Christian de Muizon

The giant bite of a new raptorial sperm whale from the Miocene epoch of Peru

The modern giant sperm whale Physeter macrocephalus, one of the largest known predators, preys upon cephalopods at great depths. Lacking a functional upper dentition, it relies on suction for catching its prey; in contrast, several smaller Miocene sperm whales (Physeteroidea) have been interpreted as raptorial (versus suction) feeders, analogous to the modern killer whale Orcinus orca. Whereas very large physeteroid teeth have been discovered in various Miocene localities, associated diagnostic cranial remains have not been found so far. Here we report the discovery of a new giant sperm whale from the Middle Miocene of Peru (approximately 12–13 million years ago), Leviathan melvillei, described on the basis of a skull with teeth and mandible. With a 3-m-long head, very large upper and lower teeth (maximum diameter and length of 12 cm and greater than 36 cm, respectively), robust jaws and a temporal fossa considerably larger than in Physeter, this stem physeteroid represents one of the largest raptorial predators and, to our knowledge, the biggest tetrapod bite ever found. The appearance of gigantic raptorial sperm whales in the fossil record coincides with a phase of diversification and size-range increase of the baleen-bearing mysticetes in the Miocene. We propose that Leviathan fed mostly on high-energy content medium-size baleen whales. As a top predator, together with the contemporaneous giant shark Carcharocles megalodon, it probably had a profound impact on the structuring of Miocene marine communities. The development of a vast supracranial basin in Leviathan, extending on the rostrum as in Physeter, might indicate the presence of an enlarged spermaceti organ in the former that is not associated with deep diving or obligatory suction feeding.

Dentition and Jaw of Kokopellia juddi, a Primitive Marsupial or Near-Marsupial from the Medial Cretaceous of Utah

We add to the knowledge of the dentition and lower jaw of the primitive marsupial or near marsupial, Kokopellia juddi, based on newly collected materials from the medial Cretaceous (Albian–Cenomanian) of central Utah. The dental formula, i4 c1 p3 m4, is primitive for (or with respect to) Marsupialia, as are a number of features of the dentary and dentition: presence of a labial mandibular foramen, ?an inflected angle, ?and a trace of the meckelian groove; lack of “staggering” of the lower incisor series; lack of “twinning” between entoconid and hypoconulid on lower molars; incompletely lingual position of lower molar paraconid; upper molar protocone relatively small and mesiodistally narrow; and conules placed about halfway between the protocone and the paracone–metacone. Other than the stylocone, cusps are lacking from the stylar shelf; we argue that this represents the primitive marsupial condition based on the economy of character change and the stratigraphic record of marsupials in the Cretaceous of North America. Recent discoveries of early marsupials, eutherians, and therians of metatherian–eutherian grade provide data indicating that some derived features of the dentary and dentition (e.g., loss of coronoid, meckelian groove, and labial mandibular foramen; acquisition of strong, “winged” conules, double rank postvallum/prevallid shearing, and stylar cusp D) probably arose independently, in some cases more than once, among the major groups of tribosphenic mammals. In turn, this suggests that a common ancestor for marsupials and placentals was more primitive than has generally been appreciated.

Exceptionally preserved North American Paleogene metatherians: adaptations and discovery of a major gap in the opossum fossil record

A major gap in our knowledge of the evolution of marsupial mammals concerns the Paleogene of the northern continents, a critical time and place to link the early history of metatherians in Asia and North America with the more recent diversification in South America and Australia. We studied new exceptionally well-preserved partial skeletons of the Early Oligocene fossil Herpetotherium from the White River Formation in Wyoming, which allowed us to test the relationships of this taxon and examine its adaptations. Herpetotheriidae, with a fossil record extending from the Cretaceous to the Miocene, has traditionally been allied with opossums (Didelphidae) based on fragmentary material, mainly dentitions. Analysis of the new material reveals that several aspects of the cranial and postcranial anatomy, some of which suggests a terrestrial lifestyle, distinguish Herpetotherium from opossums. We found that Herpetotherium is the sister group to the crown group Marsupialia and is not a stem didelphid. Combination of the new palaeontological data with molecular divergence estimates, suggests the presence of a long undocumented gap in the fossil record of opossums extending some 45 Myr from the Early Miocene to the Cretaceous.

The origin of the dog-like borhyaenoid marsupials of South America

Dog-like marsupials (superfamily Borhyaenoidea) were the largest predacious mammals during the Tertiary period in South America. They are critical to our understanding of marsupial origin, phylogeny and palaeobiogeography because they have been related to various marsupial lineages of several continents: didelphoids (mainly New World, but also Europe, Asia and Africa), pediomyid, stagodontids (North America), dasyuroids (Australia) and deltatheroidans (predominantly Asian). These relationships, based mainly on dental morphology, have been discussed and rejected several times,,,. Here we report the discovery of exceptionally well preserved skulls and skeletons, referrable to the didelphoid Andinodelphys, which shed new light on the phylogenetic and palaeobiogeographic origin of dog-like marsupials. The skulls of Mayulestes (boryhyaenoid), Andinodelphys and Pucadelphys (didelphoids), from the early Palaeocene epoch of Bolivia are the oldest known for American marsupials. Comparison of their basicranial anatomy suggests that dog-like marsupials are closely related to an early didelphimorphian radiation in South America, rather than to Asiatic (deltatheroidan), North American (stagodontid), or Australian (dasyuroid) lineages.

