Cranial anatomy of the Paleocene plesiadapiform Carpolestes simpsoni (Mammalia, Primates) using ultra high-resolution X-ray computed tomography, and the relationships of plesiadapiforms to Euprimates

Effects of binocular cue availability on leaping performance in Cheirogaleus medius: implications for primate origins

Multiple competing hypotheses attribute the evolution of the suite of traits that distinguish primates from their closest relatives, including forward-facing eyes, which create a wide field of binocular vision, to specific behavioral and ecological factors. The grasp-leaping hypothesis suggests that the evolution of these traits in basal primates was driven by the demands of a form of leaping locomotion unique to primates. Whether the grasp-leaping hypothesis provides a viable mechanism for the evolution of primates' forward-facing eyes remains untested. To determine whether grasp-leaping locomotion may have contributed to driving the evolution of primates' forward-facing eyes, the importance of vision within the binocular field for this type of leaping was evaluated experimentally in Cheirogaleus medius, one of the cheirogaleid primate species considered reasonable living analogs of the earliest primates. Availability of binocular visual cues was experimentally restricted using a head-mounted blinder that narrowed the binocular visual field without altering the total visual field. Animals altered their launch behavior, reduced their horizontal leap speed, and were significantly more likely to select paths that offered the shortest available leaps when their binocular field was restricted. Restriction of binocular cue availability also significantly increased the probability of adverse landings even when statistically controlling for potentially confounding variables such as leap distance, horizontal leap speed, learning effects, etc. These results suggest a functional mechanism by which selection for improved grasp-leaping could also have contributed to the evolution of forward-facing eyes in the earliest crown primates.

Basicranial evidence suggests picrodontid mammals are not stem primates

The Picrodontidae from the middle Palaeocene of North America are enigmatic placental mammals that were allied with various mammalian groups but are generally now considered to have close affinities to paromomyid and palaechthonid plesiadapiforms based on proposed dental synapomorphies. The picrodontid fossil record consists entirely of dental and gnathic remains except for one partial cranium of Zanycteris paleocenus (AMNH 17180). Here, we use µCT technology to unveil previously undocumented morphology in AMNH 17180, describe and compare the basicranial morphology of a picrodontid for the first time, and incorporate these new data into cladistic analyses. The basicranial morphology of Z. paleocenus is distinct from plesiadapiforms and shares similarities with the Palaeogene Apatemyidae and Nyctitheriidae. Results of cladistic analyses incorporating these novel data suggest picrodontids are not stem primates nor euarchontan mammals and that the proposed dental synapomorphies uniting picrodontids with plesiadapiforms and, by extension, primates evolved independently. Results highlight the need to scrutinize proposed synapomorphies of highly autapomorphic taxa with limited fossil records.

Virtual endocast of late Paleocene Niptomomys (Microsyopidae, Euarchonta) and early primate brain evolution

Paleogene microsyopid plesiadapiforms are among the oldest euarchontans known from relatively complete crania. While cranial endocasts are known for larger-bodied Eocene microsyopine microsyopids, this study documents the first virtual endocast for the more diminutive uintasoricine microsyopids, derived from a specimen of Niptomomys cf. Niptomomys doreenae (USNM 530198) from the late Paleocene of Wyoming. Size estimates of smaller-bodied uintasoricines are similar to those inferred for the common ancestor of Primates, so the virtual endocast of Niptomomys may provide a useful model to study early primate brain evolution. Due to the broken and telescoped nature of the neurocranium of USNM 530198, a μCT scan of the specimen was used to create a 3D model of multiple bone fragments that were then independently isolated, repositioned, and merged to form a cranial reconstruction from which a virtual endocast was extracted. The virtual endocast of Niptomomys has visible caudal colliculi, suggesting less caudal expansion of the cerebrum compared to that of euprimates, but similar to that of several other plesiadapiforms. The part of the endocast representing the olfactory bulbs is larger relative to overall endocast volume in Niptomomys (8.61%) than that of other known plesiadapiforms (∼5%) or euprimates (<3.5%). The petrosal lobules (associated with visual stabilization) are relatively large for a Paleocene placental mammal (1.66%). The encephalization quotient of Niptomomys is relatively high (range = 0.35-0.85) compared to that of Microsyops (range = 0.32-0.52), with the upper estimates in the range of values calculated for early euprimates. However, this contrast likely relates in part to the small size of the taxon, and is not associated with evidence of neocortical expansion. These findings are consistent with a model of shifting emphasis in primate evolution toward functions of the cerebrum and away from olfaction with the origin of euprimates.

