Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of
Sweden
Klaus J. Miiller and Dieter Walossek
FOSSILS AND STRATA
Editor
Stefan Bengtson, Institute of Palaeontology, Box 558, S-75 1 22 Uppsala, Sweden.
Editorial and administrative board
Stefan Bengtson (Uppsala) , Fredrik Bockelie (Bergen) , and Valdemar Poulsen (Copenhagen) .
Publisher
Universitetsforlaget, Postboks 2959, Tøyen, Oslo 6, Norway.
Fossils and Strata is an internationally distributed series ofmonographs and memoirs in palaeontology and stratigraphy. I t is issued in Numbers with individual pagination, listed cumulatively on the back of each cover; the arrangement of the numbers in volumes for binding is!eft to the individual subscriber's discretion.
Fossils and Strata forms part of the same structured publishing programme as the journals Boreas and Letkaia. These two journals are fully international and accept papers within their respective sectors of
LETHAlA
�
Editor
An International Journal of Palaeontology and Stratigraphy
Anita L6fgren, Department of Historical Geology and Palae
ontology, S61vegatan 1 3, S-223 62 Lund, Sweden.
Letkaia publishes articles of international interest in the fields of palaeontology and stratigraphy. Articles on the morphology and anatomy of fossil plants and animals should be of general interest to palaeontologists, and articles on systematie palaeontology should deal with the higher units in systema ties or key forms on which our concepts of classification are based. New features, new forms and significant changes in the known distribution of fossil organisms also constitute important criteria for the acceptance of articles. Palaeo
biology - particularly palaeoecology - and ecostratigraphy are the co re topics of the journal. Articles on stratigraphy should meet the same requirements for general interest as the palaeontological artic
les and deal with stratigraphic principles, correlations of at !east continent-wide importance, stratotype areas of key charaeter, new occurrences or revisions which establish major features in palaeo
geography, etc.
Letkaia, like its sister journal Boreas, forms part of a publishing system with special ambitions to apply and develop modern tech-
science without national limitations or preferences. Fossils and Strata, however, is an outlet for more comprehensive systematie and regional descriptions dealing with areas in the five countries of Norden, or written by palaeontologists and stratigraphers from these countries.
Contributions by colleagues in other countries may also be included as far as this series is deemed to be the appropriate medium with regard to distribution and availability. Articles can normally be accepted only if they are heavily subsidized by the national Research Council in their country of origin or by other funds. All income is re-invested in forthcoming numbers of the series.
Manuscripts should conform with the Instructions on p. 3 of this cover, which are essentially the same as for Boreas and Letkaia. If possible, the final text should be submitted in an electronic form for automatie type-setting; the editor will provide the necessary infor
mation on request.
Articles in English, German, and French are accepted; the use of the English language is preferred. A card abstraet in English should always be provided, and non-English articles should always be provided with English versions of the figure captions. Abstracts or summaries in one or more additional languages may be added.
Many regional or systema tie descriptions and revisions contain a nucleus of results which are of immediate and general interest in international palaeontology and stratigraphy. It is expected that authors of such papers will to some ex tent duplicate their publication in the form of an article for a journal, in the first place Letkaia or Boreas.
niques, methodology and standards in scientific publication. These journals are semi-specialized, intending to cover and provide infor
mation within the respective interest ranges of large groups of earth scientists. They do not duplicate the functions of ex tant, strietly specialized journals in their fidd, national series, or monograph and memOlf senes.
SUBSCRIPTION TO LETHAlA 1 98 7
Ordinary price (non-members o f IPA, institutions, libraries, etc . ) : NOK 520,00 (USD 8 7.00 1 987)
DISCOUNT O N LETHAlA AND MEMBERSHIP FOR 1 98 7 IN THE INTERNATIONAL
PALAEONTOLOGICAL ASSOCIATION
Subscribing membership for individual palaeontologists in the In
ternational Palaeontological Association (IPA, affiliated to the International Union of Geological Sciences, IUGS) may be obtained by payment of USD 45.00 to Universitetsforlaget, Box 2959, Tøyen, Oslo 6, Norway. The applicant must sign a state
ment that he undertakes to retain his discount copy of Letkaia as a personal copy and not deposit it in a public or institutional library.
Back volumes ( 1 -19, 1 968- 1 986) may be ordered on the same conditions.
Morphology, ontogeny, and life habit of Agnostus pisiJormis from the U pper Cambrian of Sweden
KLAUS]. MULLER and DIETER WALOSSEK
Contents Introduction
Miiller, Klaus J. & Walossek, Dieter 1987-08-15: Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cam brian of Sweden. Fossils and Strata, No. 19, pp. 1-124, Oslo. ISSN 0300-949 1. ISBN 82-00-075 1 1-7.
