1. Cambrocaris baltica Walossek & Szaniaswki, 1991

Type locality and age: Cambrocaris baltica has been discovered on the breaking plane of a borehole core during investigations of the alum shales in northern Poland. These shales occur approximately 2,000 m below the surface and are dated to the Upper Cambrian. Regrettably the specimen was lost during investigations.

Morphology: C. baltica must have been somewhat longer than 1 mm, but only the anterior region of the slim body, which gently tapers towards the posterior, is known. The preserved part consists of the head lacking extended margins of the head shield (in analogy to extant anostracan Branchiopoda) and a fragment of the trunk. The head is only indistinctly separated from the anterior of the four preserved trunk segments. The well-preserved locomotory antennulae (A1) are composed of elongate podomeres and are followed by a regular series of long, slender and laterally directed postantennular limbs. Remarkably the median setation on all of these limbs is rather ill-developed, not much capable for food manipulation. The mouth seems to be located at the end of a bulged structure, the hypostome, between the insertions of the second pair of post-antennular limbs, the antennae.

Life habits: Considered as a good swimmer with its slender design and the elongated, laterally directed legs (see Walossek & Szaniawski 1991)

Affinities: Besides the plesiomorphically limb-like antennula and the proximal endite on all postantennular limbs, both characteristics of the groundpattern of Crustacea (cf. Walossek & Müller 1990; Walossek 1999), the postantennular limbs have four endopodal podomeres, the distal ones being elongated and setiferous. These limbs were apparently adapted for swimming. Also the antennulae have a similar design distally and must have assisted in locomotion and food intake. Again, the annulated exopods each have setae pointing inwardly, and the proximal annulus is longer that all others, a feature that can not only be seen in C. baltica, but also in exopods of many eucrustaceans. In consequence we interpret C. baltica as a crustacean.

2. Cambropachycope clarksoni Walossek & Müller, 1990

Reconstruction not of the latest stage known.

Type locality and age: From Västergötland, Sweden, uppermost Middle Cambrian.

Morphology: Most likely much larger 1.5 mm long. Head with a small shield lacking marginal extensions (pleurotergites), a huge uniform facet eye, but no hypostome (mouth opens freely on the ventral body), antennulae (1. antennae) and 4-5 pairs of biramous limbs (ground pattern of Euarthropoda. The trunk is made of 4 segments, the anteriormost bearing a huge uniramous, paddle-shaped limb, the second a similar but smaller limb, and the last being a conical element carrying the anus mid-ventrally. One larva known (not yet published) having an eye measuring 50% of the whole body length. The SEM micrograph on the right of the most complete specimen of C. clarksoni, being somewhat laterally compressed which affected also the large, uniform compound eye. Note the huge paddle-shaped first trunk leg.

Life habits: unclear.

Affinities: Identified as a stem crustacean by its possession of proximal endites on all three postantennular head limbs, and the multi-annulate state of the exopod of limb 2 with inwardly oriented setation but lack of all labrophoran features.

Related species: C. clarksoni is closely related to Goticaris longispinosa, sharing the same type of a uniform facetted eye in front of the ill-defined head shield. By contrast, G. longispinosa lacks shield margins completely and its whole body is sack-like and tapers in a long spine. At least four trunk limbs were developed in G. longispinosa, all seemingly elongate, paddle-shaped and uniramous. Several larval stages are known starting with a stage having 4 developed limbs (see above for head larvae; pictures will be added next).

3. Goticaris longispinosa Walossek & Müller, 1990

Goticaris longispinosa , having a sack-shaped head continuing into the similarly sack-shaped trunk that ends in a long spine. We have now several ontogenetic stages of this animal. The series starts with a larva having only the head with four appendages, while the trunk fades out into the long spine (on right). During further development, the head region remains virtually unchanged, while progressively trunk limbs appear on the vetral side of the trunk. These are uniramous and elongatedly paddle-shaped. In the largest specimens at hand these are four in number.

Remarkable is the huge uniform eye in front of the body, shared with Cambropachycope, even to the detail of having a small hook anteroventrally. Only that here the eye does not grow out in vertical aspect but "remains" elongate. A pair of smaller lobes in the "neck" region, i.e. between eye and antennula insertion may represent another sense organ, possibly the protruded median eyes. Another feature shared with Cambropachycope is the lack of a well-developed hypostome, so that the mouth opening lies directy on the ventral surface.

Type locality and age: From Västergötland, Sweden, uppermost Middle Cambrian (zone 1 of former system, Agnostus-psiformis zone).

Life habits: unclear, probably a predator such as the Recent predatory water flea Bythotrephes.

Affinities: Identified as a stem crustacean by its possession of proximal endites on all three postantennular head limbs, and the multi-annulate state of the exopod of limb 2 with inwardly oriented setation but lack of all labrophoran features.

4. Henningsmoenicaris scutula (Walossek & Müller, 1990) new name for Henningsmoenia scutula Walossek & Müller 1990

This species had to be renamed because the Henningsmoenia name was pre-occupied (see Walossek & Müller 1991).

Type locality and age: From Västergötland, Sweden, uppermost Middle Cambrian.

Another representative of the stem lineage of Eucrustacea is Henningsmoenicaris scutula Walossek & Müller, 1990. This is a fancy little creature with a bowl-shaped shield and shallow tergites lacking strong subdivision into middle piece and tergopleurae. As in Martinssonia and the other "stemliners" the proximal endite is a setiferous sclerotized plate or endite at the inner proximal edge of the basipodite (NOT the coxa, which is not developed at this evolutionary level yet).

Life habits: unclear.