The largest and earliest known sample of dental caries in an extinct mammal (Mammalia, Euarchonta, Microsyops latidens) and its ecological implications

Dental cavities or caries is a common disease among modern humans, affecting almost every adult. Caries frequency has been used to study dietary change in humans over time, based on an inferred tie between the incidence of caries and a carbohydrate-rich diet. However, the disease is not unique to our species. Among non-human primates, there is also variation in caries frequency associated with diet, suggesting that this metric may provide a mechanism for studying diet in broader contexts, and across geological time. To date, very few studies have examined caries among fossil mammals, and none have done so among Eocene mammals. Here, we present our analysis of the largest sample to date of fossil caries in a single extinct mammal species, Microsyops latidens, a stem primate from the early Eocene, which is known from over a thousand specimens from the Southern Bighorn Basin of Wyoming (n = 1030). Our results show that Microsyops latidens is characterized by a high prevalence of dental caries (7.48% of individuals), with notable variation through time, reaching 17.24% of individuals from a particular interval. This interval is also associated with a change in overall dental form, as quantified by dental topographic analysis, which measures functional aspects of the chewing surface of teeth. These observations suggest that this species experienced a shift in their diet to include more fruit or other sugar rich-foods for a short period. Our analysis, therefore, suggests that the diet of M. latidens fluctuated over time, as well as providing a framework for assessing caries in other fossil taxa.

Cladogenesis and replacement in the fossil record of Microsyopidae (?Primates) from the southern Bighorn Basin, Wyoming

The early Eocene of the southern Bighorn Basin, Wyoming, is notable for its nearly continuous record of mammalian fossils. Microsyopinae (?Primates) is one of several lineages that shows evidence of evolutionary change associated with an interval referred to as Biohorizon A. Arctodontomys wilsoni is replaced by a larger species, Arctodontomys nuptus, during the biohorizon interval in what is likely an immigration/emigration or immigration/local extinction event. The latter is then superseded by Microsyops angustidens after the end of the Biohorizon A interval. Although this pattern has been understood for some time, denser sampling has led to the identification of a specimen intermediate in morphology between A. nuptus and M. angustidens, located stratigraphically as the latter is appearing. Because specimens of A. nuptus have been recovered approximately 60 m above the appearance of M. angustidens, it is clear that A. nuptus did not suffer pseudoextinction. Instead, evidence suggests that M. angustidens branched off from a population of A. nuptus, but the latter species persisted. This represents possible evidence of cladogenesis, which has rarely been directly documented in the fossil record. The improved understanding of both evolutionary transitions with better sampling highlights the problem of interpreting gaps in the fossil record as punctuations.

Virtual brain endocast of Antifer (Mammalia: Cervidae), an extinct large cervid from South America

A diverse fossil record of Cervidae (Mammalia) has been documented in the South American Pleistocene, when these animals arrived during the Great American Biotic Interchange. Using computed tomography-scanning techniques, it is possible to access the endocranial morphology of extinct species. Here, we studied the brain endocast of the extinct late Pleistocene cervid Antifer ensenadensis from southern Brazil, one of the largest forms that lived on this continent, using comparative morphology, geometric morphometrics, and encephalization quotients. The analyzed endocasts demonstrate that A. ensenadensis had a gyrencephalic brain, showing a prominent longitudinal sinus (=sagittal superior sinus), which is also observed in the large South American cervid Blastocerus dichotomus. The encephalization quotient is within the variation of extant cervids, suggesting maintenance of the pattern of encephalization from at least the late Pleistocene. Geometric morphometric analysis suggested a clear and linear allometric trend between brain endocast size and shape, and highlights A. ensenadensis as an extreme form within the analyzed cervids regarding brain morphology.

The Central Role of Small Vertical Substrates for the Origin of Grasping in Early Primates

