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Evolutionary implications of 'Orsten' fossils

1. Our View of Arthropod Evolution in General

Our view of arthropod evolution is based on three sources of evidence, extant animals and their morphologies, 3D preserved Orsten-type fossils, and flat-preserved Chengjiang- and Burgess fossils, the other big Cambrian fossil sources. Important for arthropods is one feature, the development of segmental legs. Therefore, we will present here at first a view on these:

1.1. Phylogeny Hypothesis

We recognized three major steps/levels in the evolution of arthropods: 1) the softies i.e. non-sclerotized phase of arthropod evolution = Arthropoda s. l. – here we have three taxa with extant representatives, the onychophorans, the tardigrades and the pentastomids, 2) the level of Arthropoda sensu stricto, i.e. those with sclerotized dorsum and facetted eyes (and more) and 3) the crown group, Euarthropoda with taxa having extant representatives, characterized by basipod on their biramous limbs and a head comprising eye segment, antennular segment and three more segments with biramous limbs. This last level might be seen in a more differentiated pattern in the future, when new information on the various Chengjiang, Burgess and Hunsrück fossils becomes available.

1.1.1. Arthropoda s. l. and what is the sister group to arthropods?

1.1.2. Arthropoda s. str.

1.1.3. Euarthropoda

1.2. Our View of Arthropod Limb Evolution

With the discovery of the proximal endite at the inner edge of the crustacean post-antennular limbs – a feature known from the limbs of branchiopod crustaceans already by Calman (1909, his page 51) – Dieter establish in 1990 a terminology and homologisation scheme for the limb parts of crustaceans and arthropods in general. A corresponding, standardised colour scheme he worked out – basipod yellow, endopod green, and exopod blue – was first published in 1993 and used since, expanded to all major taxa (only tracheates remain a problem so far). This scheme and its consequent terminology made clear that a proximal endite, red, is a crustacean autapomorphy, and that coxae as separate limb stem portions below the basipod originate from this endite. Much later it became evident that this coxa (red thus) developed first only on third limb of the head and subsequently on the antenna and the mandible – the Labarophora level of crustacean evolution.

NOTE: Phosphatocopina and entomostracan Eucrustacea do not have coxae on any of their post-mandibular limbs basally! Coxae on the first and second maxillae and on the first 8 thoracopods are autapomorphies of the Malacostraca.

In consequence, neither trilobites nor chelicerates have a coxa, but possess "only" a basipod and the two rami – so having only yellow, green and blue. There are no coxae, and consequently also no coxal glands either. Again, the rami of trilobites and chelicerates are homologous to those of Crustacea – already Cisne 1975 had published on this contrasting Leif Störmers interpretations, which had influenced the view for a long time and are still upheld by some authors.

Subsequently, we have expanded our study also to phylogenetically older taxa and completed the scheme from the Arthropoda sensu strictu to in-group Crustacea. These studies made clear that the original, multi-annulated limb stem (greenish) of Arthropoda s. str. is accompanied by an exopod flap but was bare of setae (Waloszek et al. 2006, PDF on request).

Subsequently the proximal articles fused to form a rigid basipod element that, from then on, bears the two rami, all themselves bearing setae or spines. Yet, even at the euarthropod level, all post-antennular limbs were completely serial, with no specialisation within the set. Such specialisation characterizes in particular the evolutionary lineage of the Crustacea.

2. Our View of Crustacean Evolution

2.1. Crustacean Phylogeny

Our view of crustacean evolution is based on mainly on our experience with extant taxa, especially entomonstracans, 3D preserved Orsten-type fossils – remarkably, Orsten has preserved numerous taxa of this kind –, and the ontogenetic stages of extant and, again, Orsten taxa, which yield an important phylogenetic signal.
Strangely, at present, no fossil from the flat-preserved Chengjiang- and Burgess fossils could be assigned to Crustacea without doubt. The affiliation of Canadaspis, Ercaia and bradoriids can be clearly refuted as being based on misunderstandings.
Likewise can be refuted that ostracodes are the Cambrian evidence of Crustacea because this is simply a misunderstanding of bradoriids and phosphatocopines as ostracodes.

Due to preservation of not only forms which can be regarded as Cambrian members of the modern taxa, Eucrustacea – also named as the crown group –, but also derivatives of earlier phylogenic branchings are represented in the Orsten fossil material. This heldped to identify, as above with the Arthropoda, three steps in the evolution of Crustacea: 1) the stem phase with a few significant novelties = Crustacea s. l., 2) the Labrophora with a large number of autapomorphies indicating that we have only a fragmentary knowledge of the diversity intil this phase, and 3) the crown group, the Eucrustacea, with taxa having extant representatives.

