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Terrestrial life

The apex predators were archosaurian reptiles, especially dinosaurs, which were at their most diverse stage. Pterosaurs were common in the early and middle Cretaceous, but as the Cretaceous proceeded they declined for poorly understood reasons (once thought to be due to competition with early birds, but now it is understood avian adaptive radiation is not consistent with pterosaur decline), and by the end of the period only azhdarchids, which included the largest flying animals to have ever existed, as well nyctosaurids are known.

After the Late Jurassic, stegosaurs substantially declined, with only a handful of records being known from the Early Cretaceous, and were presumably extinct by the Late Cretaceous. Several groups of theropod dinosaurs would radiate across most of the world during the Early Cretaceous, including the carcharodontosaurs and spinosaurs, but both groups have no records beyond the early-mid Late Cretaceous. As the Cretaceous proceeded, the dinosaur faunas became more regional, with distinctly different faunas in the northern and southern hemispheres. In North America, sauropod dinosaurs, previously abundant, would disappear completely for most of the Late Cretaceous, referred to as the "sauropod hiatus" and would substantially decline in Asia. In contrast, in the southern hemisphere sauropod dinosaurs continued to remain abundant. In the Northern Hemisphere groups such iguanodontian dinosaurs, as well as ornithomimosaurs, oviraptorosaurs, ceratopsians and pachycephalosaurs would become widespread, but were largely absent from the Southern Hemisphere. After the extinction of carcharodontosaurs in the early Late Cretaceous, abelisaurids would become the dominant predators in the southern hemisphere, including South America, Africa, Madagascar and India. In contrast, In North America and Asia, tyrannosaurs would become the dominant predators. During the Late Cretaceous, the Western Interior Seaway in North America would lead to distinct dinosaur faunas between the Laramidia and Appalachia, with Laramidia dominated by tyrannosaurids, derived hadrosaurids, ceratopsids and ankylosaurids, while Appalachia was dominated by basal tyrannosauroids, hadrosauroids and basal hadrosaurids and nodosaurs. The Late Cretaceous islands of Europe had a distinct depauperate fauna, containing both endemic elements (rhabdodontids, Struthiosaurus) as well as fauna of Gondwanan origin, like abelisaurs and laurasian elements, like hadrosauroids and hadrosaurids. There is no evidence that dinosaurs were declining prior to their extinction, with most supposed evidence likely being a result of preservational bias.

Rise of angiosperms

Archaefructus fossil from the Yixian Formation, China

Flowering plants (Angiosperms) make up around 90% of living plant species. Prior to the rise of flowering plants (angiosperms), during the Jurassic and the Early Cretaceous, the higher flora was dominated by gymnosperm groups, including cycads, conifers, ginkgophytes, gnetophytes and close relatives, as well as the extinct Bennettitales. Other groups of plants include pteridosperms or "seed ferns", a collective term to refer to disparate groups of fern like plants that produce seeds, including groups such as Corystospermaceae and Caytoniales. What plant group angiosperms originated from are uncertain, with competing hypothesis including the anthophyte hypothesis, assuming flowering plants to be closely related to gynetophytes and Bennettitales as well as the more obscure Erdtmanithecales. Benettitales, Erdtmanithecales, and gnetophytes are connected by shared morphological characters in their seed coats. Molecular phylogenetics has consistently contradicted the anthophyte hypothesis, finding all extant gymnosperms, including gnetophytes, to be monophyletic to the exclusion of flowering plants. Other hypotheses place the origin of flowering plants amongst "seed ferns", but conclusive evidence for an origin amongst any pteridosperm group is lacking. It has been several key diagnostic characters of angiosperms have poor fossilisation potential. The earliest widely accepted evidence of flowering plants are monosulcate (single grooved) pollen grains from the late Valanginian of Israel and Italy, initially at low abundance. Molecular clock estimates conflict with fossil estimates, suggesting the diversification of crown-group angiosperms during the Upper Triassic or Jurassic, but such estimates are difficult to reconcile with the heavily sampled pollen record and the distinctive tricolpate to tricolporoidate (triple grooved) pollen of eudicot angiosperms. Among the oldest records of Angiosperm macrofossils are Montsechia from the Barremian aged Las Hoyas beds of Spain and Archaefructus from the Barremian-Aptian boundary Yixian Formation in China. Tricolpate pollen distinctive of eudicots first appears in the Late Barremian, while the earliest remains of monocots are known from the Aptian. Nearly all extant flowering plant families appeared by the end of the Cretaceous. The rise of angiosperms would co-incide with the decline and extinction of several widespread vascular plant groups, including the arid adapted conifer family Cheirolepidiaceae, Bennettitales and all "seed fern" groups.

Sea life

By the end of the Cretaceous, previously widespread groups of marine reptiles such as thalattosuchian crocodyliformes, non-xenopsarian plesiosaurs (including pliosaurs) and ichthyosaurs would become extinct, with the Late Cretaceous seas being dominated by mosasaurs, elasmosaurid and polycotylid plesiosaurs, as well as dyrosaurid crocodyliformes.