The manual and pedal grasping abilities of primates, characterized by an opposable hallux, flat nails, and elongated digits, constitute a unique combination of features that likely promoted their characteristic use of arboreal habitats. These hand and foot specificities are central for understanding the origins and early evolution of primates and have long been associated with foraging in a fine-branch milieu. However, other arboreal mammals occupy similar niches, and it remains unclear how substrate type may have exerted a selective pressure on the acquisition of nails and a divergent pollex/hallux in primates or in what sequential order these traits evolved. Here, we video-recorded 14,564 grasps during arboreal locomotion in 11 primate species (6 strepsirrhines and 5 platyrrhines) and 11 non-primate arboreal species (1 scandentian, 3 rodents, 3 carnivorans, and 4 marsupials). We quantified our observations with 19 variables to analyze the effect of substrate orientation and diameter on hand and foot postural repertoire. We found that hand and foot postures correlate with phylogeny. Also, primates exhibited high repertoire diversity, with a strong capability for postural adjustment compared to the other studied groups. Surprisingly, nails do not confer an advantage in negotiating small substrates unless the animal is large, but the possession of a grasping pollex and hallux is crucial for climbing small vertical substrates. We propose that the divergent hallux and pollex may have resulted from a frequent use of vertical plants in early primate ecological scenarios, although nails may not have resulted from a fundamental adaptation to arboreal locomotion.

Macroevolutionary effects on primate trophic evolution and their implications for reconstructing primate origins

The visual-predation hypothesis proposes that certain derived features shared by crown primates reflect an insectivorous ancestry. Critics of this idea have argued that because insectivory is uncommon among extant primates it is unlikely to have been a major influence on early primate evolution. According to this perspective, the low frequency of insectivory indicates that it is an apomorphic deviation from the mostly conserved primate ecological pattern of herbivory. The present study tests two alternative hypotheses that are compatible with an insectivorous ancestor: (1) that trophic evolution was biased, such that herbivory evolved repeatedly with few shifts back to insectivory, and (2) that insectivorous lineages have diversified at a lower rate than herbivorous lineages owing to differential trophic effects on speciation and extinction probabilities. Model-based analysis conducted using trait data for 307 extant primate species indicates that rates of transition into and out of insectivory are similar, rejecting the hypothesis of biased trophic evolution. On the other hand, the hypothesis of asymmetric diversification is supported, with insectivorous lineages having a lower rate of diversification than herbivorous lineages. This correlation is mediated by activity pattern: insectivory occurs mostly in nocturnal lineages, which have a lower diversification rate than diurnal lineages. The frequency of insectivory also appears to have been shaped by repeated transitions into ecological contexts in which insectivory is absent (large body size) or rare (diurnality). These findings suggest that the current distribution of trophic strategies among extant primates is the result of macroevolutionary processes that have favored the proliferation and persistence of herbivory relative to insectivory. This conclusion implies that the low frequency of insectivory is not necessarily evidence against the visual-predation hypothesis.

The evolutionary radiation of plesiadapiforms

Very shortly after the disappearance of the non-avian dinosaurs, the first mammals that had features similar to those of primates started appearing. These first primitive forms went on to spawn a rich diversity of plesiadapiforms, often referred to as archaic primates. Like many living primates, plesiadapiforms were small arboreal animals that generally ate fruit, insects, and, occasionally, leaves. However, this group lacked several diagnostic features of euprimates. They also had extraordinarily diverse specializations, represented in eleven families and more than 140 species, which, in some cases, were like nothing seen since in the primate order. Plesiadapiforms are known from all three Northern continents, with representatives that persisted until at least 37 million years ago. In this article we provide a summary of the incredible diversity of plesiadapiform morphology and adaptations, reviewing our knowledge of all eleven families. We also discuss the challenges that remain in our understanding of their ecology and evolution.

Contextualising primate origins--an ecomorphological framework

Ecomorphology - the characterisation of the adaptive relationship between an organism's morphology and its ecological role - has long been central to theories of the origin and early evolution of the primate order. This is exemplified by two of the most influential theories of primate origins: Matt Cartmill's Visual Predation Hypothesis, and Bob Sussman's Angiosperm Co-Evolution Hypothesis. However, the study of primate origins is constrained by the absence of data directly documenting the events under investigation, and has to rely instead on a fragmentary fossil record and the methodological assumptions inherent in phylogenetic comparative analyses of extant species. These constraints introduce particular challenges for inferring the ecomorphology of primate origins, as morphology and environmental context must first be inferred before the relationship between the two can be considered. Fossils can be integrated in comparative analyses and observations of extant model species and laboratory experiments of form-function relationships are critical for the functional interpretation of the morphology of extinct species. Recent developments have led to important advancements, including phylogenetic comparative methods based on more realistic models of evolution, and improved methods for the inference of clade divergence times, as well as an improved fossil record. This contribution will review current perspectives on the origin and early evolution of primates, paying particular attention to their phylogenetic (including cladistic relationships and character evolution) and environmental (including chronology, geography, and physical environments) contextualisation, before attempting an up-to-date ecomorphological synthesis of primate origins.

