On Two Mounted Skeletons of Megalocnus Rodens
Total Page：16
File Type：pdf, Size：1020Kb
Aug.,1956 PAULA COUTO-MEGALOCNUS 423 ON TWO MOUNTED SKELETONS OF MEGALOCNUS RODENS BY CARLOS DE PAULA COUTO While in the United States, in 1951, on a fellowship of the John Simon Guggen- heim Memorial Foundation, I had my attention attracted by the important collection of ground-sloth fossil material from the Pleistocene of Cuba, stored in the Department of Geology and Paleontology of The American Museum of Natural History, in New York. Part of that collection was made by the late Dr. Carlos de La Torre, in 1910, in the locality of the Ciego Montero thermal springs (Bafnosde Ciego Montero, a well-known health resort of Cuba) Provincia de Las Villas, the same spot where a student of the University of Cuba, Jos6 de Figueroa, discovered, in 1861, the first specimens of Megalocnus rodens, type of the species. Specimens were collected also in fissure springs in the Sierra de Jati- bonico, from which de La Torre secured a considerable, although fragmentary, collection of fossil bones of Megalocnus and crocodiles. In 1911, Mr. Barnum Brown, of The American Museum of Natural History, with Dr. de La Torre's help, completed the exploration of the Casimba de Jati- bonico localities, collecting a few additional bones. From Sierra de Jatibonico they went north to Caibarien, thence southwest to Remedios (locality of former paleontological discoveries) and finally traveled south to Ciego Montero, where a large collection of fossil bones was obtained without exhausting the springs, since a great deal of material remained in the surrounding clays. The exploration of the Ciego Montero springs was finally completed by Mr. Brown in a later expedition, in 1918. In 1919, Dr. William Diller Matthew published two notes on the fossil Antil- lean mammals (Matthew, 1919a, b) and, finally, in 1931, a posthumous note on the ground-sloths of the Pleistocene of Cuba was published by The American Museum of Natural History on the basis of Matthew's incomplete manuscript (Matthew, 1931). In 1951, my interest in the collection of ground-sloth material from the Pleisto- cene of Cuba coincided with a request of the Museo Poey de la Universidad de Havana to have the greater part of the collection returned to Cuba. Through the courtesy of Dr. George Gaylord Simpson, chairman of the Department of Geol- ogy and Paleontology of The American Museum of Natural History, who was interested in having the whole collection studied as a series before being broken up, I was able to go over the collection and to complete Matthew's important manuscript on the Cuban edentates, which is now almost ready for the press. On conclusion of the study, about fifty good specimens of all the Cuban species of ground-sloths were sorted and returned to the Museo Poey de la Universidad de Havana. With authorization of Dr. Jos6 Alvarez Conde, then director of the museum of Havana, a small collection was sent to the Museu Nacional do Rio de Janeiro, to be exchanged for fossil Pleistocene material from Brazil. The fossil remains of Megalocnus rodens collected by Mr. Barnum Brown and Dr. Carlos de La Torre in Ciego Montero, although unassociated, were enough 424 JOURNAL OF MAMMALOGY Vol. 37, No. S to permit the assemblage of two composite skeletons which are shown in the exhibition halls of The American Museum of Natural History in New York, and the Museo Poey de la Universidad de Havana. They are composed of topotypes of Megalocnus rodens. The work of assembling both the skeletons and making the mounts was accomplished by Mr. A. Herrmann, former technician of the Laboratory of Vertebrate Paleontology of The American Museum of Natural History. The present paper is a preliminary note concerning these two mounted skeletons of Megalocnus rodens. I want to thank Dr. George Gaylord Simpson and The American Museum of Natural History for the opportunity given me to complete Matthew's observa- tions on that important collection. A grant from the Conselho Nacional de Pes- quisas, Rio de Janeiro, permitted me to devote full time to this research. The assistance given me by the Conselho also made possible the fine illustrations of Mr. Ulisses Bastos Freitas, whom I also want to thank for his excellent work. SYSTEMATICS Order Edentata Cuvier, 1798 Suborder Xenarthra Cope, 1889 Infraorder Pilosa Flower, 1883 Superfamily Megatherioidea Cabrera, 1929 Family Megalonychidae Zittel, 1892 Subfamily Megalocninae Kraglievich, 1923 Megalocnus Leidy, 1868. Megalocnus Leidy, 1868, p. 180. Megalonyx (Myomorphus)Pomel, 1868, p. 665. Megalochnus Ameghino, 1881, p. 303 (invalid emendation or misspelling). Megalonyx, Lydekker, 1887, p. 111 (including "Megalochnus"-Myomorphus),nec Harlan, 1825. Megalonyx (Megalochnus), Zittel, 1894, p. 136. Parocnus Miller, 1930, in part (?). Genotype: Megalocnus rodens Leidy, 1868. Diagnosis.