DNA Sequence Analysis of Familial Relationships Among Dasyuromorphian Marsupials

Although modern morphological and molecular analyses support the monophyly of the Australasian marsupial order Dasyuromorphia, there is much less certainty about relationships among its constituent families (Dasyuridae, Myrmecobiidae, and Thylacinidae). While most authors regard Dasyuridae as monophyletic, a few have suggested that thylacines, numbats, or both have their closest relatives among dasyurids. Recent morphocladistic studies have identified several basicranial characters as putative synapomorphies of dasyurids, but no features that clearly implicate thylacinids, myrmecobiids, or both, as the sister group of Dasyuridae. Only two previous DNA studies have included both thylacine and numbat sequences along with dasyurids, and neither provided strong resolution of interfamilial relationships. In this study, we report a more thorough analytical treatment of complete cytochrome b, 12S rRNA, and protamine P1 gene sequences from dasyuromorphians than has heretofore been attempted. Our results concur with previous morphological studies in showing that Dasyuridae is monophyletic and with immunological findings that thylacinids and dasyurids are sister groups, apart from myrmecobiids. However, the level of support for nodes is highly dependent on the method of phylogenetic analysis employed. Our results also suggest that partitioning of sequence data sets to account for substitutional heterogeneity within and among genes does not necessarily lead to a major reduction in the precision of estimated phylogenies.     

Post-weaning cranial ontogeny in two bandicoots (Mammalia,Peramelomorphia, Peramelidae) and comparison with carnivorous marsupials

The ontogeny of the skull has been studied in several marsupial groups such as didelphids, microbiotheriids, and dasyurids. Here, we describe and compare the post-weaning ontogeny of the skull in two species of bandicoots, Echymipera kalubu (Echymiperinae) and Isoodon macrourus (Peramelinae), analyzing specific allometric trends in both groups, describing common (and specific) patterns, and discussing them on functional and phylogenetic grounds. Growth patterns were analyzed both qualitatively and quantitatively, including bivariate and multivariate analyses of allometry. We also evaluated character transformation and phylogenetic signals of the allometric patterns in several groups of marsupials and some placentals. We identified morphological changes between juvenile and adult stages in both species of peramelids, many related to the development of the trophic apparatus. Notable differences were detected in the patterns of growth, suggesting divergences in ontogenetic trajectories between both species. Both bivariate and multivariate methods indicate that positive allometries in E. kalubu apply to longitudinal dimensions, whereas in I. macrourus, positive allometries are restricted to vertical dimensions of the skull. The comparison of the allometric trends of two bandicoots with previously studied taxa reveals that although peramelids exhibit a particularly short gestation period and divergent morphology compared to other marsupials, their pattern does not show any particular trend. Some allometric trends seem to be highly conserved among the species studied, showing weak phylogenetic signal. Marsupials in general do not show particular patterns of post-weaning skull growth compared with placentals.     

Cerebral venous hemodynamics and the basicranium of Cebus

The study of basicranial foramina has played an important role in primatological investigations. They are often the only clues for deciphering cerebral vascular evolution in fossil forms. This study illustrates the significance of considering foraminal shape when analyzing or describing basicranial anatomy in living and fossil primates. It is suggested that certain hemodynamic properties of cerebral venous drainage in Cebus represent patterns that were most likely present in ancestral catarrhines.     

Petrosal bones of metatherian mammals from the Late Palaeocene of Itaboraí (Brazil), and a cladistic analysis of petrosal features in metatherians

