The world: Late Cretaceous period, 75 million years ago
The place: A deltaic floodplain on the shores of the Inland Sea
The mission: SURVIVE!!!
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The Late Cretaceous was a pivotal time in Earth's history. Dinosaur and plant co-evolution probably triggered significant changes that would persist through the Cenozoic and shape our own time. It was also a nasty place to get into a spot. I haven't seen any rexes yet, but rest assured that Albertosaurus and Daspletosaurus make up for my loss. I'm too late to commune with Deinonychus, but its little cousins, Dromaeosaurus, keep me well occupied. No sauropods this far north, either, but no shortage of ankylosaurs, ceratopsians, hadrosaurs, pachycephalosaurs, ornithomimes, or hypsilophodonts to trample me underfoot.
The Late Cretaceous was a pivotal time in dinosaur evolution, as well, according to some theories setting up the events that would lead to a breed of furry, crepuscular vermin to speciate into politicians, spammers, Microsoft programmers, and car salesmen. It was the pinnacle of dinosaur dominance... and only 9 million years before their final extinction. Were there happenings amid the prowess that worked their downfall? Or was the dinosaur world as strong as ever... silenced only by the impact of an asteroid off the coast of Mexico?
The World
In the Late Cretaceous, sea-levels apparently were fluctuating. I'll touch on the causes and corollaries of that in a moment. For now, let me say that Earth was generally divided into six or seven "continents," which didn't exactly align with our modern ones:
Brief contacts allowed some floral and faunal interchange between the continents. Eastern and western North America connected more regularly than the other continents; related genera, such as Alabama's Lophorothon and Alberta's Kritosaurus, are evidence that the shallow Seaway occasionally allowed migrations between the biospheres. Eastern North America had a distinctly foreign character, however, compared the well-known western dinosaurs. Tyrannosaurs were replaced by dryptosaurids, whose closest relatives inhabited the northwest African landmass. Hollow-crested duckbills, such as Parasaurolophus or Corythosaurus, were apparently rare or absent. While the western half of the continent shared ceratopsians and akylosaurids with Asia, the eastern half shared nodosaurids - clubless anks - with Europe.
South America certainly remained joined to Antarctica + Australia well into the early Tertiary. Whether it remained connected to Africa is much more doubtful. African dinos have more in common in general with eastern North America, suggesting an intermittent connection through the European archipelago. Africa does, however, share titanosaurid sauropods with South America.
The two divisions of Africa probably had occasional contact much like the halves of North America. From Africa, titanosaurids apparently spread into Europe and possibly Central Asia. Europe and Central Asia likewise had irregular interchange, sharing dromaeosaurs and possibly other groups.
Asia and western North America apparently united through Alaska and Siberia as early as the Early Cretaceous days of Deinonychus and Utahraptor, and even to the present day the continental shelf is continuous. The faunas of both continents intermingled throughout the Late Cretaceous: hadrosaurs, ankylosaurs, oviraptorids, dromaeosaurs, tyrannosaurs, troodonts, therizinosaurs, ceratopsians. Only the sauropods seemed not to have spread. The Beringia connection passed well into the high latitudes. It seems fairly certain there had to be permanent populations of these groups even in the regions of 24 hour darkness to permit the flow of genes between the two continents.
The Climate
The Late Cretaceous had a generally balmy climate, with mangrove forests growing in Alberta. The equatorial regions had climates not much different from today's low latitudes, but the warmth was distributed much higher toward the poles. Even so, the varying sea-levels suggested by the intermittent draining of the shallow continental seaways suggests a limited amount of glaciation, probably in the very depths of Antarctica and the higher elevations of Alaska. The paleoclimatology record is vague enough that glaciations lasting 100,000 years or less would pass unnoticed in the rocks today.
Other factors besides biogeographical spread argue for a varying climate. Corythosaurs have been found mummified in desert conditions in Alberta, even though the area is associated with deltaic environments. Desertification could have been occasioned by the draining of the Seaway and the consequent loss of convectional precipitation.
Whether or not there were Cretaceous glaciers, it is certain that Mesozoic climate was a dynamic affair influenced by the same cycles of axis and orbit affecting modern climate.
By comparing the isotope levels in sediments, which vary by temperature, we know that Early Cretaceous dinos in Australia saw temperatures as low as 20F, while Late Cretaceous dinos at Barrow, Alaska saw temps at least as low as 40F. On the other extreme, it's likely dinos in the humid equatorial lowlands of northern Africa saw temperatures around 120F.
The Dinosaurs
The dinosaurs of western North America are the ones I have to deal with, so they're the ones I'm going to detail here. I've given you some grounding on the vital stats of my little world, so now I'm going a little deeper.
My homestead is where my computer and I chanced to dechronize (apologies to Sam Magruder): In a forest about two miles from a river in the deltas around Billings, Montana. The ground is somewhat marshy, since the land is a flat sedimentary deposition from periodic flooding; there isn't much elevation difference, so every wet season I have to abandon my computer as my compound floods with two or three feet of turbid, stinky water. There's ground much more to my liking southwest of here, where foothills of the future Big Horns poke a little out of the marshy muck, and I spend a lot of time there, particularly in the rainy season, which around here is summer. Most of the rain is convectional, boiling off the shallow superheated Seaway, so there is a noticeable dry season beginning around December.
Anyway, there is a distinct biogeographical divvying among the dinos. Anks and hollow-crested duckbills like the marshy ground of the coastal plains. Solid-crested (or sometimes crestless) hadros prefer the floodplains just inland of the deltas, while maiasaurs, pachycephalosaurs, and therizinosaurs occupy the higher ground of the foothills and mountains. Tyras, tros, droms, oviraptorids, and ceratopsians seem to just wander where they will; I haven't been here long enough to properly observe their preferences. Another biome is the inland southwest of New Mexico and vicinity, which is semiarid plateau country later populated by titanosaurids like Alamosaurus and currently by ceratopsians like Pentaceratops.
Many hadros and ceras herded, and at least several nested in vast rookeries. Migration, however, does not appear to be the continent-spanning affair visualized by mediagenic paleontologists. Sure, I've seen the herds traveling together to God knows what end, but they certainly don't travel thousands of miles twice every year. Nor do they nest every year; as with large Holocene mammals, they only reproduce every second or third year. Most nest in Spring, just before the start of the rainy season. Predators take advantage of the expected crops of eggs and juveniles by nesting themselves in the same periods, often quite near to herbivore rookeries.
If you want a real, top-notch genera list compiled by someone vastly more knowledgeable than yours truly, go to http://dinosauricon.com/genera .
Dinosaur Icon is a truly excellent site far superior to this sorry mess I've compiled...
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