January ii, 1894.] 



NATURE 



259 



The general faunal succession is marked by the sudden 

 appearance and disappearance of certain series and rise and 

 fall of great groups. In the Trias appears the remarkable pro- 

 todont or primitive-toothed Dromotherium ; we cannot deter- 

 mine its order at present. We still have no American fauna 

 corresponding to the intermediate Stonesfield of England. In 

 the Jurassic Atlantosaurus beds the three supposed repre- 

 sentatives of the Monotremes (multituberculates), Marsupials 

 (triconodonts), and Placentals (trituberculates), appear in equal 

 numbers ; the latter are generally characterised by the primitive 

 dental formula. In the Laramie the Multituberculates continue 

 in great profusion, and the Marsupials and Placentals are also 

 numerous. 



The serial succession of the Trituberculates from the Meso- 

 zoic is still an unknown chapter ; we are utterly unal)le to 

 connect the Dromatheriidae of the Trias, the Triconodontidse, 

 Amphitheriidas and Amblotheriidfs of the Jura with each 

 other, or with any Cretaceous or lower tertiary mammals. 

 The serial relations of the Multituberculates, on the other 

 hand, have been made much clearer by the discovery of 

 the Laramie fauna. Cope and Marsh in this country, and 

 Smith Woodward in England, have at last broken into the 

 long barren Cretaceous. In studying the accurate figures pub- 

 lished by Marsh and a large collection of teeth recently made 

 for the American Museum by Wortman and Peterson, I find 

 that this Laramie fauna is widely separated from the Jurassic 

 in its general evolution, and as Gaudry, Lemoine, and Cope 

 have observed, it approaches more nearly the basal Eocene 

 of the Puerco and the Cernaysian of France. The Multitubercu- 

 lates of the Laramie include the Plagiaulacidse, represented by 

 Ptilodus, the form with two premolars, and Meniscoessus, with 

 two premolars and crescentic tubercles. Meniscocssus has a 

 smaller fourth premolar, and is found to lead off to the huge 

 plagiaulacid Polymastodon of the Puerco. The only other Mul- 

 tituberculates found are those related to Bolodon of the Jurassic 

 and Chirox of the Puerco. The other mammals of the Laramie 

 range from the mouse to the opossum in size ; they have superior 

 molars of the simple tritubercular type — the low cusped or 

 bunodent molar predominating in the upper jaw, and the 

 tuberculo-sectorial in the lower. The dental formula is mostly 

 the typical p. 4, m. 3. Yet, judging by the angular region of 

 the jaws, we have here both Placentals and Marsupials. Some 

 of the teeth remind us strongly of those in the Puerco ; their 

 determination, however, is very difficult, for the jaws and teeth 

 are almost entirely isolated. From another exposure of the 

 Laramie, Cope has recently found the remarkable type Thlseodon 

 — remarkable because it is a highly specialised trituberculate 

 of typical dentition with a jaw which bears resemblance to that 

 of the Multituberculates and of Ornithorhynchus. There is 

 no placental angle nor strong marsupial inflection. This 

 raises the supposition that Thlaeodon may be one of the per- 

 sistent trituberculate Monotremes which we are now looking 

 for. 



In the Puerco or basal Eocene, a very marked change 

 occurs, for the American fauna loses some of its cosmopolitan 

 character, the multituberculates or monotremes die out and the 

 marsupials are not found at all ; in fact they do not reappear 

 in North America until the Miocene. 



Ancient and Modern Placental Differentiation. 



The Puerco is essentially an archaic fauna and is to 

 be regarded as the climax of the first period of placental 

 differentiation, a culmination of the first attempts of nature 

 to establish insectivorous, carnivorous and herbivorous groups. 

 These attempts began in the Cretaceous, and some of the 

 types thus produced died out in the Puerco, some in 

 the Wahsatch and Rridger ; only a few flesh-eaters survived 

 to the Miocene. It is most important to grasp clearly 

 the idea of this functional radiation in all directions of 

 this old Puerco fauna, resulting in forms like the modern 

 insectivores, rodents, bears, dogs and cats, monkeys, sloths, 

 bunodont and selenodont ungulates, and lophodont ungulate-;. 

 This was an independent radiation of placentals, like the 

 Australian radiation of marsupials. What was the cause of the 

 wide-spread extinction of these types? So far as the ancient 

 clawed types are concerned, their teeth and feet seem to be as 

 fully adaptive in many cases as those of the later unguiculates ; 

 the hoofed types were certainly inferior in tooth evolution, for 

 all their molars evolved on the triangular basis instead of the 

 sexitubercular ; the most sweeping defect of both the clawed 



NO. 1263, VOL. 49] 



and hoofed types was the apparent incapacity for brain-growth, 

 their bodies went on developing while their brains stood still. 