The morphology and early ontogeny of Agnostus pisiformis (Linnaeus 1757) is described on the basis of more than a hundred phosphatized, mostly enrolled specimens, etched from nodular limestones of the Upper Cambrian Agnostus pisiformis Zone, Viistergiitland, Sweden. Gradual, anameric development. with only slight morphological change suggests that most of the recognized structures can be generalized also for later developmental stages up to the adult.
The tergal components, cephalic and pygidial shields and two pleurotergites, form a highly symmetrical test, with all rims fitting tightly together when clasped together edge to edge.
Complete enclosure of the test may have protected the soft ventrai side, while Agnostus swam actively when the two shields were slightly gaping. Locomotion was elTccted by the strong exopodites of the second and third cephalic appendages. Most probably the environment in which A . pisiformis lived was a flocculent zone at the bottom of the Alum Sea, where the animals swam or floated around, as is also suggested for the other orsten arthropods. Various morphological detaiis of agnostids are simply plesiomorphic, i.e. retained from the groundplan of the Arachnata. Those features which point to affinitites with the polymerid trilobites are most clearly in the exoskeleton and in the ontogeny. The numerous modifications of the morphology of A . pisiformis from the general polymcrid trilobite body plan and features hitherto unknown from trilobites and even other arthropods confirm the separate rank of the Agnostina within the Trilobita. D Euarthropoda, A rachnata (Trilobita + Chelicerata), Agnostida (Agnostina + Eodiscina) , Agnostus, phosphatization, dorsal exoskeleton, soft integument, hypostoma, labrum, '!Yes, appendages, mode of life, environment.
Klaus J. Muller and Dieter Walossek, Institut for Palaontologie, Rheinische Friedrich- Wilhelms- Univers
itat, NujJallee 8, 5300 Bonn 1, Federal Republie of Germany; 1987 Ol 13.
Ventrai soft integument Occurrence and preservation . . . .
3 3 3 4 4 5 5 5 6 6 8 8 9 9
Inner lamella . . . . 1 1 1 1 1 1 1 l 1 3 1 4 1 4 1 4 1 4 1 6 1 6 1 7 1 7 1 8 1 8 1 9 20 20 20 Localities and lithology . . . .
Faunal association . . . . Phosphatization and preservation . . . . Material and methods . . . . Material . . . . Methods . . . . Description of the morphology . . . . General remarks . . . . Dorsal exoskeleton . . . . Cephalic shield . . . . Foramen . . . . Thoracic pleurotergites . . . . Pygidial shield . . . . Surface structures . . . . Cuticle and polygons . . . . Pores . . . . Inner surface
1 0 1 0 1 0 1 0 1 1
Body . . . . Hypostoma . . . . Paired 'frontal organ' . . . . Sternal region of cephalon . . . . Trunk . . . . Appendages . . . . Antennules . . . . Second head appendages . . . . Third head appendages . . . . Fourth head appendages . . , .. . ... . .... . . . Trunk appendages . . . . Endopodal clubs . . . . Setation . . . . Internal organs and unidentifiable structures . . Description of the ontogenetic sequence . . . .
Ontogenetic stages . . . . Meraspid degree l . . . .
Stage l a . . . . Stage l b . . . . Stage I c . . . . Meraspid degree 2 . . . � ... . Stage 2a . . . . Stage 2b . . . . Stage 2c . . . . Stage 2d . . . . Holaspid period . . . . Comparisons of the ontogenetic stages . . . . Dorsal exoskeleton and pore pattern . . . . Details of the ventrai surface . . . . Assumptions on later juvenile stages and the adults Functional morphology and life habit . . . . Enrollment . . . .
