I thought Linnorms might be a third clade that lost their hindlimbs instead of their forelimbs.
the Landwyrms lost their wings, but kept their four legs.
the Sea Serpents (Dragon 345) closely resemble aquatic Landwyrms, but advance from Wyrmling to Great Wyrm.
Though I agree with putting wyverns and drakes together.
Hmm- pseudodragon- stings like a wyvern, four legs like a dragon- maybe it could be the base of the dragon/drake clade?
EDIT:
I figure a lot of magic and a little evolution are needed for any explanation.
Maybe kobolds were "genetically engineered" so to speak, by a dragon, from dragon genes.
Those for wings, breath, etc, are suppressed.
When they revert- that's a dragonblooded kobold?