Metatherian petrosal bones were recovered from the early Late Palaeocene Itaboraí, Brazil, and are formally described. A cladistic analysis of the distribution of 56 petrosal and basicranial characters among extant and fossil metatherians was conducted, resulting in seven parsimonious trees. Relationships among metatherian ingroup taxa are congruent with current understanding of metatherian phylogeny. Metatheria is diagnosed by eight petrosal synapomorphies: stapedial artery absent in adults; reduced, intramural prootic canal; extrabullar internal carotid artery; inferior petrosal sinus between petrosal, basisphenoid, and basioccipital; cava supracochleare and epiptericum completely separated; reduction of the lateral flange; reduction of the anterior lamina; separation of the jugular foramen from the opening for the inferior petrosal sinus. The Palaeocene taxa Mayulestes , Pucadelphy s, and Andinodelphys from Tiupampa, and Petrosal Type II from Itaboraí are the sister groups of all other South American and Australian metatherians. This analysis confirms previous results showing the South American 'monito del monte' Dromiciops nested within the Australasian radiation. Within this australidelphian clade, Dromiciops is closely related to the dasyurids. The South American Caenolestes appears more closely related to the Australidelphia than to the South American didelphids. The Petrosal Types I, III, IV and V from Itaboraí are the stem taxa of the clade Australidelphia plus Caenolestes . The significant synapomorphies supporting this relationship are: enlargement of the fossa subarcuata that produces a bulbous ventral aspect of the mastoid and loss of post-temporal canal.  Journal compilation © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society , 2007, 150 , 85–115. No claim to original French government works.     

Ontogenetic and phylogenetic transformations of the ear ossicles in marsupial mammals

This study is based on the examination of histological sections of specimens of different ages and of adult ossicles from macerated skulls representing a wide range of taxa and aims at addressing several issues concerning the evolution of the ear ossicles in marsupials. Three-dimensional reconstructions of the ear ossicles based on histological series were done for one or more stages of Monodelphis domestica, Caluromys philander, Sminthopsis virginiae, Trichosurus vulpecula, and Macropus rufogriseus. Several common trends were found. Portions of the ossicles that are phylogenetically older develop earlier than portions representing more recent evolutionary inventions (manubrium of the malleus, crus longum of the incus). The onset of endochondral ossification in the taxa in which this was examined followed the sequence; first malleus, then incus, and finally stapes. In M. domestica and C. philander at birth the yet precartilaginous ossicles form a supportive strut between the lower jaw and the braincase. The cartilage of Paauw develops relatively late in comparison with the ear ossicles and in close association to the tendon of the stapedial muscle. A feeble artery traverses the stapedial foramen of the stapes in the youngest stages of M. domestica, C. philander, and Sminthopsis virginiae examined. Presence of a large stapedial foramen is reconstructed in the groundplan of the Didelphidae and of Marsupialia. The stapedial foramen is absent in all adult caenolestids, dasyurids, Myrmecobius, Notoryctes, peramelids, vombatids, and phascolarctids. Pouch young of Perameles sp. and Dasyurus viverrinus show a bicrurate stapes with a sizeable stapedial foramen. Some didelphids examined to date show a double insertion of the Tensor tympani muscle. Some differences exist between M. domestica and C. philander in adult ossicle form, including the relative length of the incudal crus breve and of the stapes. Several differences exist between the malleus of didelphids and that of some phalangeriforms, the latter showing a short neck, absence of the lamina, and a ventrally directed manubrium. Hearing starts in M. domestica at an age in which the external auditory meatus has not yet fully developed, the ossicles are not fully ossified, and the middle ear space is partially filled with loose mesenchyme. The ontogenetic changes in hearing abilities in M. domestica between postnatal days 30 and 40 may be at least partially related to changes in middle ear structures.     

