The geological periods of the world have been extensively researched and there is a general consensus about the naming and the timescale of the different ages. This can be problematical in some cases since local sites and different nations may have variations or differences in the names given to specific periods of time.
The table given below shows the main Periods Epochs and Ages of the Mesozoic Era that are generally accepted world wide. The start dates are given in Million years ago.
|Period||Epoch||Age||Start Date (Ma)|
It should be noted that each of the Ages shown will have local divisions, called Beds or Formations. It is also possible that certain sediments would have been deposited in different places at slightly different times. For this reason, precise dating is not always accurate. Stratigraphic or radiometric dates may give variations of hundreds or thousands of years where the sediments show variation. Often it is best to rely on zone fossils to give a general timescale before resorting to more precise methods.
The Triassic Pterosaurs are the earliest and most primitive species. They can often be identified from their teeth, which may have more than one cusp. Later pterosaurs have single cusped long oval shaped teeth. The bones of the Triassic pterosaurs are more massive than later species, showing a thicker cortex and smaller pneumatic openings.
The Jurassic was a period when the pterosaurs underwent a vast evolutionary development. They occupied many niches and habitats worldwide. Jurassic pterosaurs (Rhamphorhynchoids) tend to have very long teeth with a smooth oval section and a distinct enamel cap to the tooth tip. Some species show slight and fine ridging along the length of the tooth, but most have smooth teeth. Some later Jurassic pterosaurs (Pterodactyloids) start to show a reduction in teeth and an increase in the length of the wing metacarpal bone. There is a fine site at Eichstadt in Germany where many species of Jurassic pterosaurs have been preserved in the fine Lithographic Limestone.
The Cretaceous was a time when all creatures were able to grow to a large size, probably due to the higher atmospheric oxygen content at that time. Pterosaurs where no exception and some of the largest species are seen in the Cretaceous deposits. Many of the later species were widespread around the world and often very large. There were also a few small species, but far fewer than in the Jurassic. This is probably a reflection of the competition of smaller pterosaurs with the birds, which were becoming more numerous in the Cretaceous.