Through most of the Middle Ages, bread in France was simply "bread". There was no need to refer to various shapes of loaves because, for oven-baked bread, there was mainly one: round (bread baked on the hearth, which would have been flat, was rarely referred to in records). The main distinction between breads was in the grains used to make them: wheat, rye, mixed wheat and rye (maslin), spelt, barley, etc.
Around the twelfth century many more types of bread began to be mentioned in records: morning bread, varlet's bread, canon's bread, peer's bread, Knight's bread, Squire's bread, Christmas bread, etc. It is possible that at least some of these took different shapes, though many may only have been finer versions of the same round loaf seen in multiple medieval images. As a practical matter, however, that spherical or (for larger breads) hemispherical shape remained the norm for centuries.
By the seventeenth century, French bread already existed in a number of forms and these became almost uncountable in the eighteenth century. This has remained the case ever since and even ten years ago the baguette was only one of a series of breads to be found in any Parisian bakery. At the least, others would have included the ficelle, the batard, and the baguette-like pain viennois (not to be confused with its nineteenth century ancestor, which could take many shapes). Crowns, "ears of wheat", polka, split loaves were only some of the other possibilities.
Now however a strange thing has happened: the baguette has seemingly multiplied, appearing in innumerable different varieties - traditional, country, banette, spelt, whole wheat, sesame, poppy, ciabatta - and displaced numerous other varieties in the process. It is not in the least unusual in Paris today to go into a bakery and see nothing but baguettes on one row, albeit baguettes in slightly different forms or made with different grains. Meanwhile, it has become very hard to find ficelles or batards, for instance.
Ironically, this development comes in parallel with a revolt against the baguette led by artisan bakers such as Poilane (whose shop carries none). And if the baguette has one chief rival in this process it is the boule, that is the round, sour-dough leavened country bread which was once only one of many options, but now is sometimes pointed out as the "true" French bread, the root one should return to in fighting what some have called the "baguettocentrisme" of French bread baking. Yet ironically it is the very artisanal spirit that that revolt inspired that has also led to rethinking the once uniform baguette so that it is now itself a rival to the boule which had been the standard-bearer of artisanal bread.
Realistically, it is unlikely that France will ever return to the medieval state of affairs, with one form of bread massively dominant over all others. But it is striking that in 2013 the baguette, in its ever-expanding new incarnations, has already displaced loaves with which it once shared bakers' shelves, creating a tendency towards uniformity that for centuries had become unthinkable in French bread-baking.
FOR FURTHER READING:
Pierre Jean-Baptiste Legrand d'Aussy, Histoire De La Vie Privée Des Français: Depuis l'origine ..., Volume 1, Issue 1