CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



823 



6. Enlargement of the head as a whole = macrooephaly. 

 Relative diminution of the head as a whole = microcephaly 



7. Elongation of the limbs as a whole = dolichomely. 

 Elongation of the feet = dolichopody. 



8. Shortening of the limbs as a whole = brachymely. 

 Shortening of the feet = brachypody. 



PROPORTIONAL EVOLUTION IN THE TYPICAL 

 BRACHYCEPHAL PALAEOSYOPS 



As the course of the evolution of Palaeosyops from 

 generation to generation is toward solidity, the skulls 

 become progressively larger, the individual parts 

 become more massive, the sagittal and occipital crests 

 become more robust, the linea aspera of muscular 

 origin and insertion becomes more pronounced, the 

 zygomata become more widely arched and thicker in 

 section posteriorly, and the zygomatic flanges become 

 deeper. 



syops is partly ancestral or inherited (paleotely), and 

 it partly looks forward (cenotely) to a similar pro- 

 portional evolution in a certain direction or a sum of 

 proportional changes whose effects are less clearly 

 manifest in lower stages and become more and more 

 evident in higher stages. The harmonic tendency 

 toward progressive weight and brachycephaly is ac- 

 companied by the disharmonic tendency in certain 

 changes of proportion. 



BRACHYCEPHALIC ALLOMETRONS AFFECT ALL 

 BIOCHARACTERS; HARMONIC 



To a certain degree brachycephaly affects every 

 contour, relation, bone, and tooth in the skull. Even 

 in the earliest known species of Palaeosyops brachy- 

 cephaly is the dominant evolutionary trend, although 

 we still find reminiscences of less brachycephalic 

 ancestors, forms more like the mesaticephalic Lim- 



FiGUHE 740. — Proportions and flexure of skull in horses and titanotheres 



I, Horse proportions: Eguus, face elongated, cranium abbreviated; Eohippus, cranium and face equal. II, Titanothere proportions: 

 Brontotherium, face abbreviated, cranium elongated; EotUanops, cranium and face equal. Ill, Skull flexure (cyptocephaly) in ruminants 

 {EangiftTj Bubalis) and titanotheres (Palaeost/ops, brachycephalic; Bolichorhinus, dolichocephalic). 



CONTRAST BETWEEN EVOLUTION AND GROWTH 



The first impression from these modes of change is 

 that in Palaeosyops skull evolution is like a continu- 

 ous growth, as if the skulls found in the higher geo- 

 logic stages represented extremely long-lived, mature, 

 but not senescent forms of the skulls found in earlier 

 geologic stages. This impression is only partly true. 

 In evolution new proportions develop through the 

 separate rate of change of different biocharacters; 

 consequently evolution is more fully differential than 

 growth. 



DIFFERENTIAL (DISHARMONIC) ALLOMETRONS IN 

 PALAEOSYOPS 



In the evolution of the genus Palaeosyops between 

 the lower and upper Bridger beds (a geologic interval 

 of 1,500 to 2,000 feet) there is an average increase of 

 about 14 per cent in the size of the skull, but this 

 ranges in the different bones of the skull from 8 per 

 cent minimum to 73 per cent maximum. Thus there 

 is a prolonged harmonic evolution toward brachy- 

 cephaly and a separate differential evolution of bio- 

 characters. Therefore the generic character of Palaeo- 



noJiyops and Telmatherium. The modes of departure 

 from an ancestral, more generalized type toward the 

 extreme brachycephalic type are apparent in every 

 biocharacter, as shown in the eighteen structural 

 features listed below:*" 



1. Ancestral, smaller, mesaticephalic type, only partly known, 

 passes into the progressively larger, more brachycephalic type. 



2. Skull as a whole increases in massiveness, showing a more 

 rapid increase in breadth than in length — that is, as 1.5 to 1. 



3. Premaxillary symphysis becomes short and rounded. 



4. Maxillaries are increasingly abbreviated. 



5. The nasals, originally more elongate and obtuse, become 

 more abbreviate and pointed. 



6. A smooth, plane forehead develops a prominent forehead 

 convexit}'. 



7. The malar below the orbit deepens with extreme rapidity. 



8. The zygomata become more arching and deeper. 



9. Postglenoid and post-tympanic processes, originally sepa- 

 rate and leaving external auditory meatus open below, become 

 almost united, closing external auditory meatus below. 



10. Paroccipital process, distinct at base from post-tympanic 

 process, becomes confluent at the base with post-tympanic 

 process. 



11. Basifacial plane, originally more horizontal, becomes 

 displaced downw-ard. 



12. Palatines exhibit an abbreviated exposure on the palate. 



'» other progressive characters are enumerated on p. 257. 



