CHAPTER V.
FOSSIL RODENTIA.
Families of Rodents represented by Fossil Remains—State of the “Record of the Rocks”—[THE SCIURIDÆ]—Sciurine Genera now Extinct—[No Fossil ANOMALURIDÆ and HAPLODONTIDÆ]—[ISCHYROMYIDÆ]—Pseudotomus hians—Gymnoptychus—[CASTORIDÆ]—[Mr. Allen’s CASTOROIDIDÆ]—[THE MYOXIDÆ]—[No Fossil LOPHIOMYIDÆ]—[THE MURIDÆ]—[THE SPALACIDÆ]—[THE GEOMYIDÆ]—[THE DIPODIDÆ]—[THE THERIDOMYIDÆ]—[THE OCTODONTIDÆ]—[THE HYSTRICIDÆ]—[THE CHINCHILLIDÆ]—[THE DASYPROCTIDÆ]—[THE CAVIIDÆ]—[THE LEPORIDÆ]—[THE LAGOMYIDÆ]—Mesotherium cristatum—Difficulties concerning it—Mr. Alston’s Suggestion—THE HEBETIDENTATA—Teeth—Skull—Skeleton—Conclusions regarding it—Table of Rodent Families—Concluding Remarks.
THE majority of the preceding families are more or less clearly represented by fossil remains, either in the younger strata of the earth’s crust, or in those cave-deposits of comparatively recent date which have furnished so many interesting relics of the Mammals of former days. It must be remarked, however, that while a considerable number of fossil Rodents have been named and described by palæontologists, the materials upon which many of them have been established are very imperfect; in a great number of cases isolated molar or even incisor teeth furnish the sole evidence of the existence of creatures which were manifestly Rodents, but of which the other characters are rather difficult to divine from such scanty material. Still, imperfect as may be “the record of the rocks” in this as in other instances, it is in some parts sufficiently complete to enable us to trace back the existence of many forms of gnawing Mammals through a long period of geological time.
Of the SCIURIDÆ a considerable number of fossil species have been recorded. Species of the genera Sciurus, Arctomys, and Spermophilus, some of them identical with those still existing, have left their remains in Post-Tertiary deposits and in bone-caves in various parts of Europe; while species belonging to the first two genera, and to the American genus Tamias, have been detected in similar situations in North America. A few forms referred to the same genera go down still lower in the series of geological formations. True Squirrels are recorded from Miocene and Upper Eocene deposits in France, and a single species from the Tertiaries (probably Miocene) of Colorado; Marmots from Pliocene and Miocene beds in the South of France, and from a Pliocene deposit in Nebraska; and a Spermophile from the Miocene of Weisenau in Germany.
Besides these examples of known types, several fossils have been obtained both in Europe and America, which are regarded as indicating genera distinct from any now living. Plesiarctomys Gervaisii is founded on a fragment of jaw with four molars, obtained from Upper Eocene beds near Apt, Vaucluse. In its characters it appears to be intermediate between Squirrels and Marmots. Pseudosciurus suevicus, from the Upper Eocene (Bohnerz) of Würtemberg, seems to differ from all other Sciuridæ in the form of the molar teeth of the lower jaw, which are somewhat elongated, and have four tubercles arranged in two pairs, each pair being connected by a ridge. From the Tertiary deposits of the western territories of the United States, Professors Cope, Marsh, and Leidy have described several Sciurine Rodents as belonging to genera now extinct: thus Paramys has five species; Sciuravus (perhaps identical with the preceding), three; Heliscomys, Mysops, Colonymys, Taxymys, and Tillomys, one or two species each.
Of the ANOMALURIDÆ and HAPLODONTIDÆ no fossil remains are known. On the other hand, a North American fossil Rodent, described by Dr. Leidy under the name of Ischyromys typus, is regarded by Mr. Alston as the type of a distinct family, the Ischyromyidæ, nearly allied to the Sciuridæ, but also showing an affinity to the Beaver in some of its characters. The specimen described and figured by Dr. Leidy was obtained by Dr. Hayden from Miocene deposits in the “Bad Lands” of Wyoming. It was originally referred to the Sciuridæ, with which it agrees in its dentition, but is distinguished by its large infra-orbital opening, the presence of a sagittal crest, and the absence of post-orbital processes. The parietal region of the skull is much narrowed, and in this, as in the large size of the infra-orbital opening, Ischyromys resembles the Musk Rat.
