First, quantitative studies show that dinosaur diversity reached its highest point in the Late Cretaceous. According to data compiled in the Paleobiology Database, over 70 percent of all known dinosaur genera lived during the Cretaceous, with the greatest number concentrated in the Campanian and Maastrichtian stages. The Jurassic, in contrast, represents a smaller fraction of total dinosaur diversity, with fewer documented clades and less ecological specialization.

Second, the Cretaceous period witnessed the most extensive radiation of major dinosaur groups. Ceratopsians, ankylosaurs, hadrosaurids, and derived coelurosaurian theropods like tyrannosaurids and dromaeosaurids appear and diversify only in the Cretaceous. No true ceratopsids or hadrosaurids are known from the Jurassic. These groups developed complex anatomical adaptations such as advanced chewing mechanisms, kinetic skulls, and elaborate cranial ornamentation that have no precedent in the earlier Mesozoic.

Third, the Cretaceous saw the full geographic colonization of all major landmasses. By this time, continental drift had created substantial geographic isolation, driving divergent evolution and regional endemism. For example, abelisaurids thrived in South America, India, and Madagascar, while tyrannosaurids evolved exclusively in Laurasia. This pattern contrasts with the Jurassic, when the supercontinent Pangaea limited geographic separation and therefore reduced evolutionary differentiation.

The Cretaceous fossil record contains the best examples of complex food webs involving dinosaurs, mammals, birds, reptiles, and angiosperms. Hadrosaurids like Edmontosaurus and Corythosaurus exhibit highly specialized dental batteries with hundreds of closely packed teeth capable of processing fibrous plants. Ceratopsids like Triceratops show evidence of herd behavior and niche partitioning. The co-occurrence of multiple apex predators in Late Cretaceous formations, such as Tyrannosaurus rex, Dakotaraptor, and Acheroraptor, indicates trophic structures far more intricate than those observed in Jurassic ecosystems.

The rise of angiosperms during the Cretaceous also reshaped terrestrial ecology. Molecular clock estimates and palynological records suggest that flowering plants originated in the Early Cretaceous and underwent explosive diversification by the Late Cretaceous. Dinosaur herbivores adapted rapidly to this botanical revolution. The evolution of shearing jaw mechanics in hadrosaurs and complex gut contents in nodosaurids suggest an advanced digestive ecology closely tied to angiosperm consumption. These plant-dinosaur interactions were largely absent during the Jurassic, which was dominated by gymnosperms and simpler feeding strategies.

Cretaceous theropods also displayed the greatest morphological experimentation. Derived maniraptorans such as Velociraptor, Microraptor, and Troodon show evidence of feathers, semi-lunate carpal bones, and advanced neurological structures. Fossils from the Yixian Formation and other Lagerstätten in China reveal a continuum from non-avian theropods to true birds. This includes taxa such as Archaeopteryx, Confuciusornis, and Ichthyornis, which document critical stages in the evolution of flight, endothermy, and avian respiration. The evolution of powered flight and the origin of birds occurred exclusively in the Cretaceous, not the Jurassic.

The culmination of the Cretaceous in the Cretaceous Paleogene mass extinction further highlights its biological and geological significance. The Chicxulub impact event, confirmed through iridium anomalies and shocked quartz at the Cretaceous Paleogene boundary, marks one of the most dramatic biotic turnovers in Earth history. This event terminated non-avian dinosaurs, ammonites, and many marine reptiles, and created ecological space for mammalian and avian expansion in the Cenozoic. The magnitude of this extinction underscores how dominant and widespread Cretaceous dinosaurs had become.

In contrast, the Jurassic lacks this degree of diversity, ecological innovation, and evolutionary significance. Most Jurassic dinosaurs were dominated by sauropods, basal theropods, and stegosaurs. While impressive in size, they show relatively conservative anatomical designs. No true hadrosaurs, ceratopsians, or advanced maniraptorans appear in Jurassic strata. Moreover, Jurassic floras were less dynamic, dominated by cycads, conifers, and ferns, which supported less specialized herbivore diets.

I look forward to everyone's input!

The Cretaceous loses points for presiding over the fall of the dinosaurs, which is not a trivial shortcoming for a golden age. Too bad this was broken down into three periods, like hockey, instead of going to overtime.

3 hours ago, swansont said:

The Cretaceous loses points for presiding over the fall of the dinosaurs, which is not a trivial shortcoming for a golden age. Too bad this was broken down into three periods, like hockey, instead of going to overtime.

it’s not like the extinction event occurred because of internal factors on earth, it occurred some giant space rock decide to hurl itself at Earth and end glorious amount of biodiversity in the late Cretaceous 💔

Also seems to have been the golden age of teleosts, ammonites, belemnites and, not least, coccoliths, which are responsible for the chalk that gives this period its name. And then there was the takeover of the angiosperms. So perhaps a golden age of life in general.

Edited July 5Jul 5 by exchemist

2 minutes ago, exchemist said:

Also seems to have been the golden age of teleosts, ammonites, belemnites and, not least, coccoliths. And then there was the takeover of the angiosperms. So perhaps a golden age of life in general.

Agreed!

On 7/5/2025 at 1:32 AM, Sohan Lalwani said:

First, quantitative studies show that dinosaur diversity reached its highest point in the Late Cretaceous. According to data compiled in the Paleobiology Database, over 70 percent of all known dinosaur genera lived during the Cretaceous, with the greatest number concentrated in the Campanian and Maastrichtian stages. The Jurassic, in contrast, represents a smaller fraction of total dinosaur diversity, with fewer documented clades and less ecological specialization.

Here is a plug for cyanobacteria and stromatolites as they have spanned the entire geological epochs from the beginning of life to the present day.

Source Henry (Ed nature) A (very) Short History of Life on Earth

Edited July 7Jul 7 by studiot

11 hours ago, studiot said:

Here is a plug for cyanobacteria and stromatolites as they have spanned the entire geological epochs from the beginning of life to the present day.

Source Henry (Ed nature) A (very) Short History of Life on Earth

Thanks for the excerpt, it was quite nice to read +1

My apologies that poor Author is Henry Gee, not just Henry.

All good, don't sweat it