Complex sentence: These foxes display transitional features between northern red foxes and smaller fox species; their skulls possess more primitive, neotenous traits than the northern forms and are also much smaller; the maximum sizes attained by southern foxes are invariably less than the average sizes of northern foxes.
Split into simpler sentences: These foxes display transitional features from northern red foxes to smaller species; their skulls possess more primitive, neotenous traits than the northern forms. They are also much smaller than the northern forms; the maximum sizes attained by southern foxes are invariably less than the average sizes of northern foxes.