A major problem remains with the uncertatinty of the sistergroup relationship of Crustacea. We will discuss this here in more detail in due course.

2.1.1. Crustacea s. l.

A set of taxa shows a mixture of features shared with Eucrustacea and Labrophora (†Phosphatocopina + Eucrustacea), but others missing and or being features retained from Euarthropoda. From this mixture we culd identify a few characters as autapomorphies of the Crustacea in the wider sense or Crustacea s. l.:

• the exopods of the first two biramous limbs are multiannulated and have inwardly oriented swimming setae;
• at least one of the more anterior limbs has a lobate, setae bearing protrusion or endite located below the inner edge of the basipodite = 'proximal endite' or PE.

These characters are indicative of significant changes in the locomotory and feeding apparatus of the head. Plesiomorphically all postantennular limbs were serially similar in shape and function, with the antenna being the main organ for food intake towards the back and into the backward pointing mouth at the rear of the hypostome (Waloszek et al 2005). The exopods are sweep-net devices and the proximal endite aids in a separate food transport de-coupled from the locomotory action of the main limb.

Taxa considered as early derivatives of the Crustacea, likewise as derivatives of the stem-lineage of Labrophora (or Eucrustacea) are: Cambrocaris baltica, Cambropachycope clarksoni, Goticaris longispinosa, Henningsmoenicaris scutula, Martinssonia elongata and Oelandocaris oelandica.

These days, a further form, named Tanazios, has been described as a putative stem crustacean from the Silurian of Herefordshire, UK (Siveter et al. 2007), but it is remains unclear if all criteria are really fulfilled to confirm this statement. Neither the proximal endite nor the specific exopods of the second and third limbs could be demonstrated in this fossil.

Even more doubtful are the assignments of Chengjiang fossils from the Lower Cambrian, Ercaia and bradoriids to the Crustacea s. l. It is possible that, at least, Ercaia was not even a euarthropod.

2.1.2. Labrophora

The Cambrian Phosphatocopina, share much of the crown-group characters, but lack two features – the orthonauplius and the special maxillula -, are considered to be the sister group to the Eucrustacea, named Labrophora by Siveter et al. (2003, see also Maas et al. 2003), which puts emphasis on the significant new feature, the labrum. This is a fleshy outgrowth at the rear of the hypostome that was possibly already developed in the ground pattern of Arthropoda s. str. (cf. Waloszek et al. 2005). There are several other important features shared by these two, all associated with the cephalic feeding and locomotory apparatus, but missing the the stem taxa.

Up to now we identified these features as autapomorphies of Labrophora:

• labrum as a fleshy outgrowth at the rear of the hypostome, most likely originating from the distal part of the mouth membrane;
• proximal endites enlarged into coxae on the second and third head limbs, now called Antenna A2 and mandible Md; these coxae are drawn out mediall to form anditiitic protrusion with spines or a gnathobase for feeding prurposes; originally both antennae and mandibles are virtually identical in shape, so labrophorans could also be named DI-MANDIBULATA;
  AND NOTE: a mandible was NOT present in the beginning of the evolution of Crustacea, so any names reffering to a mandible, such as Mandibulata, are misleading or wrong;
• sternites of third and fourth cephalic segments fused to a sclerotic plat, the sternum;
  Note: neither the antennal sternite nor that of the fourth biramous head limb – not the maxilla originally but of trunk-limb shape – were involved in the formation of the sternum! This seems to be in contrast to, at least, insects;
• sternitic part of sternum belonging to mandibular segment with a pair of elevations = paragnaths;
  Note: this has been published by Walossek/Waloszek in several papers since 1990, also the ontogeny described for Rehbachiella in 1993 (figure 25), but ignored by Wolff & Scholtz (2006) who came to the same conclusion. They apparently wnated to make a case that Stein et al. 2005, just citing the older refernces, made a mistake in omitting the myriapods and insects from consideration, but this wasn't their intention. In fact, if these taxa have – and Wolf & Scholtz gave good evidence for that! – also paragnaths, these taxa not only would just me labrophorans but this would also be further evidence for the monophyly of the Tracheata/Atelocerata, nothing more but also nothing less............
• fine subsetules on all limbs associated with feeding;
• fine hairs on enditic surfaces, at the sides of the labrum, and on the sternum incl. the paragnaths;

and possibly:

• also a telson and furcal rami, but this has to be investigated further, and
• by having a head tagma made of the eye segment and four appendage-bearing segments (ples.: three post-antennular lead limbs).