Two other forms must be referred to here. Under the name of Pseudotomus hians, Professor Cope has described the remains of an animal which he believed to have been about the size of an Agouti, and originally thought to belong to the order Edentata. Subsequently he referred it to the Sciuridæ; but both Mr. Alston and Mr. Allen think that it may belong to the family Ischyromyidæ. In some respects the skull resembles that of Arctomys, but it has the same contraction between the orbits as Ischyromys and Fiber. The incisor teeth are separated, and Professor Cope believes that the animal had only three molars on each side in each jaw. A still more doubtful member of the family is Professor Cope’s genus Gymnoptychus, which includes four species, all said to be from the “Tertiary of the Plains.” In this genus there are five molars above and four below on each side, as in Ischyromys and the Sciuridæ; but these teeth show two crescents on the inner side in the upper, on the outer side in the lower jaw, and each crescent gives origin to a cross-ridge running to the opposite margin of the tooth.
The CASTORIDÆ, including at present only a single species common to the northern parts of both hemispheres, are represented by several peculiar fossil forms. Remains of the Common Beaver (Castor fiber) are not uncommon in peat bogs and other late superficial deposits both in Europe and America; and, according to Sir R. Owen, in association with those of the Rhinoceros, Mammoth, and Mastodon, even in the Fluvio-marine Crag (Newer Pliocene) of Norfolk. In Belgium its bones have been found in caves. Among the Mammals from the Upper Tertiaries of the Sivalik Hills, Messrs. Falconer and Cautley record a Beaver distinct from the existing species, although nearly allied to it. The skull of a great Beaver, one-fifth larger than that of the living species, was obtained many years ago by M. Fischer from sandy deposits on the shores of the Sea of Azov, and, as it differed in some peculiarities of the teeth from Castor fiber, was described by him as constituting a distinct genus under the name of Trogontherium Cuvieri. It is now regarded as a true Beaver, and named Castor Trogontherium. The British species, described and figured by Sir R. Owen from the Norfolk Forest bed under the name of Trogontherium Cuvieri, is, however, quite distinct, and belongs to the genus Diobroticus, characterised by having the third upper and first lower molar teeth with four enamel folds, and the rest only with two, most of the folds soon becoming isolated as the teeth wear down. This animal must have been nearly twice the size of the European Beaver.
At a still earlier period—namely, in the Miocene—the family Castoridæ was represented, both in Europe and America, by some small species, nearly agreeing with the Beavers in dentition, but differing in the characters of certain bones of the skull. These form the genus Steneofiber. The largest (S. viciacensis), from the Miocene of the Allier, was about half the size of the Beaver; another (S. sansaniensis), from the fresh-water limestone of Sansan, was about as large as a Rat; an American species (S. nebrascensis), from the “Bad Lands” of Dakota, was rather smaller than a Marmot, and presented some resemblance to the Agoutis in the characters of the teeth; and a fourth species (S. pansus) occurs in the Santa Fé marls. Eucastor tortus, a species rather smaller than a Marmot, is described by Dr. Leidy as very nearly related to the Beaver. Its remains were found in loose sands of the Niobrara River, Wyoming. Chalicomys and Palæomys are genera doubtfully placed here. Their species occur in the Miocene and Pliocene of Europe.
Some bones of a gigantic Rodent, indicating an animal as large as a full-grown Black Bear, obtained from Quaternary and Alluvial deposits of various parts of the United States, have been described under the name of Castoroides ohioensis, Mr. J. W. Foster, its first describer, having an idea that it was a great Beaver. It has generally been known as the “Fossil Beaver” of North America, but several authors have entertained doubts of the correctness of this designation, and Mr. Allen has lately made it the type of a special family, CASTOROIDIDÆ, which he regards as most nearly related to the Chinchillidæ. In the general aspect of the skull it resembles the Beaver, but in several details of structure approaches the Viscacha; while the structure of the molar teeth, which consist of a series of plates of dentine, completely enclosed by enamel, and held together by a thin coating of cement, occurs elsewhere only in the Chinchillidæ, and in the last molar of the Capybara.