Adaptation of the dorsal exoskeleton to
20 2 1 22 22 22 23 23 24 24 26 26 27 28 28 28
enrollment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Symmetry . . . 29
Articulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Further structures blocking outstretching . . . . 30
Adaptation of the ventrai structures to enrollment 3 1 Inner lamell a and median body . . . 3 1 Appendages . . . 33
Locomotion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Feeding . . . 36
Respiration . . . 37
Moulting and reproduction . . . 38
Sensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
Tergal pores . . . 38
Paired 'frontal organ' . . . 39
Vision . . . 39
Internal organs and unidentifiable structures . . 39
Mode of life . . . .. . . . State of knowledge prior to the discovery of ven tral soft parts . . . . Life position . . . . Environment and life habit . . . . Comparisons with other testaceous organisms . Comparative morphology . . . . Aspects of the tergal morphology . . . . Exoskeleton . . . . Somite boundaries and merocyclism . . . . Aspects of the ventrai morphology . . . . Hypostoma . . . . Comparisons with hypostomata of other trilo bi tes . . . . Agnostid hypostoma and crustacean forehead 39 39 40 42 42 43 43 43 43 44 44 44 endoskeleton . . . 46
Appendages . . . 47
Position and shape . . . . . . . . . . . . . . . . . . . . . . . 48
Number of appendages . . . 49
Tagmosis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Cephalon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1 Trunk . . . 52
Significance of A. pisiformis for the systematic position of agnostids . . . 52
Morphology and li fe habit . . . 52
Paedomorphic origin of agnostids? . . . 53
Systematic position of agnostids . . . 53
Acknowledgements . . . 54
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
Plates 1-33 . . . 57 List of abbreviations . . . 1 24
Among the Trilobita, the Agnostida represent a peculiar side branch of development. Their exoskeletal morphology seems to be characterized more by simplicity of form, di
minutive size, high degree of symmetry, and lack of some typical trilobite structures than by inherent specialized fea
tures. There are two major lines within the group, the Agnostina and the Eodiscina (Kobayashi 1 939). The latter group appears somewhat earlier in the Lower Cambrian than the former, and its morphology, having reduced the thoracic region to two or three pleurotergites (shortly terrned tergites in the following) , a regularly segmented pygidial axis, and, in some cases, compound eyes and facial sutures, is more intermediate to that of polymerid trilobites. Whereas Lauterbach ( 1 980) regards the 'Miomera' as a monophyletic unit, an alternative classification and interpretation will be presented by Robison (in press) .
The Agnostina (agnostids) lack dorsal eyes, facial sutures and a rostraI plate. Furthermore, the cephalic and pygidial shields resemble each other very much (isopygy) , which gives the whole dorsal exoske!eton a symmetrical outline from all sides. The num ber of tergites is exclusively two.
Most if not all agnostids possessed the ability of clapping their exoskeletal parts together in such a way that all rims fitted together tightly.
Agnostids have been known for more than 200 years.
Their temporaI range is from Lower Cambrian to Upper Ordovician (Harrington 1 959; according to Robison, per
sonal communication, the Devonian Pseudotrinodus aenigma, described by Kobayashi & Hamada 1 97 1 as an agnostid, is actually a polymerid trilobite). They are common in Early Palaeozoic rocks of most continents, and most genera and many species are even cosmopolitan (Robison 1 984). Local
ly they may form assemblages almost exclusively devoid of non-agnostid trilobites Qago 1 973) and even in rock forming quantities. However, nothing of their ventraI morphology has hitherto been recorded, except for a num ber of detached scleritic structures of some Middle Cambrian forms, de
scribed as hypostoma or labrum by Robison (l 972a, 1 982), Jell ( 1 975) , and Ritterbush ( 1 983).
Again, only few data are available on the ontogeny of agnostids. A summary of previous work has been given by Robison ( 1 964, p. 5 1 6). More recent references are Hunt ( 1 967) who described the ontogenetic sequence of the Mid
dIe Ordovician Trinodus elspethi (Raymond) , and Qian ( 1 982) who described the development of Pseudagnostus benxiensis. It is generally accepted that the first free-living instar of agnos
tids was a meraspid instar, having cephalic and incipient pygidial shields but no tergites. Qian's ( 1 982) report of Pseudagnostus protaspids may be based on misinterpretations
(Robison, personal communication).
Showing a wide range of exoskeletal specializations, the agnostids appear to be an ecologically heterogenous group, most probably having inhabited various different biotopes.
Notwithstanding the limited information about the ventrai body, agnostids have of ten been subject to widely varying and sometimes surprising interpretations of the ventrai mor
phology and of the mode of life, including locomotion, feed
ing habit, biotope and life strategies. Fortey ( 1 985, p. 2 1 9) observed that ' ... such a well-known group as the agnostids has been variously suggested as having been pelagic or bathypelagic (Robison 1 972) , parasitic (Bergstrom 1 973, p.
48) , benthic (e.g. Jago 1 976) , or epifaunal, attached to algal strands (Pek 1 977). Apart from encrusting in the littoral zone, this spans almost the whole range of possibilities open to marine arthropodsl'.
Extraordinary preservation of the present orsten material allows for the first time a detailed account of the morphology and morphogenesis in particular of the ventrai cuticular components of A. pisiformis to be given. Furthermore, it provides much information towards a better understanding of the mode of life of this species, which may at least partly be applicable to agnostids in general. The lack of other similarily preserved material of Paleozoic trilobites, in par
ticular the eodiscids, and other arthropods, however, limits discussion of the phylogenetic consequences of these find
mgs.
