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Rodents and Lagomorphs. Rodents and Lagomorphs. Rodents are the most successful of all mammalian orders. The order contains at least 34 families, and more than 354 genera and 1685 species.
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Rodents and Lagomorphs • Rodents are the most successful of all mammalian orders. The order contains at least 34 families, and more than 354 genera and 1685 species. • They have a worldwide distribution (excluding Antarctica), and are major components of nearly every fauna.
Rodents and Lagomorphs • Morphological diversity within the Rodentia is extreme, which makes it very difficult to discern the evolutionary relationships. This also results in a very plastic taxonomy. • Lagomorphs on the other hand have not been nearly so successful.
Rodents and Lagomorphs • The order Lagomorpha contains only 10 genera and about 63 species. • They too have a nearly worldwide distribution. However, they were introduced to Australia and the southern portion of South America by man. • Why do we consider these 2 orders together?
Rodents and Lagomorphs • Very early on, rodents and lagomorphs were considered to be part of the same order. • In fact, the Russians still consider them to be members of the same order. • We laugh at this, but Stuart Landry (SUNY and the American Museum) has noted an interesting similarity between the groups.
Rodents and Lagomorphs • If you look at the pterygoid in rodents, it extend so far back past the palate that it nearly touches the auditory bullae. We often don’t see this when we look at specimens because the processes are fragile and break off easily on museum specimens. • No other mammal group has this feature.
Rodents and Lagomorphs • If you look at the pterygoid processes of lagomorphs, you discover that they too reach back so far they nearly touch the auditory bullae. • Clearly, there are important differences between the 2 groups, but they may be more closely related than we thought.
Rodentia • The taxonomy of the Rodentia remains uncertain. • There are 3 major groups of rodents: • Sciuromorph • Myomorph • Hystricomorph • At one time, these were considered taxonomic divisions.
Rodentia • It turns out that these are not all that useful in a taxonomic sense. • The terms refer to the structure of the skull and the organization of the masseter muscles. • Regardless of its taxonomic utility, this feature gives us an important clue about the success of the rodents.
Rodent Morphology • The dental formula is usually 1/1, 0/0, 2/1, 3/3 = 22. • They have only 1 incisor in each jaw. The incisor is ever growing, and occludes with the opposite incisor to produce a sharp cutting edge. • There are no canines. Instead, they have a diastemma.
Rodent Morphology • The premolars and molars have complicated occlusal surfaces that are effective at grinding plant material. • The cheek teeth are difficult to interpret in terms of the tribosphenic tooth model. • They grow continuously, and have open pulp cavities.
A: mole ratB: muridC: old world porcupineD: chinchillaE: cuspidate muridF: folded murid
Rodent Morphology • The glenoid fossa of the squamosal is elongate, and allows both anteroposterior movement, and lateral movement of the dentary. • The mandibular symphisis is flexible, and allows the 2 halves of the dentary to move semi-independently. Thus, using the transverse mandibular muscles, the two rami can be pulled in different directions. In fact, the tips of the front incisors can be pointed towards one another.
Rodent Morphology • The masseter muscle is subdivided, and contrary to carnivores (that emphasize the Temporalis), this muscle is the primary muscle involved in dentary manipulation. • The complexity of the 3 masseters and the dentary articulation, allows complicated rotation of the jaw, and excellent ability to masticate plant material. • These features, more than any others, are responsible for their evolutionary success.
Zygomasseteric Specializations • Complex jaw architecture is associated with rotational movement of the dentary, and maintenance of crushing power. • Protogomorph = first shape • Sciuromorph = squirrel shape • Myomorph = mouse shape • Hystricomorph = porcupine shape
Zygomasseteric Specializaitons • These classifications are not without controvesy. For example, the Anomaluridae have been variously classified as Sciuromorph and Hystricomorph. • Similarly, Myomorphs could be derived from either Hystricomorph or Sciuromorph.
Zygomasseteric Specializaitons • In any event, these 4 morphs represent different solutions to the problems posed by herbivory on a small scale.
Zygomasseteric Specializaitons • Protogomorph - usually included below. • Sciuromorph • This seems to be the least derived of the 3. It is most similar to the entirely zygomatic origin of the masseters. • The masseter originates entirely from the zygomatic arch (Aplodontia = Protogomorph) or part of the masseter lateralis originates from the rostrum anterior to the zygomatic plate.
Sciuromorpha • Families: • Aplodontidae • Sciuridae • Geomyidae • Heteromyidae • Castoridae • Anomaluridae • Pedetidae
Sciuromorph Rodents • Aplodontidae • These are the ‘mountain beavers’ of the Pacific Northwest. They are not beavers as in Castor canadensis. • They are very underived morphologically. • They have a large coronoid process of the dentary. Masseters originate entirely on the zygomatic, as in paramyids. • Crown pattern on cheek teeth is unique. • Considered most ‘primitive’ of all rodents.
