EXTINCT RODENTS Among extinct rodents, only a few of the more important types may be noticed . As to the origin of the
See also:order, we are still to a
See also:great extent in the dark; and even the relations of the Duplicidentata to the Simplicidentata are not yet fully understood . With regard to the latter point, it is, however, considered probable that both are branches of a
See also:common stock, which diverged from each other before
See also:ali the typical rodent characters were acquired . As to the ancestral stock of the order, it has been suggested that this is re-presented by certain
See also:Lower Eocene
See also:European and
See also:American mammals, at one
See also:time regarded as
See also:primitive Primates . In
See also:Europe these include Plesiadapis and Protoadapis, and in North
See also:America Mixodectes, Microsyops and Cynodontomys; the last three constituting the
See also:family Mixodectidae . Possibly the European forms, in which the dental
See also:formula has been given as i . , c . 8, pi, m . I, and there is a
See also:gap between the incisors and the cheek-teeth, are more nearly related to
See also:modern rodents than the American types, and may indeed belong to the same order . On the other
See also:hand, the American forms, which have one pair of large
See also:chisel-like incisors in the lower
See also:jaw, also possess a lower canine, and show no marked gap in front of the cheek-teeth, nor any indication of the characteristic rodent backwards
See also:movement of the lower jaw . On these grounds, while admitting that they are allied to the rodents, it has been pointed out that they can scarcely be included in the
See also:Rodentia, and the order Proglires has in consequence been proposed for their reception . Whatever may be the true
See also:affinity of these problematical mammals, undoubted rodents are known from the Lower Eocene of both Europe and North America .
In Europe these
See also:form the genus Ischyromys and the family Ischyromyidae, and have premolars 't, and all the cheek-teeth low-crowned, with
See also:simple cusps or ridges . Possibly they are akin to the Sciuridae . In America, Paramys, with transversely ridged Molars, is allied ; and the European Sciuromys should perhaps find a place in the same neighbourhood . A more advanced phase is represented in the European Lower Oligocene by the Pseudosciuridae, with the genera Pseudosciurus, Sciuroides, Trechomys, Theridomys, &c., in which
See also:part of the masseter passes through the broad infra-orbital canal, and the premolars are i ; the molars being low-crowned, many-rooted and either cusped or ridged . These rodents are thought to be allied to the Anomaluridae ; and it is partly on their evidence that the family Pedetidae is placed next the latter . Here it may be mentioned that Leithia, from the
See also:Pleistocene of Malta, originally regarded as a
See also:dormouse, seems near akin to Anomalurus . In the highly specialized mastoid region of the
See also:skull, the North American Oligocene Protoptychus approaches to Dipopodomys, while the contemporary Gymnoptychus and Entoptychus likewise appear referable to the Geomyidae . The Upper Oligocene Cricetodon in Europe and Eumys in America are the earliest known forerunners of the cricetine Muridae; while at the same time primitive beavers appear in the form of Steneofiber, to be succeeded in the European Pleistocene by the gigantic Trogontherium . The still larger North American Pleistocene Castoroides, known by one
See also:species of the
See also:size of a bear, and the allied West
See also:Indian Amblyrhiza, appear to be specialized beavers, although they have been referred to a family by themselves . Near akin is the North American
See also:Miocene family Mylagaulidae, typified by Mylagaulus, but including Mesogaulus and Protogaulus . Although showing some dental characters approximating to the porcupines, these rodents are regarded as allied to the Castoridae, although forming an isolated type . The prominent feature, writes Mr E .
S . Riggs, is the unusual development of the premolar to the exclusion of the posterior teeth . Associated with this is the strength and sharpness of the lower jaw, the prominence and anterior pcsition of the masseteric
See also:ridge, and the
See also:depth of the ramus from the alveolar
See also:line to the
See also:angle . These indicate unusual capacity for crushing or grinding; while the last premolar is a crushing implement, which has reached the highest degree of specialization known in Rodentia . It is suggested that these teeth may have been employed for cracking nuts or hard seeds, although also used for grinding . The remarkable North American Ceratogaulus, with a large bony nasal
See also:horn, belongs to the same family . To discuss the remaining Miocene and later fossil Simplicidentata would be doing little more than adding to the generic names referable to the various existing families . It may be mentioned, however, that the distribution of these later
See also:Tertiary types accords very closely with that of their existing relatives; the families of South American hystricoids being represented by a number of extinct genera in the formations of
See also:Argentina and .Brazil .
See also:Special mention may be made of Megamys, from the caves of Brazil, which, while apparently allied to the living
See also:viscacha, attained dimensions approximating to those of a hippopotamus . As regards the Duplicidentata, it appears that the families Ochotonidae and Leporidae had become differentiated as early as the Lower Miocene . Titanomys is the earliest form, from the
See also:Middle Miocene, succeeded by Lagopsis, and then by the modern Ochotona . In this line there is a tendency to lose the last upper molar, but in Prolagus, which ranges in the Pliocene from
See also:Sardinia and
See also:Corsica to Spain, and forms a side-branch, the corresponding lower toothhas likewise disappeared .
In contradistinction to Titanomys, in which the cheek-teeth are rooted, is the North American Upper Oligocene Palaeolagus, where they are rootless . Ingeneral dental characters, especially the retention of three pairs of molars, this genus approximates to the Leporidae, although in the
See also:absence of
See also:post-orbital processes and the
See also:pattern of the molars it departs less widely from the modern Ochotonidae than does Prolagus .
EXTORTION (Lat. extorsio, from extorquere, to twist...
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