The Mesozoic Era: it would seem stranger to us than the Land of Oz.
The big land animals were dinosaurs, not mammals. You couldn't receive a dozen red roses; roses - or for that matter, any flowers - did not appear until the Mesozoic was more than half-way over. There were no lawns to mow because there was no grass. Until well into the Mesozoic, you couldn't hear birds singing because there were no birds. Indeed, at the beginning of the Mesozoic, the very continents themselves were connected, and they spent much of the ensuing 180 million years getting into more familiar positions.
The Mesozoic would surely seem unfamiliar to a modern visitor, but we can't really understand dinosaurs until we learn something about it. This chapter, therefore, explores a few of the physical qualities of this ancient world. We will answer questions such as "When did the dinosaurs live and how do we know?", "Where were the continents during the time of the dinosaurs?", and "What were climates like during the time of the dinosaurs?" In general, the discussion will be focused on the latter two-thirds of the Mesozoic (the Late Triassic to the end of the Cretaceous Period) because this is when there were dinosaurs. And because dinosaurs roamed continents and not oceans, we will primarily address ourselves to things terrestrial.
When did the dinosaurs live and how do we know?
Geologists dichotomize time and rocks. On the one hand, there is time that passes, regardless of what we do or what happens. On the other hand, there are sedimentary rocks, which constitute the key record on earth of the time that has passed. But sedimentary rocks are imperfect recorders: they preserve only the time during which they were deposited. Without sedimentation, there is virtually no record of time having passed.
Suppose you glance out of your window; you might see lawns, houses, apartments, open fields, something. But almost certainly, the time that is going by as you are looking outside your window - indeed, the many years represented by the lawns and houses - is not recorded by the accumulation of sediment in your area. Where, then, is that time -that is, now - recorded? Perhaps sedimentary deposition is occurring elsewhere (thereby recording that moment in time there). Perhaps, however, "now" is not represented anywhere on earth by sedimentary deposition. If so, future geologists looking back on this time interval (now) will assume, reasonably enough, that the time must have existed, but that no rocks happened to be preserved to record it. Time passes at a constant rate (because we measure it in consistent units: seconds, minutes, hours, etc.), but the rock record of it is patchy and uneven, recording here an hour and there a week of sedimentation. Nonetheless, geologists assume that time has passed, whether or not there are rocks (or fossils) preserved that represent that time. Geologists are thus affirmatively not existentialists: time is presumed to exist independently, whether or not anything was there to record it.
So the sedimentary record and hence the fossil record (because, of course, fossils are found in rocks) are crude sketches; the amount of time represented by rock deposition is orders of magnitude less than the amount of time that has actually elapsed (Figure 2.1). The challenge is to link together all the sedimentary rock records around the world with the object of developing as complete and precise a record of earth history as possible.
Recall that dinosaur remains are found in layers - or strata - of sedimentary rocks. How old or young are these layers (and the fossils they contain) is the special province of stratigraphy, the practitioners of which are called stratigraphers. Stratigraphy is divided into chrono-stratigraphy (chronos - time) or time stratigraphy, lithostratigraphy (lithos - rock) or rock stratigraphy, and biostratigraphy (bios - organisms) or stratigraphy as indicated by the presence of fossils.
Chronostratigraphy Defending religion against the supposed threat of evolution, Matthew Harrison Brady1 proclaimed, "I am more interested in the Rock of Ages than in the ages of rocks!" Regardless of creed, however, geologists are interested in the age of rocks, termed chronostratigraphy. But the ages of rocks are on a scale that is very literally completely out of our experience. Stephen Jay Gould wrote of geological time, "An abstract, intellectual
I The William Jennings Bryan character in the J. Lawrence and R. E. Lee play Inherit the Wind.
Was this article helpful?