Article by: Adam Manning
Edited by J. D. Dixon and Harry T. Jones
The Cretaceous Period was the last and longest period of the Mesozoic Era, spanning from 145 to 66 million years ago (Ma). The name is derived from the Latin word for ‘chalk’, which is a common rock type from this interval. During this time, the continents began to take on their modern appearance, with some noticeable exceptions. Africa and South America finally split apart, and the inland sea of North America started to close, but India was an island, drifting through the Pacific towards the rest of Asia. North and South America were yet to join. The previous period, the Jurassic, is arguably more famous, but the Cretaceous sees the emergence of notable characters like Tyrannosaurus rex and Triceratops horridus. It all ended in the most famous mass extinction ever, so what happened?
Dinosaurs continued to diversify, dominate, and thrive in the Cretaceous Period. Some of the biggest animals ever to walk on land roamed the Earth, like the colossal Argentinosaurus huinculensis, which weighed nearly a humongous 80 metric tons. Other famous herbivores evolved at this time, like Ankylosaurus, Triceratops, and Parasaurolophus. In fact, hadrosaurs like Parasaurolophus became the most abundant and diverse group of herbivorous dinosaurs, and the group was characterised by their upper jaw mobility and batteries of teeth that made it easier for them to digest vegetation.
Theropods also diversified into many unique groups. Dromeosaurs, like Microraptor and Velociraptor mongoliensis, stalked their prey; and predators such as abelisaurids and tyrannosaurs developed powerful bites to bring down animals. The biggest ever land predator, Spinosaurus, evolved in the Early Cretaceous of North Africa. At 17 metres long, it was a fish eater, adapted for semi-aquatic life. Some theropods even became herbivores. The bizarre therizinosaurs, like Erlikosaurus andrewsi, were a group of theropods that evolved long necks and blunt teeth adapted for eating plants. They kept the scrappy spirit of their meat-eating ancestors, however, they were armed with huge sickle-shaped claws on their forelimbs which they used to graze and for self-defence.
Giant reptiles weren’t limited to the land in the Cretaceous. The Pterodactyloids, a group of pterosaurs, began to dominate the air. Tending to be bigger than the earlier Rhamphorynchoids, the biggest creatures known to take to the air did so in the Cretaceous, like Hatzegopteryx and Quetzalcoatlus, with wingspans of more than 10 metres. Giant reptiles also swam in the sea. Mosasaurs became extremely successful in the Cretaceous, and became huge after the decline of the Ichthyosaurs, which was possibly due to ocean anoxia. The biggest mosasaur discovered is Mosasaurus hoffmanni, from the Late Cretacous, at a colossal 17 m long.
Plant life went through a huge revolution during the Cretaceous. Angiosperms, or flowering plants, first appeared in the Early Cretaceous, but by its end, they made up more than 70% of terrestrial plant species globally. Furthermore, we know from the coprolites of titanosaurs from India that modern grass had evolved before the Late Cretaceous when India became an island. However, whilst grasses were likely widespread during the Late Cretaceous, they weren’t abundant until the Cenozoic.
Mammals in the Mesozoic were traditionally thought to be small and rodent-like, but new evidence suggests otherwise. The largest discovered mammals to live in the Mesozoic are from the genus Repenomamus, from the Early Cretaceous of China. Reaching a metre long in the species R. giganticus, evidence suggests that it was carnivorous and fed on small vertebrates, including young dinosaurs. This discovery sheds light on the diversity of niches mammals filled during the Cretaceous, and they weren’t just hiding in the shadows of dinosaurs. Most importantly for us humans, the arboreal mammals of the genus Purgatorius first appeared in North America right before the end of the Cretaceous. Purgatorius is the oldest genus attributed to our order, Primates.
The Cretaceous ended in the K-PG mass extinction, the most famous mass extinction of all time. ~ 65.5 Ma, an asteroid hit what is now the Gulf of Mexico, creating a crater 180 to 200 km in diameter. The immediate impact caused huge earthquakes and tsunamis, and launched massive amounts of debris into the atmosphere that littered Earth all around the globe. Whilst devastating, these events didn’t cause the fall of the dinosaurs, but rather the climate change that came later. The asteroid impact released large quantities of water, dust, and climate-changing gasses into the atmosphere. This would have caused major damage to the global climate, causing global warming, cooling, and even blocking sunlight, shutting down photosynthesis. In the end, 75% of species became extinct, including all of the non-avian dinosaurs. But small animals like our mammal ancestors and the avian dinosaurs, the birds, managed to prevail and thrive into the next era, the Cenozoic.
 Tyrannosaurus rex by Jack Wood.
 Size comparison of different animals, both extinct and modern. Figure from Sander and Clauss (2008).
 Repenomamus robustus feeding on a Psittacosaurus hatchling. Artwork by Nobu Tamura.
Information References and Further Sources
 Buffetaut, E., Grigorescu, D., and Csiki, Z. (2002). ‘A new giant pterosaur with a robust skull from the latest Cretaceous of Romania’, Naturwissenschaften, 89, pp. 180-184. Accessed 31st December 2020. Click Here.
 Chart drafted by K. M. Cohen, D. A. T. Harper, P. L. Gibbard, and J.-X. Fan (c) International Commission on Stratigraphy, March 2020. To cite: Cohen, K. M., Finney, S. C., Gibbard, P. L. & Fan, J.-X. (2013; updated). The ICS International Chronostratigraphic Chart. Episodes 36: 199-204. Accessed 29th December 2020. Click Here.
