Modern Rodents Research Articles

Morphofunctional Transformations of the Jaw Muscles in Rodent Evolution

The paper summarizes results of long-term research on the jaw apparatus of modern rodents. The studied material includes original data on 22 families of the order Rodentia. The structure of the jaw muscles and the pathways of transformations during the evolution of the order, as well as the functional and phylogenetic aspects of its diversity in this group, are considered based on the concept of the jaw apparatus as an integral system. The structure of the jaw muscles has been studied in detail. It is shown that, despite their external diversity, aponeurotic carcasses have the same structure type; this can serve as a reliable basis for the identification and homologization of the elements of the internal muscle differentiation. The morphofunctional specialization of the jaw muscles is discussed. The morphotypes of the jaw apparatus, which reflect its adaptability to crushing and to vertical cutting are highlighted. Particular attention is paid to the description of two directions of specialization in the longitudinal grinding that is typical only for rodents. The first variant is manifested in hypertrophy of the anterior parts of the masseter, simplification of its differentiation, and a more horizontal orientation of its lateral and medial portions against the background of weakening of the temporal muscle. The second variant is associated with enhancement of internal differentiation of the masseter due to a special transformation of the lower jaw: an increase in its height, a shift of the angular process up to the dentition level, and a highly raised articular process. The correspondence of the types of jaw apparatus to the basal branches of Rodentia phylogenetic tree has been revealed. It is assumed that the typological diversity of this system evolved at the early stages of adaptive radiation of rodents. The similarity of these types within the phyletic lines results from the relatedness of the species. The functional specialization of the jaw apparatus within the basal groups was based on their structural type. It occurred mainly due to changes in the mandible and zygomatic arch and was expressed in similar spectra of its morphofunctional transformations in different rodent taxa.

Extinction rates of the Meade Basin rodents: application to current biodiversity losses

Extinction rates for terrestrial rodent species from palaeontological sites in the Meade Basin of southwestern Kansas and an archaeological site in New Mexico are compared with extinction rates for modern rodents from locations affected by anthropogenic activities. Background extinction rates are defined as global extinctions occurring over proscribed intervals in the absence of significant environmental perturbations. Background rates for the Meade Basin are estimated at 0–~1.0 E/MSY (extinctions per million species years). Elevated rates from 1.4 to 6.25 E/MSY are associated with volcanic events and Late Pleistocene environmental change. These rates are considerably less than those for rodent extinction rates promoted by human activities during the Holocene, the latter ranging from 42.3 to 50,000 E/MSY.

Cranial dimensions as estimators of body mass and locomotor habits in extant and fossil rodents

ABSTRACTEstimating body mass and locomotor habits of fossil rodents is challenging for taxa without available postcranial material. Although equations exist to estimate body mass from dental dimensions based on extant rodents, the applicability of such equations is doubtful given that modern rodents have evolved highly specialized teeth, whereas fossil taxa often exhibit a much less derived condition. For the present study, 11 cranial variables from a sample of 203 extant rodents of known body mass representing a range of taxonomic groups were assessed using reduced major axis (RMA) regression. The results show a strong correlation between body mass and each of the 11 cranial variables. The best estimators for body mass are skull length and cheek-tooth area, whereas the least reliable measures are palate length, and foramen magnum, as well as braincase and occipital condyle dimensions. We estimated body mass for specimens of five fossil Ischyromyidae rodents for which body mass had never been estimated (Paramys copei, P. delicatus, Reithroparamys delicatissimus, Rapamys atramontis, and Ischyromys typus). Principal components and canonical variates analyses based on 10 cranial dimensions for 103 members of Sciuromorpha demonstrate that a relationship exists between locomotor habits and cranial variables in this suborder. In these analyses, early ischyromyids are all placed in the terrestrial group with Aplodontia, Marmota, Cynomys, and Spermophilus. This contradicts previous hypotheses regarding early rodent locomotion, which suggested that they were arboreal or more generalized in their habits.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Bertrand, O. C., M. A. Schillaci, and M. T. Silcox. 2015. Cranial dimensions as estimators of body mass and locomotor habits in extant and fossil rodents. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2015.1014905.

