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On Tritylodon, and on the Relationships of the Multituberculata

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On Tritylodon, and on the Relationships of the Multituberculata 760 DR. R. BROOM ON TIUTYLODON [Mtty 3, Zonocypria Irevia U. 0.Gars. Fig. 4. Ahlt frmnle, viewed from right side. 6. Sitme, dorsal view. 6. Mii~iil~rimpreasions of right valve. 7. Extremity of left inferior aiiteniin. StenorJ/p,.ispeinvinntn 13rarlS. Pip;. A. Adnlt feinsle, viewed froui right side. 9. Saine, dorsal view. 10. C’nodal rami. Heteroryprie dubia G. 0.Sara. Fig. il. Adrilt female, viewed froin right side. 12. Same, dor*nl view. 1% Yiisanliir iinpreasioiis of right valvt~. 1.1.. C‘illldal ralll~ls. PLATE LXX I I r. II!j~i:yprispmpbqna G. 0.Snra. Fig. 1. Adult frmale, Tiewed from left side. 2. Sume, dorsal view. 3. Ma=ciilar iin~iressioiisof left vdve. 4. Candal ramiis. 4 Ilyorypvis nlta G. 0.Sarn. Pig. 6. Adiilt iiiale, viewed from left side. 6. Same, dorsal view. 7. Caudal ramils. Limnic.t/there obturntn G. 0. Sara. Fig. R. Adiilt fem:ile, viewed from right side. 9. Same, dorsal view. 10. Muvcnlar impressions of right valve. 11. Superior anteniia. 1% Iiifrrior antenna. 13. Right series of legs. 14. C’aiidd lobe. Pnmcypria rttrtn Q. 0.Rnrs. Fig. 16. .kdidt, inale, viewed from right side. 16. Same, dorsal viiw. 17. Caudal ramus. 6. On ‘I’rh!/loiEon, and on the 1~el:itionshipsof the iMiiltitu’uerculnta. By 11: BROOM,M.D., D.Sc., ( 3f.Z.S. [Received March 14, 1910.1 (Text-figureh 67 and 68.) In 1884 Owen described, iinder the name 5!’vit?/lmZon lonyc~~~~cs, H very reinarkable imperfect skull submitted to hini by Kk. Extoll, of the Bloeinfontein Museum. The specimen WHIS stated to have come from ‘‘ Thaba-chou, Hasutoland,” and was believed to be it Triassic mammal allied to the European Stereoynathtts. There is no locality known in Basutolancl of the name Thttba-chou, but there tire se\-er:iI nionntainh called l’haha-tsue11, and it is prohahly from one of these that the specimen came. In any ~ahewe now 1910.1 ASD THE Mb'r/rITUBBRC!ITLATA. iG1 kiiow that the whole of Basutoland is on beds of Stormberg age : and ns the Stormberg beds are situated above the Molten0 beds, which are known to be Rhstic, and as they contain remains of true Crocodiles, we may safely assume that they are of Lower Jurassic age, and not Triassic as was supposed. The specimen, which consists of the almost perfect snout with part of the interorhital region, portions of the malar arch, and the complete palate, was described by Owen at some length. and his figures have been frequently reproduced. Owen's opinion that Trit?yZodon was n niammal was supported by Cope, Obhorn, Lydekker, Seeley, and others. In 1894, however, Seeley, as the result of his examination of the Cynodont reptiles, came to the conclusion that Z'ritylodon was really a slightly modified Cynodont reptile ; and this view has been followed by a number of Inter writers. Text-fig. 67. Side view of Snout of TrifyZodon Zongmvus Owen. Nat. size. Slightly restored. Fr. Frontal. Ju. Jupl. La. Lachrymal. Hs. Maxilla. Na. Nasal. Pa. Pulntine. Pmx. Premaxilla. Sms. Septomaxilla. In 1904 I endeavoured to show that none of the arguments advanced by RPeley in favour of Z'vityZoclo?i. being a reptile were valid, hut that in all points il'ritylodo,, was an undoubted ina 111 ind 762 DR. R. BROOM OX TRITTLODON [May 3, While recently in London I made a careful study of the type, which, unfortunately, is still the only specimen known, and found that a good deal more could be made out of the original specimen than WA~supposed. In a considerable number of points my conclusions differ from those of Owen and Seeley. The flat piece of bone on the upper part of the snout, which both Owen and Seeley believe to be the frontal, I consider without doubt the upper part of the nmal. The pair of imperfect bones, which form a median ridge at the back part of the specimen as preserved, are believed by Owen and Seeley to be the parietals. I regard them as the anterior halves of the frontals. The bone which Seeley believes to be the pre- frontal I hold to be the upper part of the large lachrymal. Text-fig. 68. Upper view of snout of Tritylodon longgreaua Owen. 3 nat. sire. Fv. Frontal. Ju. Jugal. La. Lachrynd. Mz. Maxillu. Nu. Nasal. PIW.I’reninxilln. Smx. Septomuxilla. Owen gives the dental formula rn i 2, m 6. I believe there are pretty certitinly seven molars, and in front of the large incisors fhere is a socket which probably had a bmall anterior incisor. I therefore conclude that the formula is i3, mi. There is un- doubted evidence that, the lower molars ground with an antero- psterior motion against the upper. The lower incisp? most p&&ly passed together between the large upper incisors, which 1910.1 AND THE YULTITURERCULATA. 