 Clark, J. M., Perle, A., and Norell, M. A. (1994). ‘The Skull of Erlicosaurus andrewsi, a Late Cretaceus “Segnosaur” (Theropoda: Therizinosauridae) from Mongolia’, American Museum Novitates, 3115 (39), pp. 1-39. Accessed 29th December 2020.
 Grigoriev, D. V. (2014). ‘Giant Mosasaurus hoffmanni (Squamata, Mosasauridae) From The Late Cretaceous (Maastrichtian) of Penza, Russia’, Proceedings of the Zoological Institute RAS, 318 (2), рр. 148-167. Accessed 31st December 2020. Click Here.
 Hone, D., Choiniere, J., Sullivan, C., Xu, X., Pittman, M., and Tan, Q. (2010). ‘New evidence for a trophic relationship between the dinosaurs Velociraptor and Protoceratops’, Palaeogeography, Palaeoclimatology, Palaeoecology, 291 (3-4), pp. 488-492. Accessed 31st December 2020. Click Here.
 Hu, Y., Meng, J., Wang, Y., and Li, C. (2005). ‘Large Mesozoic mammals fed on young dinosaurs’, Nature, 433, pp. 149-152. Accessed 29th December 2020. Click Here.
 Ibrahim, N., Maganuco, S., Sasso, C. D., Fabbri, M., Auditore, M., Bindellini, G., Martill, D. M., Zouhri, S., Mattarelli, D. A., Unwin, D. M., Wiemann, J., Bonadonna, D., Amane, A., Jakubczak, J., Joger, U., Lauder, G. V., and Pierce, S. E. (2020). ‘Tail-propelled aquatic locomotion in a theropod dinosaur’, Nature, 581, pp. 67-70. Accessed 29th December 2020. Click Here.
 Prasad, V., Strömberg, C. A. E., Alimohammadian, H., and Sahni, A. (2005). ‘Dinosaur Coprolites and the Early Evolution of Grasses and Grazers’, Science, 310 (5751), pp. 1177-1180. Accessed 29th December 2020. Click Here.
 Sander, P. M., and Clauss, M. (2008). ‘Sauropod Gigantism’, Science, 322 (5899), pp. 200-201. Accessed 29th December 2020. Click Here.
 Sasso, C. D., Maganuco, S., Buffetaut, E., and Mendez, M. A. (2010). ‘New information on the skull of the enigmatic theropod Spinosaurus, with remarks on its size and affinities’, Journal of Vertebrate Paleontology, 25 (4), pp. 888-896. Accessed 29th December 2020. Click Here.
 Silcox, M. T. (2016). ‘Purgatorius’. The International Encyclopedia of Primatology. John Wiley & Sons, Inc. Accessed 29th December 2020. Click Here.
 Sun, G., Dilcher, D. L., Wang, H., and Chen, Z. (2011). ‘A eudicot from the Early Cretaceous of China’, Nature, 471 (7340), pp. 625-628. Accessed 29th December 2020. Click Here.
 Schulte, P., Alegret, L., Arenillas, I., Arz, J. A., Barton, P. J., Bown, P. R., Bralower, T. J., Christeson, G. L., Claeys, P., Cockell, C. S., Collins, G. S., Deutsch, A., Goldin, T. J., Goto, K., Grajales-Nishimura, J. M., Grieve, R. A. F., Gulick, S. P. S., Johnson, K. R., Kiessling, W., Koeberl, C., Kring, D. A., MacLeod, K. G., Matsui, T., Melosh, J., Montanari, A., Morgan, J. V., Neal, C. R., Nichols, D. J., Norris, R. D., Pierazzo, E., Ravizza, G., Rebolledo-Vieyra, M., Reimold, W. U., Robin, E., Salge, T., Speijer, R. P., Sweet, A. R., Urrutia-Fucugauchi, J., Vajda, V., Whalen, M. T., and Willumsen, P. S. (2010). ‘The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary’, Science, 327 (5970), pp. 1214-1218. Accessed 29th December 2020. Click Here.
 Scotese, C. R. (2004). ‘A Continental Drift Flipbook’, The Journal of Geology, 112 (6), pp. 729-741. Accessed 29th December 2020. Click Here.
 Lingham-Soliar, T. (1995). ‘Anatomy and functional morphology of the largest marine reptile known, Mosasaurus hoffmanni (Mosasauridae, Reptilia) from the Upper Cretaceous, Upper Maastrichtian of The Netherlands’, Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 347 (1320), pp. 155-180. Accessed 31st December 2020. Click Here.
 Wing, S. L., and Boucher, L. D. (1998). ‘Ecological Aspects of the Cretaceous Flowering Plant Radiation’, Annual Review of Earth and Planetary Sciences, 26, pp. 379-421. Accessed 29th December 2020. Click Here.
 Witton, M. P., and Habib, M. B. (2010). ‘On the Size and Flight Diversity of Giant Pterosaurs, the Use of Birds as Pterosaur Analogues and Comments on Pterosaur Flightlessness’, PLOS One, 5 (11). Accessed 31st December 2020. Click Here.
 You, H-L., Luo, Z-X., Shubin, N. H., Witmer, L. M., Tang, Z-L., and Tang, F. (2003). ‘The earliest-known duck-billed dinosaur from deposits of late Early Cretaceous age in northwest China and hadrosaur evolution’, Cretaceous Research, 24 (3), pp. 347-355. Accessed 29th December 2020. Click Here.