Zygomasseteric Specializaitons • Hystricomorphs • There is a very large infraorbital foramen. • The anterior portion of the masseter medialis passes through the large infraorbital foramen. • The masseter lateralis and the masseter superficialis originate entirely on the zygomatic arch.
Hystricomorph Able to generate a great deal of force on jaw because of mechanical advantage: efficient right angle attachment of medial masseter on dentary. This corresponds with a shift from less efficient temporalis and masseter lateralis to medialis. Zygomasseteric Specializaitons
Families Hystricidae Erethizontidae Caviidae Hydrochoeridae Dinomyidae Heptaxodontidae Dasyproctidae Chinchillidae Capromyidae Myocastoridae Octodontidae Ctenomyidae Abrocomidae Echimyidae Thryonomyidae Pertomyidae Bathyergidae Ctenodactylidae Hystricomorpha
Zygomasseteric Specializaitons • Myomorph • This form uses some aspects of hystricomorphs and sciuromorphs. • Origins of both the masseter lateralis and the masseter medialis have moved anteriorly. • Anterior part of the zygomatic arch is not platelike, but the masseter lateralis has a partial rostral origin. • Infraorbitat foramen is enlarged, and part of the masseter medialis, which originates partially on the rostrum on the max and premax, passes trough it. • Masster superficialis originates far forward on the rostrum.
Families Cricetidae Spalacidae Rhizomyidae Muridae Gliridae Platacanthomyidae Seleviniidae Zapodidae Dipodidae Myomorpha
Rodent Paleontology • There were rodent-like reptiles. These were the Tritylodonts of the late Triassic. They had enlarged incisor-like teeth, and a diastemma. • Tritylodonts appeard at the same time as seed-bearing vegetation. • Tritylodonts were followed by the multituberculata in the Jurassic.
a. mammal-like reptile from Jurassic.b. multituberculate from Paleocene.c. Paramys.
Rodent Paleontology • The multituberculata of the Jurassic were also rodent-like, and prototherian-like. They had enlarged incisor, a diastemma, and cheek teeth with multiple cusps (hence the name, multituberculata). • However, neither the Tritylodonts or the Multituberculata are rodents. The first rodents are the sciuromorphic Paramyids.
Rodent Paleontology • Earliest fossils are in the family Paramyidae from the late Paleocene of N. America and Eurasia. • Are these the stem group from which all rodents evolved? • Paramyids are underived in that the temporalis was large, and the masseters relatively unspecialized. Origins of the masseters were strictly on the zygomatic.
Rodent Paleontology • Example of Paramyidae is Paramys atavus. • Closest relative of Paramys is Aplodontia rufa, the mountain beaver of the Pacific Northwest.
Rodent Paleontology • Dental formula of Paramyids is 1/1, 0/0, 2/1, 3/3 = 22. • Molars and premolars are brachyodont. • Rodents underwent an adaptive radiation in the Tertiary, and suffered no mass extinction in the Pleistocene or post-Pleistocene. Consequently, today we have a rich rodent fauna.
Today, rodents are primarily herbivorous, but also consume: insects fish reptiles birds mammals Locomotor styles include: quadrupedal arboreal fossorial saltatorial semi-aquatic. Rodent Paleontology
Rodent Morphology Again • Even though rodent systmatics does not correspond to the division of Myomorph / Sciuromorph / Hystricomorph, we can divide the rodents into 2 major clades based on jaw morphology. • All rodents can be classified as Sciurognathous or Hystricognathous.
Sciurognathous and Hystricognathous Rodents • This terminology refers to the structure of the dentary - gnathous refers to ‘jaw.’ • Sciurognathous refers to dentary that are relatively simple, with insertion of the masseter directly below the molariform teeth. These rodents are in the suborder Sciurognathi. Myomorphs are considered within this group as an infraorder. • Hystricognathous rodents have a strongly deflected angular process. Insertion of the masseter is ventral and posterior to the molariform teeth. These rodents are in the suborder Hystricognathi.
a & c: sciurognathousb & d: hystricognathousarrow points to angular process: note coronoid process and flange like angular process.
Sciurognathi • Aplodontidae • Mountain beavers • Sciuridae • 50 genera and 273 species divided into the Sciurinae and the Pteromyinae (Petauristinae). Note: the flying squirrels are convergent with the Anomaluridae, Cynocephalidae, Petauridae, Pseudocheiridae, and Acrobatidae.
