Fossil Evidence
The Cambrian fossil record, above all the various lagerstätten such as the Burgess Shale, Sirius Passet, Chengjiang and Orsten faunas, has yielded a very rich record of well-preserved arthropods, including the well-known trilobites.
Many Cambrian arthropods, including the trilobites themselves, possess a single pair of slender antennae, which have been equated with either the first or second antennae of the crustaceans; and either the chelicerae of the missing appendages of the supposedly reduced deuterocerebrum in chelicerates. However, another group of arthropods, the so-called "great appendage" arthropods, including Yohoia, Leanchoilia and Alalcomenaeus, do not possess simple antennae, but rather have a robust, branched structure, which was called the "great appendage" by Harry B. Whittington in his restudy of these taxa. Yet another group of arthropods may possess two differentiated head appendages, of which the most important and controversial is the Chengjiang form Fuxianhuia. Fuxianhuia was claimed to possess a pair of short antennae anterior, followed by a robust pair of "sub-chelate" appendages. However, this assessment has been both disputed by Waloszek and colleagues, who consider that the sub-chelate appendages are in fact gut diverticulae; and supported by Graham Budd. Thus, its nature remains controverted at present. Other taxa have also been claimed to have a somewhat similar anterior appendage arrangement (e.g. Fortiforceps) but, with the exception of the well-preserved Branchiocaris from the Burgess Shale, most of them are highly equivocal.
In almost all Cambrian arthropods, the post-oral limbs show very little differentiation compared to the trunk limbs; the heads posterior to the mouth shows a considerable degree of variability, however, in the number of segments incorporated into the head.
Trilobites, in particular, possess a ventral sclerotised plate in the head called the hypostome. Whether this is homologous to the labrum or not is debated; although Waloszek and others have argued that as the phosphatocopines (upper stem-group crustaceans) seem to possess both, it cannot be.
Read more about this topic: Arthropod Head Problem
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