A remarkable arthropod fauna from the Upper Cambrian "Orsten" of ...

Transactions of the Royal Society of Edinburgh, 76, 161-172, 1985

A remarkable arthropod fauna from the Upper Cambrian "Orsten" of Sweden 5

Klaus J. Muller and Dieter Walossek ABSTRACT: Extensive preparation since 1975 has yielded thousands of specimens of various minute arthropods, mainly phosphatocopid ostracods and other crustaceans. The whole exoskeleton is exceedingly well preserved in three dimensions, showing even delicate mOlphological details. The material permits not only the description of the morphology and systematic status of the animals but also interpretation, with a high degree of confidence, of the function, life habit, and ontogeny of these early arthropods. KEY WORDS:

Crustacea, ecology, environment, function, morphology, phosphatisation.

The study of phosphatic fossils as conodonts and phosphatocopine ostracods has recently received increasing attention. Secondary phosphatisation discovered in the course of such studies caused the preservation even of small-sized softbodied arthropods.

1. Environment and fauna 1.1. Lithology of the "Orsten" Orsten is an anthraconitic limestone, forming either concretions of about 10 cm to 2 m in diameter or large, flat lenses within the alum shale which may appear as beds in small outcrops. In general, it is rather carbonaceous and often petroliferous. When dissolving a petroliferous sample in acid, oil usually concentrates on top of the liquid. Less carbonaceous beds are beige to light grey. Sandy components are lacking in the alum shale and the Orsten. The wide distribution of pyrite in many portions indicates the absence of oxygen. Because of considerable recrystallisation, the facies textures are generally not preserved. However, there are abundant shell remains and widespread, well-preserved, phosphatic fossils. The sediment was deposited under stillwater conditions. The occurrence of higher energy sediments composed of fossil "hash" is rather limited. They have not yielded specimens with preserved soft integument.

1.2. Occurrence and localities Orsten seems to be restricted to the shallower parts of the former alum shale sea. The fauna within the anthraconitic limestone may be identical to that of the adjacent shale but, in the latter, the fossils are flattened and decalcified. Arthropods with preserved soft integument are widespread in Southern Sweden (Oland, Falbygden, Kinnekulle, Hunneberg, Skane). Since the first discovery, in 1975, of specimens with soft integument, a large quantity of material has been collected from at least five trilobite zones of 22 localities. Similar material has been discovered from a borehole in Northern Poland (H. Szaniawski, pers. comm. May 1983), and in Pleistocene drift boulders from Northern Germany and Poland (Muller 1979b). Therefore, additional field work is expected to extend the recorded area of this kind of preservation.

1.3. Associated fossils Trilobite remains, mostly exuviae, are predominant, sometimes even being rock-forming. They may occur in thin layers or in beds. Based on guide trilobites, a stratigraphic sequence of six stages with more than 30 zones was established (Henningsmoen 1957; Westergaard 1922, 1947). Conodonts are also common. In some cases, they could be isolated even in their original arrangement. Compared with trilobites and conodonts, other taxa are much rarer. Chancelloriida, sponge spicules as well as horny and calcareous brachiopods are fairly widespread. The brachiopod Orusia lenticularis (Wahlenberg, 1821) occurs only in a single zone as a rock-forming element. Fossils with soft integument, however, have not been discovered from this bed. Molluscs seem to be totally absent. There are also a number of enigmatic microfossils and tubular hyolithid-like cones. Some are very rare and occur only in a few samples. Part of the collection was searched without success for acritarchs.

1.4. Composition and abundance of the non-trilobite Orsten arthropod fauna with preserved soft parts The most common arthropods are the phosphatocopid ostracods. Up to now, five genera with more than 15 species have been identified. Many of them show soft integument as well as different ontogenetic stages. Extensive searching has yielded thousands of specimens from various samples and localities which are now available for examination. Their descriptions are being prepared. Among them, Hesslandona unisulcata Muller, 1982, is predominant. Less abundant, but still in fair quantities, are Skara Muller, 1983, with more than 60 specimens each of the two recognised species (Muller & Walossek 1985), and Data peilertae Muller, 1983, also with more than 60 specimens. About 30 specimens of Bredocaris admirabilis Muller, 1983, including larval stages, have been recovered. Rehbachiella kinnekullensis Muller, 1983, is also fairly abundant (Muller 1983). A number of arthropod species, however, are represented only by one or a few individuals. For example, we have 17 specimens of Martinssonia elongata nom. nud., a new probable "pre-crustacean", belonging to five growth stages (Muller & Walossek in press) and as yet we have only a

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single individual of Oe/alldocaris degerhamnellsis Muller, 1983. The various arthropod species are quite distinct from one another morphologically. In most cases. the generic units are monospecific. Many of them are unique in their morphology, and may even belong to hitherto undescribed higher taxa. The exception is Skara Muller, 1983, with two species, S. anu/ata Muller, 1983, and Skara n.sp. occurring together in the same samples. They differ mainly in size, with S. alllliata being twice as long as the new species. Differences in construction, e.g. of the mandibular gnathobase (Fig. Ie, f), suggest these species had distinct ecological demands (Muller & Walossek 1985). On the other hand, the range of infraspecific vanatlOn seems to be quite limited. Based on more than 100 individuals of Skara, only the degree of bristliness and the number of certain setae vary slightly. Other characters have revealed no significant variability.

