THE JURASSIC PERIOD 711 



habitat, was favored by the genial climate. The frequent shiftings 

 of land- and sea-areas, without involving great relief or severe 

 climatic states, favored biological changes. 



The Marine Life 



It will be recalled that an expansional stage of epicontinental 

 sea life set in toward the close of the Trias. This continued into 

 the Jurassic, reaching a .climax late in the period, when the sea 

 attained the limit of its transgression over the land. The faunal 

 progress is less well revealed in North America than in Europe and 

 Asia, and the following general sketch of the life is based, in part, 

 on the fossils of those continents. 



The great features of the marine life of the period lay in (1) the 

 continued dominance of ammonites among the invertebrates, (2) 

 the rise of the belemnites, (3) the abundance and modernization of 

 pelecypods, (4) the rejuvenation of corals and crinoids, (5) the 

 marked development of sea-urchins, (6) the introduction of crabs 

 and modern types of crustaceans, (7) the prevalence of foraminifera, 

 radiolarians, and sponges, (8) the change in the aspect of the fishes, 

 and (9) the great sea-serpents, descended from the land-reptiles of 

 the Trias. 



(1) The ammonites were still the masters among invertebrates, 

 and were represented by many beautiful forms (Fig 481). They 

 deployed along ascending lines in some .cases, and retrograde lines 

 in others. Erratic and degenerate developments showed them- 

 selves by uncoiling and strange coiling, presaging a stage of " sport- 

 ing" and retrogression in the next period, followed by extinction. 

 Despite these adverse foreshadowings, the ammonities were yet in 

 the heyday of their luxuriance and beauty. 



(2) Ammonites and their predecessors (ceratites, goniatites, 

 and orthoceratites) were tetrabranchs, with external shells; but 

 dibranchs, with internal shells, had appeared in the Trias, and rose 

 rapidly to prominence in the form of belemnites, usually represented 

 in the fossil state by their internal shell or "pen" (Fig. 482). In 

 the course of the period the belemnites almost came to rival the 

 ammonites, and were almost as characteristic of the successive 



