Prehistoric Life 13

by Carl Strang


This sculpture of a hadrosaur from the temporary Brookfield Zoo exhibit represents the most abundant herbivore group of the Cretaceous Period in North America. The closest dinosaur fossil to Illinois, found just across the river in Missouri, belonged to a species that looked something like this.

Cretaceous Period (144-65 million years ago)

The Cretaceous Period was named for its abundant chalk deposits in parts of Europe, creta being Latin for chalk (1822). Its end (and that of the Mesozoic Era) was marked by a huge asteroid strike that caused much extinction, most notably of the dinosaurs.

Life on Earth. The primarily rudistic reefs, along with elements added by corals, were present through much of the Cretaceous, but became extinct in the latter part of the period (Science 312:857). Diatoms appeared. Ammonites and dinosaurs vanished when the Cretaceous ended. 

The flowering plants appeared in the Cretaceous Period.

A huge development was the appearance of flowering plants in the early Cretaceous (the earliest known fossil is 125 million years old, an aquatic weedy herb found in China; female parts enclose seeds and male parts have anthers, but there are no petals or sepals; Science 296: 899). The explosive diversification of flowering plants resulted in their dominating the Earth’s terrestrial flora by the end of the period. Both monocots and dicots appeared early, but by modern standards they were relatively primitive (rose-like, figs, magnolias, palms, oaks, willows, etc.). Some of these, magnolias for instance, were insect-pollinated. Until 2005, grasses were thought to have evolved later, in the Paleocene, but a study (Science 310:1177) of Cretaceous dinosaur coprolites (fossilized droppings) found bits of 5 different groups of grasses, along with dicots, conifers and palms. Studies of the genomes of modern plants indicate that there was a doubling of genes around the time these early flowering plants appeared. A volume of duplicate genes would have provided a vast amount of genetic material that would be free to mutate without causing as much harm to the plants, fostering evolutionary innovation and diversification. Another such doubling took place toward the end of the Cretaceous (Science 313:1556).

The first Lepidoptera also appeared in the Cretaceous.

As the flowering plants evolved, they provided new resources that allowed the diversification of insects as well. Lepidoptera appeared in the Cretaceous, and beetles also took advantage of the new varieties of flowering plants (however, the success of beetles was less from coevolution with plants than from beetle groups being less prone to extinction than other insects [Science 318:1913-1916]). The only remaining major groups of insects to appear in the Cenozoic Era were some bird and mammal parasites. The Cretaceous also gave rise to an explosion of new social insects (termites, bees and ants). It may be no coincidence that the earliest known orb-weaving spiders have been found at this same time (Science 312:1730), suggesting the complexity of food webs made possible by the flowering plants.

Tyrannosaurus rex is of course the emblematic dinosaur of the Cretaceous Period. This is what I call the in-the-gunsight view of Sue at the Field Museum (you also could call it the lawyer’s view if you saw Jurassic Park).

Carnosaurs trended toward larger sizes, including Gorgosaurus and Tyrannosaurus. Collagen from unusually well preserved Tyrannosaurus bone was found to be closer to that of chickens (58% similarity) than to other tested animals (T. rex would “taste like chicken?” Science 316:280). A later such study of a hadrosaur placed it closest to the Tyrannosaurus, then to birds (Science 324:626). Sauropods were relatively few in North America in the Cretaceous, but a group of them called the titanosaurs were a significant component of southern-hemisphere faunas. Ornithopods became especially diverse, including the ankylosaurs, hadrosaurs and ceratopsians (the last two, exclusively Cretaceous groups). Bone-headed dinosaurs appeared early in the Cretaceous, and spread over Europe, Asia and both parts of North America. Their diversity has been called into question by the possibility that their head structure may have changed significantly during development, so that different sized “species” may be the same species at different ages (Science 318:1236). Only a few stegosaurs survived into the early Cretaceous.

Ceratopsians, like the one represented in this Brookfield Zoo sculpture, were an exclusively Cretaceous dinosaur group.

Plesiosaurs continued, as did pterosaurs (the latter generally tail-less). A few ichthyosaurs still were around. Mosasaurs were limited to the Cretaceous. The modern crocodilian groups appeared, as did the first snakes. Birds became more distinct from other dinosaurs, diversified and became more abundant, with teeth retained by many species, though toothed birds vanished by the end of the Cretaceous. Understanding of the evolution of birds is rapidly improving thanks to ongoing study of Cretaceous fossils from China. An example is Gansus yumenensis, a bridging species from 112mya with advanced anatomical features previously not seen earlier than the Late Cretaceous. This tern-sized bird was a swimmer, but had feathers like those of flying modern birds (Science 312:1640).

