Why is there discordant diversity in sengi (Mammalia: Afrotheria: Macroscelidea) taxonomy and ecology?

The seventeen species of sengis or elephant-shrews form a well-defined clade of mammals endemic to Africa that occupy the extremes of terrestrial habitats, from coastal deserts to montane forests. Because of their isolation on Africa soon after the break-up of Gondwanaland, theoretically sengis initially evolved with little competition from other placental radiations. Their life history features include myrmecophagy, saltatorial gaits, no or limited use of nests, social monogamy, small litters of precocial young and absentee maternal care of neonates. These traits together are unique to the Macroscelidea and represent a wedding of features usually associated with either small antelopes or anteaters. Combined, these features define an adaptive syndrome that presumably has been relatively immune to competition from contemporary mammals, partially due to phylogenetic inertia. Yet paradoxically, the syndrome is well suited to a wide range of terrestrial habitats, resulting in low taxonomic diversity. Because of their unusual phylogeny and low species diversity, conservation interest is high for those sengis with relatively low densities in fragmented forests.     

Ultrastructure of Spermatozoa of the Yellow-rumped Elephant Shrew Rhynchocyon chrysopygus (Mammalia: Macroscelidea) and the Phylogeny of Elephant Shrews

The spermatozoa of the yellow-rumped elephant shrew Rhynchocyon chrysopygus have a spatulate head, tapering to a rounded proximal end with a marked narrowing at the equatorial segment. There is a short pointed subacrosome and about 24 gyres of mitochondria were found in the midpiece. The coarse fibres surrounding the axonome show enlargement of numbers 1, 5, 6 and 9. The spermatozoa of all genera of elephant shrews have now been described allowing comparisons among them. R. chrysopygus has the shortest spermatozoon and the fewest number of gyres. The pointed subacrosome is considered an ancestral feature and is shared with Macroscelides. The enlargement of coarse fibres 1, 5, 6 and 9 is found in several groups including members of the archonta and paenungulates, both postulated as being related to elephant shrews.     

Phylogeny and taxonomy of the round-eared sengis or elephant-shrews, genus Macroscelides (Mammalia, Afrotheria, Macroscelidea)

The round-eared sengis or elephant-shrews (genus Macroscelides) exhibit striking pelage variation throughout their ranges. Over ten taxonomic names have been proposed to describe this variation, but currently only two taxa are recognized (M. proboscideus proboscideus and M. p. flavicaudatus). Here, we review the taxonomic history of Macroscelides, and we use data on the geographic distribution, morphology, and mitochondrial DNA sequence to evaluate the current taxonomy. Our data support only two taxa that correspond to the currently recognized subspecies M. p. proboscideus and M. p. flavicaudatus. Mitochondrial haplotypes of these two taxa are reciprocally monophyletic with over 13% uncorrected sequence divergence between them. PCA analysis of 14 morphological characters (mostly cranial) grouped the two taxa into non-overlapping clusters, and body mass alone is a relatively reliable distinguishing character throughout much of Macroscelides range. Although fieldworkers were unable to find sympatric populations, the two taxa were found within 50 km of each other, and genetic analysis showed no evidence of gene flow. Based upon corroborating genetic data, morphological data, near sympatry with no evidence of gene flow, and differences in habitat use, we elevate these two forms to full species.     

New remains of Chambius kasserinensis from the Eocene of Tunisia and evaluation of proposed affinities for Macroscelidea (Mammalia,Afrotheria)

Abstract

Chambius kasserinensis from the late Early or early Middle Eocene Chambi locality, central Tunisia, is undoubtedly the oldest known macroscelidid and possibly the basalmost representative of the order Macroscelidea. Hence, since its discovery in 1986, Chambius has played a key role in analyses focusing on afrotherian and eutherian phylogeny; for instance, as early as 1995, Butler’s review of fossil macroscelideans highlighted the central position of Chambius in the origin of the order. Despite this, Chambius remained poorly known until recently. Here based on new mandibular fragments, well-preserved upper molars and CT scan analysis of the holotype maxilla, Chambius is revised. Its dentition is first described in detail, providing a precise characterization of the genus. Chambius is notably defined by a submolariform P₄ with a three-cusped talonid, a reduced talonid on M₂, and a prominent metaconule on M^1?2. Interestingly, the two transverse lophs of the upper molars are basically formed by preconulecristae, evoking the recently defined peculiar bilophodonty of paenungulates. Comparisons with other Paleogene and modern macroscelidids, European Louisinidae, and North American Apheliscidae are also made, allowing the various hypotheses about the origin and early evolution of macroscelidids to be reviewed.     

Morphological cranial diversity contributes to phylogeny in soft-furred sengis (Afrotheria, Macroscelidea)

Despite the well-supported Macroscelidea phylogeny proposed at the end of the 1960s, several systematic arrangements have been suggested in the last 20 years, raising doubts about the phylogeny of the Macroscelidinae; sengi inter-specific relationships are still debated to this day. The main issue of concern involves the supposed Elephantulus diphyly. To solve this persisting debate about sengi phylogeny, we examined the cranium ventral surface of 13 species using geometric morphometric techniques and neighbour-joining algorithms. This study supported the idea that the ventral side of the sengi cranium has the potential to provide important signals for reconstructing the Macroscelidea phylogeny. The phylogenetic signals seemed to differentiate between two major clades in the sengi radiation. In the first clade, the two monospecific genera (Petrodromus and Macroscelides), the two African Horn species (Elephantulus revoilii and E. rufescens), and the only North African species (E. rozeti) were clustered together. The second clade includes the remnant south-central African Elephantulus species. Our results were in agreement with both mitochondrial and nuclear data, confirmed that there is no Elephantulus monophyly and highlighted the close relationship between Petrodromus and E. rozeti. It appears that all the soft-furred sengi species are organised in two evolutionary lines: an old monophyletic clade, comprising only Elephantulus species, and a new polyphyletic clade, including P. tetradactylus, M. proboscideus, and E. rozeti. This requires a taxonomic and nomenclatural rearrangement within Macroscelidinae, where the phylogenetic position of the remnant 4 (of 12) Elephantulus species has yet to be fully defined.     

Evolutionary History of the Grey-Faced Sengi,Rhynchocyon udzungwensis,from Tanzania: A Molecular and Species Distribution Modelling Approach

Rhynchocyon udzungwensis is a recently described and poorly understood sengi (giant elephant-shrew) endemic to two small montane forests in Southern Tanzania, and surrounded in lower forests by R. cirnei reichardi. In this study, we investigate the molecular genetic relationship between R. udzungwensis and R. c. reichardi, and the possible role that shifting species distributions in response to climate fluctuations may have played in shaping their evolutionary history. Rhynchocyon udzungwensis and R. c. reichardi individuals were sampled from five localities for genetic analyses. Three mitochondrial and two nuclear loci were used to construct species trees for delimitation and to determine whether introgression was detectable either from ancient or ongoing hybridization. All species-tree results show R. udzungwensis and R. c. reichardi as distinct lineages, though mtDNA shows evidence of introgression in some populations. Nuclear loci of each species were monophyletic, implying introgression is exclusively historical. Because we found evidence of introgression, we used distribution data and species distribution modelling for present, glacial, and interglacial climate cycles to predict how shifting species distributions may have facilitated hybridization in some populations. Though interpretations are affected by the limited range of these species, a likely scenario is that the mtDNA introgression found in eastern mid-elevation populations was facilitated by low numbers of R. udzungwensis that expanded into lowland heavily occupied R. c. reichardi areas during interglacial climate cycles. These results imply that relationships within the genus Rhynchocyon may be confounded by porous species boundaries and introgression, even if species are not currently sympatric.     

A phylogenetic estimate for golden moles (Mammalia,Afrotheria, Chrysochloridae)

Background  

Golden moles (Chrysochloridae) are small, subterranean, afrotherian mammals from South Africa and neighboring regions. Of the 21 species now recognized, some (e.g., Chrysochloris asiatica, Amblysomus hottentotus) are relatively common, whereas others (e.g., species of Chrysospalax, Cryptochloris, Neamblysomus) are rare and endangered. Here, we use a combined analysis of partial sequences of the nuclear GHR gene and morphological characters to derive a phylogeny of species in the family Chrysochloridae.     

Phylogeny and conservation priorities of afrotherian mammals (Afrotheria,Mammalia)

Kuntner, M., May‐Collado, L. J. & Agnarsson, I. (2010). Phylogeny and conservation priorities of afrotherian mammals (Afrotheria, Mammalia). —Zoologica Scripta, 40, 1–15. Phylogenies play an increasingly important role in conservation biology providing a species‐specific measure of biodiversity – evolutionary distinctiveness (ED) or phylogenetic diversity (PD) – that can help prioritize conservation effort. Currently, there are many available methods to integrate phylogeny and extinction risk, with an ongoing debate on which may be best. However, the main constraint on employing any of these methods to establish conservation priorities is the lack of detailed species‐level phylogenies. Afrotheria is a recently recognized clade grouping anatomically and biologically diverse placental mammals: elephants and mammoths, dugong and manatees, hyraxes, tenrecs, golden moles, elephant shrews and aardvark. To date, phylogenetic studies have focused on understanding higher level relationships among the major groups within Afrotheria. Here, we provide a species‐level phylogeny of Afrotheria based on nine molecular loci, placing nearly 70% of the extant afrotherian species (50) and five extinct species. We then use this phylogeny to assess conservation priorities focusing on the widely used evolutionary distinctiveness and global endangeredness (EDGE) method and how that compares to the more recently developed PD framework. Our results support the monophyly of Afrotheria and its sister relationship to Xenarthra. Within Afrotheria, the basal division into Afroinsectiphilia (aardvark, tenrecs, golden moles and elephant shrews) and Paenungulata (hyraxes, dugongs, manatees and elephants) is supported, as is the monophyly of all afrotherian families: Elephantidae, Procaviidae, Macroscelididae, Chrysochloridae, Tenrecidae, Trichechidae and Dugongidae. Within Afroinsectiphilia, we recover the most commonly proposed topology (Tubulidentata sister to Afroscoricida plus Macroscelidea). Within Paenungulata, Sirenia is sister to Hyracoidea plus Proboscidea, a controversial relationship supported by morphology. Within Proboscidea, the mastodon is sister to the remaining elephants and the woolly mammoth sister to the Asian elephant, while both living elephant genera, Loxodonta and Elephas are paraphyletic. Top ranking evolutionarily unique species always included the aardvark, followed by several species of elephant shrews and tenrecs. For conservation priorities top ranking species always included the semi‐aquatic Nimba otter shrew, some poorly known species, such as the Northern shrew tenrec, web‐footed tenrec, giant otter shrew and Giant golden mole, as well as high profile conservation icons like Asian elephant, dugong and the three species of manatee. Conservation priority analyses were broadly congruent between the EDGE and PD methodologies. However, for certain species EDGE overestimates conservation urgency as it, unlike PD, fails to account for the status of closely related, but less threatened, species. Therefore, PD offers a better guide to conservation decisions.     

Bone Inner Structure Suggests Increasing Aquatic Adaptations in Desmostylia (Mammalia,Afrotheria)

Background

The paleoecology of desmostylians has been discussed controversially with a general consensus that desmostylians were aquatic or semi-aquatic to some extent. Bone microanatomy can be used as a powerful tool to infer habitat preference of extinct animals. However, bone microanatomical studies of desmostylians are extremely scarce.

Methodology/Principal Findings

We analyzed the histology and microanatomy of several desmostylians using thin-sections and CT scans of ribs, humeri, femora and vertebrae. Comparisons with extant mammals allowed us to better understand the mode of life and evolutionary history of these taxa. Desmostylian ribs and long bones generally lack a medullary cavity. This trait has been interpreted as an aquatic adaptation among amniotes. Behemotops and Paleoparadoxia show osteosclerosis (i.e. increase in bone compactness), and Ashoroa pachyosteosclerosis (i.e. combined increase in bone volume and compactness). Conversely, Desmostylus differs from these desmostylians in displaying an osteoporotic-like pattern.

Conclusions/Significance

In living taxa, bone mass increase provides hydrostatic buoyancy and body trim control suitable for poorly efficient swimmers, while wholly spongy bones are associated with hydrodynamic buoyancy control in active swimmers. Our study suggests that all desmostylians had achieved an essentially, if not exclusively, aquatic lifestyle. Behemotops, Paleoparadoxia and Ashoroa are interpreted as shallow water swimmers, either hovering slowly at a preferred depth, or walking on the bottom, and Desmostylus as a more active swimmer with a peculiar habitat and feeding strategy within Desmostylia. Therefore, desmostylians are, with cetaceans, the second mammal group showing a shift from bone mass increase to a spongy inner organization of bones in their evolutionary history.     

