Mammalian Evolution

How Mammals Conquered the Oceans

Into Landscapes of Fear: Marsupial Predators

Running Over the Same Old Ground: Stegomastodon Never Roamed South America Abstract The diversity of South American proboscideans during the Quaternary has been a subject of discussion for decades. The presence of Cuvieronius hyodon in South America is unquestioned and unanimous; however, the taxonomy of the known second South American proboscidean is still a controversy. Some authors argue that the South American species traditionally referred to Stegomastodon should instead be referred to a distinc genus, namely Notiomastodon, endemic to South America. Others authors, however, do not accept this attribution and continue to recognize the genus Stegomastodon as present in South America. In this study, we recognize several differences in the mandible, skull, dentition, and postcranial morphology of North American species of Stegomastodon and Notiomastodon, that further support the validity of Notiomastodon as a taxon separate from Stegomastodon. Indeed, a phylogenetic hypothesis of trilophodont bunodont proboscideans supports the separation between Stegomastodon and Notiomastodon, and the diversification of the common ancestor of these proboscideans may have occurred during the middle to late Miocene. No specimen with Stegomastodon diagnostic features is recognized in South America. The Stegomastodon records are restricted to the Pliocene-middle Pleistocene of North America, while Notiomastodon records are found only from the middle Pleistocene-early Holocene of South America. In this way, we recognize that Stegomastodon records are restricted to North America and that only Cuvieronius and Notiomastodon are recorded in South America.

Growth Pattern and Functional Morphology of the Cervical Vertebrae in the Gerenuk ( Litocranius walleri ): The Evolution of Neck Elongation in Antilopini (Bovidae, Artiodactyla) Abstract Long necks have evolved independently in several different taxa, but the processes underlying the evolution of this trait are not yet fully understood. In this study, we examined the skeletal mechanism underlying the neck elongation in the tribe Antilopini (Bovidae, Artiodactyla). We calculated the growth patterns of the cervical vertebrae in the gerenuk (Litocranius walleri), which possesses the longest neck in this tribe, and compared it with those in two related species. The growth rates of the vertebrae were not significantly different between species, suggesting that the long neck of the gerenuk has resulted from the elongation of the cervical vertebrae during the fetal or juvenile stage. The morphology of the cervical vertebrae of gerenuks differed from that of the closely related, relatively long-necked dama gazelle (Nanger dama), with protrusions occurring on the dorsal surface of the ventral arch of the atlas. This implies that gerenuks possess a well-developed transverse ligament of the atlas that functions to hold the dens of the axis against the atlas. We also found that the atlas lies in close proximity to the neural spine of the axis in the gerenuk, suggesting that hyperextension of the atlantoaxial joint is osteologically limited in this species. While foraging on high foliage, gerenuks flex and extend their necks freely in a bipedal posture without moving their entire body. These morphological characteristics peculiar to the gerenuk enhance the rigidity of the atlantoaxial joint and decrease the risk of subluxation of the joint during this unique foraging behavior.

Interspecific Chromosome Painting Provides Clues to the Ancestral Karyotype of the New World Monkey Genus Aotus Abstract The Neotropical monkey genus Aotus (owl or night monkeys) are among the most karyological diverse primates of the world. Their diploid numbers range from 2n = 46 to 58, but even owl monkeys with the same diploid number may have radically different karyotypes. This karyotypic variability has provided precious information for taxonomists and has a potential for aiding phylogenetic analysis of these primates. However, up to now only three out of 11 species have been analyzed with molecular cytogenetic methods. Here, we report on a fourth species, A. infulatus. Females have a diploid number of 2n = 50 while males, due to a Y/autosome translocation, have 49 chromosomes. We provide a complete map of chromosome homology between humans and A. infulatus. Comparisons with previous reports allowed us to propose a putative ancestral karyotype of the genus (2n = 52) and to deduce the rearrangements that were involved in the origin of each species chromosome complement. Integration of chromosome painting and banding analysis suggests at least three chromosomes have evolutionary new centromeres that appeared during the divergence of these four owl monkey species.

What Can an Invasive Species Tell Us about Evolution? A Study of Dental Variation in Disjunctive Populations of Microtus rossiaemeridionalis (Arvicolinae, Rodentia) Abstract The sibling vole, Microtus rossiaemeridionalis, is a North Eurasian rodent that undergoes range expansion via casual introductions by humans. The documented cases of human-mediated spread of M. rossiaemeridionalisprovide an opportunity to explore phenotypic consequences associated with the invasion events. We present an analysis of dental variability in two recently discovered invasive populations of M. rossiaemeridionalis in northern Asia (Surgut and Khabarovsk) and summarize the data on conspecifics within and outside the core range in order to uncover common and specific patterns of dental variation in the disjunctive invasive populations, and to consider the potential evolutionary significance of the phenotypic effects from neontological and paleontological standpoints. The analysis of morphotype dental patterns and inspection for rare traits suggest that the existence of invasive populations under conditions of isolation leads to a release of hidden phenotypic variation, which could be inferred from sharp increases in frequencies of reserve morphotypes and/or rare dental traits, and also from the presence of abnormalities. An atavistic anomaly of the third upper molars revealed in one individual in a fragmented habitat in Surgut recapitulates some features of the prismatic arrangement and occlusal pattern of extinct Mimomys-like arvicolines. All variants of released phenotypic variation in the invasive populations of M. rossiaemeridionalis under conditions of isolation could be interpreted as de-specialization of dentition. Such de-specialization appears to be maladaptive for a herbivore, though it might favor a transition to a more generalized diet and enhance the success during the transport, colonization, and establishment stages of the invasion.

