Siculo-norman knights of the mid-12th century are often depicted with a distinctive enclosed helm. To my understanding the enclosed helm developed near the beginning of the 13th century. Are these depictions artistic liberty or did the italo-normans develop this style of helmet earlier than western europe?
There's no evidence of the Sicilian Normans wearing fully enclosed helmets in the 12th century, but it is likely that some form of fixed visor or oversized nasal was in limited use at that time. The Monreale Capitals, carved some time in the late 12th century, do feature a single figure with a helmet that features a fixed visor that covers the face, although it doesn't cover the ears or connect round behind the neck. It is similar to a number of Spanish manuscript miniatures (example) and church carvings ranging from the start of the 12th century to the early 13th century, and by the 1180s we see it clearly in German artwork. We also have examples from Italy and England by the end of the 12th century, although it's entirely possible that type of helmet existed in those locations well before this; there is a reference to William Marshal having his head stuck in a helmet in the second half of the 12th century that has generally been written off as anachronistic, but which may point to the use of a more enclosed style of helmet prior to artistic depictions of it in English artwork.
The first two decades of the 13th century saw the proto-great helm introduced, with French, German and Italian showing the fixed visor starting to become a more streamlined part of the helmet. The general similarities in design suggest common and mutual influence, which may be indicative of a single style spreading from Spain or Italy or, as often happened, it may have been a more Western style that instead spread back to Italy (Spain, it appears, went almost entirely away from visored helmets during the 13th century).
So with all that said, it's unlikely that the Sicilian Normans were using fully enclosed helmets in the 12th century, although it is possible they were using fixed visited helmets earlier than in France, England or Germany. The fixed visor helmet likely originated in Spain, and does not appear to have had a direct Byzantine or Islamic predecessor, though. The development from the fixed visor helmet into the earliest incarnation of the great helm was seemingly simultaneous in France, Germany and Italy, and quickly became popular in Western Europe.
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