Basicranial flexion,relative brain size,and facial kyphosis in nonhuman primates

Numerous hypotheses explaining interspecific differences in the degree of basicranial flexion have been presented. Several authors have argued that an increase in relative brain size results in a spatial packing problem that is resolved by flexing the basicranium. Others attribute differences in the degree of basicranial flexion to different postural behaviors, suggesting that more orthograde animals require a ventrally flexed pre-sella basicranium in order to maintain the eyes in a correct forward-facing orientation. Less specific claims are made for a relationship between the degree of basicranial flexion and facial orientation. In order to evaluate these hypotheses, the degree of basicranial flexion (cranial base angle), palate orientation, and orbital axis orientation were measured from lateral radiographs of 68 primate species and combined with linear and volumetric measures as well as data on the size of the neocortex and telencephalon. Bivariate correlation and partial correlation analyses at several taxonomic levels revealed that, within haplorhines, the cranial base angle decreases with increasing neurocranial volume relative to basicranial length and is positively correlated with angles of facial kyphosis and orbital axis orientation. Strepsirhines show no significant correlations between the cranial base angle and any of the variables examined. It is argued that prior orbital approximation in the ancestral haplorhine integrated the medial orbital walls and pre-sella basicranium into a single structural network such that changes in the orientation of one necessarily affect the other. Gould's (“Ontogeny and Phylogeny.” Cambridge: Belknap Press, 1977) hypothesis, that the highly flexed basicranium of Homo may be due to a combination of a large brain and a relatively short basicranium, is corroborated. © 1993 Wiley-Liss, Inc.     

THE AUDITORY REGION OF EARLY PALEOCENE PUCADELPHYDAE (MAMMALIA, METATHERIA) FROM TIUPAMPA, BOLIVIA, WITH PHYLOGENETIC IMPLICATIONS

Abstract:  New petrosal bones, assigned to Pucadelphys and Andinodelphys , from the Lower Paleocene of Tiupampa, Bolivia, are described. These remains provide new information on the anatomy of the ear region of these taxa. The re-examination of characters from the petrosal and basicranium shed light on the phylogenetic relationships of the three Tiupampan genera known from complete cranial remains (i.e. Mayulestes , Pucadelphys and Andinodelphys ). The combination of dental, general cranial and basicranial characters led to two alternative hypotheses. The first is that borhyaenoids (including Mayulestes ) are nested within Notometatheria. Pucadelphyds (i.e. Pucadelphys and Andinodelphys ) are the sister group of a clade comprising MHNC 8369 (one isolated petrosal from Tiupampa) and Marsupialia. The second favours the paraphyly of 'borhyaenoids' (i.e. the exclusion of Mayulestes from borhyaenoids) and the polyphyly of 'Notometatheria'. In this case, Mayulestes and borhyaenids represent the stem group of a clade including Asiatic, American and Australian metatherians. This analysis of combined datasets (dental, general cranial and basicranial) highlighted contradictory information in the dental and cranial characters, serving to emphasize that in a large anatomical complex like an entire skull mosaic evolution of the characters is likely.     

Seasonal changes in the physiology of male Virginia opossums (Didelphis virginiana): signs of the Dasyurid semelparity syndrome?

Semelparity, which is multiplying once in a lifetime, is a rare reproductive strategy among mammals. Several species of the marsupial family Dasyuridae experience 100% male mortality following an intense mating period. We investigated seasonal physiological changes that may be associated with early mortality in the male Virginia opossum (Didelphis virginiana; Didelphidae) and compared these changes with those of semelparous, male dasyurids. Free-ranging male Virginia opossums (n=36) were collected during 2001 at the Oklahoma State University Cross Timbers Experimental Range. Seasonal data were collected on hematological, morphological, and helminth parameters of these individuals. We used one-way ANOVA to determine whether there were seasonal differences among means for each parameter. It appeared that male Virginia opossums experienced some physiological changes similar to those of male dasyurids exhibiting semelparity. All males collected in summer (August) were juveniles of the year. Lack of adult males in August suggests high mortality of this cohort during the breeding season. Opossum characteristics exhibiting the dasyurid semelparity syndrome included packed cell volume, adrenal mass, and helminth numbers. Minor lymphocytopenia, neutrophilia, and testosterone concentrations also were similar to semelparous dasyurids. However, a lack of change in serum cortisol concentration and body mass and dynamics in immunoglobulin protein, serum protein, and testes mass were not consistent with previous reports of semelparous dasyurid physiology. Evolutionary divergence and differences in breeding behavior between dasyurids and didelphids may be responsible for the lack of consistency between the taxa.     

