Zygomasseteric system
Encyclopedia
The zygomasseteric system (or zygomasseteric structure) in rodent
s is the anatomical arrangement of the masseter muscle
of the jaw and the zygomatic arch
of the skull. The anteroposterior or propalinal (front-to-back) motion of the rodent jaw is enabled by an extension of the zygomatic arch and the division of the masseter into three distinct parts.
is unmodified and the masseter originates on the ventral zygomatic surface. The Bathyergidae is considered secondarily protrogomorphous since its zygomatic condition clearly derived from a hystricomorphous ancestor.
. The masseter lateralis extends forwards onto the rostrum. The masseter superficialis extends forwards along the zygoma.
This condition is found in most of the family Sciuridae (suborder Sciuromorpha
),
and also in the Castoridae
, the Eomyidae
, and the Geomyoidea
.
to originate on the side of the rostrum. The masseter superficialis originates on the front edge of the zygoma, and the masseter lateralis extends over most of its length.
This condition is found throughout the suborders Hystricomorpha
and Anomaluromorpha
. In the suborder Myomorpha
, it is found in the superfamily Dipodoidea
and some fossil Muroidea
(such as Pappocricetodon). Hystricomorphy is also found in the African dormouse
Graphiurus
, which is a member of the suborder Sciuromorpha
.
foramen of hystricomorphy.
This condition is found in the Muroidea
(Myomorpha
) and most Gliridae (Sciuromorpha
: in the latter it is often referred to as pseudomyomorphy). Korth and Emry (1991) suggest that the infraorbital foramen of the extinct sciurid subfamily Cedromurinae may have allowed for the passage of the masseter muscle. If true, this subfamily would represent an additional example of myomorphy in the rodent suborder Sciuromorpha.
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 is the anatomical arrangement of the 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:...
of the jaw and 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...
of the skull. The anteroposterior or propalinal (front-to-back) motion of the rodent jaw is enabled by an extension of the zygomatic arch and the division of the masseter into three distinct parts.
Types of zygomasseteric system
The main types are described as protrogomorphous, sciuromorphous, hystricomorphous, and myomorphous.Protrogomorphy
This is the primitive condition, found in the family Aplodontiidae and in various fossil groups. The rostrumRostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
is unmodified and the masseter originates on the ventral zygomatic surface. The Bathyergidae is considered secondarily protrogomorphous since its zygomatic condition clearly derived from a hystricomorphous ancestor.
Sciuromorphy
The ventral surface of the zygoma tilts and broadens into a zygomatic plateZygomatic plate
In rodent anatomy, the zygomatic plate is a bony plate derived from the flattened front part of the zygomatic arch . At the back, it connects to the front root of the zygomatic arch, and at the top it is connected to the rest of the skull via the antorbital bridge. It is part of the maxillary...
. The masseter lateralis extends forwards onto the rostrum. The masseter superficialis extends forwards along the zygoma.
This condition is found in most of the family Sciuridae (suborder 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....
),
and also in the 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 :...
, the Eomyidae
Eomyidae
Eomyidae is a family of extinct rodents from North America and Eurasia related to modern day pocket gophers and kangaroo rats. The family includes the earliest known gliding rodent, Eomys -References:...
, and the Geomyoidea
Geomyoidea
Geomyoidea is a superfamily of rodent that contains the pocket gophers , the kangaroo rats and mice , and their fossil relatives.-Characteristics:...
.
Hystricomorphy
The masseter medialis is enlarged, and passes through an enlarged infraorbital foramenInfraorbital foramen
Above the canine fossa is the infraorbital foramen, the end of the infraorbital canal; it transmits the infraorbital artery, vein, and infraorbital nerve.-External links: *...
to originate on the side of the rostrum. The masseter superficialis originates on the front edge of the zygoma, and the masseter lateralis extends over most of its length.
This condition is found throughout the suborders 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 Anomaluromorpha
Anomaluromorpha
Anomaluromorpha 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...
. In the suborder 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...
, it is found in the superfamily Dipodoidea
Dipodoidea
Dipodoidea is a superfamily of rodents that includes Jumping mice and Jerboas....
and some fossil Muroidea
Muroidea
Muroidea is a large superfamily of rodents. It includes hamsters, gerbils, true mice and rats, and many other relatives. They occupy a vast variety of habitats on every continent except Antarctica. Some authorities have placed all members of this group into a single family, Muridae, due to...
(such as Pappocricetodon). Hystricomorphy is also found in the African dormouse
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...
Graphiurus
Graphiurus
The African dormice, Graphiurus, are a genus of dormouse that live throughout sub-Saharan Africa in a variety of habitats. They are very agile climbers and have bushy tails...
, which is a member of the suborder 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....
.
Myomorphy
A combination of the expanded and tilted zygoma of sciuromorphy and the enlargedforamen of hystricomorphy.
This condition is found in the Muroidea
Muroidea
Muroidea is a large superfamily of rodents. It includes hamsters, gerbils, true mice and rats, and many other relatives. They occupy a vast variety of habitats on every continent except Antarctica. Some authorities have placed all members of this group into a single family, Muridae, due to...
(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 most Gliridae (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....
: in the latter it is often referred to as pseudomyomorphy). Korth and Emry (1991) suggest that the infraorbital foramen of the extinct sciurid subfamily Cedromurinae may have allowed for the passage of the masseter muscle. If true, this subfamily would represent an additional example of myomorphy in the rodent suborder Sciuromorpha.