Large Olfactory Bulbs Research Articles

Trade-Offs in the Sensory Brain between Diurnal and Nocturnal Rodents

Introduction: Transitions in temporal niche have occurred many times over the course of mammalian evolution. These are associated with changes in sensory stimuli available to animals, particularly with visual cues, because levels of light are so much higher during the day than at night. This relationship between temporal niche and available sensory stimuli elicits the expectation that evolutionary transitions between diurnal and nocturnal lifestyles will be accompanied by modifications of sensory systems that optimize the ability of animals to receive, process, and react to important stimuli in the environment. Methods: This study examines the influence of temporal niche on investment in sensory brain tissue of 13 rodent species (five diurnal; eight nocturnal). Animals were euthanized and the brains immediately frozen on dry ice; olfactory bulbs were subsequently dissected and weighed, and the remaining brain was weighed, sectioned, and stained. Stereo Investigator was used to calculate volumes of four sensory regions that function in processing visual (lateral geniculate nucleus, superior colliculus) and auditory (medial geniculate nucleus, inferior colliculus) information. A phylogenetic framework was used to assess the influence of temporal niche on the relative sizes of these brain structures and of olfactory bulb weights. Results: Compared to nocturnal species, diurnal species had larger visual regions, whereas nocturnal species had larger olfactory bulbs than their diurnal counterparts. Of the two auditory structures examined, one (medial geniculate nucleus) was larger in diurnal species, while the other (inferior colliculus) did not differ significantly with temporal niche. Conclusion: Our results indicate a possible indirect association between temporal niche and auditory investment and suggest probable trade-offs of investment between olfactory and visual areas of the brain, with diurnal species investing more in processing visual information and nocturnal species investing more in processing olfactory information.

The virtual brain endocast of Trogosus (Mammalia, Tillodontia) and its relevance in understanding the extinction of archaic placental mammals.

After successfully diversifying during the Paleocene, the descendants of the first wave of mammals that survived the end-Cretaceous mass extinction waned throughout the Eocene. Competition with modern crown clades and intense climate fluctuations may have been part of the factors leading to the extinction of these archaic groups. Why these taxa went extinct has rarely been studied from the perspective of the nervous system. Here, we describe the first virtual endocasts for the archaic order Tillodontia. Three species from the middle Eocene of North America were analyzed: Trogosus hillsii, Trogosus grangeri, and Trogosus castoridens. We made morphological comparisons with the plaster endocast of another tillodont, Tillodon fodiens, as well as groups potentially related to Tillodontia: Pantodonta, Arctocyonidae, and Cimolesta. Trogosus shows very little inter-specific variation with the only potential difference being related to the fusion of the optic canal and sphenorbital fissure. Many ancestral features are displayed by Trogosus, including an exposed midbrain, small neocortex, orbitotemporal canal ventral to rhinal fissure, and a broad circular fissure. Potential characteristics that could unite Tillodontia with Pantodonta, and Arctocyonidae are the posterior position of cranial nerve V3 exit in relation to the cerebrum and the low degree of development of the subarcuate fossa. The presence of large olfactory bulbs and a relatively small neocortex are consistent with a terrestrial lifestyle. A relatively small neocortex may have put Trogosus at risk when competing with artiodactyls for potentially similar resources and avoiding predation from archaic carnivorans, both of which are known to have had larger relative brain and neocortex sizes in the Eocene. These factors may have possibly exacerbated the extinction of Tillodontia, which showed highly specialized morphologies despite the increase in climate fluctuations throughout the Eocene, before disappearing during the middle Eocene.

