Zoology 365

Biology of Mammals

Spring Semester, 2006

Terri McElhinny- Michigan State University

Words of the Week- Rodents

SCIUROGNATHOUS: condition in rodents where the angular process lies in the same vertical plane as the tooth row.

HYSTRICOGNATHOUS: condition in rodents where the angular process lies lateral to the vertical plane of the tooth row.

MASSETER: muscle that aids with the horizontal movement of the jaw. Large and complexly subdivided (superficial, lateral, and medial [see p. 43 in lab manual]) in rodents to provide the power necessary for mastication and gnawing.

INFRAORBITAL FORAMEN: the foramen through the zygomatic process of the maxilla.

MASSETERIC TUBERCLE: bump at the anterior end of the tooth row where the superficial masseter muscle originates.

ZYGOMATIC PLATE: in rodents, anterior portion of zygomatic arch that is flattened for attachment of the lateral masseter (found in the sciuromorphous and myomorphous conditions)

PROTROGOMORPHOUS: primitive condition in which the masseter muscle originates completely on the zygomatic arch.
infraorbital foramen: small and round
zygomatic plate: absent

SCIUROMORPHOUS: origins of the superficial and lateral masseters are shifted anteriorly. The lateral masseter attaches to the zygomatic plate.
infraorbital foramen: small and wider on the bottom than it is on the top
zygomatic plate: large

HYSTRICOMORPHOUS: medial masseter is enlarged and its origin is shifted anteriorly to the side of the rostrum anterior to the z-arch.
infraorbital foramen: huge! and round
zygomatic plate: absent

MYOMORPHOUS: combo of sciuromorph and hystricomorph where the lateral masseter originates on the anterior edge of the z-arch and the medial masseter passes through a small to moderate infraorbital foramen (often keyhole- shaped) originating anterior to the z-arch.
infraorbital foramen: small to middle-sized and wider on the top than it is on the bottom
zygomatic plate: small

RE-ENTERENT FOLD: invagination (along the side of a cheek tooth)