Two cases of osteoid osteoma in skulls dating from the 13–14th centuries from St. Elisabeth’s Church in Wrocław, Poland

In the study two human skulls recovered from archaeological excavations at St. Elizabeth’s Church in Wrocław, dating from the 13-14th centuries were assessed. Direct measurements of each skull were recorded, and X-ray images in P-A, lateral and basal projections were taken. The skulls represented adult males. Large, bony, lobular tumours were found on the palatine bones of both skulls. X-ray examination identified these tumours as osteoid osteomas, which are benign bone tumours that may originate in the periosteum or may be located inside the bone, distorting the maxilla or mandible. However, osteoid osteoma of the palatine is very rare. This study extends our knowledge regarding the health and diseases of historical populations.

Virtual endocasts of Eocene Paramys (Paramyinae): oldest endocranial record for Rodentia and early brain evolution in Euarchontoglires

Understanding the pattern of brain evolution in early rodents is central to reconstructing the ancestral condition for Glires, and for other members of Euarchontoglires including Primates. We describe the oldest virtual endocasts known for fossil rodents, which pertain to Paramys copei (Early Eocene) and Paramys delicatus (Middle Eocene). Both specimens of Paramys have larger olfactory bulbs and smaller paraflocculi relative to total endocranial volume than later occurring rodents, which may be primitive traits for Rodentia. The encephalization quotients (EQs) of Pa. copei and Pa. delicatus are higher than that of later occurring (Oligocene) Ischyromys typus, which contradicts the hypothesis that EQ increases through time in all mammalian orders. However, both species of Paramys have a lower relative neocortical surface area than later rodents, suggesting neocorticalization occurred through time in this Order, although to a lesser degree than in Primates. Paramys has a higher EQ but a lower neocortical ratio than any stem primate. This result contrasts with the idea that primates were always exceptional in their degree of overall encephalization and shows that relative brain size and neocortical surface area do not necessarily covary through time. As such, these data contradict assumptions made about the pattern of brain evolution in Euarchontoglires.

The first major primate extinction: An evaluation of paleoecological dynamics of North American stem primates using a homology free measure of tooth shape

OBJECTIVES

The disappearance of the North American plesiadapoids (stem primates, or plesiadapiforms) in the latest Paleocene has been attributed to competition with rodents over dietary resources. This study compares molar morphology of plesiadapoids and early rodents to assess whether all taxa were adapted to consuming foods of the same structural properties with similar mechanical efficacy.

MATERIALS AND METHODS

Micro-CT scans of second mandibular molars (M2 s) of plesiadapoids (n = 181) and ischyromyid (early fossil) rodents (n = 13) were evaluated using Dirichlet normal energy (DNE), a dental topographic metric that quantifies the curvature of a tooth's occlusal surface, independent of the orientation of the occlusal plane; this metric can be used to infer diet.

RESULTS

Comparisons of DNE values for plesiadapoids and rodents show that rodents shared functionally similar dental morphology with at least some plesiadapid plesiadapoids and thus were likely adapted to processing foods with similar physical properties. However, the DNE values for rodents contrast markedly with those for the other two plesiadapoid families, the Carpolestidae and Saxonellidae.

CONCLUSIONS

It is unlikely that direct competition over food resources with rodents played a major role in the extinction of carpolestids and saxonellids, as members of these families were capable of consuming a range of foods that were not accessible to rodents. Although several plesiadapid species overlap with rodents in their range of DNE values, only three overlap in time. One of these (Plesiadapis cookei) may have been too large to be in direct competition with rodents, another (Plesiadapis dubius) has DNE values substantially different (higher) than those of rodents, whereas the third, Chiromyoides, has teeth of both a similar size and DNE value to those of Clarkforkian rodents. If dietary niche overlap with rodents played a direct role in the decline of plesiadapiforms, it can only have potentially done so for Chiromyoides.

Life history of the most complete fossil primate skeleton: exploring growth models for Darwinius

Darwinius is an adapoid primate from the Eocene of Germany, and its only known specimen represents the most complete fossil primate ever found. Its describers hypothesized a close relationship to Anthropoidea, and using a Saimiri model estimated its age at death. This study reconstructs the ancestral permanent dental eruption sequences for basal Euprimates, Haplorhini, Anthropoidea, and stem and crown Strepsirrhini. The results show that the ancestral sequences for the basal euprimate, haplorhine and stem strepsirrhine are identical, and similar to that of Darwinius. However, Darwinius differs from anthropoids by exhibiting early development of the lower third molars relative to the lower third and fourth premolars. The eruption of the lower second premolar marks the point of interruption of the sequence in Darwinius. The anthropoid Saimiri as a model is therefore problematic because it exhibits a delayed eruption of P2. Here, an alternative strepsirrhine model based on Eulemur and Varecia is presented. Our proposed model shows an older age at death than previously suggested (1.05-1.14 years), while the range for adult weight is entirely below the range proposed previously. This alternative model is more consistent with hypotheses supporting a stronger relationship between adapoids and strepsirrhines.