2. Preservation As with silicification, there are considerable differences in phosphatisation between the various systematic groups. Chitinous or similar matter seems to be predominant. Accordingly, the phosphatised fossils recovered most probably represent only a small part of the original alum-shale sea fauna. The study of them therefore gives only limited evidence for the whole faunal community. Trilobita are most abundant in the rock. However, though certain sites yielded partly phosphatised exoskeletons. neither appendages nor larvae of trilobites have been found in this type of preservation with the exception of several agnostid larvae in which soft integument can be found. Phosphatisation occurs either by coating or by replacement of the body wall. Both types can be found in the same sample or even in the same specimen. Many specimens remained entirely three-dimensional and inflated. Further-

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more, the preservation is not restricted to strongly sclerotised portions, such as the head shield or trunk segments. It also permits recognition of even the finest details, such as hairs, bristles and membranous areas (see Section 4), which facilitates a detailed examination. The wrinkled preservation of some specimens can be explained neither by compression during diagenesis nor by the loss of turgor pressure prior to entombment (Fig. 1d). On certain specimens the carapace is wrinkled whereas other parts are well preserved. In this case, the individual possibly died at the time of moulting or immediately afterwards (Fig.1b). In general, interior organs or tissues have not been fossilised, and the insides of the specimens are empty. It is possible that a former calcitic filling has been etched away during the preparation process (Fig. la). But in some cases, the whole inner space was filled secondarily with a phosphatic granular matter which has shrunk due to recrystallisation leaving a small gap between cuticle and internal cast. Thin, columnar fillings which have been observed, mainly in the trunk region of Skara, may represent the remains of the gut. But because of its coarse phosphatisation, further details have not been preserved. Among the arthropods with preserved soft integument, the phosphatocopid ostracods are quite abundant, particularly at certain localities. Because of their smooth and shiny carapaces, they can be easily recognised with the naked eye on a split surface even in the field. Nevertheless, it may take several days of fieldwork to discover a productive locality. Phosphatisation can be regarded (Muller & Walossek 1985), as one important factor that caused an upper size limit of preserved specimens. Complete individuals and fragments with preserved soft integument range from about 0.2 to 2 mm. However, isolated appendages of more than 1 mm have been found which must have been derived from individuals much longer than 2 mm.

3. Present state of work Figure 1 SEM-photos of various morphological details. (a) Skara anu/ata MUller. Posterior view of the trunk showing its annular segments (ts); at the top, some of the marginal spines (sp) of the seventh to ninth segments can be seen; the broken telson permits a view into the almost empty interior of the trunk. (ST 4156, UB 760; Gum/Kinnekulle, zone 1). (b) Hesslandortidae sp. View of the anterior portion with preserved carapace (cs) and two appendages (all = antenna, md = mandibula); the carapace is strongly crumpled which might have been caused by fossilisation at or immediately after moulting while the cuticle was still pliable. (ST 954, UB 761; Stenasen/Falbygden, z. 5). (c) Ventral view of an unidentified nauplius with prominent labrum (1), the mouth (m) being on its posterior surface, and three pairs of appendages: antennula (atl), antenna (all), and mandibula (md); most of the short marginal setae (s) on the limbs are preserved; the body terminates in two furcal spines (fsp) ventrally (the dorso-caudal spine is disguised). (ST 2860, UB 762; Gum, z. 1). (d) The same type of larva as in (c). The specimen has a strongly crumpled cuticula; as all appendages are stretched out, it is concluded that a loss of turgor could not have led to the shrivelled surface. (ST2850. UB 763; Gum. z. 1). (e) Skara allu/ala. View of the mandibular coxal endite (md ce); two thick and spine-like masticatory setae (sp) arise from its median surface distally adorned with numerous delicate setulae (stl); the setae may have aided in the forward-directed transport of food particles and stuffed them into the recessed mouth (m) below the labrum (I); at the top right, the antennal coxal endite (all ce) flanks the posterior end of the labrum. (ST 3559, UB 695; Gum, z. 1). (f) Mandibular coxa of Skara n.sp. In contrast to the former species, the coxa (md cox) is elongated and developed as a gnathobase, medially armed with fine spines or denticles (dt) (cs = cephalic shield). (ST 3074, UB 709; Gum, z. 1).

Large collections have been prepared by a considerable amount of processing and sorting. Often the number of rare or exceedingly well-preserved species was increased by the preparation of additional rock from the same concretions and second sortings from the washed residues. This stage of the study is now complete. Apart from the ostracods, more than ten different genera of arthropods have been identified. Description of the collection has begun but careful studies by scanning electron microscope are still necessary before detailed information about the various taxa can be presented. The first monographs, on the new order of Crustacea, Skaracarida, and on Martillssollia elollgata appear in Muller and Walossek (1985, and in press).

4. Recognition of morphological details The extraordinary preservation permits not only the recognition of gross morphology, i.e. carapace, body or limbs, but also identification of even the finest details on the outer surface, such as setae, bristles, muscle scars, pores, and less sclerotised areas like membranes and joints.

4.1. Eye structures and other sensory organs Eye structures are developed in several forms. They range from simple protruberances and naupliar eyes to stalked and even compound ones. Bredocaris admirabilis, Rehbachiella killnekullellsis, Walossekia quillquespinosa Muller, 1983, and Oelandocaris degerhamnensis exhibit eyes with two lobes or

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bulges, differing in size and shape in each species (Muller 1983), The Hesslandonidae have tripartite median eyes (Muller 1979a, 1982b; Fig. 21). Skara allulata and Skara n.sp. were most probably blind, but, in contrast to the other forms, these two species bear a movable, tubular frontal process which is assumed to represent a sensory organ (Fig. 2b). Apart from probably sensory bristles on the limbs of Dala peilertae (see 4.5; Fig. 2c), other sensory organs have not yet been identified.

4.2. Glandular and excretory exits In Skara, the excretory pore at the base of the antenna (,antennal gland'; Fig. 2d) as well as a pore of still unknown function below the maxillular coxa (Fig. 2e) have been identified. The labrum of Vestrogothia spinata bears two circular pores on its posterior surface, close to the mouth (Fig. 2f). Probably they are exits of salivary glands that produced slime to facilitate food intake, comparable to Recent Crustacea (Kastner 1967, p. 931).

4.3. Pliable areas of the cuticle Head shield, labrum, trunk segments, or portions of the limbs, may have been rather sclerotised, but various areas of the cuticula were obviously much softer. These parts can be easily distinguished by their position and appearance. For example, eyes (Fig. 2a), arthrodial membranes and the depression for the maxillipeds on the anterior trunk segments in Skara (Fig. 3a) may be more or less recessed or collapsed. The thoracic region and the origins of the filter appendages in Bredocaris (Fig. 3b) as well as the mouth of Martinssollia (Fig. 3c) are also little sclerotised and delicately folded.

Figure 2 SEM-photos of morphological details. (a) Hesslandona unisulcata Muller. Anterior head portion; the carapace valves (cv) are mostly broken away which permits a view of the forehead (fh) with the median eye (eye), the labrum (I), the short antennulae (atl), and the proximal parts of the antennae (all) (m = mouth, st = sternum). (ST 2906, UB 659; Gum, z. 1). (b) Skara n.sp. Lateral view of the forehead (fh) with a tube-shaped frontal process which may represent a sensory organ (fo = 'frontal organ') (\ = labrum). (ST 4108, UB 716; Gum, z. 1). (c) Dala peilertae Muller. Terminal, paddle-shaped endopodal podomere of a thoracopod; the surface is covered with groups of tiny bristles (br); short, pinnate bristles (ss), probably with sensory function, occur on several parts of the limb; most of the marginal setae (s) arc broken off. (ST 1513, UB 764; StenstorpDala/Falbygden, z. 5d-e). (d) Skara allltlata. View of the anterior head portion with forehead (fh) and labrum (I); the appendages (atl, all, md) and the frontal organ (fo) are broken off; just ventral to the origin of the antenna the exit of the 'antennal gland' (crustacean excretory organ) can be seen (po all); note the shallow depression, a muscle scar (ms), at the posterior third of the labrum. (ST 2853, UB 696; Gum, z. 1). (c) Skara aflu/ata. View of the posterior surface of a maxillula; the most proximal part (sh = shaft) is pliable and strongly folded; the coxa (cox) is divided into a proximal well-sclerotised portion with one median endite (end) and a soft distal one with two endites (on the anterior side, however, the coxa is uniform!); a circular pore (po) of unknown function is positioned on the border between shaft and coxa; muscle scars (ms) can be seen on various parts of the limb (bas = basipod, en = endopod). (ST 3095, UB 719; Gum, z. 1) (f) Vestrogotlzia spinata Muller, 1964. Posterior view of the projecting labrum (I); circular pores (po) on both sides most probably represent exits of salivary glands which produced slime to facilitate the food intake; the rows of bristles (br) on both sides may have guided the food particles into the mouth (m). (ST 950, UB 600: Stcnascn, z. 5).

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4.4. Muscle scars Due to the slight sclerotisation, the muscle scars can be seen as shallow depressions on the outer surface. In Skara, for example, they can be seen on the head shield and the adjacent trunk tergite, close to the lateral attachment points of the trunk segments, as well as on the labrum and on various parts of the limbs (Fig. 2d, e, 3d).

4.5. Hairs or hair-like structures Skara exhibits different types of hairy structures. The long and thick setae of the antennal and mandibular exopodites (major force-generating elements) are suitable for swimming. Those of the paddle-shaped exopodites are somewhat thinner but bear additional rows of subordinate setulae in order to enlarge the surface and to gain more efficiency during the posteriorly-directed power stroke (Fig. 4a). The enditic setae are also adorned with subordinate setulae, and similar thin bristles cover the surface around the mouth. Thin hairs of moderate length are situated on the podomeres of the antennulae. The plate-like postmandibular exopoditcs of Waldoria sp. also bear marginal setae with subordinate setulae but they are thinner than in Skara (Fig. 4c). In Bredocaris, the median surfaces of mandibulae and maxillulae are adorned with numerous setulae (Fig. 3e). In contrast to the other forms, Bredocaris additionally bears rows or clusters of tiny bristles, not only on the lateral surface of the three anterior limbs, but also on the anterior side of the labrum, the abdomen and the furcal rami. No special function of these bristles could be recognised. Dala exhibits numerous densely-spaced bristles on the enditic surfaces of the large thoracopods (Fig. 3f). Small, pinnate hairs are developed on several parts of the same limbs, arising from slight depressions (Fig. 2c, 4f). These may represent sensory bristles and probably served a hydrodynamic function. Dala and both species of Skara have rows of fringes at the posteroventral margins of the trunk segments, but they are differently developed. In Skara, most of the ring-shaped apodous trunk segments bear tiny fringes which may have protected the arthrodial membranes from soil. The seventh to ninth trunk segments each have, in addition, three setulose and caudally-directed spines (Fig. 4d). Their position indicates that they were used to clean the appendages. In Dala, the four apodous abdominal segments have different fringe types: long, distally divided hair-like ones and short bristles (Fig. 4e). The latter comb-like bristles may have mainly protected the pliable arthrodial membranes from soil, while the long fringes increased buoyancy and stabilised the body during swimming. Other Orsten arthropods like Rehbachiella, Bredocaris, and the phosphatocopid ostracods seem to lack comparable structures. This might well be due to their large, protective carapaces.

5. Reconstruction of the biology The reconstruction of the fossils as living animals is based on certain aspects of the lithology, the preservation, the gross morphology and shape of the forms, and morphological details indicating ecological adaptations. Furthermore. the various components of the faunal assemblage can be compared either with one another or with Recent organisms and their life strategies.

5.1. Environment and life habit The associated fauna, particularly the trilobites, indicates a marine environment for the Orsten. The soft bottom

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sediment was rich in organic, detrital matter and poor in oxygen. Additionally, the widespread occurrence of conodont clusters preserved in their original arrangement in the sediment indicates prevailing stillwater conditions (see 1.3). On the other hand, no sessile, burrowing, interstitial, or infaunal organisms within the Orsten arthropod assemblage have been recognised. The prevalence of a mud bottom might explain the rare occurrence of crawling or walking forms. The majority of the Orsten arthropods had well-developed swimming exopodites, at least on antennae and mandibulae, but this does not necessarily imply a pelagic life habit. Based on other details it is more likely that they were epibenthic or benthic. As shown in Figure 5, the Orsten arthropod assemblage comprised a wide range of ecotypes. Among them there were forms without a projecting head shield, with well-developed cephalic swimming appendages and a long annular and apodous trunk, such as Skara and Martinssonia (Fig. 5a, b). Skara had well-developed large annular and setose exopodites on its antennae and mandibulae (Fig. 3a, d). They obviously served for swimming as compared with those of Recent crustaceans (Kastner 1967, pp. 888-9). Crawling aids, such as claws, were not developed. The function of the setose furcal rami was mainly to stabilise the long trunk during swimming. Several features, chiefly the trunk, the absence of eyes, and the fringes of the trunk segments (see 4.5) indicate an epibenthic life habit for Skara (Miiller & Walossek 1985). At first sight, Martinssonia resembles Skara in having a long annular trunk and lacking a distinct head shield. But in contrast to Skara, its lack of a cephalic filter apparatus and the development of strong spinose endites on the anterior cephalic limbs indicate a different feeding habit, probably as a bottom dweller. The mouth is surrounded by a pliable area Figure 3 SEM-photos of morphological details. (a) Skara anulata. Ventral view of the anterior body (head region to upper trunk) with labrum (I) and partly-preserved appendages (all, md, mx1 = maxillula, mx2 = maxilla); the maxi 1liped (mxp) is broken off; anteriorly, the cuticle of the second trunk segment is pliable and deeply recessed (rec) providing space for the limbs when the trunk was flexed ventrally. (ST 4332, UB 765; Gum, z. 1). (b) Bredocaris admirabilis Muller. Lateral view of the posterior part of the thoracic region (Th) with some of the paddle-shaped limbs (app) preserved; while the thorax is finely transversely folded, the adjacent abdomen (Abd) is almost smooth (a thick arrow points anteriorly). (ST 2109, UB 766; Kestad/Kinnekulle, z. 5c-d). (c) Larva of Martinssonia elollgata Muller & Walossek nom. nud. Ventral view of the pliable and slightly protruded mouth opening (m); to the right the sternum (st) with several furrows can be recognised, as well as parts of the anterior appendages (atl, all, md); from the upper and lower margins of the micrograph antennal enditic spines (sp) point towards the mouth. (ST 3221, UB 756; Gum, z. 1). (d) Skara n.sp. Lateral view of an S-curved specimen; the antenna (all) is stretched anteriorly; all other limbs are broken off; several muscle scars (ms) can be seen on the labrum (I), shield (cs) and tergite of the first trunk segment (tsl); pliable arthrodial membranes (am) connect the successive trunk segments. (ST 4318, UB 767; Gum, z. 1). (e) Maxillula of Bredocaris admirabilis. On the protruding enditc the surfaces around the setae (s) are covered with numerous projecting, fine bristles; at the lower right some of the trunk limbs (tl) can be partly seen; a thick arrow points anteriorly (cs = cephalic shield, ex md = mandibular exopodite). (ST 1417, UB 640; Stenstorp-Dala, z. 5d-e). (f) Distorted thorax fragment of Dala peilertae displaying few limb details. However, even the delicate enditic bristles and subordinate sctulae (stl) on the setae are well preserved. (ST 1831, UB 768; Stenstorp-Dala, z. 5d-e).

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and is shifted to a subterminal position at the posterior end of the labrum. This feature, which suggests a sucking function, can be recognised already on the larvae (Fig. 3c). The long caudal segment with its bifurcate end is very similar to the pleotelson of Recent decapod larvae. The way the limbs are attached to the body suggests that MartillSsollia was less a wriggler type such as Skara but may have swum in a manner similar to Rect!nt copepods or shrimps, probably also with sudden ventral strokes of the caudal segment. The endopodites are much longer than the exopodites and terminate in a long soft seta. This may indicate that Martinssonia was able to crawl to some extent (Miiller & Walossek in press). Dala is well known only from its thoracic and abdominal region (Miiller 1983); the shape and size of the head shield are still unknown. The thorax bears faintly-developed sclerites. The eight thoracic filter appendages are very large with numerous setose enditic lobes and leaf-like terminal podomeres on the endo- and exopodites (Fig. 3f, 6f). The abdomen is composed of four apodous, annular segments and a tel son with paddle-shaped furcal rami. These features, together with the fringes and the probably sensory bristles on the limbs (see 4.5), suggest that Dala represents the free swimming type among the Orsten arthropods (Fig. 5c). It is likely to have swum upside down, as to Recent anostracans and several other crustaceans (Kastner 1967). Walossekia and Bredocaris have well-developed univalved head shields, paired eyes and swimming appendages (Miiller 1983). This indicates that they may mainly have been swimmers in the flocculent bottom layer (Fig. 5d, e). The antennal and mandibular expodites of Bredocaris are similar to Skara but slightly smaller than the corresponding endopodites. The maxillular exopodite is reduced to one small tubular segment. The following eight appendages are unjointed with endopodites and exopodites' being flat paddles (Fig. 3b). The development of thin spinules on the inner margins and pliable shafts suggest that these limbs served for both food transport and locomotion (quick paddling, similar to the thoracopods of cope pods or the pleopods of shrimps). A fairly large pair of eye lobes is positioned far frontally below the gaping shield rims. According to these features, Bredocaris might mainly have been swimmers, perhaps somewhat off the bottom. However, the detailed study of this form has not yet been completed (Miiller & Walossek in prep. b). The large thick head shield or carapace of Rehbachiella and the phosphatocopid ostracods mostly encloses the whole body. Again, the locomotory appendages do not extend far beyond the margins (Fig. 5f, g) which suggests that these arthropods lived near the bottom. As the thoracopods from adults of Rehbachiella are obviously filter limbs (Fig. 6c), and structures indicating a walking ability are absent, this species may have lived close to, but not on the bottom. Hesslandonids have a well-developed median eye anterior to the big labrum and rather short antennulae (Miiller 1979a, 1982b; Fig. 2a). The large protopodites of the antennae and mandibulae with their strong gnathobases are suitable for grinding even large food particles (Fig. 6a, b). The postmandibular appendages are developed as filter limbs having hairs and setulae on their inner surfaces (Fig. 4b). The exopodal podomeres are fused to a single plate with marginal setae (Fig. 6a, b). Subordinate setulae on these setae enlarge the surface of the exopodites (Fig. 4c). They are likely to have functioned as vibratory plates in order to produce a respiratory and alimentary water current along the pliable inner lamella. These features give further evidence for a life close to the bottom.

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5.2. Articulation and flexibility The delicate prese rva tio n offers the uniqu e chance to study the original context of body parts. Many of the individuals are fossilised as if fixed in their o riginal life positions (e.g. as if just swimming or filtering ; Fig. 3a, ct , 6a, b) . Even the smallest larvae of the nauplius type have been we ll preserved in the Orsten , with their body inflated and the limbs stretched out (Fig. Ie, d). This facilitates reconstruction of the tagrnosis, articu lation, flexibility, the range of extension of joints, and even the mode of motion with a high degree of confidence.

The majority of the OTsten arthropods are little sclerotised and , most probably due to this , have only poo rly·developed articulations. The body wa s most probably kept in shape by turgor pressure, similar to comparable small-sized Recent crustaceans such as Mystacocarida or Cephalocarida (e.g. Kastner 1967 ; Sanders 1963) . Martillssollia may have used turgor for the expansion and contraction of the exopodal se tae . The lobate podo meres bear one se ta each on their inner surface. Laterally, all podomeres are fused. When the turgor was increased , the softer lobes expanded to a certain extent. This led to a flexure of the whol e exopodite and a fanning of the setae (Fig. 6d). The successive trunk segments are sometimes only faintl y divided , as can be seen e.g. on Bredoearis (Fig. 3b). Often the appendages a ri se from pliable folded shafts (Fig. 2e, 5b) or originate from broad bases (Fig. 4f). However, the limbs of phosphatocopid ostracods have distinct joints at their bases. Skara has well-developed arthrodial membranes between

Figure 4 SEM-photos o f mo rphological de tails. (a) Skara n.sp . Exopodal se tae (s) of the maxilliped with long subordinate setulae (st l) di stall y; at the lowe r right , the margin of the seventh trunk segment , some of the marginal fringes (fr) and o ne of the spines (sp) ,Ire visible. (ST2351, UB732 ; Kestad, z. 2a). (b) Hesslandonidae sp. Isolated postmandibular limb ; the endites (end) of the pro topodite (prot) as well as the endopodal (en) podomeres arc much elongated and bear numerous stiff, short se tae; the exopodite (ex) is flattened and leaf-like having long marginal setae; at the top middle, the area where the limb had inserted can be see n (ins). (ST2201, UB 769; Gum , z. I). (c) Waldoria sp. (Hesslandonidae). High magnification o f the marginal se tae fro m a post mandibular exopodite; the setae are adorned with nume rous del icate setulae . (ST 4267 , UB 770; Gum, z. 1). (d) Skara atllllata. Spin es and fr inges from the ventrocaudal rims of the sevcnth and eighth trunk segments; the cuticle between the spines is somewhat softe r and slightJy collapsed (a thick arrow points ante riorly). (ST 4006. UB 727; Gum . z. 1). (e) Dolo peilerrae. Enlarged view of the rows of frin ges from the ventro·caud al margin of an abdo min al segme nt ; within the outer row lo ng, pinnate brist les alternate with sho rt bristles; the space between them is again occupied by minute bristles; a second row below is composed of sho rt brist les only. (Same specimen as in Fig. 3f.) (f) Isolated thoracopod of Data peilertae. The median surface of the laterally compressed protopodit e is divided into several setose lobate endites (e nd); both rami (e n, ex) terminate in padd le-shaped podomeres with a marginal row o f setae each; note the row of probably sensory bristles (ss) on the surface (ins = inse rtion area). (ST959, UB 639; Stenasen, z. 5). (g) Skora n. sp. Ventra l view of the cephalic filter apparatus; the appendages insert almost in a circle and enclose a small food chamber; the distal parts of the limbs are broken o ff; the endit ic setae o f the post mandibular limbs (mx I, mx2 , mxp) point anter· io rly while the mandibular gnathobases (md) are almost adaxially directed to stuff food particles into the mouth (m); the antennal endites (a ll) flank the labrum (I) and close off the food chamber anteriorly ; at the top left, the forehead (fh ) is visible. (ST2856, UB 647; G um . z. t ).

