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The Youngest Species of the Aquatic Sloth Thalassocnus and a Reassessment of the Relationships of the Nothrothere Sloths (Mammalia: Xenarthra)

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The Youngest Species of the Aquatic Sloth Thalassocnus and a Reassessment of the Relationships of the Nothrothere Sloths (Mammalia: Xenarthra) Journal of Vertebrate Paleontology 24(2):387±397, June 2004 q 2004 by the Society of Vertebrate Paleontology THE YOUNGEST SPECIES OF THE AQUATIC SLOTH THALASSOCNUS AND A REASSESSMENT OF THE RELATIONSHIPS OF THE NOTHROTHERE SLOTHS (MAMMALIA: XENARTHRA) CHRISTIAN DE MUIZON1, H. GREGORY MCDONALD2, RODOLFO SALAS3, and MARIO URBINA3 1UMR 8569 CNRS, DeÂpartement Histoire de la Terre, MuseÂum national d'Histoire naturelle, 8, rue Buffon, F-75005 Paris, France, [email protected]; 2National Park Service, 7333 West Jefferson Ave, P.O. Box 25287, Denver, Colorado 80225, U.S.A.; 3Departamento de PaleontologõÂa de Vertebrados, Museo de Historia Natural (UNMSM), Av. Arenales 1256, Lima 14, Peru ABSTRACTÐTwo new specimens of the aquatic sloth Thalassocnus from the Pliocene of Peru are described, one of T. carolomartini McDonald and Muizon, 2002 and the other T. yaucensis, sp. nov. Comparisons with the type species of Thalassocnus, T. natans, demonstrates that T. carolomartini and T. yaucensis are more similar morphologically to each other than to other species of the genus and are more derived. For example, both have a more elongated rostrum (premaxillae and mandibular spout) and more robust, quadrate to circular molariform teeth. The aquatic sloth Thalas- socnus is suf®ciently distinct morphologically that it is placed in a new subfamily, the Thalassocninae, and the Noth- rotheriinae is raised to family rank. The morphological features shared by T. carolomartini and T. yaucensis indicate these species were better adapted to grazing on marine vegetation than earlier species of Thalassocnus. INTRODUCTION GEOLOGY Two specimens recently discovered in the Pisco Formation The Pisco Formation is a Neogene marine unit present along of the Sacaco Region, Peru, provide additional insight into the the southern coast of Peru. It has yielded an abundant fauna of evolutionary history of the Thalassocnus lineage of nothrothere marine vertebrates including ®shes, reptiles, birds and mam- sloths (Muizon and McDonald, 1995; McDonald and Muizon, mals. Hoffstetter (1968), Muizon (1981), Muizon and DeVries 2002; Muizon et al., 2003). One is a partial skull and mandible (1985), and Marocco and Muizon (1988) reported abundant re- along with associated hyoid elements, foot bones, and a femur. mains of sloths, which were regarded by Muizon and Mc- This specimen is from the locality of Sacaco and was discov- Donald (1995) as having semi-aquatic habits. This interpreta- ered in the SAO (Sacaco) vertebrate-bearing horizon. It is, tion was initially based on the taphonomy of the specimens. therefore, late early to early late Pliocene in age (Muizon and The Peruvian coast was a desert during the late Neogene (SeÂ- DeVries, 1985). Although slightly smaller and more gracile, it brier et al., 1984; Todsal et al., 1984; Alpers and Brimhall, is otherwise very similar to the holotype of Thalassocnus car- 1988). These data, combined with the extreme abundance and olomartini McDonald and Muizon, 2002, also from the SAO excellent preservation (articulated skeletons) of the sloth re- Horizon, and is referred to this species. The second specimen mains and the fact that virtually no other land mammals have was discovered at Yauca, 10 km south of Sacaco, in a verte- been found in the Pisco Formation, led to the conclusion that brate-bearing horizon that was initially regarded as equivalent they regularly entered the ocean to feed on marine vegetation, in age to the SAO Horizon. No geologic correlation has been either seaweed or sea grass. Many of the anatomical features established between the two localities and, so far, no biostrati- of the Pisco nothrothere, Thalassocnus natans, are compatible graphic evidence indicates that the Yauca beds are the same age with an interpretation of an aquatic or semi-aquatic life (Muizon as the SAO Horizon. The Yauca specimen is a partial skeleton and McDonald, 1995). with an exceptionally well-preserved skull and mandible, much McDonald and Muizon (2002) and Muizon et al. (2003) de- better than that of the holotype of T. carolomartini and one of scribed three additional species of Thalassocnus that were re- the best-preserved specimens of the Thalassocnus lineage. Comparison of the Yauca specimen and those of T. carolomar- covered from horizons above and below that in which the ho- tini with the older species of Pisco sloths (T. antiquus, T. na- lotype of T. natans was found. The oldest species, Thalassocnus tans, T. littoralis) indicate that T. carolomartini and the Yauca antiquus, is from the Aguada de Lomas Horizon (AGL), ca. 7 specimen are more similar morphologically