Cranial endocast of Anagale gobiensis (Anagalidae) and its implications for early brain evolution in Euarchontoglires

AbstractAnagalids are an extinct group of primitive mammals from the Asian Palaeogene thought to be possible basal members of Glires. Anagalid material is rare, with only a handful of crania known. Here we describe the first virtual endocast of an anagalid, based on the holotype of Anagale gobiensis (AMNH 26079; late Eocene, China), which allows for comparison with published endocasts from fossil members of modern euarchontogliran lineages (i.e. primates, rodents, lagomorphs). The endocast displays traits often observed in fossorial mammals, such as relatively small petrosal lobules and a low neocortical ratio, which would be consistent with previous inferences about use of subterranean food sources based on heavy dental wear. In fact, Anagale gobiensis has the lowest neocortical ratio yet recorded for a euarchontogliran. This species was olfaction‐driven, based on the relatively large olfactory bulbs and laterally expansive palaeocortex. The endocast supports previous inferences that relatively large olfactory bulbs, partial midbrain exposure and low encephalization quotient are ancestral for Euarchontoglires, although the likely fossorial adaptations of Anagale gobiensis may also partly explain these traits. While Anagale gobiensis is a primitive mammal in many aspects, some of its derived endocranial traits point towards a new, different trajectory of brain evolution within Euarchontoglires.

Olfaction and reaction: The role of olfactory and hypothalamic investment in the antipredator responses to chemical alarm cues by northern redbelly dace.

Neuroplasticity enables teleosts to promote or downregulate the growth of their brains regionally. To compensate for the effects of predation pressure, teleosts may alter their brain morphology and behavioral responses to mitigate its impact on individual fitness. High-predation environments often promote specific patterns of brain growth and produce bolder and more proactive populations. Owing to the expense of maintaining neural tissue, relative size indicates the regions most relied upon. In northern redbelly dace Chrosomus eos, as little as 2 weeks of elevated predation pressure, resulted in increased investment in their olfactory bulbs and optic tecta, while the imposition of captivity produced smaller, less symmetric hypothalami. Taken together, these results suggest that an individual could potentially become better able to detect a threat, and simultaneously less inclined to react to it, making the impact of either change in isolation is difficult to discern. Here, we compared interindividual variation in gross brain morphology, risk-taking tactics in a novel arena (shy-bold personality), and responding to olfactory cues (proactive/reactive stress-coping style). We hypothesized that olfactory investment would positively correlate with response intensity to predator cue concentration and respond across a wider range of cue concentrations, while hypothalamus size would correlate with shyness and reactivity. Exposure to heightened risk produced more bold/proactive individuals, with larger olfactory bulbs and smaller hypothalami. However, the direction of the correlation between hypothalamus size and behavior varied by treatment, and olfactory investment only corresponded with response intensity amongst proactive individuals. Our findings illustrate the potential pitfalls of relating gross brain morphology to complex behavior and suggest that stress-coping style is a relevant consideration in future studies.

The endocast of Euparkeria sheds light on the ancestral archosaur nervous system

AbstractUnderstanding the evolution of the tetrapod brain is essential to trace the history of ecomorphological diversification of modern clades. While previous studies focused on the morphological transformation of the nervous system along the dinosaur–bird transition, little is known about the brain anatomy of archosauriformes and early archosaurs. Here, we describe the endocast of Euparkeria capensis, a small bodied, terrestrial archosauriform closely related to Archosauria, with the goal of resolving the current uncertainties surrounding the ancestral condition of the archosaurian nervous system. The endocast of Euparkeria is sigmoidal, with large olfactory bulbs, an expanded cerebral hemisphere and an elongated flocculus. We suggest that this pivotal taxon was an active predator with a remarkable olfactory acuity. Overall, the endocast of Euparkeria resembles the ones observed in phytosaurs, crocodilians and early dinosaurs, implying that modern crocodilians retain an archosaurian plesiomorphic brain morphology.

Cranial osteology and palaeobiology of the Early Cretaceous birdJeholornis prima(Aves: Jeholornithiformes)

AbstractJeholornis is a representative of the earliest-diverging bird lineages, providing important evidence of anatomical transitions involved in bird origins. Although ~100 specimens have been reported, its cranial morphology remains poorly documented owing to poor two-dimensional preservation, limiting our understanding of the morphology and ecology of the key avian lineage Jeholornithiformes, in addition to cranial evolution during the origin and early evolution of birds. Here, we provide a detailed description of the cranial osteology of Jeholornis prima, based primarily on high-quality, three-dimensional data of a recently reported specimen. New anatomical information confirms the overall plesiomorphic morphology of the skull, with the exception of the more specialized rostrum. Data from a large sample size of specimens reveal the dental formula of J. prima to be 0–2–3 (premaxillary–maxillary–dentary tooth counts), contrary to previous suggestions that the presence of maxillary teeth is diagnostic of a separate species, Jeholornis palmapenis. We also present evidence of sensory adaptation, including relatively large olfactory bulbs in comparison to other known stem birds, suggesting that olfaction was an important aspect of Jeholornis ecology. The digitally reconstructed scleral ring suggests a strongly diurnal habit, supporting the hypothesis that early-diverging birds were predominantly active during the day.

Gross Morphology of the Brain and Spinal Cord of the African Pygmy Hedgehog (Atelerix albiventris)

Abstract The African pygmy hedgehog (Atelerix albiventris) is an insectivorous animal, native to Africa. The central nervous system (CNS) consists of the brain and the spinal cord, protected by the cranium and vertebral column respectively. Assessment of the gross appearance and morphometries of the African pygmy hedgehog CNS were carried out using six adults (3 males and 3 females). The gross examination showed the brains to be lissencephalic, with relatively large olfactory bulbs, similar to that observed in some rodents. The rootlets of the first cervical spinal nerves were observed to emerge before the foramen magnum. Linear measurements were obtained from both the brain and spinal cord. The mean weight of the animals was 199.00 ± 16.09 g, with the males having an average body weight of 183.50 ± 12.02 g and the females 206.80 ± 11.95 g. Although not statistically significant, the males had a higher encephalisation quotient (0.40 ± 0.08) relative to the females 0.36 ± 0.04). The values for the brain weight, length of spinal cord and heights of the telencephalon and diencephalon at different points were higher in the males, while the spinal cord weight, length of brain and cerebellar height were higher in the females. The spinal cord showed slight enlargements at the cervical, thoracic, lumbar and sacral segments. This study aimed to provide baseline data for the study of the gross appearance and neuromorpho-metrics of the hedgehog, with possible application in regional anaesthesiology and comparative wildlife neuroanatomy.

Better Olfactory Performance and Larger Olfactory Bulbs in a Mouse Model of Congenital Blindness.

It is well established that early blindness results in enhancement of the remaining nonvisual sensory modalities accompanied by functional and anatomical brain plasticity. While auditory and tactile functions have been largely investigated, the results regarding olfactory functions remained less explored and less consistent. In the present study, we investigated olfactory function in blind mice using 3 tests: the buried food test, the olfactory threshold test, and the olfactory performance test. The results indicated better performance of blind mice in the buried food test and odor performance test while there was no difference in the olfactory threshold test. Using histological measurements, we also investigated if there was anatomical plasticity in the olfactory bulbs (OB), the most salient site for olfactory processing. The results indicated a larger volume of the OB driven by larger glomerular and granular layers in blind mice compared with sighted mice. Structural plasticity in the OB may underlie the enhanced olfactory performance in blind mice.

Cranial endocast of the stem lagomorph Megalagus and brain structure of basal Euarchontoglires.

Early lagomorphs are central to our understanding of how the brain evolved in Glires (rodents, lagomorphs and their kin) from basal members of Euarchontoglires (Glires + Euarchonta, the latter grouping primates, treeshrews, and colugos). Here, we report the first virtual endocast of the fossil lagomorph Megalagus turgidus, from the Orella Member of the Brule Formation, early Oligocene, Nebraska, USA. The specimen represents one of the oldest nearly complete lagomorph skulls known. Primitive aspects of the endocranial morphology in Megalagus include large olfactory bulbs, exposure of the midbrain, a small neocortex and a relatively low encephalization quotient. Overall, this suggests a brain morphology closer to that of other basal members of Euarchontoglires (e.g. plesiadapiforms and ischyromyid rodents) than to that of living lagomorphs. However, the well-developed petrosal lobules in Megalagus, comparable to the condition in modern lagomorphs, suggest early specialization in that order for the stabilization of eye movements necessary for accurate visual tracking. Our study sheds new light on the reconstructed morphology of the ancestral brain in Euarchontoglires and fills a critical gap in the understanding of palaeoneuroanatomy of this major group of placental mammals.