Pterosaurs, like this fossil in a Field Museum exhibit, reached amazing diversity in the Cretaceous. Some stood nearly as tall as giraffes, and yet they could launch themselves from the ground and fly!

Mammals were few and small relative to the dominant dinosaurs, but took the important step of evolving the first marsupials and placentals. The earliest known fossil marsupial is a small arboreal animal, Sinodelphys, from early Cretaceous China, 125 mybp (million years before present; Science 302:1934). The most ancient known placental mammal, shrew-sized Eomaia, is from the same deposit in China (Science 296: 637). There was also an early platypus 112 mybp (Science 318:1237). The largest known Mesozoic mammals were triconodonts (a primitive group, neither marsupial nor placental), Repenomamus giganticus and R. robustus, the size of a coyote and of a large house cat, respectively. These Chinese predators included baby dinosaurs in their diet (Science 307:192). Placental mammals were diversifying in the Cretaceous, however. For example, the primates are estimated to have diverged from their nearest relatives, gliding mammals called colugos, in the Cretaceous (Science 318:792). The ungulates have been pushed back to the Late Cretaceous thanks to a condylarth fossil found in central India (Science 318:937).

Local landscape. The climate was warm and subtropical. The nearest Cretaceous deposits are in west central Illinois, and are a matrix of sand with some larger stones that never lithified, in an unglaciated zone. They are like deposits from the same time period in Minnesota, Iowa and Kansas, and indicate that the landscape was heavily eroded but of low relief. Throughout the Cretaceous our area was low land, and at about today’s latitude, though the climate was subtropical as the Earth generally was warmer then. Eastern North America was connected with the West in the early Cretaceous, but the seas advanced again to separate east from west in the middle and late Cretaceous. The connection between North America and Europe became narrower and more northern as the fissure opening the Atlantic spread north. Having an ocean just west of us probably insured enough moisture for forests. Global sea level reached the all-time high for the Phanerozoic eon during the Cretaceous, with subsequent drops resulting from the growth of ice at the poles (Science 310: 1293).

Local life. Both monocots and dicots are known in eastern North America by the mid-Cretaceous. Illinois’ limited Cretaceous deposits contain plant fossils, but so far no dinosaurs, though the same deposits in Missouri and Tennessee have produced dinosaur bones. The birch and walnut families were characteristic plants of the late Cretaceous to early Oligocene, and magnolias and persimmons were well southeast and northeast of us (floras are better known for coastal areas than for our inland location). Representatives of the maple and basswood families were in North America as well, along with a diverse coniferous flora and palms. Angiosperms had become dominant at the end of the Cretaceous, though conifers remained important.

Members of the walnut family were important components of North American plant communities in the late Cretaceous.

The sea just west of us had various plesiosaurs, mosasaurs, the giant sea turtle Archelon, the large toothed diving bird Hesperornis and the more generalized Icthyornis. Pteranodon is known from both Kansas and Europe, but may have been a coastal genus. Many of the larger species of pterosaurs ate fish, some may have been scavengers, and smaller ones may have been more widely distributed consumers of insects.

That sea separated us from known fossils of the largest carnosaurs, including Tyrannosaurus. However, the 20-foot-plus Dryptosaurus, a late Cretaceous member of the tyrannosaurid family, occurred in both New Jersey-Maryland and Colorado-Montana. Ricardoestesia gilmorei, another late Cretaceous theropod, was distributed from Alberta to Texas to Hungary. Deinonychus, a close relative of Velociraptor, has been found in Maryland. The ornithopod Iguanodon is known from both the eastern U.S. and Europe, and hadrosaurs were widespread in Europe and both eastern and western North America. The hadrosaur Edmontosaurus occurred in both Alberta and New Jersey. The fossil dinosaur found closest to Illinois is an unidentified hadrosaurid found just across the Mississippi River in Missouri. The early Cretaceous ornithopod Hypsilophodon occurred from England to Portugal to South Dakota, and its possible descendant Camptosaurus has been found in both western North America and western Europe. The late Cretaceous sauropod Hypsibema has been found in both North Carolina and Missouri. An early Cretaceous ankylosaur, Priconodon, was found in Maryland.


Get every new post delivered to your Inbox.

Join 66 other followers

%d bloggers like this: