Non-Crustacean Arthropods (to be completed)

Upper Middle Cambrian Agnostus pisiformis (L.) (formerly zone 1 of the Swedish Upper Camrian series)

SEM of an early larval specimen of Agnostus pisiformis viewed from the anterior. Shield party gaping. Antennules and most other limbs broken off distally permitting to view on the huge balloon-shaped hypostome (not a crustacaean labrum!). One of the left trunk limbs fully preserved.

Reconstruction of the head region of Agnostus pisiformis
Note the possibly raptorial antennae and 2nd and third appendages with swimming exopod. Last, fourth head limb designed as the trunk limbs having a short exopod but well-developed endopod with peculiar, banana-like outgrowths laterally. No proximal endite as in stemgroup crustaceans, but position of 2nd limb at least paroral and posteromedially directed as in crustaceans.


Reconstruction of larval stages and morphology. Stages include the pattern of pores on the tergum. Cross section shows the orientation of limbs when the body is artificially streched. Lower left: possible orientation during swimming.

On lower right: Agnostus in the enrolled position.

Traditionally the agnostids are combined with the eodiscids, another group of minute arthropods of trilobite design and together placed within the Trilobita, considered as diminished forms. Yet no limbs have been found for eodiscids to confirm this. At least the ventral morphology of Agnostus differs considerably from what is known from trilobites. In some ways, Agnostus seems to be closer to Crustacea than to Trilobita, particularly with regard to its specialized anterior head limbs and its round setae, while those of trilobites are lammelar (for this character see e.g., Hou & Bergström 1997).

Morphology:

• Head and tail were covered by large shields. Trunk tergum composed of two free tergites and a large so-called pygidium. Median, axial region much humped. No eyes dorsally (name-giving features).
  Hypostome bulging, located in the center of the head shield, carrying the mouth at ist rear, soft except of a system of sclerotic bridges. A pair of soft humps anteriorly of the hypostome lacking a counterpart in trilobite morphology (this region is covered by the large hypostomal plate in these), possibly respresenting light-sensitive structures. Special articulations between tergites and trunk shield hindered a wide gaping of the shield and prohibited streching, as reconstructed in papers before the knowledge of the soft parts. Anus opening at about two thirds of the trunk shield. Dorsal tergum with many delicate pores (1-3 µm), also on inner edge of shields (doublure), distinctive set on a node of the head shield.
• Appendages – All appendages and ventral structures had to fit into the nut-like shell during enrollment (see picture 3), and the antennae (antennulae, a1) were s-shaped and crossed in front of the large globular hypostome. The second appendages even had lost their endopod, while that of the third limbs was smaller than in all posterior limbs. Endopods of last head limb and all trunk limbs bearing peculiar 'banana-shaped' outgrowths at their outer edges (one for each article), which may have had respiratory or osmoregulatory function. Outer edge of basis and exopod with soft, finely wrinkled feathered setae.
  Trunk bearing 5 pairs of limbs which are successively less developed. Large abaxially oriented joints and the small space for musculature between ventral surface and dorsal cuticle suggest very limited movability of the trunk limbs.

Life habits: Most likely drifting around while rowing with the exopods of appendages 2 and 3. Complete opening of the shell was blocked by a special mode of articulation between the tergites on the trunk. Agnostus, as the other agnostids, was able to enroll, i.e. to close its shell completely, most likely to rest or withstand unfavourable conditions as is known from ostracodes.

Upper Middle Cambrian Cambropycnogon klausmuelleri

SEM of one of the larger specimens and reconstruction

Morphology: Approx. 210-270 µm long, apparently immature form with egg-shaped body terminating in a pair of long bristulous outgrowths. Two stages known, differening mainly in size (210, 250-270). Front with a pore-like subterminal mouth opening and small hook-like structures laterally of it. Ventrolaterally a pair of large cheliphores is developed with immobile and mobile 'finger'. Two more pairs developed, consiting of a gnathobasic portion (basipod) and a finely annulated ramus (distally broken off). No anus present. Body - actually the head - with a weakly defined heda shield ist margin indicated by nomerous fine furrows starting at the chliphores and running around the head dorsal to the limbs.

Shape very similar to the earliest instars, protonymphs, of extant Pantopoda. The first 2 stages of Pycnogonum litorale also grow only in size. Life style and affinities of the fossil uncertain, not least since there are no larger specimens providing more informaiton on the fate of structures, and there are no other undisputed chelicerates in the Cambrian (the Middle Cambrian Sanctacaris has neither chelicerae/ cheliphores, nor other clear chelicerata characters).

The main outcome from the phylogenetic analysis is that the Pycnogonida are proposed as the sister-group of Euchelicerata embraced within the Chelicerata. Within the Pycnogonida, the new form is suggested as the sister-group to all remaining pycnogonids, including the Devonian (see e.g., Stürmer et al. 1980) and extant taxa.

Upper Middle Cambrian Lobopodian

This first lobopodian in 3D preservation, the youngest of its kind and the smallest, will be published in the Chinese Science Bulletin soon. The most remarkable aspect of this beautifully preserved form is the single ventral gonopore, which proovides evidence that Arthropoda s. l. started already out having this feature. Other striking aspects concern the replication of the cellular pattern on the whole body surface, telescoping spines and other spines that are build of several cells, and limbs stretched out laterally to be able to crawl. This was supported by the different spines pointing ventrolaterally. More soon.

Tardigrade from the Middle Cambrian of Siberia

SEM of stemline tardigrade in lateral view (this will become the holotype). SEM of same specimen, viewed from ventral side. Note the fine pustules on the surface, representing enlarged pillars. Reconstruction. Note the different claws, anterior one having three hooks.

