Robert C. Switzer

Phylogeny through Brain Traits: More Characters for the Analysis of Mammalian Evolution

We have assembled data on nine brain traits, in addition to the fifteen we have previously described, which provide new evidence for assessing mammalian relationships. States of these characters are tabulated as they occur in each of 152 mammalian species, providing data in numerically ordered form, useful for multiple analyses of phylogenetic relationships in programs which take into account variations in several different characters simultaneously. Derived states of each of the nine traits are characteristic of certain restricted groups of mammals; (1) mirroring of the complete SI body representation in isocortex (anthropoid primates); (2) loss of the accessory olfactory bulbs (sirenians, cetaceans, most bats, catarrhine primates); (3) Rindenkerne, clumps of cell bodies in layer 6 of cerebral cortex (sirenians); (4) posteriorly-pointing digits in the SI body representation (bats, both mega- and micro-); (5) equivalent tectopetal connections to the anterior colliculus of one side from both retinas, rather than predominantly from the contralateral retina (primates and megabats); (6) loss of lamination in dorsal cochlear nuclei (anthropoid primates, bats, seals, sirenians, cetaceans); (7) separation of claustrum from cerebral cortex (diprotodont marsupials, carnivores, artiodactyls, perissodactyls, hyracoids, cetaceans and primates), (8) presence of a complete secondary (SII) somatic sensory region of cerebral cortex (therians-all extant mammals other than monotremes), and (9) presence of a distinct external cuneate nucleus among the nuclei of the dorsal columns (all mammalian groups except monotremes and sirenians). Two examples of phylogenetic trees derived from these data are presented. These sample trees maintain the segregation of the monotremes and the marsupials, and the basic dichotomy of placentals seen in our earlier trees based entirely on brain data. They also show: an orderly sequence of bifurcations (rather than the commonly seen multifurcation near the base of the radiation) in the reconstruction of placental relationships; extremes of derivation for the Cetacea, the Chiroptera, and the Sirenia (in concordance with trees based on other data); a ferungulate association of Carnivora, Perissodactyla, Artiodactyla, Hyracoidea and Sirenia; and an assemblage of related Dermoptera, Primates, Scandentia, and Chiroptera which in this model also includes Insectivora and Macroscelidea. Analyses based on brain characters can reinforce conclusions based on other data, while at the same time introducing new ideas about relationships. Neural traits provide a source of data independent of those commonly used in phylogenetic analysis, and are extremely valuable for testing old hypotheses and for introducing new ones.(ABSTRACT TRUNCATED AT 400 WORDS)

Absence of mitral cells in monolayer in monotremes. Variations in vertebrate olfactory bulbs.

An invariant feature of the olfactory bulb in placental and marsupial mammals is the arrangement of the perikarya of mitral cells in a monolayer. Contrasting with this is the arrangement found in the olfactory bulbs of the monotremes, platypus and echidna, where the large perikarya are not only absent from the position of a monolayer (usually forming the external boundary of the internal plexiform layer) but occupy a region which would characterize them as tufted cells. In other classes of amniote vertebrates, reptiles and birds, the placement of large perikarya in the olfactory bulb ranges from a compact layer to a broad band. Such an overview among several vertebrate classes suggests that a monolayer of mitral cells may be a specialized subset of the tufted-mitral cell population. The accessory olfactory formation among mammals also exhibits variation in the compactness of the large perikarya: a broadband in most but a compact layer in a few others such as the chinchilla and the capybara. Such specialized alignment of perikarya (and, consequently, of their dendritic and axonal elements) may enable more refined signal processing than does random alignment of these elements. Such speculations can be tested using appropriate phylogenetic sampling, and monotremes provide particularly advantageous test cases.

Brain Traits through Phylogeny: Evolution of Neural Characters

We have previously derived a hypothetical tree of the lines of mammalian descent, based upon a comprehensive numerical taxonomic cross-analysis of primitive and derived states of 15 brain traits in 38 representative species. In this communication we use this tree to describe the probable sequence of changes that have taken place in phylogenetic history. 2 characters proved to be multiply convergent, occurring in parallel in several disparate lines of descent. The remaining 9 characters each appeared in ancestors of one or another of the lineages and characterize related progeny.

Somatosensory Nuclei of the Manatee Brainstem and Thalamus

Florida manatees have an extensive, well‐developed system of vibrissae distributed over their entire bodies and especially concentrated on the face. Although behavioral and anatomical assessments support the manatees reliance on somatosensation, a systematic analysis of the manatee thalamus and brainstem areas dedicated to tactile input has never been completed. Using histochemical and histological techniques (including stains for myelin, Nissl, cytochrome oxidase, and acetylcholinesterase), we characterized the relative size, extent, and specializations of somatosensory regions of the brainstem and thalamus. The principal somatosensory regions of the brainstem (trigeminal, cuneate, gracile, and Bischoffs nucleus) and the thalamus (ventroposterior nucleus) were disproportionately large relative to nuclei dedicated to other sensory modalities, providing neuroanatomical evidence that supports the manatees reliance on somatosensation. In fact, areas of the thalamus related to somatosensation (the ventroposterior and posterior nuclei) and audition (the medial geniculate nucleus) appeared to displace the lateral geniculate nucleus dedicated to the subordinate visual modality. Furthermore, it is noteworthy that, although the manatee cortex contains Rindenkerne (barrel‐like cortical nuclei located in layer VI), no corresponding cell clusters were located in the brainstem (“barrelettes”) or thalamus (“barreloids”). Anat Rec, 290:1138–1165, 2007.