A Reexamination of Proposed Morphology-Based Synapomorphies for the Families of Dasyuromorphia (Marsupialia). I. Dasyuridae

The validity of eight morphological features previously advanced as synapomorphic for Dasyuridae is investigated in the light of new fossil and molecular data. Results indicate that one of these features (alisphenoid–periotic enclosure of the foramen ovale) is common to outgroups for Dasyuromorphia. Another feature (loss of intestinal cecum) is a likely synapomorphy for Dasyuromorphia. Two features (development of a hypoconulid notch, enlargement of stylar cusp D) may represent shared–derived characters within Dasyuromorphia but not at the family level for Dasyuridae (i.e., probably unite Dasyuridae–Thylacinidae). Another two features (loss of posterolateral palatine foramina, reduction of P₃) are also apomorphic within Dasyuromorphia but unite specialized clades within Dasyuridae. Only two previously treated features are probable synapomorphies for the family (enlargement of the alisphenoid tympanic wing and development of a distinct periotic hypotympanic sinus). An additional feature is identified as a dasyurid synapomorphy (presence of a distinct tubal foramen). Of all putative synapomorphies proposed to date, only the presence of a periotic hypotympanic sinus and tubal foramen are unique for Dasyuridae among dasyuromorphians. Results suggest considerable homoplasy for basicranial features within Dasyuromorphia. Independent acquisition for alisphenoid enclosure of the foramen ovale, development of secondary foramina ovale and loss of posterolateral palatal foramina has occurred in derived thylacinid and dasyurid clades. Convergence is also indicated for hypertrophy of the alisphenoid tympanic wing shown for dasyurids and myrmecobiids, and the development of a squamosal epitympanic sinus in Thylacinidae, Dasyuridae, and Myrmecobiidae. The finding of plesiomorphy for alisphenoid–periotic enclosure of the foramen ovale within Dasyuromorphia undermines the strongest morphology-based synapomorphy uniting a monophyletic Dasyuridae–Myrmecobiidae. Phylogenetic placement for some plesiomorphic fossil dasyuromorphians, known only from dental material, within Dasyuridae is currently untenable, with no dental synapomorphies uniting the family. The value of identifying morphoclines within clades known from robust phylogenetic data for consideration in character analysis is stressed, as is the importance of form–function and ontogenetic data.     

Functional-adaptive analysis of the hindlimb anatomy of extant marsupials and the paleobiology of the Paleocene marsupials Mayulestes ferox and Pucadelphys andinus

This article analyzes the adaptations of the hindlimb of two Early Paleocene marsupials, Mayulestes ferox and Pucadelphys andinus. This analysis is based on detailed comparisons with various extant marsupials, both South American and Australian. In the case of the South American opossums, original myological data were collected and osteological-myological associations were related to their locomotor behavior. The use of Australian genera helped to improve the appraisal of the locomotory habits of the fossil taxa. Several features are indicative of the ability of Mayulestes to climb or walk on uneven surfaces (e.g., very mobile hip joint, astragalocalcaneal joint pattern), and some other features emphasize a relative agility (e.g., strongly everted iliac blades, morphology of the distal epiphysis of the femur, medially stabilized cruroastragalar joint). Pucadelphys exhibits a hindlimb relatively similar morphologically to that of Mayulestes, but with features indicating slightly increased agility and a terrestrial component that is more emphasized than in Mayulestes. The Tiupampa fossils were therefore more agile than most living didelphids and resembled the condition observed in living dasyurids more. These conclusions complement a previous study performed on the forelimb of these fossils.     

