On the Malagasy Rodent Genus Brachyuromys; and on the Mutual Relations of some Groups of the Murida (Hesperomyirue, Microtinae, Murinte, and“Spalacidæ”) with each other and with the Malagasy Nesomyina.
Authors: Major C. I. Forsyth 
Published: August 2010

The Malagasy Muridæ have, by common accord, been placed amongst the“Cricetinæ” (“Sigmodontinæ”); this is true in a general sense, at least with regard to the great majority of them.

In the foregoing pages I have tried to define more precisely their position, as well as their relationships with Muridæ from other regions. Although the present paper deals essentially only with one genus, and though it will be necessary for me to take the Malagasy Eodents (forming, as I consider, a special subfamily, the Nesomyince) up again as a whole,“when all the forms will have been fully worked out, I could not, for comparison's sake, avoid anticipating somewhat the future publication, by throwing side‐glances on some of the other genera.

The Malagasy Eodents have to be considered as the lowest of the Muridæ–lower even than the Miocene members of the family, so far as these are known,–because in cranial and dental characters they approach more than any other Muridæ such more primitive groups of Eodentia as the Dipodidæ and Winge's Anomaluridæ. These characters are: large infraorbital foramen–well‐developed jugale–absence of advanced hypselodontism– intermediate cusps of molars less reduced than in Heyperomyinæ and in Old World Cricetinæ, and external cusps (of upper molars) less reduced than in the Murinæ–greater agreement in pattern and size of m. 1 and m. 2 than in other Muridæ; very often all three molars agree with each other.

If I had to regret that in my excavations I came upon but scanty remains of extinct fossil Mammalia, I have had some compensation by these living fossils. Were any justification necessary for having divided my time between neontological and palæontological exploration, this result would amply justify my proceeding.

The light which the investigation of the Malagasy Rodentia has thrown on their relatives outside the Island has been somewhat unexpected to me. From what Peters had noted concerning the affinities of one of the genera 1 and from other considerations, I was prepared to meet with the nearest and perhaps the only close affinities amongst the American Hesperomyinæ. These affinities certainly exist, and I have endeavoured to put them in their true light. However, other affinities, apart from those just mentioned, are very remarkable.

The germs Brachytarsomys, which, as stated on aformer occasion2, stands somewhat apart from the other Malagasy Rodents, proves to be a forerunner of the Microtinæ. It is, however, certainly not a member of the genus Microtus, nor of an)r of the other genera included in the subfamily; it cannot even, in my opinion, be placed within this subfamily, for it lacks the specializations which characterize the latter. Apart from the molars being not only rooted, but even perfectly brachyodont, neither the last upper nor the first lower molar show any additional increase to the normal Muridine form: the skull, too, differs from the Microtine cranium in all the characters, which in these are the direct outcome of the increased vertical size of the molars and the adaptation to a subterranean life. But otherwise the teeth as well as the cranium (size and shape of the jugal, form of the rostrum, of the outer wall of the infraorbital foramen and of the foramen itself, general conformation of the upper region of the skull and its crests) are precisely such as we might expect them to have been in the forerunners of the Microtinæ.

Next as to the genus Nesomys. The large size and breadth of the foramina incisiva, and, what is still more to the point, the large size of the infraorbital foramen, and the strong development of the jugal–which characters this genus shares with most of the other Malagasy Rodents–show it to be a very low member of the Muriclæ, approaching the Dipodidæ. The two anterior molars, agreeing in size and general form with each other, tell the same tale. The intimate structure of the molars, as compared with the Hesperomyinæ and the Murinæ, might induce us to consider Nesomys as a connecting‐link between these two groups. But the relationship to them will be more rightly expressed by considering it to be ancestral to both; especially if we bear in mind that the characters of both the cranium and teeth are less specialized than in the two subfamilies mentioned.

The present paper deals chiefly with a third genus, Braehyuromys. Its affinities with some fossorial Rodents, viz. Tachyoryctes from Abyssinia, Rhizomys from the Oriental Region, Spalax and Siplineus from the Palaearctic, have been fully discussed, and as one of the results these four genera are classed amongst the lowest Muridæ. Retirement under the earth and adaptation to fossorial habits have done for these four genera what isolation has done for

Brachyuromys&c, i. e. the preservation of primitive types of Muridæ1. Theirs is a parallel to that of the African insectivorous family Chrysochloridæ, as compared with the more generalized memebrs of the Malagasy family Centetidæ.

Whilst the somewhat closer agreement with Brachyuromys than with the rest of Malagasy Bodents may in the case of Rhizomys, Spalax, and Siphneus be due to the circumstance that in each the molars are hypselodont (though in a much lesser degree in Brachyuromys), the case seems to be different with regard to Tachyoryctes. This latter is in the pattern of its molars almost identical with one of the species of Brachyuromys (B. ramirohitra).

The mutual relations of the Hesperomyinæ, Microtinæ, Murinæ, and Spalacidæ with each other and with the Malagasy Nesomyinm have thus to a certain extent been cleared up by a better acquaintance with the latter.