Affinities: Identified as a stem crustacean by its possession of proximal endites on all three postantennular head limbs, and the multi-annulate state of the exopod of limb 2 with inwardly oriented setation but lack of all labrophoran features.

Ontogeny: At least 11 consecutive stages, starting with an ovoiv stage with four pairs of appendages and a huge bulged hypostone with a tiny mouth at its back, see images on upper right. The anus is located ventrally of a small caudal spine and subterminally (plesiomorphic condition). Possibly feeding from the beginning – in contrast to Martinssonia. Gradually eyes are developed anterolaterally, which eventually become stalked (see on right side).

Henningsmoenicaris and the other stem taxa will be worked up by Joachim Haug in the next future, who is also investigating the larval stages of all of these taxa in an evolutionary perspective.

5. Martinssonia elongata Müller & Walossek, 1986

Presumed early representative of the Crustacea, i.e., a representative of the stem lineage of the Eucrustacea, also named crown-group of Crustacea. Up to now, five instar stages are known, but even the last stage might not be the adult.

Type locality and age: From Västergötland, Sweden, uppermost Middle Cambrian.

Morphology: Largest stage with 7 trunk metameres, approx. 1.7 mm long. head shield short, no laterally extended margins, anteriorly protruded by the forehead region, compound eyes missing. Segment of maxilla 2 'not yet' fully added to the head, no differentiation of postantennal head limbs, as in the Labrophora (Phosphatocopina + Eucrustacea, but also in contrast to the other putative stem taxa. Trunk with long tail piece with a bifid caudal margin. Anus ventrally, well in front of caudal margin (pre-terminally). One smaller stage with only 5 trunk somites, othewise similarly designed. First stage egg-shaped with a small anterior and posterior spine, antennae and 3 limbs present, as in the ground pattern of Euarthropoda = head larvae, in contrast to the (ortho)nauplius of Eucrustacea = short-head larva). Second stage more spindle-shaped due to enlargement of the spines (more pointed cones). First two stages non-feeding. Mouth and anus appear at stage 3.

Life habits: Possibly a benthic bottom-dweller, stirring up food by its anteriorly held appendages.

Affinities: M. elongata was the first record of a Cambrian representative of the stemline of the Eucrustaca, recognizable in the:

• development of a 'proximal endite' at the inner edge of the limb basis (basipodit, which is an autapomorphy of Crustacea, see Walossek & Müller 1990, Walossek 1993 and following papers)
• position of the exopod setation at the inner edge (= autapomorphy of Crustacea)

Plesiomorphically retained are the:

• limb shape of the antennula as an organ for locomotion and feeding
• a large hypostome (in contrast to its shortening and development of a labrum at its rear as developed in the Labrophora)
• mouth positioned at the rear of the hypostome (in contrast to the mouth in an atrium oris in the Labrophora),
• lack of a special fused sternum (as developed in the Labrophora)
• lack of paragnaths (as developed in the Labrophora)
• lack of bristles on the sternum and all associated structures (as developed in the Labrophora)
• the maxilla-2 segment being +/- free from the head (included apparently later in phylogeny)
• head larva (as developed in the Euarthropoda; in contrast to the nauplius developed in Eucrustacea)

Difficult to interpret: the postantennular limbs +/- homonomous, and, accordingly, no differentiation of a2 and md (as in phosphatocopines), nor mx1 (as in Eucrustacea s. str.), in Henningsmoenicaris the 2nd and 3rd appendages have, at least, exopods different from the posterior limbs, a feature also known from Agnostus.

6. Oelandocaris oelandica Müller, 1983

The original description of Oelandocaris oelandica was based on a single fragmentary specimen from the Isle of Öland, Sweden, dated to the Upper Cambrian. A first survey of five additional specimens preserved with appendages and other ventral details from Västergötland, Sweden (Stein et al. 2005) led to identify O. oelandica as a member of the early phase of crustacean evolution. Reconstruction of O. oelandica and its life attitudes benefited from the application of different 3D modelling techniques.

Though the new specimens vary in size, which correlates with different trunk-segment numbers ranging from 1 to 5, morphological variation is insignificant. The holotype has four trunk segments and has been assigned to a younger instar of the set. We presume that even the largest specimens of O. oelandica did not exceed an estimated length of 1 mm indicating that our material may not contain adults.

Based on appendage morphology, we also have tentatively assigned a 400 µm long specimen to this species, as an early developmental stage. The most conspicuous feature of O. oelandica is its extremely long antennula branching into three long rods with regular spikes along these, most likely having served as the major structure to sweep in food. Another characteristic feature are minute pores on the outer edges of the posterior limbs and on the trunk tergites, possibly having born sensilla originally and serving as water current detectors.

The very insignificant proximal endite occurring only on the third head appendage points to a rather basal position of this species, because all other stem crustaceans known so far, have more of these endites – at least during late developmental stages. In line with this, this animal has still exclusively the hypostome and the mouth at its rear and neither a well-defined sternum, not paragnaths.

These helped, e.g., in identifying the combination of the plesiomorphic feeding and locomotory habit of the antennulae with an enormous modification of these for sweeping in line with the specialisation of the exopods of the next two head limbs and, hence, development of a sweep-net feeding mode as one of the key novelties in the evolution of Crustacea s. l. Such mode of feeding coupled with feeding is still practiced in the naupliar and metanaupliar phase of many extant eucrustaceans, or even adults, and led us to put forward the hypothesis that, different from Arthropoda s. str. and Euarthropoda, crustacean evolution started at small scale, i.e. in the millimetre range.

A larger paper on this interesting animal is almost finished and will, hopefully be submitted soon.