Foramen magnum position in bipedal mammals

The anterior position of the human foramen magnum is often explained as an adaptation for maintaining balance of the head atop the cervical vertebral column during bipedalism and the assumption of orthograde trunk postures. Accordingly, the relative placement of the foramen magnum on the basicranium has been used to infer bipedal locomotion and hominin status for a number of Mio-Pliocene fossil taxa. Nonetheless, previous studies have struggled to validate the functional link between foramen magnum position and bipedal locomotion. Here, we test the hypothesis that an anteriorly positioned foramen magnum is related to bipedalism through a comparison of basicranial anatomy between bipeds and quadrupeds from three mammalian clades: marsupials, rodents and primates. Additionally, we examine whether strepsirrhine primates that habitually assume orthograde trunk postures exhibit more anteriorly positioned foramina magna compared with non-orthograde strepsirrhines. Our comparative data reveal that bipedal marsupials and rodents have foramina magna that are more anteriorly located than those of quadrupedal close relatives. The foramen magnum is also situated more anteriorly in orthograde strepsirrhines than in pronograde or antipronograde strepsirrhines. Among the primates sampled, humans exhibit the most anteriorly positioned foramina magna. The results of this analysis support the utility of foramen magnum position as an indicator of bipedal locomotion in fossil hominins.     

J. P. Hill and Katherine Watson's studies of the neural crest in marsupials

In the early part of the 20th century, J. P. Hill and K. P. Watson embarked on a comprehensive study of the development of the brain in Australian marsupials. Their work included series from three major groups: dasyurids, peramelids, and diprotodonts, covering early primitive streak through brain closure and folding stages. While the major part of the work was on the development of the brain, in the course of this work they documented that cellular proliferations from the neural plate provided much of the mesenchyme of the branchial arches. These proliferations are now known to be the neural crest. However, except for a very brief note, published shortly after Hill's death, this work was never published. In this study, I present Hill and Watson's work on the development of the early neural plate and the neural crest in marsupials. I compare their findings with published work on the South American marsupial, Monodelphis domestica and demonstrate that patterns reported in Monodelphis are general for marsupials. Further, using their data I demonstrate that in dasyurids, which are ultra-altricial at birth, the neural crest migrates early and in massive quantities, even relative to other marsupials. Finally, I discuss the historical context and speculate on reasons for why this work was unpublished. I find little support for ideas that Hill blocked publication because of loyalty to the germ layer theory. Instead, it appears primarily to have been a very large project that was simply orphaned as Watson and Hill pursued other activities.     

The unusual cranial attributes of KNM-ER 1805 and their implication for studies of sexual dimorphism inHomo habilis

A principal components analysis (PCA) of basicranial measurements (Thompson 1991) isolated KNM-ER 1805 as having the highest Principal Component (PC) score on PCI of all the fossil hominids. Two measurements with high loadings on PCI were B12 and B13 and these two measurements indicate the relative positions of the foramina ovale (FO) and infratemporal crests (IT) to the tympanic bone (TP). The object of this study was to compare the two measurements of KNM-ER 1805 with those of other early fossil hominids as well as a sample of extant hominoids. The comparison involved the raw measurements, the index of the two measurements, the coefficient of variation, and a t-test. The results of this comparison showed that KNM-ER 1805 had more forwardly placed foramina ovale than any of the comparative specimens. KNM-ER 1805 possesses a number of other unique features which differentiate it from other hominids including a persistent metopic suture, the form of the premolar roots, and the form of the asterionic region. These apparent unique features mean that KNM-ER 1805 is unlikely to represent an “average” maleHomo habilis and so is an inappropriate model for the male morph of that species.     

Relationships among orders and families of marsupials based on 12S ribosomal DNA sequences and the timing of the marsupial radiation