Impact of Orogeny and Environmental Change on Genetic Divergence and Demographic History of Dipus sagitta (Dipodoidea, Dipodinae) since the Pliocene in Inland East Asia Abstract Inland East Asia encompasses particular landscapes, including discontinuous large deserts isolated by mountains, and these landscapes have been greatly impacted by the uplift of the Qinghai-Tibetan Plateau (QTP) since the Pliocene. However, little research has been performed on the impact of desertification on the evolutionary history of animals in this area. We examined a widespread desert rodent species, Dipus sagitta, to better understand the influences of geological events on the evolutionary history and phylogeographic patterns. We sequenced two mitochondrial genes and three nuclear genes from 237 individuals collected from 43 populations across inland East Asia. Phylogenetic, network, intraspecific delimitation, and population structure analyses identified a structured pattern of geographic differentiation with six well-defined evolutionary clades. High mountains (such as the Tianshan Mountains) and important climate demarcation lines (such as the 200 mm isohyet) were inferred as genetic barriers among the six clades by BARRIER analysis. The most recent common ancestor of D. sagitta was estimated to have existed in late Miocene, and the first clade was estimated to have diverged at c. 7.57 Ma, whereas the later clade diversified rapidly at approximately 2.56–1.53 Ma during the Pleistocene. Demographic analyses suggested different demographic histories among the distinct clades, and the continuous expansion that occurred during the climate oscillation period appeared to be more closely related to the increasing aridification caused by orogeny rather than climate oscillation.

Cryptoprocta spelea (Carnivora: Eupleridae): What Did It Eat and How Do We Know? Abstract The extent to which Madagascar's Holocene extinct lemurs fell victim to nonhuman predators is poorly understood. Madagascar's Holocene predator guild included several now-extinct species, i.e., crocodiles, carnivorans, and raptors. Here we focus on mammalian carnivory, specifically the roles of Cryptoprocta spelea and its still-extant but smaller-bodied sister taxon, C. ferox, the fosa. Cryptoprocta spelea was the largest carnivoran on Madagascar during the Quaternary. We ask whether some extinct lemurs exceeded the upper prey-size limits of C. spelea. We use univariate and multivariate phylogenetic generalized least squares regression models to re-evaluate the likely body mass of C. spelea. Next, we compare characteristics of the forelimb bones of C. ferox and C. spelea to those of other stealth predators specializing on small, mixed, and large-bodied prey. Finally, we examine humeri, femora, crania, and mandibles of extinct lemurs from six sites in four ecoregions of Madagascar to identify damage likely made by predators. We test the relative prevalence of carnivory by mammals, raptors, and crocodiles at different sites and ecoregions. Our data reveal that crocodiles, raptors, and the largest of Madagascar's mammalian predators, C. spelea, all preyed on large lemurs. Cryptoprocta opportunistically consumed lemurs weighing up to ~85 kg. Its forelimb anatomy would have facilitated predation on large-bodied prey. Social hunting may have also enhanced the ability of C. spelea to capture large, arboreal primates. Cryptoprocta carnivory is well represented at cave and riverine sites and less prevalent at lake and marsh sites, where crocodylian predation dominates.

New Bovid Remains from the Early Pleistocene of Umbria (Italy) and a Reappraisal of Leptobos merlai Abstract The extinct bovid Leptobos is one of the most characteristic elements of Eurasian faunal assemblages during most of the Villafranchian Land Mammal Age (i.e., from the late Pliocene to most of the early Pleistocene). Several species of this genus have been established since the end of XIX Century, but their taxonomic status and phylogenetic relationships remain unclear due to the fact that most of them are described on the basis of scanty material. European species are divided into two groups or lineages. The first includes L. stenometopon, L. merlai, and the poorly known L. furtivus, the second L. etruscus and L. vallisarni. While the last two species are well documented in the Italian early Pleistocene fossil record, very little is known on the L. stenometopon-merlai-furtivus group and especially on L. merlai, whose richest sample is from the French locality of Saint Vallier. Here, we describe new material of L. merlai from the early late Villafranchian of Umbria (central Italy), including a nearly complete female cranium and a male neurocranium with horn cores. These remains constitute the best-preserved and complete sample of L. merlai in the Italian Peninsula and bring new consistency to the fossil record of this species. In addition, they serve to confirm the extension of the spatial distribution of L. merlai to the south and of the chronological distribution of this taxon from the middle to the early late Villafranchian. Finally, we offer critical remarks on some not well defined Leptobos species.

Newly Discovered Crania of Nyanzachoerus jaegeri (Tetraconodontinae, Suidae, Mammalia) from the Woranso-Mille (Ethiopia) and Reappraisal of Its Generic Status Abstract Suids are among the most common mammalian groups in the Plio-Pleistocene vertebrate fossil record of Africa and the most studied largely due to their significance as biochronological indicators. However, despite their abundance in the fossil record, the remains are mostly isolated teeth and fragmentary crania and mandibles. As a result, disagreements have persisted in terms of their taxonomy and phylogenetic relationships. Here, we present for the first time a detailed description of the cranial anatomy of Nyanzachoerus jaegeri based on two crania recovered from middle Pliocene deposits of the Woranso-Mille paleontological study area, Afar region, Ethiopia. Understanding the cranial morphology of this species is particularly significant given the recent reclassification of Nyanzachoerus jaegeri to the genus Notochoerus based largely on the incisor and symphyseal morphology of specimens from Kanapoi, Kenya. Here, we show that the two genera are clearly distinguished from each other by distinct morphological features of the cranium such as the shape of the braincase, orientation of the zygomatic arches, and premolar/molar ratio, among others. Furthermore, we show that the mandibular and dental morphological features identified by some workers as characteristic of Notochoerus are variable among tetraconodont species and that Nyanzachoerus jaegeri best fits within the genus Nyanzachoerus.