Anamniote Skeletons

INTRODUCTION
Dermal bones are derived from dermal scales that sank deep & joined the internal skeleton. These bones form as connective
tissue membranes that later ossify. Endochondral or replacement "elements" begin as cartilage & may ossify or remain cartilaginous
in the adult.
The axial skeleton elements are on the midline or mid-sagittal axis of the body. The axial skeleton includes the notochord,
vertebrae, ribs & median fins. The notochord is a centrally located, gel-filled rod that supports the body of a developing embryo. In
most vertebrates, vertebrae replace it, partly or completely. The vertebral body or centrum is the large solid disk that takes the
compressive forces during body movement. Some centra are biconcave (amphicoelous) or flat (acoelous). In fishes, amphicoelous
vertebrae are tightly held together with sheathing of connective tissue. Other centra have rounded cavities on the anterior (procoelous)
or posterior (opisthocoelous) surface with a corresponding rounded opposite side that articulates with adjacent vertebrae. The
vertebrae enclose the spinal cord with neural (or vertebral) arches. Caudal vertebrae have hemal arches that enclose caudal arteries
& veins in the tail. All vertebrae have several areas of muscle attachment via neural spines, transverse processes & hemal spines.
Tetrapod vertebrae have zygapophyses (zygo= yoke) to support the body & keep the trunk from sagging. The zygapophyses
allow vertebrae to articulate with each other, and limit the total range & direction of body movement. The pre-zygapophyses are on the
anterior side of vertebra & have articulating facets that face upward. The post-zygapophyses are on the posterior side & have
articulating facets that face downward. Post-zygapophyses fit on top of pre-zygapophyses.
Fishes have 2 regions in vertebral column: trunk vertebrae have ribs and caudal vertebrae form the tail. Anamniote tetrapods
have 4 regions in vertebral column. They have 1 cervical or neck vertebra, called the atlas. This vertebra has large facets (prezygapophyses)
that articulate with the paired occipital condyles seen in living amphibians. Tetrapods have a single sacral vertebra
that attaches to the pelvic girdle. In frogs, the caudal vertebrae are fused into a rod-like urostyle.
A sternum is present only in tetrapods, and even then may be missing or very small in a few species. The sternum acts as a
brace to attach to support the rib cage or an additional brace for the pectoral girdle. Fishes have unpaired, median fins located along the
mid-sagittal axis. These include dorsal, anal & caudal fins. Fish may have 1 or more dorsal fins. Caudal fins vary in design.
Heterocercal caudal fins are present in the Chondrichthyes and some primitive Actinopterygians. In these tails, the caudal vertebrae
extend into the dorsal & larger lobe of the caudal fin. Some Actinopterygians have a hemi-homocercal design where the vertebrae just
turn dorsally at the very tail tip & do not extend far into the tail fin. The tail fin doesn’t appear to be asymmetrical, but the asymmetry can
be seen in the skeleton & in the body profile. Most living Actinopterygians have a homocercal caudal fin that forms both a symmetrical
profile & symmetrical caudal fin.
The appendicular skeleton refers to the elements that form the paired, laterally placed appendages for the paired front & hind
fins or limbs. The pectoral girdle of fishes is a combination of dermal & endochondral elements. The dermal pectoral girdle bones of
ray-finned fish form an arc just behind the opercular bones of the skull. These dermal bones include the cleithrum, clavicle & other
bones that attach the pectoral girdle to the skull. The sharks lack these dermal bones as they lack all dermatocranium in the skull as
well. Tetrapods lose many dermal bones when the pectoral girdle detaches from the skull. Thus in tetrapods, the head moves
independently of the legs. Some tetrapods retain the clavicles as paired bones that extend from the sternum towards the humerus. One
new dermal pectoral girdle bone, the interclavicle, appears in Sarcopterygians & some tetrapods. This is an unpaired bone that may be
in the midline between the paired clavicles.
The endochondral elements of the pectoral girdle are called scapulocoracoid or coracoscapula, if they don’t ossify. In
derived Actinopterygians (e.g. Perch) these ossify into a ventrally positioned anterior coracoid (procoracoid) & a more dorsally
positioned scapula. The tetrapods may add a suprascapula dorsal to the scapula. Amniotes gained a new element, the posterior
coracoid (metacoracoid). In
Fish pelvic girdles are suspended in muscle: ischiopubic or pubioischiatic plates. In derived Actinopterygians this girdle &
its associated fins are anterior in the body. This position may improve the braking ability of those fins. In some ray-finned fish, the pelvic
girdle attaches to the pectoral girdle but is still ventral to the pectoral fins. All tetrapods (with legs) have 3 bones that form the pelvic
girdle. The pubic bone or pubis is directed forward & down & remains cartilaginous in many anamniotes. The ischium faces
downward & posteriorly. The ilium is the new bone in the tetrapod pelvis. It is oriented dorsally & attaches the pelvic girdle to the sacral
vertebra.
The median & paired fins of fishes contain the same types of support elements. The most proximal support elements are
called basal pterygiophores & they are typically larger and fewer in number than the more distal radial pterygiophores. Different
materials in fish support the fin membranes. Ceratotrichia are made of keratin & support the fins of sharks, skates & rays.
Lepidotrichia are tiny, overlapping dermal scales in Actinopterygian fins. Actinotrichia are bony spines that may be present at the
anterior edge of a fin or replace lepidotrichia throughout an entire fin.
The paired limbs of tetrapods (with limbs) comprise a standard pattern with 1 bone, followed by a pair of bones then many
bone in series. The forelimb has a humerus; radius & ulna; carpals; metacarpals; phalanges. The hind limb has a femur; tibia & fibula;
tarsals; metatarsals; phalanges.