Part of the mitochondrial 12S ribosomal RNA gene was amplified and sequenced for 26 marsupials. Multiple alignments for these sequences as well as seven additional sequences taken from GenBank were obtained using CLUSTAL. PAUP was used for phylogenetic analysis and to obtain random tree-length distributions. Analyses were performed with and without phylogenetic constraints. Our results clearly show that 12S rDNA contains phylogenetic signal at and above the ordinal level and is thus appropriate for addressing phylogenetic questions deep in the mammalian tree. Standard parsimony analyses provide some support for a clade containing diprotodontians, dasyurids,Dromiciops, andNotoryctes; transversion parsimony analysis suggests the possible inclusion of peramelids as well. Within the Diprotodontia, vombatids and phascolarctids cluster together on transversion parsimony and phalangerids may be associated with this clade. The enigmatic tarsipedids are apparently part of a clade that also contains pseudocheirids, petaurids, and acrobatids. The 12S sequences suggest that the origination of extant marsupial orders peaked 15 million years later than the equivalent taxonomic diversification of extant placental orders and may be entirely post-Cretaceous. Families of diprotodontian marsupials originated during the Eocene and early Oligocene, which is consistent with previous single-copy DNA hybridization results.     

The scaling of basicranial flexion and length.

Based on correlations between the cranial base angle (CBA) and the index of relative encephalization (IRE, calculated as the cubed root of brain volume divided by basicranial length), several recent studies have identified relative brain size as the factor most responsible for determining basicranial flexion in primates. IRE, however, scales with positive allometry relative to body mass, unlike the negatively allometric relationship between brain volume and body mass. This poses new questions concerning the factors underlying the correlation between IRE and CBA. Specifically, if basicranial flexion represents a spatial solution to the problem of housing a large brain within a neurocranium of limited size, then why is it that the problem is greatest in those species whose brains are smallest relative to body mass? To address this question, the scaling relationships of IRE and the measurements used to calculate it were examined in 87 primate species. It was found that the positive allometry of IRE is due to the fact that its denominator, basicranial length (BL), scales with very strong negative allometry relative to body mass. The scaling relationship of BL may reflect the fact that the noncortical components of the brain (i.e., diencephalon, mesencephalon, medulla) also scale with strong negative allometry relative to body mass, perhaps because of energetic constraints. Importantly, BL and these three brain components scale isometrically against each other. Thus, although cranial base flexion may be an adaptation to accommodate the size of the brain relative to basicranial length, the reason why that adaptation is necessary is not the evolution of a large brain, but rather the evolution of a short cranial base. In so far as basicranial length is affected by the strong negative allometry of the diencephalon, mesencephalon and medulla, the scaling relationships of these brain components are therefore indirectly responsible for the evolution of basicranial flexion.     

Evolution of post-weaning skull ontogeny in New World opossums (Didelphidae)

Quantification of mammalian skull development has received much attention in the recent literature. Previous results in different lineages have shown an effect of historical legacy on patterns of skull growth. In marsupials, the skull of adults exhibits high variation across species, principally along a size axis. The development keys of the marsupial skull are fundamental to understanding the evolution of skull function in this clade. Its generally well-resolved phylogeny makes the group ideal for studying macroevolution of skull ontogeny. Here, we tested the hypothesis that ontogenetic similarity is correlated with phylogeny in New World marsupials, so that developmental patterns are expected to be conserved from ancestral opossums. We concatenated our previously published ontogenetic cranial data from several opossum species with new ontogenetic sequences and constructed an allometric space on the basis of a set of comparable cranial linear measurements. In this ontogenetic space, we determined the degree of correspondence of developmental patterns and the phylogeny of the group. In addition, we mapped ontogenetic trajectories onto the opossum phylogeny, treating the trajectories as composite, continuously varying characters. Didelphids differed widely in the magnitude of skull allometry across species. Splanchnocranial components exhibited all possible patterns of inter-specific variation, whereas mandibular variables were predominantly allometrically “positive” and neurocranial components were predominantly allometrically “negative.” The distribution of species in allometric space reflected the compounded effect of phylogeny and size variation characteristic of didelphids. The terminal morphology of related species differed in shape, so their ontogenetic trajectories deviated with respect to that of reconstructed common ancestors in varying degree. Phylogeny was the main factor structuring the allometric space of New World marsupials. Didelphids inherited an ancestral constellation of allometry coefficients without change and retained much of it throughout their lineage history. Conserved allometric values on the nodes splitting placental outgroups and marsupials suggest a developmental basis common to all therians.     