Quantification of neocortical ratios in stem primates

Extant euprimates (=crown primates) have a characteristically expanded neocortical region of the brain relative to that of other mammals, but the timing of that expansion in their evolutionary history is poorly resolved. Examination of anatomical landmarks on fossil endocasts of Eocene euprimates suggests that significant neocortical expansion relative to contemporaneous mammals was already underway. Here, we provide quantitative estimates of neocorticalization in stem primates (plesiadapiforms) relevant to the question of whether relative neocortical expansion was uniquely characteristic of the crown primate radiation. Ratios of neocortex to endocast surface areas were calculated for plesiadapiforms using measurements from virtual endocasts of the paromomyid Ignacius graybullianus (early Eocene, Wyoming) and the microsyopid Microsyops annectens (middle Eocene, Wyoming). These data are similar to a published estimate for the plesiadapid, Plesiadapis tricuspidens, but contrast with those calculated for early Tertiary euprimates in being within the 95% confidence intervals for archaic mammals generally. Interpretation of these values is complicated by the paucity of sampled endocasts for older stem primates and euarchontogliran outgroups, as well as by a combination of effects related to temporal trends, allometry, and taxon-unique specializations. Regardless, these results are consistent with the hypothesis that a shift in brain organization occurred in the first euprimates, likely in association with elaborations to the visual system.

Endocranial morphology of Palaeocene Plesiadapis tricuspidens and evolution of the early primate brain

Expansion of the brain is a key feature of primate evolution. The fossil record, although incomplete, allows a partial reconstruction of changes in primate brain size and morphology through time. Palaeogene plesiadapoids, closest relatives of Euprimates (or crown-group primates), are crucial for understanding early evolution of the primate brain. However, brain morphology of this group remains poorly documented, and major questions remain regarding the initial phase of euprimate brain evolution. Micro-CT investigation of the endocranial morphology of Plesiadapis tricuspidens from the Late Palaeocene of Europe--the most complete plesiadapoid cranium known--shows that plesiadapoids retained a very small and simple brain. Plesiadapis has midbrain exposure, and minimal encephalization and neocorticalization, making it comparable with that of stem rodents and lagomorphs. However, Plesiadapis shares a domed neocortex and downwardly shifted olfactory-bulb axis with Euprimates. If accepted phylogenetic relationships are correct, then this implies that the euprimate brain underwent drastic reorganization during the Palaeocene, and some changes in brain structure preceded brain size increase and neocortex expansion during evolution of the primate brain.

Eye size and visual acuity influence vestibular anatomy in mammals

The semicircular canals of the inner ear detect head rotations and trigger compensatory movements that stabilize gaze and help maintain visual fixation. Mammals with large eyes and high visual acuity require precise gaze stabilization mechanisms because they experience diminished visual functionality at low thresholds of uncompensated motion. Because semicircular canal radius of curvature is a primary determinant of canal sensitivity, species with large canal radii are expected to be capable of more precise gaze stabilization than species with small canal radii. Here, we examine the relationship between mean semicircular canal radius of curvature, eye size, and visual acuity in a large sample of mammals. Our results demonstrate that eye size and visual acuity both explain a significant proportion of the variance in mean canal radius of curvature after statistically controlling for the effects of body mass and phylogeny. These findings suggest that variation in mean semicircular canal radius of curvature among mammals is partly the result of selection for improved gaze stabilization in species with large eyes and acute vision. Our results also provide a possible functional explanation for the small semicircular canal radii of fossorial mammals and plesiadapiforms.

