790 



TITANOTHERES OF ANCIENT Wl'OMING, DAKOTA, AND NEBRASKA 



Bony exostoses. — All that we see in fossil skulls are 

 the bony exostoses, or thickenings of the outer bony 

 layer, with expansion of the cancelous bony tissue 

 beneath, as in the horn rudiment of PaJaeosyops in the 

 accompanying figures. 



In the titanotheres, as in the Amblypoda {Baihy- 

 opsis, Coryphodon, Vintaiherium) , the bony exostosis 

 expands into a bony horn covered with a thickened 

 dermal pad lacking any evidence of a horny epidermal 

 sheath and certainly without a distinct epidermal 

 horn composed of agglutinated hairs of the rhinoceros 

 type. The bony horns of titanotheres and amblypods 

 become unique among ungulates as great fighting 

 weapons covered with thickened skin pads, rounded, 

 triangular, oval, and finally platelike in form. The 

 absence of horny sheaths is indicated by the absence 

 of the channels for nutrient blood vessels such as 

 surround the bony horn cores of ruminants and of 

 rhinoceroses. The dermal pad of the titanotheres 

 may have been similar originally to the "wart" of 

 the wart hog (Phacochoerus), which we may observe 

 as partly protecting the ears when two wart hogs are 

 butting each other. ^- The rounded bony horns of the 

 giraffes are probably degenerate because partly 

 covered with hairs and seldom used." 



MODES OF ORIGIN OF THE HORNS OF THE TITANOTHERES 



1. Horns originate independently at different periods 

 in four Eocene titanothere phyla. 



2. Horns originate invariably above or slightly in 

 front of the orbits, on the line of the nasofrontal suture. 

 The frontal element more or less completely overlaps 

 the underlying nasal elements. 



3. Horns apparently have a predisposition or pre- 

 determination to originate in this particular region of 

 the face; no evidence of bony horn rudiments is 

 observed in any other part of the skull; the rudiments 

 are invariably continuous with the frontals pos- 

 teriorly, with the nasals anteriorly. 



4. Rudiments of horns are smooth, rounded, not 

 primarily rugose, with no surface indications of 

 channels for large blood vessels. They are totally 

 unlike the vascular horn rudiments of rhinoceroses, such 

 as Aceratherium incisivum. 



5. The evidence indicates the existence of thickened 

 epidermis rather than of superficial horny sheaths. 



6. Horns are equally developed (so far as known) 

 in both sexes in the original Eocene stages. 



7. Horns are secondarily more prominently devel- 

 oped in males and less prominently in females. 



8. Horns secondarily shift in position on the skull 

 from a point immediately above the orbits to a point 

 more or less above the canines. 



" Memorandum by W. K. Gregory. See also note on the "horn of Phacoclioaus" 

 in review of 0. C. Marsh's "The ' Brontotheridae,' a new family of fossil mammals.'' 

 Nature. Jan. 22, 1874, p. 277. 



'J Valuable papers on the genesis of horns are those of Hans Gadow, 1902.1; E. 

 Ray Lankester, 1902.1; J. ririeh Durst, 1902.1; and especially Max Weber, 1904.1, 

 p. 23, all cited in full in the bibliography of this chapter. 



9. Horns are originally elongate and oval in form, 

 secondarily of triangular form {Menodus), of rounded 

 form {Megacerops), of transversely oval form {Bronto- 

 therium). 



10. No evidence is found that the bony horn core 

 develops an independent center of ossification. 



From the beginning the titanothere horns are paired 

 and supraorbital in position, as in the Cervidae, 

 Antilopinae, and Giraffidae, among the artiodactyls. 

 and as in Colonoceras among the perissodactyls. 



PHYLETIC DIVERGENCE IN TIME OF ORIGIN OF HORNS 



As is shown in the accompanying diagram (fig. 712), 

 the horns are precocious in origin in the Dolichorhin- 

 inae — that is, they appear in Mesafirhinus of Bridger 

 C as elongate oval swellings overhanging the orbits, 

 at a pei'iod when no trace of horn swellings can be 

 observed on the face of the contemporary Palaeo- 

 syops leidyi. These elongate oval horns of Mesati- 

 rJiinus develop into the prominent oval horns of 

 DolicJiorJiinus Tiyognathus of Washakie B. 



Contemporary with MesatirJiinus is the phylum 

 Manteoceras manteoceras, the "prophet horn" titano- 

 there, also found near the base of Bridger C. The 

 apparent successor of Manteoceras, namely, Protitano- 

 therium emarginatum of Uinta C 1, exhibits a horn 

 type which is directly successive to that of Manteo- 

 ceras and similar in form except that it is moved 

 much farther forward over the narial opening. 



ACCELERATED DIRECT EVOLUTION OF HORNS IN THE 

 "PROPHET HORN" PHYLUM 



In Figure 712 are illustrated five direct stages 

 of evolution, namely, (1) the rudiment of the horn 

 in Manteoceras of Bridger C and its shifting forward 

 in Protitanotherium. of Uinta C 1, (2) its ontogenesis in 

 Brontops hrachycepJialus of Chadron A, (3) its adult 

 form in Brontops hrachyceplialus of Chadron A, (4) its 

 adult form in AUops marshi of Chadron B, and (5) its 

 ultimate form in Brontops robustus of Chadron C, 

 final stage of evolution. 



RETARDED EVOLUTION OF HORNS IN THE PALAEOSYO- 

 PINAE AND TELMATHERIINAE 



In contrast to the early geologic appearance of the 

 bony horn in the Eometarhinus, Mesatirhinus, Doli- 

 chorhinus, and Manteoceras phyla we observe the 

 extreme retardation of their appearance in the Palae- 

 osyops and Telmatherium phyla. No horn rudiments 

 are present on the sides of the face in the species of 

 Limnohyops and Palaeosyops of Bridger C, contem- 

 porary with Manteoceras and MesatirJiinus. As late 

 as Bridger D Palaeosyops roiustus shows a rounded 

 bony horn rudiment, which becomes quite conspicu- 

 ous in aged individuals. Skulls of the contemporary 

 Telmatherium are unknown, so we can not state 

 positively that Telmatherium at this stage lacks the 

 horn rudiment. This rudiment is first directly 



