PHYLOGENY THROUGH BRAIN TRAITS - MORE CHARACTERS FOR THE ANALYSIS OF MAMMALIAN EVOLUTION

We have assembled data on nine brain traits, in addition to the fiftee n we have previously described, which provide new evidence for assessi ng mammalian relationships. States of these characters are tabulated a s they occur in each of 152 mammalian species, providing data in numer ically ordered form, useful for multiple analyses of phylogenetic rela tionships in programs which take into account variations in several di fferent 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 (anthr opoid primates); (2) loss of the accessory olfactory bulbs (sirenians, cetaceans, most bats, catarrhine primates); (3) Rindenkerne, clumps o f cell bodies in layer 6 of cerebral cortex (sirenians); (4) posterior ly pointing digits in the SI body representation (bats, both mega- and micro-); (5) equivalent tectopetal connections to the anterior collic ulus 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 (diprot odont marsupials, carnivores, artiodactyls, perissodactyls, hyracoids, cetaceans and primates), (8) presence of a complete secondary (SII) s omatic sensory region of cerebral cortex (therians - all extant mammal s other than monotremes), and (9) presence of a distinct external cune ate nucleus among the nuclei of the dorsal columns (all mammalian grou ps except monotremes and sirenians). Two examples of phylogenetic tree s derived from these data are presented. These sample trees maintain t he segregation of the monotremes and the marsupials, and the basic dic hotomy of placentals seen in our earlier toes 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 t he reconstruction of placental relationships; extremes of derivation f or the Cetacea, the Chiroptera, and the Sirenia (in concordance with t rees based on other data); a ferungulate association of Carnivora, Per issodactyla, Artiodactyla, Hyracoidea and Sirenia; and an assemblage o f related Dermoptera, Primates, Scandentia, and Chiroptera which in th is model also includes Insectivora and Macroscelidea. Analyses based o n brain characters can reinforce conclusions based on other data. whil e at the same time introducing new ideas about relationships. Neural t raits provide a source of data independent of those commonly used in p hylogenetic analysis, and are extremely valuable for testing old hypot heses and for introducing new ones. They also provide interesting sugg estions about convergent or parallel evolution of characters: for exam ple, two characters, loss of accessory olfactory formations and loss o f lamination in the dorsal cochlear nuclei, together occur convergentl y in several lines (bats, monkeys, seals, dolphins and manatees); one common feature in the evolution of these diverse groups is departure f rom a terrestrial habitat. This is consistent with the functional loss of the accessory olfactory formation, which is activated by airborne olfactory pheromones; it establishes a basis for more informed specula tion about the still mysterious role of the dorsal cochlear nuclei in the auditory system.