FORAMEN PSEUDOVALE AND QUASI-MAMMALS.

Abstract

In the true, or therian, mammals, the mandibular division of the trigeminal (fifth cranial) nerve usually leaves the skull via a hole perforating the alisphenoid bone, the foramen ovale (Figs. 1G, 2G). That this was the primitive condition in placental mammals has been suggested and strongly supported by Edinger and Kitts (1954). That paper presents evidence that the foramen ovale may vanish as such by secondary union with the foramen lacerum (variously called the foramen lacerum medium or f. 1. anterior and primitively located at the alisphenoid-periotic-basisphenoid-basioccipital junction). Such a union (Fig. 2F) is widespread among perissodactyls, artiodactyls, and rodents; it appears to be secondary, however, because the primitive members of these orders, particularly in the fossil record, show a proper foramen ovale (Fig. 2G), as do most (but not all, e.g. Phascogale and Marmosa; see Fig. 2H) marsupials. By Late Cretaceous time, some therian mammals had attained a fully independent foramen ovale perforating the alisphenoid in essentially the same relationships to the other major foramina of this bone as seen in Canis today. One specimen of the material collected at Bug Creek, Montana, (Fig. 3), by expeditions under the direction of Dr. Robert Sloan of the University of Minnesota (Sloan, 1964; Sloan and Van Valen, 1965) shows this clearly. That specimen, as well as other fossils in the same collection, indicate a predictably strong affinity in ear region anatomy between known Paleocene mammals and their Cretaceous predecessors (MacIntyre, MS.). The information in the present paper is presented separately, because the special tonic of the triLeminal foramen is DeriDheral to the evolution of the ear region per se, but it is of considerable importance to the general problem of mammalian origins, the much discussed homology of the mammalian alisphenoid, the perennial puzzle of the relationships of monotremes, and the still unsettled question: What is a mammal? The initial stimulus for this paper was the statement by Kermack and Mussett (1958, 1959) and Kermack (1963) that the foramen for the mandibular division of the trigeminal (fifth cranial) nerve in the morganucodontids of the Rhaetic (Latest Triassic) and in the monotremes (known only from Pleistocene and Recent) passes through a foramen in the periotic called the foramen pseudovale. Although Kermack (1963) has retreated from the claim that this single resemblance demonstrates a relationship between morganucodontids and monotremes, no one has yet observed that the very facts upon which that claim was originally based have been misstated or misinterpreted. A closer study of the relevant literature has brought out two important points. First of all, the foramen for the mandibular nerve does not pass through the periotic in monotremes. Secondly, no evidence has yet been published which demonstrates clearly that such a relationship between the mandibular nerve and the periotic bone really existed in morganucodonts (or, indeed, in any known reptile or mammal, living or fossil). The converse has likewise not been established, however, and if such a peculiar condition did exist, it would be interesting to see just how many of the near-mammalian groups of non-therians (ictidosaurs, monotremes, etc.) show it unequivocally.