EVOLUTION: BONY SKELETONS

SKELETONS

The earliest skeletons were hydrostatic skeletons with the body or cell fluid inflating the organism to its characteristic shape determined by the outer covering.  The support and protection functions of a skeleton were the original values of a hydrostatic skeleton  The exoskeleton of arthropods was a successful adaptation that added movement to the support function.  The shells of mollusks and several other phyla were primarily of value for protection, but some groups added movement to their value along with support.

Bony skeletons of vertebrates add values that are not related to the skeletal functions of support, protection, and movement.  Mineral storage is one of those additional functions.  In mammals the blood-forming tissue is primarily the marrow of bone cavities.  So the bones are therefor important in enabling the blood to transport oxygen and carbon dioxide, produce an immune response to foreign organisms, and trigger clotting of blood to close wounds.

The earliest chordates lacked a bony skeleton, but they did have a skeletal rod of dense tissue, the notochord, above which the central nerve cord developed.  The notochord is still an important skeletal structure of lampreys, jawless fish without paired fins.  Among the diversity of fishes in the Paleozoic were Ostracoderms and Placoderms, close to freshwater ancestors of the lungfishes and coelacanths preceding amphibians.

The sharks and other fish with cartilaginous skeletons were marine and did not develop bony skeletons.  In freshwater ponds and streams fish were able to survive with mineral deposits converting cartilage to bone without the danger of making them sink to the bottom of the ocean if they stopped swimming.  The bony fish in the ocean are descendants of freshwater fish with lungs that gave them buoyancy which compensated for the heavy bony skeleton.  Eventually the lungs developed into the swim bladder of modern fishes found in both freshwater and the ocean.

"Crossopterygians* in the same branch as the lungfish progressed toward the legged amphibians by greater development of paired, lobed fins.  They were thought to have become extinct at the end of the Devonian, until an unusual deep-water form, a coelacanth, in the Indian Ocean was first caught in 1939.  During the Devonian some of the freshwater relatives made the transition to primitive amphibians."

[*A reference included for the above quotation from Evolution Insights, 2010, was as follows.
Gorr, Thomas, and Traute Kleinschmidt.  1993.  Evolutionary relationships of the coelacanth.  Am. Scientist, 81:72-82.  Their hemoglobin is closest to frog tadpole hemoglobin.  Lamprey and hagfish have monomeric hemoglobin, other vertebrates have tetrameric hemoglobin (with 2 alpha and 2 beta chains) and most have two forms of hemoglobin- juvenile and adult.]


SKELETONS AND ANIMAL SIZE

As noted in the (January 28, 2016) post on muscle, strength of bone scales up proportional to the cross sectional area (or square of linear dimensions), whereas the strength a skeleton needs to support an animal scales up with the mass (or cube of linear dimensions).  Thus terrestrial dinosaurs and elephants are about as massive as animals can become on land.  The support function of whale skeletons is much less because of the buoyancy water provides negating much of the gravitational forces acting on their bodies.

Birds have less weight than might be expected  because many bones are hollow and air filled.  They further economize on energy needed for flight by having only one functional ovary in females, as well as having shelled eggs to lay in nests.  Laying one egg at a time, instead of numerous ones as a turtle does, means less weight during flight.  The tremendous energy demands for flight are provided by a lung with a flow through design rather than the in and out flow to and from the sack-like lungs of other terrestrial vertebrates.

BONE

Bone is living tissue.  Bone cells are widely separated but connected by microscopic canals.  Long bones usually begin with one central and two terminal centers of ossification.  Mature long bones have layers of bone around the exterior.  As the bone gets bigger inner layers are dissolved along additional vessels that then produce fresh layers, leaving fragmented arcs of bone layers between the new canals with fresher bone.  The length of bones increases until cartilaginous centers of ossification are completely converted to bone.

The method of continuing bone renewal is a slow process, but a broken bone that heals, with poor match of the two portions fusing, will eventually be much closer in shape to the original bone.

In addition to genetic control of bone development, there is an effect of orientation and activity that may contribute to bone development.  Bone development appears to be increased by exercise and/or stress placed on the bone.  The upright posture of humans results in much less development of bony anchorage for muscles attached to the back of skull for holding up the head, as compared to primates using arms to aid walking.

Natural selection of unusual bony formation favoring survival is illustrated by the skull of the Eastern Kingbird, Tyrannus tyrannus.  It is a flycatcher that frequently perches on the topmost twigs of trees or other high perches.  From there it will fly out and chase a much larger crow or hawk.  You would think its perch would make it vulnerable to a hawk, but it probably has 360 degree vision of the horizon without turning it head.  It is possible because its skull behind the orbits for the eyes on each side has a deep groove.  So it appears that vision is not obscured in any horizontal direction.


Joseph G. Engemann    Kalamazoo, Michigan     February 22, 2016