The term Sciuromorpha has referred to numerous groups of rodent

Rodent

Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....

s, but the only family common to all variations is the Sciuridae, the squirrels. Most definitions also include the Mountain Beaver

Mountain Beaver

The Mountain Beaver is the most primitive extant rodent. Not to be confused with the North American beaver Castor canadensis, or its relative the Eurasian beaver, Castor fiber, it has several common names including Aplodontia, Boomer, Ground Bear, and Giant Mole...

.

Traditionally the term has been defined on the basis of the shape of the infraorbital canal

Infraorbital canal

Not to be confused with the infraorbital groove and infraorbital foramen, which are on opposite ends of the canal.One of the canals of the orbital surface of the maxilla, the infraorbital canal, opens just below the margin of the orbit...

. A sciuromorphous zygomasseteric system

Zygomasseteric system

The zygomasseteric system in rodents is the anatomical arrangement of the masseter muscle of the jaw and the zygomatic arch of the skull...

 is characterized by attachment of the lateral masseter muscle

Masseter muscle

In human anatomy, the masseter is one of the muscles of mastication.In the animal kingdom, it is particularly powerful in herbivores to facilitate chewing of plant matter.-Origin and insertion of the two heads:...

 along the side of the rostrum. Unlike hystricomorphous and myomorphous rodents, the medial masseter muscle does not pass through the infraorbital canal. Among extant rodents, only the families Sciuridae, Castoridae

Castoridae

The family Castoridae contains the two living species of beaver and their fossil relatives. This was once a highly diverse group of rodents, but is now restricted to a single genus, Castor.- Characteristics :...

, Heteromyidae

Heteromyidae

The family of rodents that include kangaroo rats, kangaroo mice and rock pocket mice is the Heteromyidae family. Most heteromyids live in complex burrows within the deserts and grasslands of western North America, though species within the Heteromys and Liomys genera are also found in forests and...

, and Geomyidae are truly sciuromorphous. Some authorities would exclude the Geomyidae and Heteromyidae from that list due to the attachment of the medial masseter directly behind the zygomatic arch

Zygomatic arch

The zygomatic arch or cheek bone is formed by the zygomatic process of temporal bone and the temporal process of the zygomatic bone , the two being united by an oblique suture; the tendon of the Temporalis passes medial to the arch to gain insertion into the coronoid process...

.

Carleton and Musser (2005) redefined rodent suborders on the basis of a host of both morphological

Morphology (biology)

In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 and molecular

Molecular phylogeny

Molecular phylogenetics is the analysis of hereditary molecular differences, mainly in DNA sequences, to gain information on an organism's evolutionary relationships. The result of a molecular phylogenetic analysis is expressed in a phylogenetic tree...

 reasons. They defined the Sciuromorpha as including three families, Sciuridae, Aplodontiidae, and Gliridae. Of these, only Sciuridae is truly sciuromorphous. Aplodontiidae is protrogomorphous and Gliridae is myomorphous. The connection between Aplodontiidae and Sciuridae has been proposed on numerous times in the past. The two families have been united into a common infraorder (Sciurida) or superfamily

Taxonomic rank

In biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...

 (Sciuroidea). It has long been suggested that dormice

Dormouse

Dormice are rodents of the family Gliridae. Dormice are mostly found in Europe, although some live in Africa and Asia. They are particularly known for their long periods of hibernation...

 (Gliridae) are not particularly related to the Myomorpha

Myomorpha

Suborder Myomorpha contains 1,137 species of mouse-like rodents, nearly a quarter of all mammal species. Included are mice, rats, gerbils, hamsters, lemmings and voles. They are grouped according to the structure of the jaw and the structure of the molar teeth. Both their medial and lateral...

, and their zygomasseteric structure has been termed "pseudomyomorphy". The connection between squirrels and dormice has been almost exclusively suggested through genetic studies, and to a lesser degree via the fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 rodent Reithroparamys.

Current families

The suborder Sciuromorpha contains 307 living species in 61 genera and seven families.

• †Allomyidae
• Aplodontiidae - mountain beaver
• †Mylagaulidae
  Mylagaulidae
  The Mylagaulidae or mylagaulids are a prehistoric family of sciuromorph rodents. They are known from the Neogene of North America and China...
  
  
• Sciuridae - squirrels, chipmunks, marmots, flying squirrels, etc.
• †Reithroparamyidae
• Gliridae - dormice

Historical families

• Castoridae
  Castoridae
  The family Castoridae contains the two living species of beaver and their fossil relatives. This was once a highly diverse group of rodents, but is now restricted to a single genus, Castor.- Characteristics :...
  
   - beavers
• Geomyidae - pocket gophers
• Heteromyidae
  Heteromyidae
  The family of rodents that include kangaroo rats, kangaroo mice and rock pocket mice is the Heteromyidae family. Most heteromyids live in complex burrows within the deserts and grasslands of western North America, though species within the Heteromys and Liomys genera are also found in forests and...
  
  - kangaroo rats and mice