Biotic and Abiotic Forcing During the Transition to Modern Grassland Ecosystems: Evolutionary and Ecological Responses of Small Mammal Communities Over the Last 5 Million Years

Understanding the origin of modern communities is a fundamental goal of ecology, but reconstructing communities with durations of 103–106 years requires data from the fossil record. Early Pliocene to latest Pleistocene faunas and sediments in the Meade Basin and modern soils and rodents from the same area are used to examine the role of environmental change in the emergence of the modern community. Paleoenvironmental proxies measured on modern surface soils and paleosols are described, and faunal dynamics of fossil rodents are discussed. Mean annual precipitation (MAP) was estimated from elemental concentrations and magnetic properties, and warm-season temperature and δ18O of soil water was estimated using carbonate isotope paleothermometry on pedogenic nodules. MAP and temperature estimates from paleosols exhibit no short-term variability, no long-term trends, and generally bracket modern values. Estimated soil water δ18O values increased through time, suggesting aridification played a role in the evolution of the regional grassland ecosystem. Carbon isotope analyses of biomarkers are used to examine the abundance of C4 grasses, which suggest more C4 biomass and more variability in C4 biomass than carbonate proxies. Rodent species richness remained constant due to balanced rates of extinction and immigration, both of which show episodic spikes consistent with a balance between forcing mechanisms that result in equilibrium on long time scales. Overall, these results suggest that different mechanisms of faunal change may be acting at different time scales, although the stratigraphic resolution of paleoenvironmental proxies needs to be increased, and body size and dietary distributions of rodents need to be determined before which processes of change are most important can be decided.

Comparative cytogenetics of rodents

Here, we present analysis of data on comparative chromosome painting produced using various chromosome-specific libraries for members of different Glires groups. Based on the results of comparative cytogenetic and molecular studies, the modern rodents can be conventionally classified into two groups with sharply differing directions and tempoes of karyotypic evolution. One group (suborders Sciuromorpha, Castorimorpha, and Anomaluromoprpha) preserved conserved genomes, which are probably close in structure to the genome of the ancestor of all mammals. The genomes of the other group (suborder Myomorpha) underwent "catastrophic evolution," which resulted in numerous breaks and fusions of the ancient chromosomes. The current data do not allow unambiguously assigning the order Hystricomorpha to any of these groups.

Divergence of imprinted genes during mammalian evolution.

BackgroundIn contrast to the majority of mammalian genes, imprinted genes are monoallelically expressed with the choice of the active allele depending on its parental origin. Due to their special inheritance patterns, maternally and paternally expressed genes might be under different evolutionary pressure. Here, we aimed at assessing the evolutionary history of imprinted genes.ResultsIn this study, we investigated the conservation of imprinted genes in vertebrate genomes and their exposition to natural selection. In a genome-wide comparison, orthologs of imprinted genes show a stronger divergence on cDNA and protein level in mammals. This pattern is most pronounced for maternally expressed genes in rodents in comparison to their non-rodent orthologs. The divergence is not attributable to increased mutation of CpG positions. It is contrasted by strong conservation of paternally expressed genes in mouse and rat. Interestingly, we found that the early divergence of imprinted genes was accompanied by an unusually strict conservation of their paralogs.ConclusionsThe apparent degeneration of maternally expressed genes may reflect a relaxation of selective pressure due to counteracting effects on maternal and embryonic fitness. Functional redundancy provided by the presence of highly conserved (non-imprinted) paralogs may have facilitated the divergence. Moreover, intensification of imprinting in modern rodents seems to have shifted the evolutionary fate of imprinted genes towards strong purifying selection.

Strontium isotope ratios in fossil teeth from South Africa: assessing laser ablation MC-ICP-MS analysis and the extent of diagenesis

This study tests the precision and accuracy of laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) for measuring strontium isotope ratios ( 87Sr/ 86Sr) in fossil teeth from Swartkrans and Sterkfontein in South Africa, and explores the extent of diagenesis in these specimens. Compared to solution MC-ICP-MS or thermal ionization mass spectrometry (TIMS) methods, LA-MC-ICP-MS can be faster and less expensive, but has lower precision and accuracy. We found that 87Sr/ 86Sr ratios in fossil rodent teeth measured by LA-MC-ICP-MS were offset from solution values by a mean of 0.0005 ± 0.0010 (2 σ). Because of the very wide spread of biologically available 87Sr/ 86Sr ratios (∼0.710 to ∼0.790) in the region, LA-MC-ICP-MS is more than sufficient for assessing the geographic origins of the fossil rodents. The concentration of strontium in modern and fossil enamel from the area is statistically indistinguishable, as is the range of 87Sr/ 86Sr ratios in modern rodents and fossil rodents from Swartkrans. Thus, there is little evidence of diagenetic Sr in the fossil enamel. In contrast, the fossil dentine may contain up to ∼50% diagenetic strontium, although many dentine 87Sr/ 86Sr ratios are still far removed from the background cave value of 0.729. We propose that LA-MC-ICP-MS will also be of use for investigating the geographic origins of other, non-rodent fossil teeth from the Sterkfontein and Swartkrans collections, and may prove of similar utility at other suitable sites.

