Surprise: A Cretaceous Ice Age

Scientists know the Cretaceous Age (140 million years BP-66 million years BP) as a greenhouse world when glacial ice existed nowhere on the planet for millions of years at a time and water inundated 85% of earth’s surface.  But several studies suggest that about 91.2 million years ago, an Ice Age occurred, lasting for approximately 200,000 years.  Scientists noticed the oxygen isotope ratios found in fossil foraminifera dating to this time period indicated the presence of cooler ocean temperatures and the presence of polar ice.  They believe this ice cap grew in the southern hemisphere on the continents of Australia and Antartica which made up one combined continent 91.2 million years ago.  The polar ice cap was only about half the size of the present extent of ice covering the south pole today.  It’s unlikely any ice covered the north pole then.  There’s also evidence of a dramatic lowering of sea level during this 200,000 year episode.  Low sea levels may explain how crocodylian species common to both Western North America and Appalachia (the eastern part of North America) colonized both sides of the Western Interior Seaway.  The WIS was too wide during most of the Cretaceous for even saltwater species of crocodylians to cross, but if a chain of islands existed due to low sea levels, an expansion of their range was possible.

Map of North America during the Cretaceous.

The robust crocodylian (Deinosuchus rugosus) left fossils on both sides of the Western Interior Seaway.  Although no Deinosuchus fossils have been found that date to before 83.5 million years BP, I think they must have colonized Western North America during the Cretaceous Ice Age, 91.2 million years BP , when lowered sea levels allowed a chain of islands to emerge in the WIS facilitating range expansion.  As the illustration depicts, Deinosuchus ate tyrannosaurs.

The Cretaceous Polar Cap grew on the continents of Antartica and Australia which combined to form a single continent then.

Ironically, scientists believe intense global warming caused the formation of the Cretaceous south pole glacier.  The heat increased evaporation, therefore increasing moisture in the atmosphere, resulting in heavy snowfall at high latitudes and altitudes as well as in Antartica.  At high latitudes in Asia average annual temperature during the Cretaceous Ice Age was just 50 degrees F, compared to an average of 64 degrees F during the late Cretaceous.

One of the dominant predators of the Cretaceous Ice Age was Yutyrannus huali; a 27 foot long, 3000 pound feathered tyrannosaur.  It is the largest feathered dinosaur known to science.  The presence of feathers suggests it lived in a cool climate.  Large animals such as elephants, rhinos, and humans evolved toward a loss of fur in tropical climates, while closely related species such as mammoths and wooly rhinos did have fur in cooler climates.  Dinosaurs evolved feathers as a thermoregulatory adapatation, and the feathers originally served the same purpose as fur.  But feathers or fur would contribute to overheating in large animals living in tropical climates.  This means later tyrannosaurs living in the warm climate of the late Cretaceous probably didn’t have feathers, and there’s no evidence they did.

Yutyrannus, an early feathered tyrannosaur adapted for cold-temperate conditions.  During the early Cretaceous tyrannosaurs competed with allosaurs and megalosauroids.  Despite the similarity in appearance, tyrannosaurs were not closely related to allosaurs.

Yutyrannus may have competed with allosaurs and megalosauroids which were contemporeneous but didn’t necessarily occur in the same geographical area.  Allosaurs looked like tyrannosaurs, but were not closely related.  The former had 3 fingers while the latter had 2.  Tyrannosaurs generally were more massively built.  Tyrannosaurs didn’t become extinct until the K-T impact; Allosaurs became extinct well before then.  It’s likely tyrannosaurs eventually outcompeted allosaurs.  The evolution of feathers on yutyrannus may have allowed them to dominate cooler regions and evolve further into a larger more powerful predator, that later, when the Cretaceous Ice Age ended, colonized the warmer regions and drove allosaurs into extinction.

Late Cretaceous fossil outcroppings do occur in southeastern North America.  The following links are my past articles on Cretaceous dinosaur and non-dinosaur fossils found in Georgia and Alabama

https://markgelbart.wordpress.com/2011/02/04/cretaceous-dinosaur-fossils-discovered-in-georgia-and-alabama/

https://markgelbart.wordpress.com/2011/02/11/cretaceous-non-dinosaur-fossils-found-in-georgia-and-alabama/

References:

Macarthur, J.M. et. al.

“Paleotemperatures, Polar Ice Volume, and Isotope Stratigraphy: The early Cretaceous”

Paleogeography, Paleoclimatology, and Paleoecology  248 2007 pp. 391-430

Xu, Xing, et. al.

“A Gigantic Feathered Dinosaur from the Lower Cretaceous of China”

doi 10.1028/nature/0901

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