Glires Rodents Rabbits And Relatives

Rodents and rabbits share numerous derived characters of the skull and dentition, such as the large open-rooted incisor teeth. This and other shared characters have suggested that rodents and rabbits are grouped together as the clade Glires (Novacek et al., 1988; Meng et al., 2003). Molecular evidence for this grouping was at first mixed, but it is now generally accepted (see Box 10.6), although one recent study failed to confirm the mono-phyly of Glires (Misawa and Janke, 2003).

The success of the rodents is legendary. They are a diverse and widespread order of mammals with just over 2000 living species (40% of all living mammals). Their adaptability seems to know no bounds, as can be seen from the way in which mice, rats and squirrels have modified their behaviour in order to coexist in a human landscape. Rodents are characterized by their extaordi-nary teeth and jaws, which formed the basis of their rapid evolutionary radiation.

10.13.1 Rodent teeth and jaws

Rodents have deep-rooted incisor teeth, one pair in the upper jaw and one in the lower, which grow continuously throughout life, an unusual feature among mammals. In cross-section a typical rodent skull (Figure 10.42(a)) seems to be largely occupied by the deep open roots of the incisors that curve back round the snout region and fill up most of the lower jaw. The incisors are used to gnaw wood, nuts, husks of fruit and so on. They are triangular in cross-section and bear enamel only on the front face, so that the dentine behind wears faster and gives a sharp enamel cutting edge. Behind the incisors is a long diastema, a gap representing the missing second and third incisors and a canine, followed by at most a single premolar and three molars. In many forms the molars are hypsodont (high-crowned) or hypselodont (ever-growing).

The main jaw actions of rodents are proal, that is, the lower jaw may be protruded for gnawing, and the cutting action is from back to front. Forwards movements are produced by the pterygoideus muscle, which runs from the palate to the inside ofthe jaw, and the masseter muscle, whose main portions originate generally in the snout area and run back to the outside of the lower jaw (Figure 10.42(b)). The strength and effectiveness of the propalinal movements depend on the size and angle of the masseter muscle in particular. Four patterns occur in rodents (Figure 10.42(c-f)):

1 protrogomorph, seen in primitive forms, in which the middle and deep layers of the masseter attach to the zygomatic arch;

2 hystricomorph, seen in porcupines, in which the deep masseter passes through the infraorbital foramen to attach to the side of the snout in front of the eye;

3 sciuromorph, seen in squirrels and others, in which the middle masseter attaches in front of the eye;

Hystricomorph Skull

medial temporalis masse,er medial temporalis masse,er

Hystricomorph Skull
(b)

infraorbital foramen

Fig. 10.42 Rodent teeth and jaw muscles: (a) cross-section of a beaver skull showing the deeply rooted cheek teeth and evergrowing incisors in black; (b) main jaw muscles of the living porcupine Erethizon, showing the temporalis muscle and the masseter muscle, which fall into three main portions; (c—f) the main lines of action of the segments of the masseter muscle in rodents with the (c) protrogomorph, (d) hystricomorph, (e) sciuromorph and (f) myomorph patterns; in the last three, the medial masseter invades further and further forwards on the side of the snout. (Based on several sources.)

4 myomorph,seen in rats and mice, in which the middle masseter is attached in front of the eye (as in sciuromorph) and the deep masseter passes up into the orbital area and through the infraorbital foramen.

The four muscle patterns appear to have arisen independently several times and (except for myomorphs) they do not characterize unique monophyletic groups.

10.13.2 Rodent evolution

Equipped with their ever-growing incisors and powerful low-angle masseters, the rodents have chewed their way through wood, tough plant fibres and nuts for the past 60 Myr. The first rodents, the ischyromids of the upper Palaeocene and Eocene of North America and Eurasia, such as Paramys (Figure 10.43(a)), show primitive characters in the protrogomorph jaw muscle pattern and in the teeth. The cheek teeth (Figure 10.43(b)) still have mound-like cusps instead of the ridges of some later rodents (Figure 10.43(c)) and the last molar is not fully part of the grinding dental battery.

The oddest rodents were the mylagaulids of the

Miocene of the Great Basin, USA. Epigaulus (Figure 10.43(d)) has broad paddle-like hands with long claws, used in digging, and small eyes, so it probably lived underground in burrows. It has a pair of small horns on the snout just in front of the eyes, whose function is a mystery, unless they were used in pre-mating fights; not all specimens have the horns, so they may have been restricted to males only. Alternatively, the horns might have been used for digging.