The World at Their Feet

PALEONTOLOGY Although mammals and dinosaurs evolved at about the same time about 200 million years ago, the fossil record of mammals is scant. Because mammals tended to be small in numbers and stature, their delicate bones are rarely preserved and the record consists mostly of teeth. One way to sidestep this gap in the fossil record is to look for tracks made by mammals. Lockley and Foster have discovered several sets of mammal tracks from the latest Cretaceous (about 75 to 65 million years ago) in Colorado. The three- to five-toed footprints resemble those of modern rodents; however, rodents did not evolve until later. The footprints probably belong to a multituberculate or a marsupial. From the tracks it is clear that these mammals were small and agile and lived among birds, dinosaurs, and other reptiles on a fertile coastal plain. More work will be needed to connect the partial record from head to toe to determine more about how mammals evolved in the Mesozoic. — LR Ichnos 10 , 269 (2003).

A new, unusual castorid (Rodentia) from the earliest Miocene of Nebraska

ABSTRACT Migmacastor procumbodens, gen. et sp. nov. is described from the late Arikareean of Nebraska based on a nearly complete skull. The new taxon exhibits various incisal specializations (e.g., incisors elongated along longitudinal axis, procumbent upper incisors) that are associated with tooth-digging behavior in modern rodents. Migmacastor does not belong in the Palaeocastorinae, the previously recognized fossorial clade of beavers, nor in any other castorid subfamily. Phylogenetic analysis indicates it is more derived than Agnotocastorinae and is a sister taxon to a monophyletic clade of beavers composed of both fossorial and semiaquatic taxa (Castorinae + Palaeocastorinae + Castoroidinae). Migmacastor is united to the latter unnamed clade by five unambiguous characters: lack of P3; upper tooth rows that diverge posteriorly; P4 larger than molars; smooth enamel on cheek teeth; and longitudinally grooved palate. Preliminary phylogenetic and morphological evidence suggest that tooth-digging behavior arose at least twice in North American beavers.

The human genome has 49 cytochrome c pseudogenes, including a relic of a primordial gene that still functions in mouse

Using a computational approach, we have identified 49 cytochrome c (cyc) pseudogenes in the human genome. Analysis of these provides a detailed description of the molecular evolution of the cyc gene. Almost all of the pseudogenes are full-length, and we have concluded that they mostly originated from independent retrotransposition events (i.e. they are processed). Based on phylogenetic analysis and detailed sequence comparison, we have further divided these pseudogenes into two groups. The first, consisting of four young pseudogenes that were dated to be between 27 and 34 Myr old, originated from a gene almost identical to the modern human cyc gene. The second group of pseudogenes is much older and appears to have descended from ancient genes similar to modern rodent cyc genes. Thus, our results support the observation that accelerated evolution in cyc sequence had occurred in the primate lineage. The oldest pseudogene in the second group, dated to be over 80 Myr old, resembles the testis-specific cyc gene in modern rodents. It is likely that the mammalian ancestor had both the somatic and the testis-specific cyc genes. While the testis-specific gene is still functional in modern rodents, the human has lost it, retaining only a pseudogene in its place. Thus, our study may have identified a pseudogene that is a dead relic of a gene that has completely died off in the human lineage.