7G;q I believe to be i', and their tips probably rested against the small i'. Between the nasals and the premaxillaries is a pair of well- developed septoniaxillaries which have not previously been observed. The position of these bones is seen in the figures given of both side and upper views. The septomaxillary com- pletely separates the premaxillary from the nasal and passes down into the anterior nrtsitl region. There is no evidence that the nares have been beparated by a complete internasal process of the premaxillaries. When the back part of the skull and the lower jaws are restored there is presented a singularly Rodeiit-like appearawe. As regards the habits of Tritylodon, I think it most probable that it was a root-eating animal, the large tusk-like 2nd incisors being well adapted for digging out and breaking of the roots, and the molars with their rows of crescentic tubercles admirably suited for grinding. I'ritylodon i8 the oldest Multituberculate mammal the skull of which is known even partly, and the only other Multituber- culate of which much besides the lower jaw is known is one of the latest Eocene types, Ftilodz~s. Fortunately there has recently been discovered ant1 described by Gidley a good skull with the lower jaw in position and a number of the more important bones of the skeleton of Ptilodus yracilis. From the study of Ytilodus, Gidley comes to the following very interesting conclusions, which may be stated in his own words :- id A fwt of first importance is that neither in the skull nor skeleton of the Montana specimen are there any indications of affinities suggesting the Monotremes, while every character is niarsopial, as shown in the general arrangement and function of the teeth and the developnient of the skull and skeleton. The unequal development of the f0i.e and hind limbs, the character of the inciaors, the form of the palate, and the position of the cheek-teeth all indicate definite affinities with the Diprotodonts. At the same time the reduction in numbers of the molars and the extreme specidisation of the premolars confirms Osborn's conclusion regarding the philetic position of the group, namely, that the Multitubmxhts may be the last representatives of a very ancient pliylurn that became extinct in the early Tertiary. 6' From the foregoing therefore it appeal's that the Allotheria represent an extinct group of niultituberculate Eutherian mammals closely related with but not ancestral to the Diprotodont division of the Marsupialia, with which division they rniiy now be classed as an Infraorder, or Superfamily, their relationship dating back to H common ancestry sonlewhere in the Jurassic or even to earlier Triassic times, as was suggested by Cope." While everyone must be most grateful for the great addition made by Gidley to our knowledge of the Multituberculate struc- ture, I should like to suggest that I think the case is by no 764 DR. R. l3ROOM 0s TRITTLO1)OX [Mn:\. 3, 1iiea.m proven that they are at all closely allied to the Diplototlolit Marsupials. There is a little ambiguity in his 1angua.ge w1tet.e he speaks of “ inultittiberculate Eutheriun mamnials ” forniiug an Infruorder of the Miirsupialia. Presumably he means ‘‘ Met:i.- therian.” But it is manifest he rega,rcls the Multituberculitta RS forming a division of the Marsupialia. Let us consider in sonie detail the points that are urged in favour of this conclusion. “ Every character,” it is said, “ is marsupial, as shown in the general arrangement and function of the teeth and the develop- ment of the skull and skeleton.” If by arrangement is memt the occurrence of incisors. premolars, and molars, without canines, the character is not more marsupial than rodent, and a somewtl;lt siruilar arrangement is found in the other Eutherian orders. l3iit further the nuniber of the molars seeins to me opposed to any close association of the Multitnbercnllttes with the Marsupials. Xo living marsupial has more than three prernolitrs arid almost every one has four molars. The Multituberculittes have apparently as the normal series four premolars and three iiiolrtrs as in the Eutheria. The whole seven are seen in dh‘odon, antl wlien the number is reducer1 to six as in Ptilodzcs there seems little doirltt that the loss is in frolit and not behind. From the position of the lower teeth I should incline to the view that the large grooved tooth in Ptilodtis and the posterior of the grooved teeth in Pluyi- (su,Zaz and Ctethacodon are 1st true molars. But whatever difference of opinion may be held on the interpretation of the teeth, there can be little doubt that in structure the molars are cluite unlike anything known aniong Marsupials.
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