From a few hundred grams of "Orsten"-type rock of a locality in Siberia, Russia (Kuonamka formation, Lena area) Middle Cambrian in age (approx. 530 Mio years), four 250-350 µm long fossils have been found considered as tardigrades (only fossil record before is from Cretaceous amber). These creatures are not only similar in size and shape particularly to marine arthotardigrades, but they share fascinating details with Tardigrada, such as in the presence of frontal sensorial organs - cephalic papillae and cirri -, 6 folds in the mouth region, columnar structures in the cuticle (pillars), and details of the limbs with their claws (lunular area, peduncle, hooked claw and simple claw). There are only 3 pairs of stumpy legs in alle specimens which belong to two different instars, but furrows at the rear end of one of the specimens may indicate the development of a 4th limb as known from recent tardigrades. All legs possess a pair of claws (heteronych = different claws).

The simple claw of the pair resembles the claws of Onychophora and Cambrian 'lobopodians' which are currently discussed as their fossil precursors (e.g. Microdictyon, Onychodictyon, Hallucigenia, Aysheaia, Xenusion, Paucipoda, Cardiodictyon, Luolishania). However, a differentiated analysis is needed since those available (Budd 93, 96, Mongo-Najera 95, or Hou & Bergström 95, Ramsköld, as examples) refer to outdated information, exclude taxa from analysis (e.g., tardigrades, arthropods), or make subjective descisions to build up trends toward the onychophorans. Yet there are still too many uncertainties, such as the repeated head- and tail reversals by Ramsköld and recently H&B, or conflicting characters (list of references can be requested from me). The find of a Cambrian tardigrade closes a further gap in the knowledge of animales diverged from the early stem line of Arthropoda. 'Regrettably' the fossils are already as specialized and diminished as today's tardigrades, so leaving us again with speculations on their larger and longer ancestors.

A detailed investigation and formal description is under way, in co-operation with Reinhardt M. Kristensen from Copenhagen.

Furongian = Late Cambrian Stem-lineage Representatives of the Pentastomida

See the images on the right:

SEM of Boeckelericambria pelturae, representing a round-headed larva having 'palps', a mouth and well-separated, three-segmented limbs.

Ventral view of same specimen

Head region of the same specimen viewed from lateroventrally. Note the well-separated limbs and the wide ventral inter-limb area missing in the hammer-headed types of larvae. Margin of mouth (on left) lip-like enhanced and finely wrinkled. Terminal setae of trunk-limb vestigia broken off.

Hammer-headed larva Heymonsicambria kinnekullensis showing the well-segmented limbs but lacking mouth opening of this larval type (trunk rear broken off; arrows point to frontal papillae, missing mouth and limb articles).

Hammer-headed larva Heymonsicambria repetskii, lacking the vestigial trunk limbs, Note fusion of the limbs which are held far laterally.

Apparently larval fossils, with lengths ranging from 200-700 µm. Morphology: Body comprising a prominent head and more slender trunk. Head with or without a mouth, with or without frontal palps, in the latter case with two pairs of frontal sensory papillae, and with two pairs of stumpy, tripartite limbs for attachment. Trunk made of four portions, segments 2 and 3 bearing short, vestigial limbs with a distal tuft of setae.

We have assigned these forms to the stem line of Pentastomida because they share various details with these, such as in particular: the presence of either palps or frontal papillae on the forehead and caudal sensory papillae, only two pairs of tripartite limbs for attachment, a trunk made of three segments and a caudal end, pores on articles 1 (slit) and 2 (rounded) of the attachment limbs, and complete segmentation from the beginning.

Plesiomorphic is, e.g., the elongation of the last three trunk portions, shape of limbs, and lack of boring structures of the hatching larva present in all Recent forms. These characters clearly mark the stemline position of the fossils. Again, Recent Porocephalida and Cephalobaenida can be clearly separated by the different fate of the proximal article of their head limbs: in the P., the median side dissolves, which produces the so-called fulcrum (for muscle attachment), while in C. this occurs lateral, leading to an U-shaped article underlying the hook plate.

Detailed view of the limbs of Boeckelericambria pelturae to show the slit pore of the proximal artice and the round pore of the second article. Distal article small and finger-like (functioning as a subchela).

Same view of a limb of the Recent larval cephalobaenid pentastomid Reighardia sternae having also both a slit pore and a round pore, indicating the same subdivision of the limbs in three articles as in the fossils. Different is only the fusion of second and third articles and the occurrence of a secondary hook on the second article (= hook plate) in this larval stage (other 'secondary hooks' are either a split off of the distal end of the hook or devlope from the membrane between first and second article).

Life habits: Material comprising various taxa showing variability in design and orientation of the limbs, in the trunk segmentation and in the development of limb vestigia, ranging from two pairs to none. This plasticity is interpreted as variability in host attachments. The hosts are still unknown, but the variability of attachment strategies suggests a diverse host group, possibly the early chordates or vertebrates. At least the conodonts as tooth structures of early chordates are not only very abundant in the Orsten material, but also similarly variable and also occurring worldwide.

Taxa: hammer-headed larvae: Heymonsicambria kinnekullensis Walossek & Müller, 1994; H. scandica Walossek & Müller, 1994; Heymonsicambria repetskii Walossek & Müller, 1994; H. taylori Walossek, Repetski & Müller, 1994, described from Newfoundland (Upper Cambrian/Ordovician boundary beds). Round-headed larvae: Boeckelericambria pelturae.

A detailed investigation of new material has been finished by Christopher Castellani, and the ms will be submitted in due course.