Coalescence methods reveal the impact of vicariance on the spatial genetic structure of Elephantulus edwardii (Afrotheria, Macroscelidea)

Within the Macroscelidea 15 species of elephant-shrews are recognized, of which nine occur in the southern African subregion. The Cape rock elephant-shrew (Elephantulus edwardii) is the only strictly endemic South African elephant-shrew species. Recent distribution data suggest that E. edwardii is continuously distributed from Namaqualand in the Western Cape Province to Port Elizabeth in the Eastern Cape Province. Molecular sequences from the mitochondrial cytochrome b gene and variable control region indicate significant substructure within the Cape rock elephant-shrew across its distribution. Our data unequivocally showed the presence of a northern Namaqua and central Fynbos clade with four evolutionary lineages identified within the latter. The geographical delimitation of the northern and central clades corresponds closely with patterns reported for other rock-dwelling vertebrate species, indicating a shared biogeographical history for these taxa in South Africa. A coalescent method revealed the effects of ancestral polymorphism in shaping the Namaqua and Fynbos populations since their divergence ~1.7 million years ago. Furthermore, our analyses uncovered a distinct Karoo lineage(s) that does not correspond to any of the previously described and/or currently recognized species, and we therefore argue for the possible recognition of a new sister taxon to E. edwardii. The taxonomic affinities of this clade were examined by sequencing corresponding regions from the type specimens of species described in the past, but which presently are synonimized within E. edwardii. Our results reveal the morphological misidentification of one of these types, accentuating the problems of field identification.     

Homoplasy in the ear region of Tethytheria and the systematic position of Embrithopoda (Mammalia,Afrotheria)

The ear region of mammals has long been considered as morphologically very conservative and accordingly, phylogenetically useful. In this study, the anatomy of the petrosal and bony labyrinth (osseous inner ear) of Numidotherium (Proboscidea) and Arsinoitherium (Embrithopoda) are investigated and compared in order to assess the evolution of ear region characters in proboscideans and embrithopods. Using a cladistic analysis across Paenungulata based on ear region characters only, we found that Arsinoitherium is surprisingly best placed as a crown proboscideans to the exclusion of Numidotherium and Phosphatherium, which results in the paraphyly of proboscidean. The clade Proboscidea is actually well supported by dental and post-cranial characters, and we propose that this result underlines the great amount of morphological convergences in the ear region of Embrithopoda and Proboscidea, possibly due to convergent evolution of capabilities toward infrasonic hearing.     

Comparative Endocranial Anatomy,Encephalization, and Phylogeny of Notoungulata (Placentalia,Mammalia)

Journal of Mammalian Evolution - Cranial endocasts are one of the most direct tools available to obtain information about the endocranial cavity of fossil mammals, but few anatomical comparisons...     

Body Weight and Relative Brain Size (Encephalization) in Eocene Archaeoceti (Cetacea)

Calibration of the Brownian diffusion model of Felsenstein indicates that phylogeny may have an influence on body length and other phenotypic measures in Cetacea for as many as 10,000 generations or about 180,000 years, which is negligible in the 35 million year history of extant Cetacea. Observations of phenotypic traits in cetacean species living today are independent of phylogeny and independent statistically. Four methods for estimating body weight in fossil cetaceans are compared: (1) median serial regression involving a set of multiple regressions of log body weight on log centrum length, width, and height for core vertebrae; (2) regression of log whole body weight on log body length for individuals; (3) regression of log whole body weight on log body length for species means; and (4) regression of log lean body weight on log body length for individuals. These yield body weight estimates for the Eocene archaeocete Dorudon atrox of 1126, 1118, 1132, and 847 kg, respectively, with consistency and applicability to partial skeletons favoring the first approach. The whole-body weight expected, P_e (in kg), for a given body length, L_i (in cm), is given by log₁₀ P_e?=?2.784 ? log₁₀ L_i???4.429. Negative allometry of body weight and body length (slope 2.784?<?3.000) means that small cetaceans are shorter and more maneuverable than expected for their weight, while large cetaceans are longer and more efficient energetically than expected for their weight. Encephalization is necessarily quantified relative to a reference sample, most mammals are terrestrial, and terrestrial mammals provide a logical baseline. The encephalization residual for living terrestrial mammals as a class (ER_TC), is the difference between observed log₂ brain weight (E_i in g) and expected log₂ brain weight (E_e in g), where the latter is estimated from body weight (P_i in g), as log₂ E_e?=?0.740 ? log₂ P_i???4.004. ER_TC is positive for brains that are larger than expected for a given body size, and negative for brains that are smaller than expected. Base-2 logarithms make the ER_TC scale intuitive, in uniform units of halving or doubling. Encephalization quotients (EQ) are unsuitable for comparison because they are proportions on a non-uniform scale. Middle Eocene archaeocetes have ER_TC values close to ?2 (two halvings compared to expectation), while late Eocene archaeocetes have ER_TC values close to ?1 (one halving compared to expectation). ER_TC is not known for fossil mysticetes, but living mysticetes have ER_TC values averaging about ?2. Oligocene-Recent odontocetes appear to have ER_TC values averaging about +1 (one doubling compared to expectation) through their temporal range. Definitive interpretation of the evolution of encephalization in Cetacea will require better documentation for Oligocene–Recent mysticetes and odontocetes.     