Toe‐bud clipping of juvenile small marsupials for ecological field research: No detectable negative effects on growth or survival

Toe clipping is widely used to permanently mark many species of small vertebrates including marsupials, particularly didelphids and dasyurids. Small marsupials are marked as juveniles, by removing the tip of developing toe buds. It has recently been shown that survival and/or recapture probability decreases with increasing number of toes clipped in frogs. Because of this and other animal welfare concerns, toe clipping of adult vertebrates is increasingly being discouraged. The short‐ and long‐term effects of toe‐bud clipping have not been evaluated in marsupials. We used an experiment to test if marking more toes results in slower growth or higher mortality in the brown antechinus (Antechinus stuartii, Dasyuridae) in the short or long term. We found no harmful effects of toe‐bud clipping. There were no infections associated with clipping, marking more toes did not reduce growth in young or adults, and did not affect survival of young in captivity, survival of independent animals in the wild, or recapture probability. Toe‐bud clipping is done at an extremely immature stage, when the area cut is tiny and perception and memory of pain is unlikely to be a problem. We suggest that toe‐bud clipping is a humane and benign method of permanently marking antechinuses, and probably also the young of other morphologically similar small marsupials.     

Yet Another Interpretation of the Coelacanthiform Basicranial Muscle and its Innervation

Abstract A compilation of newly published information on the innervation of the basicranial muscle in the living tufttail Latimeria chalumnae and data obtained from other craniates, extant as well as extinct, suggest that the coelacanthiform basicranial musculature and the nerve supply which pertains to it are trimetameric in character rather than monometameric, as usually reported. This result supports the available data on the basic composition of the head of craniate animals and gives no support to a recently suggested homology between the tufttail basicranial muscles and the tetrapod retractor bulbi musculature.     

The circumorbital foramina in primates

This paper documents the diversity and variation in the circumorbital foramina in extant primates. A qualitative and quantitative analysis of the circumorbital foramina, comprising the supraorbital foramen, the infraorbital foramen, and the malar foramen, was carried out using representative species from nine extant families of primates. The information obtained from the study is used to reconstruct ancestral morphotypes, and to make inferences about evolutionary changes that may have taken place in the major primate lineages. In addition, the analysis provides useful comparative data for interpretation of the phylogenetic significance and paleobiological implications of the circumorbital foramina in fossil primates.     

Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales

The origin of baleen in mysticete whales represents a major transition in the phylogenetic history of Cetacea. This key specialization, a keratinous sieve that enables filter-feeding, permitted exploitation of a new ecological niche and heralded the evolution of modern baleen-bearing whales, the largest animals on Earth. To date, all formally described mysticete fossils conform to two types: toothed species from Oligocene-age rocks ( approximately 24 to 34 million years old) and toothless species that presumably utilized baleen to feed (Recent to approximately 30 million years old). Here, we show that several Oligocene toothed mysticetes have nutrient foramina and associated sulci on the lateral portions of their palates, homologous structures in extant mysticetes house vessels that nourish baleen. The simultaneous occurrence of teeth and nutrient foramina implies that both teeth and baleen were present in these early mysticetes. Phylogenetic analyses of a supermatrix that includes extinct taxa and new data for 11 nuclear genes consistently resolve relationships at the base of Mysticeti. The combined data set of 27,340 characters supports a stepwise transition from a toothed ancestor, to a mosaic intermediate with both teeth and baleen, to modern baleen whales that lack an adult dentition but retain developmental and genetic evidence of their ancestral toothed heritage. Comparative sequence data for ENAM (enamelin) and AMBN (ameloblastin) indicate that enamel-specific loci are present in Mysticeti but have degraded to pseudogenes in this group. The dramatic transformation in mysticete feeding anatomy documents an apparently rare, stepwise mode of evolution in which a composite phenotype bridged the gap between primitive and derived morphologies; a combination of fossil and molecular evidence provides a multifaceted record of this macroevolutionary pattern.