and mikrosaurs also became more diverse during this time. Ankylosaurs also went extinct everywhere except on a few islands where competition is scarce.
In the end, the age of mammals would never come in this alternate timeline. Most mammal groups from our home timeline, including humans, would never evolve. Eutherian mammals would get no larger than cougars while metatherians and monotremes would be much more diverse than in our home timeline. Most ceratopsians would also stay small since the niches were already held by larger dinosaurs. Almost all modern bird groups failed to evolve since pterosaurs were still ruling the skies. However, birds would still be very diverse. Mosasaurs, oviraptorosaurs, pachycephalosaurs and grass never evolved. The lack of volcanic activity after the Tithonian-Berriasian glaciation in this alternate timeline completely changed the course of evolution.
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Dinosaurs of the Ice Age
|Sauropsida||Archosauria||Theropoda||Abelisauroidea • Aves • Dromaeosauridae • Gigaraptornae (Dinosaurs of the Ice Age) • Ornithomimosauria • Therizinosauria • Tyrannosauroidea|
|Ornithischia||Ankylosauria • Ceratopsia • Ornithopoda|
|Lepidosauromorpha||Plesiosauria • Squamata|
|Mammalia||Eutheria • Metatheria • Monotremata|