Organization of the Cerebral Cortex in Monotremes and Marsupials

The view that monotreme, marsupial, and placental orders of mammals formed an orderly progression in mammalian evolution arose in the 19th Century. Monotremes were designated the Prototheria, or first mammals, based largely on their reptilian-like reproductive practice of laying eggs, while the marsupials were designated the Metatheria, or changed mammals, and were thought to form the next stage of mammalian evolution toward placental mammals—the Eutheria or complete mammals. However, these notions of a simple hierarchy in mammalian evolution are misguided in that each of the three orders has under-gone its independent evolutionary development with perhaps major transformations having taken place within each order from the earliest forebears of that order (for reviews, see Tyndale-Biscoe, 1973; Clemens, 1977, 1979a,b; Griffiths, 1978; Archer, 1982; Dawson, 1983; Tyndale-Biscoe and Renfree, 1987). Furthermore, the reptilian-like precursors of these three mammalian orders may bear little resemblance to surviving reptiles as both the reptilian and the mammalian lines have evolved over independent courses for approximately 300 million years since the Carboniferous period (Dawson, 1983). The evolutionary line that was to lead to mammals probably underwent a reptilian-to-mammalian transition about 200 million years ago in the late Triassic Period (Fig. 1). The monotreme ancestors formed a separate line of mammalian evolution at about that time or, even earlier, emerged through the reptile-to-mammal barrier on a separate path from the therian mammals. Thus, mammalian evolutionary progress may have come from two broad stem lines, the therian mammals, from which placental and marsupial species have arisen, and the nontherian mammals that are now represented by present-day monotremes (Fig. 1). A more detailed representation of the marsupial radiation is given in Fig. 2 from Kirsch (1977).