This is it. Arcane magic is my favorite aspect of D&D. I revel in the wondrous things that can be wrought (or unmade) with magic. So, without further ado...

Stonebones
Transmutation

Level: Earth 5, Wiz/Sor/Mag 5
Components: V,S,M
Casting Time: 1 standard action
Range: Medium (100 ft + 10 ft/level)
Target: One creature
Duration: Instantaneous
Save: Fort partial (see text)
Spell Resistance: Yes

This lesser variant of flesh to stone is the result of studies in the past made by the chronomancer Jareth Moonscriber. He discovered that many fossils were not fashioned over time, as many sages thought, but were created instantly by the gaze attack of a strange little quilled dinosaur that may have been an ancestor of the modern cockatrice.
Now, rather than turning a creature's full mass to stone, this insidious spell turns to stone only those parts that are already the most mineral-rich--the skeleton in vertebrates, the exoskeleton in invertebrates (hereafter, the terms are used interchangeably). While this does not kill a creature outright, it slows it down and prevents it from healing naturally, while making it more resistant to bludgeoning blows and other magic that would alter its form.
When a creature fails its save, a number of things happen. First and foremost, the creature's skeleton approximately triples in mass (not volume). This means:

• The abrupt shift in mass and material causes 1d4 points of damage per caster level.
  
• Weight increases by 50%.
  
• Since this weight gain is not only in conveniently central places, but in the extremities as well, Dexterity-based skill and ability checks suffer a -4 penalty, and the target loses its Dex modifier (if positive) to AC.
  
• The target loses the benefits of Dexterity-based feats and feats requiring any degree of physical flexibility, nimbleness or speed, including, but not limited to: Acrobatic, Agile, Combat Expertise (Improved Disarm, Improved Feint, Improved Trip, Whirlwind Attack), Combat Reflexes, Deflect Arrows (Snatch Arrows), Deft Hands, Dodge (Mobility, Spring Attack), Improved Initiative, Lightning Reflexes, Nimble Fingers, Point-Blank Shot (Far Shot, Precise Shot [Improved Precise Shot], Rapid Shot [Manyshot, Improved Manyshot], Shot on the Run), Quick Draw, Rapid Reload, Skill Focus (any Dexterity-based skill), Stealthy, Two-Weapon Fighting (Two-Weapon Defense, Improved Two-Weapon Fighting, Greater Two-Weapon Fighting), and Weapon Finesse.
• Speed is reduced by half, rounded down. This includes a creature's swim speed, if any. Burrowing speed is unaffected.
  
• If the creature has a fly speed, it can no longer fly.
  
• The creature loses any ability to alter self, polymorph, shapechange or wild shape.
• The creature suffers a -4 penalty to all attacks.
• The creature can no longer heal naturally, and a creature with the fast healing quality loses it. These are both dependent on the body's ability to manufacture blood, which occurs in the bones. Magical healing and regeneration work normally.
  
• The creature automatically fails checks made to stabilize once it drops below 0 hp, unless magic is involved. Since the body cannot create more blood, once it has leaked out, it's gone (barring magical healing).