New insights into the ear region anatomy and cranial blood supply of advanced stem Strepsirhini: evidence from three primate petrosals from the Eocene of Chambi, Tunisia

We report the discovery of three isolated primate petrosal fragments from the fossiliferous locality of Chambi (Tunisia), a primate-bearing locality dating from the late early to the early middle Eocene. These fossils display a suite of anatomical characteristics otherwise found only in strepsirhines, and as such might be attributed either to Djebelemur or/and cf. Algeripithecus, the two diminutive stem strepsirhine primates recorded from this locality. Although damaged, the petrosals provide substantial information regarding the ear anatomy of these advanced stem strepsirhines (or pre-tooth-combed primates), notably the patterns of the pathway of the arterial blood supply. Using μCT-scanning techniques and digital segmentation of the structures, we show that the transpromontorial and stapedial branches of the internal carotid artery (ICA) were present (presence of bony tubes), but seemingly too small to supply enough blood to the cranium alone. This suggests that the ICA was not the main cranial blood supply in stem strepsirhines, but that the pharyngeal or vertebral artery primitively ensured a great part of this role instead, an arterial pattern that is reminiscent of modern cheirogaleid, lepilemurid lemuriforms and lorisiforms. This could explain parallel loss of the ICA functionality among these families. Specific measurements made on the cochlea indicate that the small strepsirhine primate(s) from Chambi was (were) highly sensitive to high frequencies and poorly sensitive to low frequencies. Finally, variance from orthogonality of the plane of the semicircular canals (SCs) calculated on one petrosal (CBI-1-569) suggests that Djebelemur or cf. Algeripithecus likely moved (at least its head) in a way similar to that of modern mouse lemurs.

First record of the genus Microchoerus (Omomyidae, Primates) in the western Iberian Peninsula and its palaeobiogeographic implications

In this paper we describe new material of Microchoerus (Microchoerinae, Omomyidae, Primates) from Zambrana (Miranda-Trebiño Basin, northern Iberian Peninsula, Spain), a locality assigned to Reference Level MP18 (middle Headonian, Late Eocene). The specimens studied consist of two mandibular fragments, bearing p3-m3 and p4-m3. The teeth resemble in size and morphology those of Microchoerus erinaceus from Hordle Cliff, England, although some differences prevent us from making a definitive ascription to this species. We therefore refer the material from Zambrana to Microchoerus aff. erinaceus. Some traits, such as the development of the mesoconid and hypoconulid in the m1 and m2, and the shape of the hypoconulid lobe in the m3, are intermediate between those of M. erinaceus and Microchoerus edwardsi. Thus, the material from Zambrana is very similar to other species of Microchoerus present in Europe, representing a transitional form between M. erinaceus and M. edwardsi. The described material represents the first discovery of a primate from the Miranda-Trebiño Basin, and also the westernmost record of the genus Microchoerus in the Iberian Peninsula. Moreover, the identification of this microchoerine, with clear similarities to the representatives of this genus described from other European sites, reinforces the idea of the existence of connections between western Iberia and the rest of Europe in the Late Eocene, previously hypothesized after the discovery of typical European artiodactyls in the site of Zambrana.

The placental mammal ancestor and the post-K-Pg radiation of placentals

To discover interordinal relationships of living and fossil placental mammals and the time of origin of placentals relative to the Cretaceous-Paleogene (K-Pg) boundary, we scored 4541 phenomic characters de novo for 86 fossil and living species. Combining these data with molecular sequences, we obtained a phylogenetic tree that, when calibrated with fossils, shows that crown clade Placentalia and placental orders originated after the K-Pg boundary. Many nodes discovered using molecular data are upheld, but phenomic signals overturn molecular signals to show Sundatheria (Dermoptera + Scandentia) as the sister taxon of Primates, a close link between Proboscidea (elephants) and Sirenia (sea cows), and the monophyly of echolocating Chiroptera (bats). Our tree suggests that Placentalia first split into Xenarthra and Epitheria; extinct New World species are the oldest members of Afrotheria.

Cochlear labyrinth volume in euarchontoglirans: implications for the evolution of hearing in primates

Using high resolution X-ray computed tomography data we examined the relationship between cochlear labyrinth volume and body mass in extant, non-primate euarchontoglirans, and in two fossils, to allow for comparison with the results of Kirk and Gosselin-Ildari (2009). Modern primates have significantly higher cochlear labyrinth volumes relative to body mass than other euarchontoglirans, which may be related to a downward shift in the highest and lowest audible frequencies over the course of primate evolution, and to the relative increase in brain size observed in Euprimates.