Paramys, and most other Eocene rodents, have a primitive jaw arrangement in which the area of attachment of the masseter muscle on the dentary is a vertical surface in the same plane as the incisor tooth. This is the sciurognathous jaw pattern (Figure 10.43(e)). A second pattern is seen in porcupines and the South American rodents in which the masseter insertion is deflected outwards,the hystricognathous (Figure 10.43(f)) condition, that seemingly arose once only.

The sciurognathous rodents are the largest group

Sivacanthion

Fig. 10.43 Early rodents:(a,b) the early Eocene ischyromid Paramys,skeleton and cheek teeth from the upper (top) and lower (bottom) jaws, seen in occlusal view; (c) upper cheek teeth of the modern mouse Theridomys in occlusal view; (d) the horned Miocene mylagaulid Epigaulus; (e) the sciurognathous lower jaw with vertical sides; (f) the hystricognathous jaw, with a deflected horizontal bony flange. [Figures (a,b) after Wood, 1962; (c,e,f) after Savage and Long, 1986; (d) after Gidley, 1907.]

Fig. 10.43 Early rodents:(a,b) the early Eocene ischyromid Paramys,skeleton and cheek teeth from the upper (top) and lower (bottom) jaws, seen in occlusal view; (c) upper cheek teeth of the modern mouse Theridomys in occlusal view; (d) the horned Miocene mylagaulid Epigaulus; (e) the sciurognathous lower jaw with vertical sides; (f) the hystricognathous jaw, with a deflected horizontal bony flange. [Figures (a,b) after Wood, 1962; (c,e,f) after Savage and Long, 1986; (d) after Gidley, 1907.]

and include today the Sciuromorpha (squirrels and beavers) and Myomorpha (dormice, hamsters, mice, rats and voles). The Sciuromorpha date back to the early Eocene, with the oldest squirrels and beavers being late Eocene in age.

While modern beavers are known for their dambuilding and tree-felling activities, some fossil forms excavated remarkable burrows. Large helical burrows named Daimonelix have been known for some time from the Oligocene and Miocene of Nebraska, USA. They extend to 2.5 m deep and have an upper entrance pit, a middle vertical spiral and a lower living chamber (Figure 10.44(a)).The burrow diameter is constant and the helix may be dextral or sinistral in the same locality. These burrows have been ascribed to Palaeocastor (Figure 10.44(b)), an early beaver, on the basis of complete and incomplete skeletons found in the living chamber (Martin and Bennett, 1977).

The myomorphs arose in the early Eocene, but they radiated dramatically from the Miocene onwards. The eomyids were important early myomorphs, known from the middle Eocene to the Pleistocene of Europe, Asia and North America. Fossils from the oil shales of the Oligocene locality Enspel in Germany include perfectly preserved examples of Eomys with skin and hair, and these show that it was a gliding form (Figure 10.44(c)) with a narrow skin membrane along the side of the body and between the hindlegs (Storch et al., 1996). This is not the only gliding rodent group: others include certain modern squirrels (Sciuridae), scaly-tailed flying squirrels (Anomaluridae) and dormice (Gliridae). But for the exceptional preservation, there would have been little clue that Eomys was a glider too.

Most of the later myomorph radiation consisted of mice and rats, the Family Muridae, which rose from its origins in the Eocene to over 1100 living species. Much

Sivacanthion

Fig. 10.44 Diverse rodents: (a) spiral burrows, termed Daimonelix,made by (b) the Miocene beaver Palaeocastor; (c) restoration of the Oligocene gliding myomorph Eomys; (d) restoration of the Miocene porcupine Sivacanthion; (e) relative size of the giant caviomorph Telicomys and a small rhinoceros. [Figure (a) altered from Martin and Bennett, 1977; (b) after Zittel, 1925; (c) modified from Storch et al., 1996; (d,e) after Savage and Long, 1986.]

Fig. 10.44 Diverse rodents: (a) spiral burrows, termed Daimonelix,made by (b) the Miocene beaver Palaeocastor; (c) restoration of the Oligocene gliding myomorph Eomys; (d) restoration of the Miocene porcupine Sivacanthion; (e) relative size of the giant caviomorph Telicomys and a small rhinoceros. [Figure (a) altered from Martin and Bennett, 1977; (b) after Zittel, 1925; (c) modified from Storch et al., 1996; (d,e) after Savage and Long, 1986.]

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