xix. 4 SYNAPSIDS 539 



years ago. However, it has been realized in recent years that many 

 features in marsupial organization are special developments, not 

 'primitive' characteristics carried over from the ancestral condition. 



The piecing together of all this evidence to give a reliable picture 

 of the history of mammals is a valuable exercise in zoological and 

 geological method, as well as a means of becoming familiar with a 

 fascinating story. We may now return to the Carboniferous times to 

 examine the evidence more in detail (see Fig. 221). 



4. Mammal-like reptiles, Synapsida 



All our evidence about the origin of mammals must of course be 

 based upon the study of hard parts, which can become fossilized, and 

 in particular on the skull. The characteristic feature of the skull in the 

 populations that led to the mammals is the development of a hole in 

 the roof, low down on the side of the temporal region, the lower 

 temporal fossa (Fig. 223). This hole was at first bounded above by the 

 post-orbital and squamosal bones, below by the jugal and squamosal. 

 It was at one time supposed that this single fossa was formed by union 

 of the two present in diapsid reptiles, and thus the whole group 

 acquired the inappropriate name Synapsida. Animals having this 

 characteristic appear in the rocks at the same time as the cotylosaurs, 

 with no hole in the skull roof (Anapsida), from which they were pre- 

 sumably derived. We have only fragmentary knowledge of anapsids 

 from the Carboniferous ; most of our information comes from lower 

 Permian forms, such as * Seymouria ; yet there are quite well-preserved 

 synapsids of Upper Carboniferous date. Therefore we have no com- 

 plete series of successive types to show how the earliest mammal-like 

 types arose; nevertheless we can see something of the probable stages 

 of this progress within the Permian Anapsida. One of the charac- 

 teristics of the skull of * Seymouria was the presence of an 'otic notch', 

 in which lay the tympanic membrane, at the back of the skull (Fig. 

 220). In *Captorhinus and similar anapsid fossils from the Lower 

 Permian this notch has disappeared (Fig. 222). The tympanum 

 apparently lay behind the quadrate, leaving the whole side of the skull 

 as a rigid support for the jaw. A long series of subsequent evolutionary 

 changes led to one of the most characteristic features of the skeleton 

 of mammals, namely, the conversion of the hinder jaw bones (quad- 

 rate and articular) into ossicles for the transmission of vibrations to the 

 ear (p. 550). 



*Captorhinus and its allies were two or more feet in length and 

 showed some development of the limbs towards the mammalian 