Neurosensory and Sinus Evolution as Tyrannosauroid Dinosaurs Developed Giant Size: Insight from the Endocranial Anatomy of Bistahieversor sealeyi.

Tyrannosaurus rex and other tyrannosaurid dinosaurs were apex predators during the latest Cretaceous, which combined giant size and advanced neurosensory systems. Computed tomography (CT) data have shown that tyrannosaurids had a trademark system of a large brain, large olfactory bulbs, elongate cochlear ducts, and expansive endocranial sinuses surrounding the brain and sense organs. Older, smaller tyrannosauroid relatives of tyrannosaurids developed some, but not all, of these features, raising the hypothesis that tyrannosaurid-style brains evolved before the enlarged tyrannosaurid-style sinuses, which might have developed only with large body size. This has been difficult to test, however, because little is known about the brains and sinuses of the first large-bodied tyrannosauroids, which evolved prior to Tyrannosauridae. We here present the first CT data for one of these species, Bistahieversor sealeyi from New Mexico. Bistahieversor had a nearly identical brain and sinus system as tyrannosaurids like Tyrannosaurus, including a large brain, large olfactory bulbs, reduced cerebral hemispheres, and optic lobes, a small tab-like flocculus, long and straight cochlear ducts, and voluminous sinuses that include a supraocciptal recess, subcondyar sinus, and an anterior tympanic recess that exits the braincase via a prootic fossa. When characters are plotted onto tyrannosauroid phylogeny, there is a two-stage sequence in which features of the tyrannosaurid-style brain evolved first (in smaller, nontyrannosaurid species like Timurlengia), followed by features of the tyrannosaurid-style sinuses (in the first large-bodied nontyrannosaurid tyrannosauroids like Bistahieversor). This suggests that the signature tyrannosaurid sinus system evolved in concert with large size, whereas the brain did not. Anat Rec, 303:1043-1059, 2020. © 2020 American Association for Anatomy.

Paleoneurology, auditory region, and associated soft tissue inference in the late Oligocene notoungulates Mendozahippus fierensis and Gualta cuyana (Toxodontia) from central-western Argentina

ABSTRACT Noninvasive computed tomography (CT) scanning techniques performed on the high-quality preserved holotypes of Mendozahippus fierensis (grouped within the paraphyletic ‘Notohippidae’) and Gualta cuyana (Leontiniidae) allowed a thorough description of both endocasts, not only associated with the brain but also with closely related endocranial spaces, which proved to be highly diagnostic among Notoungulata and relevant from a paleobiological perspective. The specimens come from Quebrada Fiera (Mendoza, Argentina), an upper Oligocene locality extensively studied, especially during the last decade. Regarding brain endocast proportions and neocortical complexity, M. fierensis shows morphological affinities to coeval ‘notohippids,’ whereas G. cuyana resembles Leontinia gaudryi. An acute olfaction is inferred from the large olfactory bulbs and piriform lobes. Conversely, evidence concerning the development (and implications) of some necortical areas (i.e., somatosensory cortex and auditory cortex) were inconclusive. The encephalization quotients are around those previously reported for ‘notohippids’ and Toxodontidae Nesodontinae, and slightly lower than in Typotheria (except for Mesotheriidae). Other endocranial traits previously unexplored (such as the transverse diploic communication between the temporal sinuses in M. fierensis and the large temporal sinuses inferred for G. cuyana) show that there is relevant variability within Toxodontia that deserves further research. Extending similar approaches to early diverging taxa will certainly contribute to a better understanding of the morphological evolution of the brain, special sense organs, and associated blood circulation pattern in toxodontians and notoungulates in general.

Virtual endocranial and inner ear endocasts of the Paleocene 'condylarth' Chriacus: new insight into the neurosensory system and evolution of early placental mammals.