Endocasts of Microsyops (Microsyopidae, Primates) and the evolution of the brain in primitive primates

We describe a virtual endocast produced from ultra high resolution X-ray computed tomography (CT) data for the microsyopid, Microsyops annectens (middle Eocene, Wyoming). It is the most complete and least distorted endocast known for a plesiadapiform primate and because of the relatively basal position of Microsyopidae, has particular importance to reconstructing primitive characteristics for Primates. Cranial capacity is estimated at 5.9 cm(3), yielding encephalization quotients (EQ) of 0.26-0.39 (Jerison's equation) and 0.32-0.52 (Eisenberg's equation), depending on the body mass estimate. Even the lowest EQ estimate for M. annectens is higher than that for Plesiadapis cookei, while the range of estimates overlaps with that of Ignacius graybullianus and with the lower end of the range of estimates for fossil euprimates. As in other plesiadapiforms, the olfactory bulbs of M. annectens are large. The cerebrum does not extend onto the cerebellum or form a ventrally protruding temporal lobe with a clear temporal pole, suggesting less development of the visual sense and a greater emphasis on olfaction than in euprimates. Contrasts between the virtual endocast of M. annectens, and both a natural endocast of the same species and a partial endocast from the earlier-occurring Microsyops sp., cf. Microsyops elegans, suggest that the coverage of the caudal colliculi by the cerebrum evolved within the Microsyops lineage. This implies that microsyopids expanded their cerebra and perhaps evolved an improved visual sense independent of euprimates. With a growing body of data on the morphology of the brain in primitive primates, it is becoming clear that many of the characteristics of the brain common to euprimates evolved after the divergence of stem primates from other euarchontans and likely in parallel in different lineages. These new data suggest a different model for the ancestors of euprimates than has been assumed based on the anatomy of the brain in visually specialized diurnal tree shrews.

Virtual endocast of Ignacius graybullianus (Paromomyidae, Primates) and brain evolution in early primates

Extant primates are distinctive among mammals in having relatively large brains. As stem primates, Paleogene plesiadapiforms provide direct information relevant to the earliest stages in the evolution of this characteristic. Here we describe a virtual endocast reconstructed from ultra high resolution X-ray computed tomography data for the paromomyid plesiadapiform Ignacius graybullianus (USNM 421608) from the early Eocene of Wyoming. This represents the most complete endocast known for a stem primate, allowing for an unprecedented study of both size and fine details of anatomy. Relative to fossil and extant euprimates, I. graybullianus had large olfactory lobes, but less caudal development of the cerebrum and a poorly demarcated temporal lobe, suggesting more emphasis on olfaction and a less well developed visual system. Although its brain was small compared to those of extant primates, the encephalization quotient of I. graybullianus is higher than that calculated for Paleocene Plesiadapis cookei and overlaps the lower portion of the range documented for fossil euprimates. Comparison to the basal gliroid Rhombomylus suggests that early primates exhibited some expansion of the cerebrum compared to their ancestors. The relatively small brain size of I. graybullianus, an arboreal frugivore, implies that neither arboreality nor frugivory was primarily responsible for the expanded brains of modern primates. However, the contrasts in features related to the visual system between I. graybullianus and fossil and extant euprimates suggest that improvements to these portions of the brain contributed to increases in brain size within Euprimates.

Intrinsic hand proportions of euarchontans and other mammals: implications for the locomotor behavior of plesiadapiforms

Arboreal primates have distinctive intrinsic hand proportions compared with many other mammals. Within Euarchonta, platyrrhines and strepsirrhines have longer manual proximal phalanges relative to metacarpal length than colugos and terrestrial tree shrews. This trait is part of a complex of features allowing primates to grasp small-diameter arboreal substrates. In addition to many living and Eocene primates, relative elongation of proximal manual phalanges is also present in most plesiadapiforms. In order to evaluate the functional and evolutionary implications of manual similarities between crown primates and plesiadapiforms, we measured the lengths of the metacarpal, proximal phalanx, and intermediate phalanx of manual ray III for 132 extant mammal species (n=702 individuals). These data were compared with measurements of hands in six plesiadapiform species using ternary diagrams and phalangeal indices. Our analyses reveal that many arboreal mammals (including some tree shrews, rodents, marsupials, and carnivorans) have manual ray III proportions similar to those of various arboreal primates. By contrast, terrestrial tree shrews have hand proportions most similar to those of other terrestrial mammals, and colugos are highly derived in having relatively long intermediate phalanges. Phalangeal indices of arboreal species are significantly greater than those of the terrestrial species in our sample, reflecting the utility of having relatively long digits in an arboreal context. Although mammals known to be capable of prehensile grips demonstrate long digits relative to palm length, this feature is not uniquely associated with manual prehension and should be interpreted with caution in fossil taxa. Among plesiadapiforms, Carpolestes, Nannodectes, Ignacius, and Dryomomys have manual ray III proportions that are unlike those of most terrestrial species and most similar to those of various arboreal species of primates, tree shrews, and rodents. Within Euarchonta, Ignacius and Carpolestes have intrinsic hand proportions most comparable to those of living arboreal primates, while Nannodectes is very similar to the arboreal tree shrew Tupaia minor. These results provide additional evidence that plesiadapiforms were arboreal and support the hypothesis that Euarchonta originated in an arboreal milieu.