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the successive trunk segments (Fig. 3d) . As the latter are attached to one an other by lateral dicondylic joints, the tail could only be Hexed widely up and down. The movement was slightly limited in the posterior region by caudally extended ventral margins. Preserved specimens of Skara show seve ral degrees of fle xibility as is shown in Figure 7. Again , in Skara the coxa of the three post mandibular appendages is divided into a proximal, we ll-scle rotised and a dista l, pliable portion on the posterior side. On the othe r hand , the joints between basipod and rami , as well as between the endo podal podomeres , are only faintly developed (the exopod ite is a uniform paddle); the pliable distal-portion of the coxa represe nted the main joint of the whole distal part (Fig. 2e) . The postmaxillular limbs of Bredocaris lack any joints. Both rami are Hat paddles wh ich might have been swung to and fro to a certain extent when moving the basal portions (Fig. 3b).

. 5.3. Appendages and their function Corresponding to their different ecological adaptations, the postantennular appe ndages in particular show considerable d.ifferences related to : the ex istence of pliable shafts;

Figure 5 Schemes from different ecotypes o f the Orsten arthropods (a ll about the sa me size). (a) Martillssollia e/ollgaUl Miill er & WaJossek nom. nud . (b) Skara anu/ala Mull er. 1983. (c) Data peilertae Mi..iller, 1983 (anterior part unknown) . (d) Walossekia qU;IIquespillOS(I Milller , 1983. (e) Bredocaris admirabilis MUlier , 1983. (f) Rehbachiella killllekllllensis Mi..iller , 1983. (g) Hesslalldolla k itlllckllllellsis MUlier, 1964.

171


scale-like sclerotisations on the surface; setation and bristliness; the division of the protopodite; the development of enditic projections, gnathobases or precoxal endites; the shape of the distal rami with different degrees of fusion of their podomeres. Annulated outer rami with one seta on each ringlet functioned mainly as force-generating elements. This is particularly the case with antennal and mandibular exopodites. The terminal podomere or the entire ramus may be developed as a flattened paddle with marginal setae. It may have served for both locomotion and feeding (postmandibular limbs of Skara) , as a vibratory plate (postmandibular limbs of Hesslandonidae), and/or for closing the filter chambers (Walossekia, Hesslandonidae, Rehbachiella). Whereas soft limb shafts (postmandibular limbs of Skara) may have permitted rotating movements, scales at the base (antennae and mandibulae of Skara) or on the whole outer surface of appendages (anterior three limbs of Bredocaris) reduced the flexibility. Several forms have filter appendages with setae arrange in rows or combs to sieve nutrient particles from the water (Skara, Rehbachiella, Walossekia, Dala, Hesslandonidae). On the other hand, the limbs of Martinssonia were not obviously adapted for that purpose. Because of their large protopodites having few spines and setae medially, it is more likely that Martinssollia was a bottom-dweller feeding on detrital matter which it gathered together. Structures that indicate a predatory life habit could not be identified in any form.

5.4. Size and maturity None of the specimens with preserved soft parts exceeds a length of about 2 mm. This size limit might be due to the special mode of preservation (see 2). The existence of larger arthropods within the Orsten assemblage is indicated by findings of isolated limbs of more than 1 mm in length and large empty valves of phosphatocopid ostracods (Muller 1964). Martinssollia may be regarded as immature; likewise, because of the low number of trunk segments and rudimentary limbs, Walossekia may be represented by larval specimens. On the other hand, forms like Bredocaris, Rehbachiella, Dala and Skara are about 2 mm long or smaller but are

Figure 6 SEM-photos of morphological details. (a) Waldo ria sp. The broken carapace valves (cv) permit a view of the body with the large median eye (eye). the labrum (I). and the almost fully-preserved appendages; the antennulae (atl) are longer than in the other hesslandonids; the postantennular limbs (antenna = all. mandibula = md, postmandibular limbs = pm!) all aid in feeding; most of the enditic and exopodal setae on the postmandibular limbs are still present. (Same specimen as in Fig. 4c.) (b) The same specimen as in (a), lateral view. The large protopodites of antenna and mandibula (prot all, prot md) beside the labrum (I) have lost their exopodites, holes indicating their positions; the segments of the postmandibular exopodites are fused to vibratory plates (vp), except on the more anterior limbs where the most distal podomeres remain segmented. (c) Rehbachiella kil/I/ekullel/sis Muller. Ventral view of a fragment with fully-expanded postmandibular filter appendages; the inner surface of the limbs is divided into a series of bulbolls endites (end) which are armed with subcircular rows of setae; in the upper middle the deeply recessed sternum (st) can be seen. (ST 4214, UB 771; Gum, z. 1). (d) Martinssonia elollgata. Exopod of the fourth appendage with one large proximal podomere (pp) followed by lobate podomeres (lp); the latter bear one seta (s) each medially and are fused laterally. (ST 4216, UB 750; Gum, z. I).