The end-Cretaceous mass extinction allowed placental mammals to diversify ecologically and taxonomically as they filled ecological niches once occupied by non-avian dinosaurs and more basal mammals. Little is known, however, about how the neurosensory systems of mammals changed after the extinction, and what role these systems played in mammalian diversification. We here use high-resolution computed tomography (CT) scanning to describe the endocranial and inner ear endocasts of two species, Chriacus pelvidens and Chriacus baldwini, which belong to a cluster of 'archaic' placental mammals called 'arctocyonid condylarths' that thrived during the ca. 10 million years after the extinction (the Paleocene Epoch), but whose relationships to extant placentals are poorly understood. The endocasts provide new insight into the paleobiology of the long-mysterious 'arctocyonids', and suggest that Chriacus was an animal with an encephalization quotient (EQ) range of 0.12-0.41, which probably relied more on its sense of smell than vision, because the olfactory bulbs are proportionally large but the neocortex and petrosal lobules are less developed. Agility scores, estimated from the dimensions of the semicircular canals of the inner ear, indicate that Chriacus was slow to moderately agile, and its hearing capabilities, estimated from cochlear dimensions, suggest similarities with the extant aardvark. Chriacus shares many brain features with other Paleocene mammals, such as a small lissencephalic brain, large olfactory bulbs and small petrosal lobules, which are likely plesiomorphic for Placentalia. The inner ear of Chriacus also shares derived characteristics of the elliptical and spherical recesses with extinct species that belong to Euungulata, the extant placental group that includes artiodactyls and perissodactyls. This lends key evidence to the hypothesized close relationship between Chriacus and the extant ungulate groups, and demonstrates that neurosensory features can provide important insight into both the paleobiology and relationships of early placental mammals.

New Virtual Endocasts of Eocene Ischyromyidae and Their Relevance in Evaluating Neurological Changes Occurring Through Time in Rodentia

Little is known about the early evolution of the brain in rodents. We report on nine new virtual endocasts for one of the most primitive family of rodents, Ischyromyidae, based on five specimens of Pseudotomus and Notoparamys (Paramyinae) and four specimens of Reithroparamys and Rapamys (Reithroparamyinae), dating from the early Eocene to the late middle Eocene of North America (Colorado, Wyoming, Utah and Montana). The virtual endocasts were obtained from high-resolution X-ray micro-computed tomography data. Comparisons with previously described ischyromyid virtual endocasts allow us to make inferences about the ancestral condition of the brain in rodents. Since Reithroparamyinae are suggested to be more closely related to the squirrel-related clade than other Ischyromyidae, comparisons were also made with the oldest virtual endocast for a squirrel, which gave us the opportunity to look at finer neurological changes occurring in the early evolution of squirrels. These new data permit a preliminary assessment of the endocranial diversity in Ischyromyidae. The results do not show evidence for a clear increase in Encephalization Quotient through time for early rodents. Instead, variation among species could be due to ecological factors (e.g., locomotion). Significant expansion in the neocortex and increase in paraflocculi ratios may have occurred in the transition from Ischyromyidae to Sciuridae, as previously hypothesized. Large olfactory bulbs and exposed midbrain are inferred to have been features present in the common ancestor of rodents, while neocortical expansion is reconstructed as having occurred twice independently within Ischyromyidae.

Anatomical evidence for scent guided foraging in the turkey vulture

The turkey vulture (Cathartes aura) is a widespread, scavenging species in the Western Hemisphere that locates carrion by smell. Scent guided foraging is associated with an expansion of the olfactory bulbs of the brain in vertebrates, but no such neuroanatomical data exists for vultures. We provide the first measurements of turkey vulture brains, including the size of their olfactory bulbs and numbers of mitral cells, which provide the primary output of the olfactory bulbs. Comparative analyses show that the turkey vulture has olfactory bulbs that are 4× larger and contain twice as many mitral cells as those of the sympatric black vulture (Coragyps atratus), despite having brains that are 20% smaller. The turkey vulture has the largest olfactory bulbs in absolute terms and adjusted for brain size among birds, but the number of mitral cells is proportional to the size of their olfactory bulbs. The combination of large olfactory bulbs, high mitral cell counts and a greatly enlarged nasal cavity likely reflects a highly sensitive olfactory system. We suggest that this sensitive sense of smell allowed the turkey vulture to colonize biomes that are suboptimal for scavenging birds and become the most widespread vulture species in the world.

