NO. 7 EARLY CENOZOIC VERTEBRATES — FRIES ET AL. 1 5 



MEASUREMENTS OF HOLOTYPE OF PARADIPSOSAURUS MEXICANUS, NEW GENUS AND 

 SPECIES (U.S.N.M. NO. 20667) 



Mm. 



Projected horizontal length of skull roof from anteromesial limit of nasal 



to posteromesial border of parietal 22.0 



Projected vertical depth of skull at level of posterior orbital border 14.3 



Maximum width of skull across jugals 18.2 



Maximum length of frontal 1 1 .0 



Maximum width of frontal across posterolateral processes 1 1 .0 



Minimum interorbital width of frontal 4.0 



Average height of labially exposed tooth crowns i.i 



Average anteroposterior diameter of tooth crowns i.o 



Average transverse diameter of tooth crowns i.i 



Discussion. — In general structure, the presently described skull is 

 essentially iguanid in character (Camp, 1923; Cope, 1892; Gilmore, 

 1927). Although incomplete and not revealing the method of tooth 

 replacement, the dentition also would appear to be basically of the 

 iguanid type. For the present, therefore, Paradipsosaunis is con- 

 sidered an extinct representative of the family Iguanidae. 



From comparative study Paradipsosaunis would appear to ap- 

 proach more closely to the northern crested lizard Dipsosaurus than 

 to any of the other iguanids that presently live in Mexico and the 

 southwestern United States (Smith, 1946; Smith and Taylor, 1950). 

 The most noticeable points of resemblance are : the broad, flat parietal 

 table elevated well above the level of the supratemporal arch ; the 

 unrestricted supratemporal fossa ; the deep, broad snout without pro- 

 nounced nasolachrymal ridges ; and the forward-opening external 

 nares. These writers have been, as yet, unable to determine whether 

 these and other more detailed osteological similarities denote genetic 

 relationship between Dipsosaurus and Paradipsosaurus or merely 

 adaptive convergences. Certainly, however, such demonstrable con- 

 trasts between the two genera as the dentition and splenial elements, 

 among other differences, are sufficient reasons by current taxonomic 

 criteria for separating the fossil from the living genus. 



The oldest recognized lizard remains derive from rocks of late 

 Jurassic age (Romer, 1945). Hov/ever, they do not become a common 

 component of continental faunas until the late Cretaceous, by which 

 time it may be presumed that most of the major lacertilian categories 

 had appeared. Fossils referred to the Iguanidae are known in every 

 geologic epoch from the late Cretaceous to the Recent. Unfortu- 

 nately, this family representation is meager in terms of taxonomic 

 entities and collectively displays no progressive morphologic trend 

 into which the unique Mexican form can be fitted. Paradipsosaurus, 