and are more de- to 8 Ma; T. natans from the Montemar Horizon (MTM), ca. 6 rived. Despite the presence of numerous shared features, the Ma; Thalassocnus littoralis from the Sud-Sacaco (SAS) Hori- morphology of the mandibular spout in the Yauca specimen zon, ca. 5 Ma; and T. carolomartini from the Sacaco (SAO) differs signi®cantly from that of the two specimens of T. car- Horizon, ca. 4 to 3 Ma. The stratigraphy of the Pisco Formation olomartini and, because of this feature, is referred here to a used here follows Muizon and DeVries (1985). Based on the new species of Thalassocnus. above four species, McDonald and Muizon (2002) and Muizon The purpose of this paper is to describe the skull and man- et al. (2003) raised the hypothesis of the existence of a Thal- dible of the new referred specimen of T. carolomartini and the assocnus lineage spanning four different vertebrate horizons Yauca specimen. They are compared to the holotypes of T. car- and at least 4 Ma. However, Muizon et al. (2003) also admitted olomartini and the genotypic species T. natans, which is the that Thalassocnus might have had a more complex evolutionary best preserved of the other three species in the genus. The Yau- history. ca specimen was closely associated with several bones of a Abbreviations MNHN, MuseÂum national d'Histoire natu- second smaller individual indicating a fetal or newborn onto- relle, Paris, France; MUSM, Museo de Historia Natural de la genetic stage. This is regarded as an indication that the skeleton Universidad Nacional Mayor de San Marcos, Lima, Peru; is probably that of a female. SMNK, Staatliche Museum fuÈr Naturkunde, Karlsruhe, Ger- 387 388 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 24, NO. 2, 2004 many; UPCH, Universidad Peruana Cayeteno Heredia, Lima, Megatheriidae, but the validity of these characters was ques- Peru. tioned by De Iuliis (1994). Patterson et al. (1992) included the Nothrotheriinae within the Megatheriidae using characters of SYSTEMATIC PALEONTOLOGY the ear region and noted that the Megatheriinae and Nothroth- eriinae shared a number of derived characters. McKenna and Order XENARTHRA Cope, 1889 Bell (1997) altered this classi®cation by creating the tribe Noth- Suborder PHYLLOPHAGA Owen, 1842 rotheriini, within the subfamily Megatheriinae, family Megath- Superfamily MEGALONYCHOIDEA Simpson, 1931 eriidae, thus eliminating the subfamily Nothrotheriinae alto- Family NOTHROTHERIIDAE Ameghino, 1920, new rank gether. Gaudin and De Iuliis (1999) used the designation Noth- Included subfamilies Nothrotheriinae Ameghino, 1920; rotheriidae in their discussion of the relationship of Nothropus Thalassocninae, subfam. nov. priscus and considered the group to be monophyletic based on Revised Diagnosis Skull elongated anterior to orbits and three shared characters: separation of the patellar trochlea from muzzle smaller than posterior part of skull; supraorbital fora- the medial and lateral femoral condyles; an enormous vomerine men present; zygomatic arch incomplete with jugal separated keel in the nasopharynx; and an anteriorly directed zygomatic from zygomatic process of squamosal; anteriorly directed zy- root. gomatic root; enormous vomerine keel in nasopharynx; man- It should be noted that the family Nothrotheriidae as used dibular symphysis elongated into a spout. Dentition either 5/4 here does not include the Miocene genera Hapalops, Synha- or 4/3; when dentition is 5/4, anteriormost tooth in skull and palops and Diheterocnus, which have sometimes been included mandible modi®ed into a caniniform positioned midway be- within the nothrotheres. These genera may be considered equiv- tween anterior of skull and molariform cheek teeth. Molari- alent to the Schismotherium group identi®ed by Patterson et al. forms square in cross section with a well-developed transverse (1992) and formally identi®ed as the subfamily Schismotheri- loph on the mesial and distal edges of occlusal surface; molar- inae by McKenna and Bell (1997). While there may be a close iforms have a longitudinal groove on labial and lingual sides. relationship between these genera and the Nothrotheriidae (as Ungual on digit two of manus semicircular in cross section de®ned here), and although McKenna and Bell (1997) included while all other unguals on the manus and pes are an isosceles the genera Synhapalops and Diheterocnus in their Nothrother- triangle; third trochanter present on femur; patellar trochlea of iini, as of yet there is insuf®cient evidence to establish exactly femur separated from articular surfaces of distal condyles; tuber what this relationship is. While further work will certainly elu- calcis of calcaneum expanded equally both anteroposteriorly cidate these relationships, such an analysis is beyond the scope and mediolaterally; astragalus with medial trochlea modi®ed of this paper. into an odontoid process and lateral trochlea anteroposteriorly Recent research on the Thalassocnus lineage and
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