Chromosome painting among Proboscidea, Hyracoidea and Sirenia: support for Paenungulata (Afrotheria, Mammalia) but not Tethytheria

Despite marked improvements in the interpretation of systematic relationships within Eutheria, particular nodes, including Paenungulata (Hyracoidea, Sirenia and Proboscidea), remain ambiguous. The combination of a rapid radiation, a deep divergence and an extensive morphological diversification has resulted in a limited phylogenetic signal confounding resolution within this clade both at the morphological and nucleotide levels. Cross-species chromosome painting was used to delineate regions of homology between Loxodonta africana (2n=56), Procavia capensis (2n=54), Trichechus manatus latirostris (2n=48) and an outgroup taxon, the aardvark (Orycteropus afer, 2n=20). Changes specific to each lineage were identified and although the presence of a minimum of 11 synapomorphies confirmed the monophyly of Paenungulata, no change characterizing intrapaenungulate relationships was evident. The reconstruction of an ancestral paenungulate karyotype and the estimation of rates of chromosomal evolution indicate a reduced rate of genomic repatterning following the paenungulate radiation. In comparison to data available for other mammalian taxa, the paenungulate rate of chromosomal evolution is slow to moderate. As a consequence, the absence of a chromosomal character uniting two paenungulates (at the level of resolution characterized in this study) may be due to a reduced rate of chromosomal change relative to the length of time separating successive divergence events.     

Petrosal and Bony Labyrinth Morphology Supports Paraphyly of Elephantulus Within Macroscelididae (Mammalia,Afrotheria)

Interest in the phylogeny of Macroscelididae (sengis or elephant shrews) has been prompted by molecular studies indicating that Elephantulus rozeti is best placed as the sister group of Petrodromus tetradactylus (this clade being in turn the sister taxon to Macroscelides proboscideus) than among other species of the genus Elephantulus. Until now, no discrete morphological characters have been proposed to support the grouping of E. rozeti, Petrodromus, and Macroscelides into this single so-called ‘Panelephantulus’ clade. Here, we employed μCT scanning in order to investigate the petrosal and bony labyrinth (bony capsule of the inner ear) morphology of most species of extant Macroscelididae. We performed a cladistic analysis on ear traits and found that despite some convergences (e.g., concerning the bony arterial canals in Macroscelides and Rhynchocyon) the middle and inner ear morphology furnishes significant support for the ‘Panelephantulus’ clade. In our analysis, this clade is unambigously supported by the presence of a fully ossified stapediofacial tube. Two additional characters (the presence of a bony septum at the mouth of the fenestra cochleae dividing the D3 sinus into two distinct cavities and the absence of an accessory lateral pneumatic fossa) could also support ‘Panelephantulus.’ These newly discovered morphological characters support the molecular phylogenies published and highlight the importance of coding hitherto difficult to sample morphologies within cladistic analyses using micro-CT techniques. Taxonomic implications are briefly discussed.     

Histamine Turnover in the Brain of Different Mammalian Species: Implications for Neuronal Histamine Half-Life

The turnover of neuronal histamine (HA) in nine brain regions and the spinal cord of the guinea pig and the mouse was estimated and the values obtained were compared with data previously obtained in rats. The size of the neuronal HA pool was determined from the decrease in HA content, as induced by (S)-alpha-fluoro-methylhistidine (alpha-FMH), a suicide inhibitor of histidine decarboxylase. The ratios of neuronal HA to the total differed with the brain region. Pargyline hydrochloride increased the tele-methylhistamine (t-MH) levels linearly up to 2 h after administration in both the guinea pig and the mouse whole brain. Regional differences in the turnover rate of neuronal HA, calculated from the pargyline-induced accumulation of t-MH, as well as in the size of the neuronal HA pool, were more marked in the mouse than in the guinea pig brain. The hypothalamus showed the highest rate in both species. There was a good correlation between the steady-state t-MH levels and the turnover rate in different brain regions. Neither the elevation of the t-MH levels by pargyline nor the reduction of HA by alpha-FMH was observed in the spinal cord, thereby suggesting that the HA present in this region is of mast cell origin. The half-life of neuronal HA in different brain regions was in the range of 13-38 min for the mouse and 24-37 min for the guinea pig, except for HA from the guinea pig hypothalamus, which had an extraordinarily long value of 87 min. These results suggest that there are species differences in the function of the brain histaminergic system.     

