Vertebrate Skeletal Anatomy I

John Merck

Outline of the Vertebrate skeleton

So far, our discussions have been of the ancestral vertebrate morphotype, a creature lacking any bony skeleton. Before we embark on our climb up the vertebrate tree, it is useful to have in our heads, a decent vocabulary of skeletal terms, and an idea of the direction in which evolution is headed. Therefore, in this lecture, we consider the skeletons of some more advanced vertebrates, mostly the lobe-finned fish Eustheropteron and the land vertebrate Ophiacodon.

Anatomical directions:

Standard terms used by anatomists for orienting themselves to the non-human vertebrate body.

Bone Types:

Skeletal elements form in two general ways:

In primitive vertebrates, these bone types tend to be distinct, however over evolutionary time, they interact in complex ways.

"Finny tribes"

Before we proceed, know the names of the fins of a proper fish. Each tells an interesting evolutionary story:

Caudal fin morphology:

At the animal's rear end, a caudal fin, supported by a post-anal tail, aided in propulsion. Among early vertebrates, caudal fins generally took one of three forms, depending on the orientation of the notochord:

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Regions of the skeleton:

The Axial Skeleton:

Elements of the vertebral column: The schematic (right) shows these elements as they might have appeared in an aquatic sarcopterygian.

Different lineages of vertebrates have modified these elements in various patterns:

Rhachitomous vertebrae
Rhachitomous: Among the earliest land vertebrates, the centra retained their geometric relationships but were more robust, articulating with one another and restricting the notochord. From here, it was possible to go in many directions.

Stereospondylous vertebrae
Stereospondylous: In Stereospondyli, the pleurocentra are reduced or lost and the intercentra become spool-shaped.

Embolomerous vertebrae
Embolomerous: In Embolmeri, the pleurocentra and intercentra become similar, in effect giving each neural arch two spool-shaped centra.

Gastrocentrous vertebrae
The gastrocentrous pattern: In amniotes and their relatives, the pleurocentra are enlarged and fused, forming spool-shaped centra while the intercentra are reduced to small tangerine-segment-shaped wedges or are completely lost.

Holospondylous vertebrae
Holospondylous: I several vertebrate lineages, neural arches and centra coossify, yielding a unified holospondylous vertebra. Without a clear evolutionary context, in such cases it is difficult to know if the centrum is derived from the pleurocentrum, intercentrum, or both. The notochord remains in the form of intervertebral disks.

Mammalian dorsal vertebra
Intervertebral articulations: In land vertebrates, the neural arch not only protects the spinal cord, but also facilitates the articulation of adjacent vertebrae and provides points of insertion for the muscles that move the vertebral column. As a result we regularly see:

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