Extract of Aspects from my Paper Held at the Symposium on Arthropoda Phylogeny in London, 1996

2) Concerning head segmentation, it becomes clear from the various Chengjiang, Burgess, and Orsten fossils that the ground pattern of Arthropoda cannot include more than at most the antennular (a1) and three limb-bearing somites. A head constructed with four limb-bearing somites has already been challenged by Cisne (1974), but this fact has never been taken seriously into account, though repeatedly confirmed for more taxa thereafter - e.g. also for the Aglaspida, which cannot be Chelicerata accordingly. Nonetheless Lauterbach and others, denying the published evidence have continued to claim an original arthropod and euarthropod head with a larger set of somites, a view upheld in most recent textbooks too.

Recent studies now demonstrated the a1 nature of all anterior appendages, so anything from the antennae of Fuxianhuia, the chelicerae of Chelicerata to the "great appendages" of anomalocarids and allied (see Chen et al. 2004), the locomotory and feeding antennulae of crustaceans (ground pattern) or the feelers of trilobites.

4) The situation of the incorporation of a fourth limb-bearing somite into the head seems to have taken place in the stem line to the Eucrustacea – we know now that is was even before the Labrophora, see Maas et al. 2003). Still within Eucrustacea the limb of this somite, the so-called second maxilla, is well separated ontogenetically in several taxa, and is of the shape of a trunk limb in Cephalocarida. This situation is also observable in Upper Cambrian crustaceans such as Bredocaris, Dala, and Rehbachiella (not in Skara, which in this character shares the specific modification of maxillae and first trunk leg to a maxilliped with mystacocarids and copepods = autapomorphy of the copepod lineage). Also the Lower Devonian Lepidocaris rhyniensis Scourfield, 1926 may exhibit this. Even more, the problems of ostracode limb terminology might arise from this peculiar phylogenetic problem.

The ground pattern of Crustacea reveals this situation too, as do the Chelicerata (Fig. 3A, D, E for Naraoia and Limulus). Within the stem line of Eucrustacea, i.e. as an autapomorphy of the Crustacea, a separate, setiferous proximal endite is formed at the proximal inner edge of the basipod (Fig. 3B, 4C), together with a change in the exopod and the first antenna (a1), features all associated with a change in locomotory and feeding attitudes. We are currently studying Oelandocaris from the Orsten material, and, strikingly, this form has only a single proximall endite on the third limb, so-called mandible.

A coxa is present in the mandible of phosphatocopines and in the antenna (a2) and mandible of the groundpattern of Eucrustacea, but basically not in any subsequent limb. Subsequent studies could unravel that the coxa originates clearly from the proximal endite (see images of Phosphatocopina). This co-occurrence of a coxa in Phosphatoocopina and Eucrustacea is one of the various autapomorphies of their common stem species = characterizing the monophylum Labrophora.

With this the specific fate of the proximal endites on each limb and within each crustacean line is of interest for the phylogenetic approach. This hypothesis implies that one cannot reconstruct a general ground pattern for the crustacean limb, but only for each limb separately, but that one can recognize the different limb portions, i.e. the arthrodial membrane, the basipod either with a proximal endite or coxal element, and the two rami endopod and exopod which have a different phylogeny and thus are not equivalents (see above for Fuxianhuia).

• left side: entomostracans showing the proximal endite and the enlargment of the basipod with lobate enditic protrusions medially; endopod was 5-segmented originally, but is less segmented in other ingroups (4 and less in all maxillopods and branchiopods).
• right side: biramous limb of a syncarid to show complee organisation, with a small coxa and basipod, 5-segmented endotpod and exopods; tehre are also epipods laterally at the connection of basipod and coxa; the right pictures shows a uniramous (exopod-less) stenopodial leg of a crab; malacostracan trunk limbs have a coxa and basopod, but both are short, while the endopod is 5-segmented and long.

The limbless 15th somite, present in Phyllocarida (Leptostraca) has vanished in its sister group, the Eumalacostraca (autapomorphy). It may have been added to either pleon segment 6 or the telson, which by this would no longer be the equivalent of the true telsons found in Phyllocarida and the ground pattern of Entomostraca.

The ground pattern of the Entomostraca comprises also a trunk made of 14/15 somites, but here, as it seems to me, the limbless somite has undergone multiplication to the formation of a purely limb-less abdomen. Cephalocarida have 10 (not 11) abdominal segments and 9 (not 8) limb-bearing thoracomeres. Branchiopoda have a varying number of apodous abdominal somites ranging from nothing visible (in taxa with polysegmentation such as Spinicaudata or reduction such as Cladocera) to 6 (Anostraca) and a variable number in Calmanostraca (Notostraca and Kazacharthra). Number of thoracomeres varies from 11 to 13 in Kazacharthra, Laevicaudata and Anostraca up to many in notostracans and few in Cladocera.

Maxillopoda have 4 true abdominal somites in their ground pattern. Within the different lines these number may be lowered, and, most likely at least twice, the last thoracomere bearing the genitalia as modified 7th pair of thoracopods, has shifted to the abdomen in the course of the formation of a longer tail. Even more, the last abdominal somite (no. 4) may have fused with the telson, while the groundpattern situation can be seen only in fossil taxa such as Skara and Dala.