Variance in Brain Volume with Advancing Age: Implications for Defining the Limits of Normality

Background

Statistical models of normal ageing brain tissue volumes may support earlier diagnosis of increasingly common, yet still fatal, neurodegenerative diseases. For example, the statistically defined distribution of normal ageing brain tissue volumes may be used as a reference to assess patient volumes. To date, such models were often derived from mean values which were assumed to represent the distributions and boundaries, i.e. percentile ranks, of brain tissue volume. Since it was previously unknown, the objective of the present study was to determine if this assumption was robust, i.e. whether regression models derived from mean values accurately represented the distributions and boundaries of brain tissue volume at older ages.

Materials and Methods

We acquired T1-w magnetic resonance (MR) brain images of 227 normal and 219 Alzheimer’s disease (AD) subjects (aged 55-89 years) from publicly available databanks. Using nonlinear regression within both samples, we compared mean and percentile rank estimates of whole brain tissue volume by age.

Results

In both the normal and AD sample, mean regression estimates of brain tissue volume often did not accurately represent percentile rank estimates (errors=-74% to 75%). In the normal sample, mean estimates generally underestimated differences in brain volume at percentile ranks below the mean. Conversely, in the AD sample, mean estimates generally underestimated differences in brain volume at percentile ranks above the mean. Differences between ages at the 5^th percentile rank of normal subjects were ~39% greater than mean differences in the AD subjects.

Conclusions

While more data are required to make true population inferences, our results indicate that mean regression estimates may not accurately represent the distributions of ageing brain tissue volumes. This suggests that percentile rank estimates will be required to robustly define the limits of brain tissue volume in normal ageing and neurodegenerative disease.     

Species tree of a recent radiation: the subfamily Delphininae (Cetacea, Mammalia)

Lineages undergoing rapid radiations provide exceptional opportunities for studying speciation and adaptation, but also represent a challenge for molecular systematics because retention of ancestral polymorphisms and the occurrence of hybridization can obscure relationships among lineages. Dolphins in the subfamily Delphininae are one such case. Non-monophyly, rapid speciation events, and discordance between morphological and molecular characters have made the inference of phylogenetic relationships within this subfamily very difficult. Here we approach this problem by applying multiple methods intended to estimate species trees using a multi-gene dataset for the Delphininae (Sousa, Sotalia, Stenella, Tursiops, Delphinus and Lagenodelphis). Incongruent gene trees obtained indicate that incomplete lineage sorting and possibly hybridization are confounding the inference of species history in this group. Nonetheless, using coalescent-based methods, we have been able to extract an underlying species-tree signal from divergent histories of independent genes. This is the first time a molecular study provides support for such relationships. This study further illustrates how methods of species-tree inference can be very sensitive both to the characteristics of the dataset and the evolutionary processes affecting the evolution of the group under study.     

Endocranial Volume of Mid-Late Eocene Archaeocetes (Order: Cetacea) Revealed by Computed Tomography: Implications for Cetacean Brain Evolution

The large brain of modern cetaceans has engendered much hypothesizing about both the intelligence of cetaceans (dolphins, whales, and porpoises) and the factors related to the evolution of such large brains. Despite much interest in cetacean brain evolution, until recently there have been few estimates of brain mass and/or brain–body weight ratios in fossil cetaceans. In the present study, computed tomography (CT) was used to visualize and estimate endocranial volume, as well as to calculate level of encephalization, for two fully aquatic mid-late Eocene archaeocete species, Dorudon atrox and Zygorhiza kochii. The specific objective was to address more accurately and more conclusively the question of whether relative brain size in fully aquatic archaeocetes was greater than that of their hypothesized sister taxon Mesonychia. The findings suggest that there was no increase in encephalization between Mesonychia and these archaeocete species.