Sciurognathi
Encyclopedia
Sciurognathi is a suborder of rodent
s that includes squirrel
s, chipmunk
s, beaver
s, and many types of mice
. The group is characterized by a specific shape to the lower jaw
. In sciurognaths, the angular process of the jaw is in the same plane as the root of the incisors. This is in contrast to the suborder Hystricognathi
where the angular process is outside of the plane formed at the root of the incisor due to the presence of a shelf for muscle attachment.
The sciurognathous condition is considered to be the primitive condition in rodents, and is therefore not a good character according to cladistic
philosophy
. Although hystricognaths are almost universally accepted as representing a real evolutionary grouping
, most researchers do not consider Sciurognathi as an equally valid group. In particular, gundi
s are thought to be more closely related to the hystricognathous rodents than to other sciurognaths.
In spite of this, most texts continue to use these two suborders due primarily to a lack of a viable alternative.
Alternatively, some texts group rodents into three suborders on the basis of the shape of the infraorbital canal
. According to this taxonomy the rodents are divided into the suborders Sciuromorpha
, Hystricomorpha
, and Myomorpha
.
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 that includes squirrel
Squirrel
Squirrels belong to a large family of small or medium-sized rodents called the Sciuridae. The family includes tree squirrels, ground squirrels, chipmunks, marmots , flying squirrels, and prairie dogs. Squirrels are indigenous to the Americas, Eurasia, and Africa and have been introduced to Australia...
s, chipmunk
Chipmunk
Chipmunks are small striped squirrels native to North America and Asia. They are usually classed either as a single genus with three subgenera, or as three genera.-Etymology and taxonomy:...
s, beaver
Beaver
The beaver is a primarily nocturnal, large, semi-aquatic rodent. Castor includes two extant species, North American Beaver and Eurasian Beaver . Beavers are known for building dams, canals, and lodges . They are the second-largest rodent in the world...
s, and many types of mice
Mouse
A mouse is a small mammal belonging to the order of rodents. The best known mouse species is the common house mouse . It is also a popular pet. In some places, certain kinds of field mice are also common. This rodent is eaten by large birds such as hawks and eagles...
. The group is characterized by a specific shape to the lower jaw
Mandible
The mandible pronunciation or inferior maxillary bone forms the lower jaw and holds the lower teeth in place...
. In sciurognaths, the angular process of the jaw is in the same plane as the root of the incisors. This is in contrast to the suborder Hystricognathi
Hystricognathi
Hystricognathi is an infraorder of rodents. Hystricognaths are distinguished from other rodents by the bone structure of their skulls. The masseter medialis passes partially through the infraorbital foramen and connects to the bone on the opposite side...
where the angular process is outside of the plane formed at the root of the incisor due to the presence of a shelf for muscle attachment.
The sciurognathous condition is considered to be the primitive condition in rodents, and is therefore not a good character according to cladistic
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...
philosophy
Philosophy
Philosophy is the study of general and fundamental problems, such as those connected with existence, knowledge, values, reason, mind, and language. Philosophy is distinguished from other ways of addressing such problems by its critical, generally systematic approach and its reliance on rational...
. Although hystricognaths are almost universally accepted as representing a real evolutionary grouping
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
, most researchers do not consider Sciurognathi as an equally valid group. In particular, gundi
Gundi
Gundis are a group of small, stocky rodents found in Africa. They live in rocky deserts across the northern parts of the continent. The family comprises 4 living genera and 5 species , as well as numerous extinct genera and species...
s are thought to be more closely related to the hystricognathous rodents than to other sciurognaths.
In spite of this, most texts continue to use these two suborders due primarily to a lack of a viable alternative.
Alternatively, some texts group rodents into three suborders 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...
. According to this taxonomy the rodents are divided into the suborders Sciuromorpha
Sciuromorpha
The term Sciuromorpha has referred to numerous groups of rodents, but the only family common to all variations is the Sciuridae, the squirrels. Most definitions also include the Mountain Beaver....
, Hystricomorpha
Hystricomorpha
The term Hystricomorpha has had many definitions throughout its history. In the broadest sense it refers to any rodent with a hystricomorphous zygomasseteric system. This includes the Hystricognathi, Ctenodactylidae, Anomaluridae, and Pedetidae...
, and 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...
.
See also
- SciuromorphaSciuromorphaThe term Sciuromorpha has referred to numerous groups of rodents, but the only family common to all variations is the Sciuridae, the squirrels. Most definitions also include the Mountain Beaver....
- MyomorphaMyomorphaSuborder 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...
- CastorimorphaCastorimorphaCastorimorpha is the suborder of rodents containing the beavers, the pocket gophers, and the kangaroo rats and mice.-Taxonomy:*Suborder Castorimorpha**Superfamily Castoroidea***Family †Eutypomyidae***Family Castoridae - beavers...
- AnomaluromorphaAnomaluromorphaAnomaluromorpha is the name given to a clade that unites the anomalures with the springhare. It has alternately been designated as either a suborder or infraorder...
- HystricomorphaHystricomorphaThe term Hystricomorpha has had many definitions throughout its history. In the broadest sense it refers to any rodent with a hystricomorphous zygomasseteric system. This includes the Hystricognathi, Ctenodactylidae, Anomaluridae, and Pedetidae...