 Thus the stupid giant fauna, the Dinocerata, which rose outjo 

 this period, gave way to the small but large-brained modern 

 types. It is noteworthy that the latest survivors of this wreck 

 of ancient life were the large-brained Hya^nodons. 



Some of the least specialised spurs of this radiation appear to 

 have survived and become the centres of the second or mid- 

 Tertiary radiation from which our modern fauna has evolved. 

 Yet we have not in a single case succeeded in tracing the 

 direct connection. To sum up, we find on the North American 

 continent evidence of the rise and decline and disappearance of 

 monotremes and marsupials, and two great periods of placental 

 radiation, the ancient radiatio7t beginning in the mesozoic, 

 reaching a climax in the Puerco and unknown post-Puerco, and 

 sending its spurs into the higher tertiary, and the tnodern radia- 

 tion reaching its climax in the Miocene, and sending down to 

 us our existing types. 



Another Eocene centre was lower South America, which has 

 of late dimmed the prestige of North America in yielding 

 strange form.s of life. One theory of this Patagonian fauna is 

 that it was an independent centre of functional radiation like 

 the Puerco and Australian, full of adaptive parallels, but not 

 yielding to Europe or America any of their older types. But 

 Ameghino, to whose energetic researches we are chiefly in- 

 debted, believes that he finds a lower Eocene life zone- — a sort 

 of south polar centre — which supplied both America and 

 Europe. The Puerco he believes is no older than the Santa- 

 cruzian, which in turn is very much older than the Parana and 

 Pampean formations, which Burmeister has made so well 

 known. This yields the Homunculus Patagonicns which paral- 

 lels Cope's Anaptomorphus in presenting a dentition as 

 advanced in reduction as that of man. Ameghino finds here the 

 ancestors of the Macrauchenidse ; he believes the Homolo- 

 dontotheridse are the ancestors of the Chalicotheriida — thus 

 deriving a buno-selenodont from a lophodont type ; the 

 Proterotheriidas, he believes, replace the Condylarthra and 

 Hyracotherium in the ancestry of the horses. Similarly the 

 Microbiotheriidje are the stem of the creodonts and carnivores. 

 I cannot coincide with any of these views. The Multitubercu- 

 lates are far older and widely different from the Abderites to 

 which Ameghino traces their ancestry. I fully concur with 

 the opinion of Cope, Zittel, Scott and others that this fauna is 

 of somewhat later age, that it was directly connected with 

 Australia and somewhat later with North America, supplying 

 us, as has always been supposed, with our sloths. 1 quote 

 from a recent address by Scott : — 



"The oldest mammals from South America are those from 

 Patagonia, which Ameghino has referred to the Eocene, but 

 which are more probably Oligocene or Miocene. This fauna is 

 of extreme peculiarity and isolation ; it is made up chiefly of 

 edentates, rodents and ungulates of those very aberrant types 

 known as Litopterna and Toxodontia, which are so widely 

 different from the hoofed mammals of the northern hemisphere ; 

 together with some primitive forms of primates, creodonts and 

 marsupials. The marsupials are of extraordinary interest, for 

 they comprise not only forms allied to the opossums, but also to 

 recent Australian forms such as Thylacinus, Dasyurus and Hypsi- 

 prymnus. This is a most unexpected fact, and seems to point 

 unmistakably to a great southern circumpolar continent." 



The Puerco thus remains the most extensively known and most 

 productive lower Eocene centre, yet we have very slender threads 

 of positive evidence to connect its fauna with the later 

 placental radiation. 



The Creodonts of Cope occupy the same relation to the modern 

 insectivores and carnivores that the Condylarthra do to the 

 ungulates. The American group has been recently enriched by 

 the discoveries of Wortman, and the literature by the careful 

 revision of Scott. This author has divided them into eight 

 families, placing the forms which most resemble the Insecti- 

 vora in the new family, Oxyclsenidae. These families illus- 

 trate superbly the same law of functional radiation later 

 repeated in the placental and marsupial carnivores. The 

 Mesonyx family presents some analogies to the Thylacines. 

 The modern bears are paralleled in the Arctocyons, with their 

 low tubercular molars ; Wortman and myself, with fresh 

 materials, have recently added Anacodon to this family, 

 a genus which was doubtfully regarded by Cope as an ancient 

 ungulate. The Cats and Hyaenas are imitated in the Oxyeenas 

 and Hysenodons, some of the Miocene forms of which Scott 



