384 STRATIGRAPHICAL GEOLOGY 



for the ancestors of these we must search the marine Trias of 

 Southern and Central Europe. 



The rate of change in the organic world and the differentiation 

 of species appears to have become much more rapid in Jurassic 

 times than it had been in Palaeozoic times, so that any given group 

 of species has a much less vertical range, and consequently a much 

 smaller set of strata requires separate description. It will, therefore, 

 be convenient to describe the prevalent genera and the characteristic 

 species of each series separately, but in the first place the distinguish- 

 ing features of the Jurassic fauna as a whole may be indicated. 



In the first place it was an " age of Reptiles" and it is specially 

 characterised by the remarkable development of the orders Dino- 

 sauria, Pterosauria, Ichthyopterygia, and Sauropterygia. These orders 

 survived into the Cretaceous period and then became extinct. The 

 Dinosauria were land animals, and many were of gigantic size (from 

 30 to 70 feet long) ; in many cases their hind limbs were much 

 longer and stronger than the fore legs, so that they could walk in 

 an upright position like a kangaroo. Some were carnivorous (as 

 Megalosaurus) and some herbivorous (as Cetiosaurus}. The Ptero- 

 sauria are reptiles adapted for the purpose of flight in the air, and 

 thus possess many bird-like characters, such as pneumatic bones, 

 keeled sternum, and more or less bird-like skull ; their membranous 

 wings were attached to the sides of the body and to the greatly 

 elongated fifth digit. The Ichthyopterygia and Sauropterygia were 

 specially adapted for life in the sea, both pairs of limbs being modified 

 into the form of paddles or flippers. Representatives of the orders 

 Crocodilia and Chelonia also occur. 



Birds also make their appearance, but are known by two 

 specimens only, found in the lithographic limestone at Solenhofen 

 in Bavaria. They are referred to the genus Archceopteryx, which 

 exhibits strong reptilian affinities, possessing teeth and a long 

 narrow tail of twenty separate vertebrae, each of which apparently 

 carried a pair of feathers ; the wings are bird-like, and the creature 

 was about the size of a common pigeon. 



It is in Jurassic strata also that the earliest mammals have been 

 found, and these all belong either to the sub-class Prototheria (which 

 includes the living duck-billed mole, Ornithorhynchus, and the spiny 

 ant-eater, Echidna) or to the sub-class Metatheria (which includes 

 the opossums and kangaroos). No placental mammals have been 

 found in the Jurassic System. Mammalian remains have been 

 found at three horizons ; the oldest (Microlestes) in the Rha3tic 

 Beds at the very base of the system, others (e.g. Phascolotherium, 

 Amphitherium), in the Great Oolite, and others (10 genera, e.g. 

 Triconodori) in the very highest group (Purbeck Beds). 