The Evolution of Olfactory Capabilities in Wild Birds: A Comparative Study

Modern birds possess an olfactory apparatus similar to that of other vertebrates, yet the major evolutionary forces that drove the evolution of diversity in olfactory capabilities in birds remain elusive. Several non-mutually exclusive evolutionary scenarios for the evolution of olfactory capability in birds have been proposed. Olfactory capability may have evolved due to its role in recognition in social contexts. In addition, olfactory capability may have evolved because it provides a selective advantage in foraging or navigation. Finally, olfactory capability could be favored whenever the ecological conditions of species hindered the use of other senses and/or favored olfaction. Here we evaluate predictions from these hypotheses in a comparative study using a species-level phylogeny of 142 bird species and considering the indirect effects of some predictors on others. We find an interactive effect of aquatic dependence and diet on the size of the olfactory bulb: vegetarian and omnivore birds living in aquatic environments have larger olfactory bulbs than terrestrial birds, whereas species with an animal diet, aquatic and terrestrial species have similar-sized olfactory bulbs. In addition, the size of the olfactory bulb was weakly related with social complexity, with colonial species having relatively larger olfactory bulb than solitary breeders or those forming small aggregations. Our results suggest that the role of foraging in driving enhancements in the avian olfactory apparatus is contingent on ecological conditions that may affect the transmission of odor-based signals. This provides evidence for the largely neglected possibility that the evolution of the olfactory apparatus of birds has been driven by the interaction between ecological and behavioral traits, rather than being solely due to main effects of these traits.

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.

Gross and Microscopic Anatomy of the Olfactory Bulbs in the Chinchilla (Chinchilla lanigera)

Chinchillas are primarily used as laboratory animals in auditory pathways research, it is necessary to acquire detailed neuroanatomical data on other encephalon segments, in order to assess whether this species might also contribute as olfactory pathways experimental models. Thus, our research focused on providing a gross and microscopic description of the chinchilla’s olfactory bulbs. Five chinchilla encephalon samples were harvested from commercially slaughtered young adults. The pairs of olfactory bulb were isolated by dissection, macroscopically examined while still attached, then detached, fixed in buffered formalin, serially cut and stained using a modified Luxol-Cresyl protocol. Gross and microscopic features were assessed and compared to literature descriptions of related species such as the rat and the rabbit. Chinchillas presented large olfactory bulbs with an average maximal diameter of 3 mm each, positioned in two corresponding deep grooves of the cribriform plate, orally to the anterior limit of the cerebral hemispheres. They decreased in girth ventro-aborally towards their base, which was located on the ventral side of the telencephalon. Histologically, they presented a laminar structure: an olfactory nerve layer, a glomerular layer, an external plexiform layer, a mitral cell layer, an internal plexiform layer and a granule cell layer. The proportionally well developed olfactory bulbs and the existence of the accessory olfactory bulbs demonstrates that C. lanigera is a macrosmatic species. The olfactory bulbs in the chinchilla present the gross anatomy and histological outline concurring with literature rodent descriptions, with certain particularities (accessory bulb topography, olfactory ventricle topography, glomerular size and shape).

Endocranial morphology of Microchoerus erinaceus (Euprimates, Tarsiiformes) and early evolution of the Euprimates brain.

