by Carl Strang
This year’s winter series is a review of the prehistoric life and geologic history of northeast Illinois. Each chapter will summarize current understanding, gleaned from the literature, of what was going on with life on Earth in a particular span of time, what we know about the local landscape, and what we can say about local life. I include some references, particularly to papers published in the journal Science which commonly is available at public libraries. Contact me if you need sources for other items. The Earth is so old that every imaginable environment was here at some point, from ocean depths to mountaintops, from equatorial tropics to tundra, and from wetlands to desert.
Jurassic Period (208-144 million years ago)
The Jurassic Period was named in 1795 for layers of rocks in the Jura Mountains between France and Switzerland.
Life on Earth. Reef-forming corals went extinct at the end of the Triassic Period. New ones evolved 8-10 million years later, but by the late Jurassic, bivalves called rudists had begun to displace corals as the primary reef formers, and retained that status through the rest of the Mesozoic Era (Science 312:857).
Gymnosperms including conifers and cycads were the dominant woody plants. No flowering plants appeared, yet. However, there is evidence that insect-mediated pollination by a suite of scorpionflies was taking place in non-angiosperm seed plants (Science 326: 840).
Ants were a major new insect group. A side-necked turtle is known from the late Jurassic of Europe. Phytosaurs went extinct in the early Jurassic, but the similar crocodilians diversified.
Dinosaurs became the dominant terrestrial animals, and ranged even into deserts. Prosauropods left the most abundant vertebrate fossils of the early Jurassic. Their presumed offshoot group the sauropods, the familiar enormous four-legged dinosaurs with long necks and long tails, appeared in the early Jurassic, and by the late Jurassic reached their peak diversity, having become the dominant herbivores. They had several advantages that allowed them to have such large body sizes and numerical abundance. These included a large gut capacity enabling a long residence time for contents, which made chewing or grinding of plant food unnecessary and allowed digestion of large masses of food. Without a heavy chewing apparatus the head could be relatively small and the neck long, allowing a higher reach (in fact, sauropod skulls were so fragile and light that they seldom are found; complete skulls are known for only 8 of 120 sauropod species). The efficient, avian style air exchange system complete with bone-lightening air sacs was an important large size enabler. Other advantages were a high metabolic rate, and the production of large numbers of small young rather than a few large ones (Science 322:200-201). Sauropod genera from one famous Jurassic fossil source, the Morrison formation, include Camarasaurus, Apatosaurus (“Brontosaurus;” O.C. Marsh, an overzealous paleontologist, found and named Apatosaurus, then 2 years later found a larger sauropod he named Brontosaurus. The latter fossil turned out to be yet another Apatosaurus, which remains the correct name because of priority), Diplodocus (the size of five elephants), Ultrasaurus (approached blue whale size), Supersaurus, and Brachiosaurus. The last also is known from Portugal, and north and east Africa.
Predatory carnosaurs were few in mid-Jurassic times, but had become a large and varied group by the late Jurassic. The Morrison formation includes Ceratosaurus and the larger Allosaurus. Tooth marks from the latter are known on Apatosaurus bones, but scavenging is a possibility. Ornitholestes, also from the Morrison formation, had long grasping front legs. Compsognathus is known from the forested eastern seashore of our then continent in the late Jurassic of Germany. Early ornithopods (2-legged ornithischian herbivores) appeared in Asia in the early Jurassic, and there was a major dispersal into North America in the mid-Jurassic. The stegosaurs appeared in the middle Jurassic in Europe and Asia, also spread to North America, and peaked during the late Jurassic Period with only a few surviving into the Cretaceous. Ankylosaurs appeared in late Jurassic Europe, China, and the western U.S. and Canada.
An ecological study of late Jurassic fossil assemblages found that arid regions were dominated by large-bodied, high-reaching herbivores and their likewise large predators. Wet, densely vegetated forest regions had more abundant dinosaurs of small to medium size. Intermediate environments had a diversity of large and small forms.
Ichthyosaurs flourished early, but declined during the Jurassic. Plesiosaurs were represented throughout, and beyond, the Jurassic. Pterosaurs (winged reptiles) were diverse in Europe through the Jurassic, with long-tailed ones dominant for most of the period. They declined in the late Jurassic, and were replaced by short-tailed ones that appeared then. These continued into the Cretaceous. Only a few Jurassic pterosaur fossils are known from North America, but conditions for preserving them here were poor. Pterosaurs now are known to have been fur-covered (though some suspect this “fur” was closer to feathers than to hairs), and probably were warm-blooded.
Birds (Archaeopteryx) appeared in the late Jurassic, and the concensus now is that they were an offshoot of the two-legged, predatory theropod dinosaurs. In fact, Archaeopteryx and some feathered members of the dromaeosaur family of theropods were very similar to one another (Paleobiology 32:417). The range of fossils indicates that the dromaeosaurs, their close relatives of the troodontid family, and birds all started out small (Science 317:1378). One small troodontid may have had flight capability with 4 wings (the forelimbs, plus winglike clusters of quill feathers on the legs; Nature 461: 640). The larger, later members of those two dinosaur groups such as Velociraptor were derived from small ancestors. But Velociraptor retained quill feathers on its forelimbs (Science 317:1721). Early in 2009 an ornithischian dinosaur was found with fuzz that well may have been proto-feathers (Nature 458:333). The sauropods and theropods both had air sac systems which connect them to the birds as well. Mammals diversified, but still had produced none of the groups we see today.
Local landscape. There are no Jurassic deposits in Illinois. Surrounding geology informs us that this area remained low land. The closest Jurassic bedrock is in small areas of the Oklahoma panhandle, and the Black Hills of western South Dakota. Pangaea began to break up in the early Jurassic, with southern Gondwana and northern Laurasia separating by the middle Jurassic. A tilting of the continent permitted a sea to flood the center of North America, reaching from the current Gulf of Mexico to the Arctic, and our nearest west coast was somewhere in central to western Iowa. By the late Jurassic, eastern North America was separate from western North America and all other continents except western Europe. Our area continued to drift north, though the worldwide climate remained warm.
Local life. As in the Triassic, we don’t know what kind of vegetation was here, though again this area was dry land. Elsewhere at our then latitude, conifers, ginkgos and ferns were dominant plants. We were a little farther north of the equator thanks to continental drift, but the climate was still warm. Astrodon, a brachiosaurid, has been found in Maryland and Texas, and so is a good candidate for our local dinosaur fauna. The large predator Allosaurus has been found both in the western U.S. and in Portugal, and so may well have been here as well. The prosauropods Anchisaurus and Ammosaurus have been found in the middle Jurassic of Massachusetts, along with the theropod Podokesaurus. Camptosaurus is an early ornithopod that has turned up on all sides of this part of North America. Our area then was connected with Europe, so pterosaurs and early birds also are a possibility. Late Jurassic dinosaur assemblages typically include “three or more sauropods, two or more theropods, one or two stegosaurs, and one or two ornithopods.”