-Teeth 5/4, the first upper and lower pair enlarged and spaced as in Megal- onyx, but approximated medially, flattened into a scalproform type, broadly convex an- teriorly, concave posteriorly; the cheek teeth like those of Megalonyx but longer. Palate greatly depressed in relation to the basicranial axis, much as in the Glyptodontia. Condyles much elevated above tooth row; anterior border of coronoid process between second and third molars. Limb bones as in Santacruzean megatherioids. Humerus with large entepi- condylar foramen. Manus as in Santacruzean forms, but metacarpals less differentiated, and unguals long, slender, comparatively straight and but little compressed (mainly after Matthew, in Matthew and Paula Couto's "The Cuban Edentates," ms.). The skeleton in The American Museum of Natural History is mounted in quadrupedal or walking position (Fig. 1), and the skeleton exhibited in the Museo Poey de la Universi- dad de Havana is in a semi-erect position, having its fore feet applied against the stock of a tree. The skull and mandible of Megalocnus rodens resemble particularly those of the tree- sloths of today, and the glyptodonts, by their general shape. They are more globose, in conjunction, than elongate. The zygomatic arch is widely open, the jugal bone being slender and reduced (the figure of the skull, in Scott, 1937: 664, Fig. 396, is erroneous in showing a closed zygomatic arch). The teeth are very like those of Megalonyx, but the front ones are more specialized, somewhat rodent-like. The neck is relatively elongated, the body some- what heavy. The ilium is broad, but much less expanded anteroposteriorly than in Megal- Aug., 1956 PAULA COUTO--MEGALOCNUS 425 VW __1- - !. V'Utr_ FIG. 1.-TOP: Megalocnus rodens Leidy. Skeleton from Ciego Montero, Cuba. (After a photograph of the composite specimen shown in The American Museum of Natural History.) BoTToM:Megalocnus rodens Leidy. Restoration based on the skeleton in The American Museum of Natural History. onyx and Paramylodon, for example, recalling that of Nothrotheriumin this aspect. The tail is rather short. The limb and foot characters are much as in the Miocene (Santacruzean) relatives (Hapalops, etc.) but with elongate ungual phalanges. They are very primitive and unspecialized. The morphology of the astragalus shows that the animal, like the ant- eaters (Myrmecophaga),had no tendency to walk upon the outer side of the pes, as is the case with the continental Pleistocene relatives; the pes, as the manus, was plantigrade (Fig. 1). The skeleton, in general, is most like that of the Santacruzean megalonychids, but somewhat more massive, although not nearly so much as in Megalonyx. Megalocnus is the most specialized genus of the West Indian group, and of its family in general. It includes the largest and most abundant of the Antillean species of the group, Megalocnus rodens, the size of which was about, or a little larger than, that of an American black bear (Ursus americanus). Its weight would have been about 600 pounds (270 kilos) in the fully adult condition. According to Matthew, the West Indian ground-sloth genera may all be derived from a single invading ancestral type, Eucholoeops or some near ally, reaching the Antilles by passive way in Miocene or early Pliocene times, and there specializing into the several 426 JOURNAL OF MAMMALOGY Vol. S7, No. S genera of the Pleistocene (so far as one may judge from the diversity in its late Pleistocene descendants, and from their structural relations to the progressive evolutionary stages of their continental relatives in the Tertiary and Pleistocene history). It seems to me that this is, indeed, the most plausible hypothesis for the origin of the Antillean genera, since all them (Acratocnus, Mesocnus, Microcnus, and Megalocnus), although very specialized present, as it is known, primitive features closely recalling the Miocene (Santacruzean) ancestors, such as Eucholoeops, Hapalops, and Schismotheriumof Patagonia. Their evolution, as generally true with the evolution of all the insular or in- sulated animal groups, was bradytelic relative to that of the continental ground sloths. In the ground sloths the evolving rate, although not so slow, was nevertheless slower than the common or standard rate, as exemplified by the evolution of the Holarctic and Sonoran mammalian groups in general. Otherwise, all the characteristic mammalian groups of the Cenozoic of the Neotropical Region in South and Central America, where the ecological roles were divided between marsupials and more or less archaic placentals (edentates, protungulates, and paenungulates of primitive aspect) although differentiated in a rela- tively large number of zones of adaptive radiation, seem to have evolved at a slower rate than the contemporary groups in the Holarctic and Sonoran regions. These latter were more successful, "probably because they were already the selected survivors of a long series of intermigrations with Eurasia while South American mammals had been evolving in isolation" (Simpson, 1953a: 26). South America, we must remember, was an isolated continent during most of Tertiary times, developing in that time a particular fauna. This was derived, at least in its greater part, from pre-Cenozoic primitive North American ancestors.