Again these features indicate significant changes in the locomotory and feeding system of Crustacea, clearly separating them off from the Euarthropoda level and the Chelicerata. The next step will be to check these features against the tracheate taxa, Chilopoda, Symphyla, Pauropoda, Diplopoda and Hexapoda, and against several fossil taxa that might be related in these surroundings to test the hypotheses of closer alliance of the one or other or sets of them with Crustacea and/or in-group taxa.

2.1.3. Eucrustacea

NOTE:

• A maxillula, as a specialised limb for food intake, did not yet exist at the level of Labrophora
• A maxilla, as a further specialised limb for food intake, did not yet exist at the level of Eucrustacea and Entomostraca. At these levels, these were still normal trunk limbs, though being cephalised! Only Malacostraca had, in their stem species a maxilla, which looked similar to the maxillula.

The new crustacean Yicaris dianensis we recently described from the Lower Cambrian of China is another fossil to test our methodology. Indeed, using our published blocks of autapomorphies for Crustacea, Labrophora, Eucrustacea, Entomostraca and Malacostraca, we have a large number of characters available to compare in a phylogenetic analysis the new form with and to identify its systematic position:

• Yicaris shares the exopods and the possession of proximal endites with Crustacea, so is a crustacean;
• It shares the labrum, coxae on A2 and MD, sternum with paragnaths, the labrum and proximal endites on all posterior limbs with Labrophora, so is at least a labrophoran;
• It shares the
  • maxillula with four endites medially (PE + 3 on basipod), the
  • enlarged, fleshy basipod of postmaxillulary limbs having
  • 6 to 8 setiferous endites along its inner edge and the
  • endites having three sets of setae (anterior setae, medial spines and posterior crescent of setulate setae)
    
    with Entomostraca, but lacks all malacostracan features, so is at least an entomostracan;
• It shares the plesimorphically flat sternal area between the trunk limbs, as in cephalocarids, so is not a branchiopod, i.e. is lacking the median sternal filter food groove.
• It lacks the characteristic tagmosis of Maxillopoda, i.e. 7 thoracomeres and 4 abdominal segments plus a telson, so is not a maxillopod.

That's all for the moment being, a better resolution can't be offereed so far but it is important to see that the method of comparing these features works fine and to a high degree of resolution. Yicaris is still immature in its largest stage, estimated to be about 3 mm by then, but we still have to work up all details stage by stage.

Two more features of Yicaris dianensis are noteworthy:

• the presence of three flap-like epipodites (more on this feature following the link) on the maxillae and trunk limbs – maxillulae have only two setae there; epipodites have to be discussed in more detail here soon because many authors in the field have regrettably mismatched the structure of function of these organs, limb parts ro whole limbs, so that much confusion exists about gills and epipodites. Even worse, people that made the statement that insect wings originated from epipodites. Hence, our restricted use must therefore be explained. In any way, it is clear that epipodites arise/arose from fine setae, which blow up, become flaps and eventually only the tip of the seta the remains in one of the edges of the flap. We know that Boxshall (2007) has published something against this view, but apart from offensive, rude and uncolleaguial comments about our scientific qualification (which we regret and regard as completely off-way in a scientific paper and refute here – but a problem with this person that has a 20 years tradition), there was nothing substantial that would necessitate to change our view.
  
  
• the presence of 6 endopodal podomeres. This is one more than expected because Malacostraca have 5 in the ground pattern, cephalocarids 5 or 6. All other entomostracans have smaller numbers, i.e. 4 maximally. Phosphatocopines have at most 3, and also the "stem staxa" have five or fewer podomeres. Again this is in accord with the above list.

More soon........

Also to be added step by step in due course:

The development of the locomotory and feeding apparatus as a key event in arthropod evolution

Details of limb morphologies

The hypostome and labrum

Phylogeny within Crustacea

Notes on some key taxa such as Cephalocarida –––––––––––––––––––––––>

Are Crustacea really "paraphyletic"? Where to put the myriapods and hexapods in?

2.2. Eucrustacean in-group relationships

3. Chelicerate Phylogeny

The lineage toward Chelicerata s. str.

Pycnogonida and Pantopoda, fossil record and notes on their special morphology

Euchelicerata and the phylogeny and evolution within the group

4. Metazoan Phylogeny

The autapomorphies of the Metazoa

The bilaterian stem species

Gastroneuralians versus Radialia, why not "protostomes and deuterostomes"

Notes on the evolution of body cavities and coeloms