Innovations in brain structure and increase in brain size relative to body mass are key features of Primates evolutionary history. Surprisingly, the endocranial morphology of early Euprimates is still rather poorly known, and our understanding of early euprimate brain evolution (Eocene epoch) relies on a handful of specimens. In this article, we describe the endocranial cast of the tarsiiform Microchoerus erinaceus from the late Early Eocene of Perrière (Quercy fissure filling, France) based on a virtual reconstruction extracted from CT scan data of the endocranial cavity of the complete, undeformed specimen UM-PRR1771. The endocast of M. erinaceus shows the derived features observed in other Euprimates (e.g. sylvian fissure and temporal lobe), with limited neocortical folding, and a telencephalic flexure comparable to that of extant primates. Comparison with the endocasts of other available late Eocene primates shows that they already exhibited a variety of brain morphologies, highlighting the complex history of the external features of the primate brain, as early as the Eocene. M. erinaceus was a fruit and gum eater considered as nocturnal based on its orbit size. However, its brain showed small olfactory bulbs--smaller than in the coeval diurnal taxa Adapis parisiensis--and a neocorticalization similar to folivorous taxa. These observations contrast with patterns observed in primates today where nocturnal taxa have larger olfactory bulbs than diurnal taxa, and call into question a direct correlation between frugivory and neocorticalization increase in primates.

Diversity in olfactory bulb size in birds reflects allometry, ecology, and phylogeny.

The relative size of olfactory bulbs (OBs) is correlated with olfactory capabilities across vertebrates and is widely used to assess the relative importance of olfaction to a species’ ecology. In birds, variations in the relative size of OBs are correlated with some behaviors; however, the factors that have led to the high level of diversity seen in OB sizes across birds are still not well understood. In this study, we use the relative size of OBs as a neuroanatomical proxy for olfactory capabilities in 135 species of birds, representing 21 orders. We examine the scaling of OBs with brain size across avian orders, determine likely ancestral states and test for correlations between OB sizes and habitat, ecology, and behavior. The size of avian OBs varied with the size of the brain and this allometric relationship was for the most part isometric, although species did deviate from this trend. Large OBs were characteristic of more basal species and in more recently derived species the OBs were small. Living and foraging in a semi-aquatic environment was the strongest variable driving the evolution of large OBs in birds; olfaction may provide cues for navigation and foraging in this otherwise featureless environment. Some of the diversity in OB sizes was also undoubtedly due to differences in migratory behavior, foraging strategies and social structure. In summary, relative OB size in birds reflect allometry, phylogeny and behavior in ways that parallel that of other vertebrate classes. This provides comparative evidence that supports recent experimental studies into avian olfaction and suggests that olfaction is an important sensory modality for all avian species.

Digital cranial endocast of Pseudoplohophorus absolutus (Xenarthra, Cingulata) and its systematic and evolutionary implications

ABSTRACTGlyptodonts are a group of extinct xenarthrans with several anatomical features that make them one of the most bizarre groups of mammals. By the late 19th century, some authors began to analyze the brain of Pleistocene glyptodonts using natural endocranial casts. These studies revealed the small size of the brain of the large Pleistocene forms. However, the evolution of the brain in glyptodonts and how it fits in a phylogenetic context has not been analyzed. In order to evaluate the evolution of the brain in this group, we described the first digital endocranial cast of the late Miocene glyptodont Pseudoplohophorus absolutus and compared it with digital endocranial casts of the Pleistocene glyptodonts Glyptodon, Doedicurus, and Panochthus and the extant armadillos Dasypus, Euphractus, Chaetophractus, and Zaedyus. The endocast morphology of P. absolutus is similar to that of Pleistocene glyptodonts: large olfactory bulbs, a small cerebrum with a single neocortical sulcus, and a large cerebellum. However, the relative brain size is larger than in the Pleistocene forms, with values of the encephalization quotient (EQ) close to that of extant armadillos. A comparison between xenarthrans orders shows that Cingulata (glyptodonts and armadillos) have lower EQ values than Pilosa (sloths and vermilinguas). This could possibly be related to certain restrictions and benefits imposed by the presence of the carapace in cingulates. Furthermore, because the carapace restricts the development of the cervical musculature that supports the skull, the small size of the brain in glyptodonts could be a trade-off (along with others) to reduce the weight of the skull.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP