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Polydactyly in early tetrapods should be understood as having more than five digits to the finger or foot, a condition that was the natural state of affairs in the very first tetrapods. Early groups like Acanthostega had eight digits, while the more derived Ichthyostega had seven digits, the yet-more derived Tulerpeton had six toes.
The acromion is a bony ridge on the outer distal end of the scapula that functions in providing an attachment for the clavicle. [2] [3] Nodosaurids develop a pronounced spur known as the pseudoacromion, which probably formed an attachment site for the Musculus scapulohumeralis anterior, and therefore is analogous with the acromion of mammals. [4]
A small projection of bone is present on the acetabulum, a depression in the pelvis that attaches with the end of the femur. This projection is also seen in the Late Devonian tetrapod Ichthyostega . The femur of Antlerpeton is much more robust than that of Ichthyostega , and has prominent ridges like the fourth trochanter that are attachment ...
Limbs are attached to the torso via girdles, either the pectoral girdle for the forelimbs, or the pelvic girdle for the hindlimbs.In terrestrial tetrapods, the pectoral girdles are more mobile, floating over the rib cage connected only via the clavicles (to the sternum) and numerous muscles; while the pelvic girdles are typically fused together anteriorly via a fibrocartilaginous joint and ...
The humerus and femur of Acanthostega contain evidence of greater development of the appendicular muscles compared to more aquatic tetrapods. [3] Acanthostega has a total lack of dermal fin rays and displays the presence of two or more spool-shaped bones or cartilages articulating individually in antero-posterial sets on the distal end of its ...
The caudofemoralis attachment crest first separated from the femoral head in the Erythrosuchidae, large basal archosauriform predators of the early Triassic period. Shortly afterwards, eucrocopodan archosauriforms (such as Euparkeria ) evolved, losing the intertrochanteric fossa and acquiring a symmetrical fourth trochanter.
A small Ossinodus femur recovered, postulated to belong to a juvenile and compared to the Horton Bluff femur, suggests younger specimens were more aquatic than older specimens. [11] In addition to the fractured radius, the enclosed sensory canal synapomorphy is also indicative of tetrapods moving away from aquatic perceptions. These events are ...
In effect, "tetrapod" is a name reserved solely for animals which lie among living tetrapods, so-called crown tetrapods. This is a node-based clade , a group with a common ancestry descended from a single "node" (the node being the nearest common ancestor of living species).