184 POPULATIONS OF THE SEA 



Tethys and Other Mesozoic Seaways 



Repeatedly, from the beginning of latest Paleozoic time, the 

 thick-shelled benthos and nekton of the world ocean encircled it 

 o^■er a route through the present Mediterranean depression and 

 Middle Eastern and central Indian lowlands, except as hindered 

 by transverse obstacles, mainly in the region of Central America. 

 This meridional seaway is called Tethys, and it would function 

 anew with moderate rise of the sea or subsidence of the land. 



Tethys reached its peak after an interxal of temporary blockage 

 coincident with the dessication transitional from Paleozoic to 

 Mesozoic times. The rich warm-water shelly faunas and reef biotas 

 of later Mesozoic time spread from India to western North 

 America (e.g., Smith, 1910, p. 482) and reached northward along 

 more temporary routes to high latitudes, and even beyond the 

 Arctic and Antarctic circles. Dasycladacean algae, denoting warm 

 water in the upper photic zone, are common associates of some 

 faunal facies. 



The Jurassic (middle Mesozoic) has the best-documented record 

 of the systems with intimate Tethyan connections. Uhlig (1911) 

 showed that Neumayr's (1883, 1885) pioneer efforts toward a 

 worldwide zonation of the Jurassic marine faunas were based on 

 faulty analogy, but he sustained the idea of broad latitudinal 

 zonation parallel to the present one. He extended the warm and 

 temperate climatic zones northward and dispensed with Neumayr's 

 cold zones. He also recognized nine separate marine provinces in 

 the Jurassic, all with channels to the Tethyan seaway. I'hlig's 

 scheme in turn has undergone revision by Arkell (1956, pp. 606- 

 618), who reduced the provinces to three, Tethyan, Pacific, and 

 Boreal. On the basis of his master work on the Jurassic geology of 

 the world, Arkell concluded that the Early Jurassic (Lias) fauna 

 was essentially universal and Tethyan; the divisions evolved later, 

 with much shifting of boundary zones. Limestone is rare or lacking 

 in high latitudes, but a conspicuously shelly fauna is recognized 

 as far north as Greenland. Arkell (1956, p. 616) logically concludes 

 that even the subpolar waters of Liassic time "were at least as 



