176 INTRODUCTION TO EVOLUTION 



152); they are still represented by a few species dwelling in modern seas, 

 including the pearly nautilus (Fig. 8.8, p. 154). 



Belemnites 



The modern squids and octopi had a host of Mesozoic relatives, the 

 belemnites. In general appearance they resembled the modern squid (Fig. 

 9.2). Their cigar-shaped internal skeletons are extremely abundant in 

 Mesozoic deposits. They ranged in length from a few inches to 5 or 6 

 feet. Fortunate occurrence of fossils having the outlines of the body indi- 

 cated by a carbonized film informs us that there were six tentacles and 

 that the latter were provided with hooks, in place of the sucking discs 

 possessed by modern squids. Remnants of the "ink" by means of which 

 belemnites, like modern squids, formed a "smoke screen" to facilitate es- 

 cape are preserved with rare fossils. 



The internal skeletons of belemnites were divided into chambers sug- 

 gestive of those of the uncoiled nautiloids (Fig. 8.9, p. 155) that were prob- 

 ably their ancestors. There are indications that the internal skeleton was 

 a remnant of an external, chambered shell possessed by nautiloid, or pos- 

 sibly ammonoid, ancestors. 



EVOLUTION OF DINOSAURS AND 

 THEIR RELATIVES 



On preceding pages we have traced the rise of land- 

 dwelling vertebrates from crossopterygian fishes. We noted that the im- 

 mediate descendants of the latter were the labyrinthodont amphibians, 

 which, in turn, gave rise to the cotylosaur reptiles. All this occurred before 

 the beginning of the Mesozoic. As noted on page 170, cotylosaurs are im- 

 portant as the ancestors of higher reptiles (Fig. 9.3). 



The cotylosaurs had skulls solidly roofed in the region back of the eyes, 

 the temporal region. From the cotylosaurs arose several lines of descend- 

 ants, each line characterized by presence or absence of one or both of 

 two openings in the temporal region: the temporal fossae. The Therap- 

 sida or mammal-hke reptiles have already been mentioned (p. 171); they 

 had a single temporal fossa — a single opening on each side, placed poste- 

 rior to the eye and rather widely spaced from the midline of the skull. 

 Therapsid and other reptiles so characterized are classed together as 

 Synapsida (Fig. 9.4). 



Another line is known as the Diapsida because of the possession of two 