\J.!I.'/~ t

.~

1_

~.,~

Figure 7 Flexibility of Skara Muller. 1983; each drawing is based on one or two actual specimens (after Muller & Walossek 1985).

obviously adult stages. It seems rather unlikely that much larger individuals of these species originally existed. This minute size may be correlated with different morphological adaptations, such as a low degree of sclerotisation, less developed articulation and the shape of appendages. Also the development of eye structures and other sensory organs may be influenced by size. There are no special respiratory organs, which suggests that the soft body wall must have fulfilled this function, as in small Recent crustaceans (Kastner 1967, p. 902). The phosphatocopid ostracods may have used their pliable inner lamellae for respiration with their vibratory plates providing a continuous oxygen supply.

5.5. Ontogeny Larval stages are widespread in the study material. While some of them cannot be referred to larger forms (e.g. those presented in Figure lc, d; Muller & Walossek in prep. a), others could be definitely identified as belonging to recognised species. For example, a continuous series of larval stages could be assigned to Bredocaris. This ranges from a metanauplial larva with rudimentary maxillula, through stages showing a gradual increase in size and the addition of rudimentary trunk limbs, up to an instar with four well developed cephalic limbs and five rudimentary ones on the trunk (Muller & Walossek in prep. b). Several major morphological features such as the carapace, the eyes, the labrum, the three anterior limbs, and the furcal rami, do not change their shape markedly during development. This may be interpreted as a very conservative and primordial method of anameric development. Five different growth stages of Martillssollia have been found. Again, several succeeding instars of Rehbachiella have been recognised. In the latter species, the earliest larval stages have much larger eyes than the adults. On the other hand, the shield successively increases in size during ontogenesis. These developmental changes may have been correlated with a change in the life habit, with the free-living larvae gradually sinking to the soft bottom layer. However, the full clarification of all details has not yet been completed.

6. References Henningsmoen, G. 1957. The trilobite family Olcnidae. SKR NOR VIDENSK-AKAD OSLO MATH NATURWISS KL 1, 1-303.

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A. 1967. Lehrbuch der Speziellell Zoologie, Part I. Wirbellose 2, Crustacea, 2nd edn. 847-1242. Stuttgart: Fischer. Muller, K. J. 1964. Ostracoda (Bradorina) mit phosphatischen Gehausen aus dem Oberkambrium von Schweden. N JB GEOL PALAONT ABH 121, 1-46. Muller, K. J. 1979a. Phosphatocopine ostracods with preserved appendages from the Upper Cambrian of Sweden. LETHAIA 12, 1-27. Muller, K. J. 1979b. Ostracoden mit erhaltenen GliedmaBen aus einem Oberkambrischen Stinkkalk-Geschiebe. DER GESCHIEBESAMMLER 13, 91-4. Muller, K. J. 1982a. Weichteile von Fossilien aus dem Erdaltertum. NATURWISSENSCHAFTEN 69, 249-54. Muller, K. J. 1982b. Hesslandolla wlisulcata sp. nov. (Ostracoda) with phosphatized appendages from Upper Cambrian 'Orsten' of Sweden. III Bate, R. H. et al. (cds) A research manual of fossil and Recent ostracods, 276-307. Chichester: Ellis Horwood. Muller, K. J. 1983. Crustacea with preserved soft parts from the Upper Cambrian of Sweden. LETHAIA 16, 93-109.

Muller. K. J. & Walossek, D. 1985. Skaracarida. a new order of Crustacea. from the Upper Cambrian of Vastergiltland, Sweden. FOSSILS STRATA 17, in press. Muller. K. J. & Walossek, D. in press. Martillssol/ia dOl/gata gen. et sp.n., a crustacean-like arthropod from the Upper Cambrian 'Orsten' of Sweden. ZOOL SCR. Muller. K. J. & Walossek. D. in preparation a. Arthropod larvae from the Upper Cambrian 'Orsten' of Sweden. Muller. K. J. & Walossek. D. in preparation b. Bredocaris admirabilis Muller, 1983, from the Upper Cambrian of Sweden and its ontogeny. Sanders. H. L. 1963. The Cephalocarida. Functional morphology, larval development, comparative external anatomy. MEM CONNECTICUT ACAD ARTS SCI 15, 1-80. Westergaard, A. H. 1922. Sveriges Olenidskiffcr. SVER GEOL UNDERS SER CA 18, 1-205. Westergaard, A. H. 1947. Supplementary notes on the Upper Cambrian trilobites of Sweden. SVER GEOL UNDERS SER C 489, 1-35.

KLAUS J. MOLLER and DIETER WALOSSEK, Rheinische Friedrich-Wilhclms-Universit~it Bonn. Institut fUr PaHiontologie. NuBallee 8. D-5300 Bonn I, B.R.D.