Late Cretaceous Mammal Tracks from North America

Mammal tracks from the latest Cretaceous (Campanian-Maastrichtian) of Colorado are described as Schadipes crypticus ichnogen. et ichnosp. nov., on the basis of material from the Laramie Formation in Golden, eastern Colorado. This ichnospecies, and a closely related form (Schadipes sp.) from the Mesaverde Group of western Colorado, represent the only mammal tracks so far identified from the Upper Cretaceous. A possible mammal track from North Africa (Agadirichnus elegans) was originally attributed to a lizard/lacertilian. Other purported Lower Cretaceous mammal tracks are based on isolated specimens of materials that are dubious or as yet undescribed in detail. Morphologically, Late Cretaceous mammal tracks resemble those of some modern rodents. However, based on the dominant mammalian elements of faunas at that time they are probably of marsupial or multituberculate affinity.

Craniofacial development of the rat with respect to vestibular orientation.

The vestibular orientation was applied in the present work. Its purpose was to contribute to the knowledge of the craniofacial development of the growing rat. Sagittal cuts were made in 91 crania from Wistar rats (44 males and 47 females) at 8, 15, 30 and 60 days of age. The x and y coordinates of the sagittal points were determined. The sagittal intersection of the vestibular plane was employed as the abscissa and its perpendicular at the mid point of the external semicircular canal (mesovestibion) as the ordinate. Cranial diagrams were constructed in which the length of the sagittal projection of each bone and its angulation with respect to the abscissa were measured. It was confirmed a progressive evolution towards orthocrania due to the bone rotation complexes, which characterized three cranial regions: the anterior region (frontal-ethmiodal-facial complex) with trigonometric rotation; the mid region (parietal-sphenoidal complex) remained stable, and the posterior region (interparietal-occipital complex) with clockwise rotation. Each complex had its own pattern of rotation. The causal factors reside in the differential increment between the functional components: (a) neurocranium-splanchnocranium, (b) neurocranium-cervical column, (c) dorsal neurocranium-basilar neurocranium and (d) dorsal splanchnocranium-basilar splanchnocranium. Its etiology obeys to the functional requirements induced by the adaptation of the modern rodents.

On the morphology of Syphacia callosciuri n. sp., pin-worm parasite of a Malaysian squirrel and on the origin of Rattus pin-worm, Syphacia muris (Yamaguti, 1935) (author's transl)

Description of a new species of Oxyurid: Syphacia (Syphatineria) callosciuri n. sp. parasite of a Sciurid rodent Callosciuris caniceps in Malaysia. In rodents belonging to the genus Callosciurus in Malaysia two species: S callosciuri n. sp. and S. owyangi Quentin, 1975, show morphological characteristics which are intermediate between the primitive Syphacia and two different lines of species parasite of recent rodents. These observations appear to indicate that the adaptation of Syphacia of Sciurids to the modern rodents has occured in South-East Asia.

Mesozoic Mammalian Evolution

The Mesozoic history of the Mammalia is approximately twice as long as the Cenozoic. Because of their small size the remains of early mammals were often overlooked. In the last two decades development of new collecting techniques has greatly augmented their fossil record. Now a large number of fossils document the evolutionary transition from mammallike reptiles to earliest mammals that occurred in the Late Triassic, approximately 190-200 M.Y.B.P. (million years before the present, I-Iowarth 34). During the Jurassic and Cretaceous periods--which had a total duration of over 120 million years--mammals formed a small element of the terrestrial fauna, but major evolutionary changes, including the origin and early differentiation of the Eutheria (Placentalia) and Metatheria (Marsupialia), place. With the extinction of the dinosaurs at the close of the Cretaceous, approximately 64 M.Y.B.P., the mammalian fauna underwent a major radiation and for the first time began to occupy niches available only to large terrestrial vertebrates. THE FOSSIL ~tECOEV ffurassie and Cretaceous mammals were small creatures; the largest yet discovered was no bigger than a terrier. Most were in the size range of modern small rodents and shrews. The minute bones and teeth of these animals frequently have been overlooked by paleontologists in search of skeletons of dinosaurs to dominate museum exhibit halls. Only in the last two decades have collecting techniques adapted for recovery of mammalian and other vertebrate microfossils been developed and consistently employed (McKenna 58). The result has been a multifold increase in the available record, but, as will become apparent, many gaps remain between well-sampled local faunas. The evolutionary history of Triassic terrestrial vertebrate faunas is one of change from dominance of the mammallike reptiles, therapsids, in the Early Triassic to archosaurian dominance at the end of the period. The earliest mammals, descendants of therapsids, have been collected from a series of localities of the Rhaetic (Late Triassic) age. Of these, the largest samples are from deposits formed within fissures and caverns in Carboniferous