A new carnivorous marsupial from the Palaeocene of Bolivia and the problem of marsupial monophyly

THE distinctive soft anatomy and reproductive biology of marsupials sets them apart as a unique group within mammals. These features are, of course, absent in fossils, so it is difficult to determine marsupial origins and evolution: many of the proposed dental and skeletal characters are controversial1 or primitive1,2. A contribution of the alisphenoid bone to the tympanic bulla of the skull has long been considered a reliable diagnostic feature of the group1–5. But this feature is lacking both in the borhyaenoid marsupial Mayulestes ferox, which I describe here, and the didelphoid Pucadelphys andinus6,7, both from the early Palaeocene of Bolivia7–9. Comparison with younger taxa suggests that this feature appeared several times independently within the group, and is thus an inappropriate defining character. What, then, is a marsupial?

THE YOUNGEST SPECIES OF THE AQUATIC SLOTH THALASSOCNUS AND A REASSESSMENT OF THE RELATIONSHIPS OF THE NOTHROTHERE SLOTHS (MAMMALIA: XENARTHRA)

Abstract Two 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 Thalassocnus is sufficiently distinct morphologically that it is placed in a new subfamily, the Thalassocninae, and the Nothrotheriinae 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.

The evolution of feeding adaptations of the aquatic sloth Thalassocnus

Abstract The aquatic sloth Thalassocnus is represented by five species that lived along the coast of Peru from the late Miocene through the late Pliocene. A detailed comparison of the cranial and mandibular anatomy of these species indicates different feeding adaptations. The three older species of Thalassocnus (T. antiquus, T. natans, and T. littoralis) were probably partial grazers (intermediate or mixed feeders) and the transverse component of mandibular movement was very minor, if any. They were probably feeding partially on stranded sea weeds or sea grasses, or in very shallow waters (less than 1 m) as indicated by the abundant dental striae of their molariform teeth created by ingestion of sand. The two younger species (T. carolomartini and T. yaucensis) were more specialized grazers than the three older species and had a distinct transverse component in their mandibular movement. Their teeth almost totally lack dental striae. These two species were probably feeding exclusively in the water at a greater depth than the older species.

Late Cretaceous vertebrates, including mammals,from Tiupampa, Southcentral Bolivia

Abstract A taxonomically diverse fossil fauna with small and medium size vertebrates was recovered from the Late Cretaceous (Maestrichtian) age El Molino Formation at Tiupampa, southcentral Bolivia. Among the known taxa, many new to science, are selacians, actinopterygians, lungfish, amphibians, lizards, snakes, turtles, crocodiles, and several mammals. This finding provides a unique and previously unavailable opportunity to understand debated aspects of the phylogenetic and biogeographic histories of these groups.

The cranial anatomy of Thalassocnus (Xenarthra, Mammalia), a derived nothrothere from the Neogene of the Pisco Formation (Peru)

Abstract The Pisco Formation (Miocene–Pliocene) of Peru contains a diverse marine vertebrate fauna. Among the more unusual members of the fauna is the semi-aquatic to aquatic nothrothere sloth, Thalassocnus. Continued fieldwork indicates that, in addition to the late Miocene type species, Thalassocnus natans, the genus is also represented by two additional species in higher horizons of the Pisco Formation. The skull morphology of T. natans is described along with that of Thalassocnus littoralis, sp. nov. and Thalassocnus carolomartini, sp. nov. from the earliest and late early to early late Pliocene, respectively. Many aspects of the cranium of the oldest species of Thalassocnus, T. natans, compare well to those of terrestrial nothrotheres. The terminal species, T. carolomartini, has the most derived features of the skull, absent in all other nothrotheres, that are related to feeding in a marine environment.

Walrus-like feeding adaptation in a new cetacean from the Pliocene of Peru

THE recent discovery in the Southern Hemisphere (15.5 °S) of a walrus-like skull of a toothed whale (odontocete) from the Pisco Formation of southern Peru presents a startling example of convergence and specialization unprecedented among cetaceans. In contrast to other toothed whales, Odobenocetops peruvianus has no elongated rostrum but has large, ventrally directed premaxillary alveolar processes housing asymmetrical tusks. The dorsally facing orbits indicate the possibility of dorsal binocular vision which could compensate for the absence of the melon, a rostral organ involved in echolocation. Strong muscle scars on the anterior edge of the premaxillae suggest the presence of a powerful upper lip. It is this feature, together with the morphology of the deep vaulted palate devoid of maxillary teeth, that enables us to hypothesize a convergent feeding adaptation with the walrus which feeds mainly on thin-shelled bivalve molluscs, sucking out the foot and/or siphon and ejecting the shell. The structure of the face and basicranium indicates it was a delphinoid cetacean, probably related to the living beluga and narwhal (Monodontidae).