MERRIAM — THE THAIrATTOSAT'inA 'It 



Relationship to Squamata. — Althougli tliu Tluilattt)saurs evidently possessed 

 two temporal openings, with a complete lower temporal bar, their resemblance 

 to the lacertilian and mosasaurian branches of the S([uaiiuita arc so prominent 

 as to demand consideration of possible relationship to them. The general aspect 

 of the mandilile is that of a rej)resentative of the S(jnamata, though it is also 

 approached in the Rhynchocephalia. The form, situation, and prominence uf 

 the coronoid are such that the posterior portion of tli» jaw taken alone could 

 scarcely l)e considered as other than lepidosaurian. 



( )n the superior side of the skull there is a noticeable similarity to the 

 Mosasauria. In both the Thalattosaurs and Mosasaurs the rostral region is 

 narrow ; it is generally somewhat elongated and acute terminally. The nares 

 are set w(dl back and the premaxillary is produced posteriorly as a stem or bar 

 which meets tlie frontals. There are no discrete nasals in the Mosasaurs and they 

 are generally supposed to be united with the posterior end of the premaxillary 

 bar as in ]"aranus. Should it appear that they have been lost and the fronto- 

 premaxillary connection be immediate, the structure in this region would be 

 very similar in the two groups. 



In the posterior part of the Mosasaur skull the l)road frontals are often 

 partly arched around the anterior ends of tlie parietals as in the Thalattosaurs, 

 while the postfrontal and postorbital have partly united. So far as can be 

 determined, there is room to suppose tbat tlie upper portion of the temporal 

 region may not differ greatly in the two groups. The quadrate of the Thalatto- 

 saurs is in general of rhynchocephalian type and the widely extended anterior 

 wing is not seen in the Sc^uamata, but the presence of a strong external wing 

 and the incipient development of a posterior hook make it begin to show resem- 

 blance to the type seen in the Mosasaurs and Lizards. 



In the characters of the limbs and arches there is much that is similar. 

 The vertebrae and ribs differ, Init perhaps not so fundamentally as would a])])ear 

 from superficial examination. 



On the whole the resemblance to the aquatic Squamata is v<'rv striking; 

 but it can not ove~rbalance such characters as the evident presence of a lower 

 temporal arch, the existence of a strongly developed vomerine dentition, an 

 immovably lixed mainly rhynchocephalian (quadrate, abdominal ribs and very 

 })rimitive vertelirae. 



While it is not possible to place the Thalattosaurs in the S(|uamata. as that 

 grou}) is at present defined, it is not improl)able that the primitive Lacertilians 

 were closely related to or derived from the land or shore forms from which the 

 Thalattosavirs were deriveil. Adaptation to a(|uatic life would tend to make the 

 Thalattosaurs take on rapidly a certain set of characters which would appear 