Frontal fusion: collapse of another anthropoid synapomorphy

We test the hypothesis that the fused interfrontal suture of anthropoids is a uniquely distinguishing feature and a derived characteristic indicative of their monophyletic origin. Our survey of nonanthropoid primates and several archontan families indicates frontal fusion is widespread. It is most variable (fused, open or partially fused) inter- and intra-specifically among strepsirhines. The frontal bone is more commonly fused in living lemuroids and indrioids than among lorisoids. It appears to be fused regularly among Eocene adapids. Among nonanthropoid haplorhines, the interfrontal is fused in Tarsius, even in neonates and invariably in adults, probably also in all fossil tarsiiforms preserving the frontal bone, and in the late Eocene protoanthropoid Rooneyia. The plesiadapiform pattern remains uncertain, but fusion is ubiquitous among living tree shrews, colugos and bats. Distributional evidence implies that interfrontal fusion was present in the last common ancestor (LCA) of haplorhine primates and possibly in the LCA of euprimates as well. Anthropoids, therefore, cannot be defined cladistically by interfrontal fusion, not out of concern for homoplasy but because it is probably a primitive feature inherited from other taxa related to anthropoids. Fusion of the large anthropoid frontal bone, which was extended anteriorly to roof the orbits and expanded laterally in connection with a wide forebrain in the LCA of anthropoids and protoanthropoids, may have been preadaptive to the evolution of the postorbital septum. The zygomatico-frontal suture of the septum may provide an alternative mechanism for dissipating the calvarial strains of mastication formerly taken up by an open interfrontal suture.

Craniofacial growth in fetal Tarsius bancanus: brains, eyes and nasal septa

The tarsier skull has been of particular interest in studies of primate taxonomy and functional morphology for several decades. Despite this, there remains no comprehensive data on how the tarsier skull develops, especially in relation to the soft-tissues of the head. Here we have documented for the first time fetal development of the skull and brain as well as the nasal septum and eyes in T. bancanus. We have also tested for the possible influence of these tissues in shaping skull architecture. Nineteen post-mortem specimens were imaged using high-resolution magnetic resonance imaging and magnetic resonance microscopy. Landmarks and volume data were collected and analysed. Findings demonstrated massive increases of brain size and eye size as well as flattening of the midline cranial base, facial projection and orbital margin frontation. Little evidence was found to support the notion that growth of the brain or nasal septum physically drives the observed changes of the skull. However, increases in the size of the eyes relative to skull size were associated with orbital margin frontation. With the possible exception of the results for eye size, the findings indicate that rather than forcing change the soft-tissues form a framework that physically constrains the morphogenetic template of the skeletal elements. This suggests, for example, that the degree of cranial base angulation seen in adulthood is not directly determined by brain expansion bending the basicranium, but by brain enlargement limiting the extent of cranial base flattening (retroflexion) in the fetus.

New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates

Plesiadapiforms are central to studies of the origin and evolution of primates and other euarchontan mammals (tree shrews and flying lemurs). We report results from a comprehensive cladistic analysis using cranial, postcranial, and dental evidence including data from recently discovered Paleocene plesiadapiform skeletons (Ignacius clarkforkensis sp. nov.; Dryomomys szalayi, gen. et sp. nov.), and the most plesiomorphic extant tree shrew, Ptilocercus lowii. Our results, based on the fossil record, unambiguously place plesiadapiforms with Euprimates and indicate that the divergence of Primates (sensu lato) from other euarchontans likely occurred before or just after the Cretaceous/Tertiary boundary (65 Mya), notably later than logistical model and molecular estimates. Anatomical features associated with specialized pedal grasping (including a nail on the hallux) and a petrosal bulla likely evolved in the common ancestor of Plesiadapoidea and Euprimates (Euprimateformes) by 62 Mya in either Asia or North America. Our results are consistent with those from recent molecular analyses that group Dermoptera with Scandentia. We find no evidence to support the hypothesis that any plesiadapiforms were mitten-gliders or closely related to Dermoptera.