Occurrence and preservation
Localities and lithology
The fossiliferous rock material was collected in the Upper Cam brian Agnostus pisiformis Zone only at Kinnekulle, Vastergotland, in southern Sweden. The vast majority of samples came from a small quarry near Gum, southeast of Blomberg. The productive nodules were exposed in the bottom bank of the quarry and were collected individually from about the same leve! over a lateral distance of less than 50 m. All nodules exposed in the outcrop at the time of fie!d work were sampled and etched for their fossil con tent. Addi
tional material was collected from the same leve! in another small quarry south of Kake!ed and west of Vesterplana at the SSW slope of Kinnekulle. Many other outcrops of the same leve! at Kinnekulle and e!sewhere in Falbygden, Oland and Skåne were investigated in detail for similar material, but without success.
4 Klaus J. Miiller and Dieter Walossek
The productive lithology is a medium- to fine-grained, anthraconitic limestone, which occurs either as beds or i!1 layers of numerous nodules in the black .alum shale. The nodules have a diameter of 0. 1-2.0 m and are locally called orsten. In general, they are abundantly fossiliferous and con
tain Agnostus in rock-forming quantities. Many of the nod
ules are distinctly banded by fossiliferous layers or by alter
nation of fine-grained, blackish limes tone and grey to beige, commonly more sparry limes tone. It is assumed that the lighter-coloured bands were deposited under more aerated conditions than the darker ones; however, there is no appar
ent difference between their faunal composition. Neverthe
less, the preservation of the soft integument of A. pisiformis in these outcrops seems to be concentrated in the light-beige, medium-grained limes tones, having small brownish to blackish spots.
During field work, the lighter, more sparry limes tone was preferentially collected, because its residue is less bulky and easier to sort after etching. In addition, phosphatized fossils from the more dense, black limestone are mostly contami
nated by attached, undissolved matrix particles that may conceal details.
Faunal association
The fauna is almost exclusively composed of various growth stages of Agnostus. The fossils are mostly detached shields and tergites. Enrolled specimens are rare, and the vast majority are calcitic. Maximum size of the individuals varies between samples. Other megafossils are very rare; a single sample (64 1 7) yielded Acrocephalites stenometopus (Angelin) , a trilobite previously unknown from Vastergotland (G. Hen
ningsmoen, personal communication) .
Ostracodes are fairly widespread. Most of them belong to the hesslandonid phosphatocopines, while Bradoriina are rare. Various phosphatized arthropods are associated with A. pisiformis, such as Skara anulata Muller, 1 983, and S. minuta Miiller & Walossek, 1 985, Martinssonia elongata Muller &
Walossek, 1 986, Rehbachiella kinnekullensis Muller, 1 983, Wa
lossekia quinquespinosa Miiller, 1983, severaI arthropod larvae, and other yet undescribed forms (Muller 1 979, 1 982a, b, 1 983; Miiller & Walossek 1 985a, b, 1 986a, b) . The whole arthropod fauna appears to be composed of miniaturized forms which were adapted to a special soft-bottom environ
ment.
Conodonts are an important component of the fauna (Muller 1 959) . At Gum they are abundant and diverse in species. Furthermore, they are larger and thicker than in similar associations of the same age.
Phosphatization and preservation
Calcitic remains of all developmental stages of A . pisiformis with shield lengths up to 6 mm, are rock forming in the orsten. However, the soft ventrai cuticle of later stages than the first holaspid stage has not been recognized in the mate
rial. Detached phosphatic shields of other species of the same genus are present in the material, but occur in different samples and at different levels.
No more than two or three phosphatized individuals could be sorted from the residues of l kg of limes tone. It is not
FOSSILS AND STRATA 19 (1987)
clear, however, why only such a small proportion of enrolled specimens was permineralized by phosphatic matter. Phos
phatized instar specimens of A. pisiformis are much rarer than the soft-integumented phosphatocopine ostracodes, but in contrast to the latter forms, the cuticular structures of the ventraI body wall in Agnostus were preserved on ly in the enrolled condition. Specimens with gaping but connected shields or outstretched individuals are very rare. Disj unct organs, such as hypostomata or appendages have not been found, except when disconnected by breaking during the preparation process. It is assumed that this unusual preser
vation has occurred prior to the final embedding of the fossils, probably in the deeper zones of a flocculent environ
ment on the sea floor. Further aspects of preservation have been discussed by Miiller ( 1 985) .
The relative completeness of preservation is very variable.
In severai specimens, the interior of the enrolled exoskele
ton, formerly inhabited by the body, may either have been almost empty ( PIs. 9:2; 1 0:9) or contained matrix material ( Pl. 28:5) . The preservation of ventraI structures ranges from the presence of on ly the hypostoma ( Pl. 7: l ) or parts of the inner lamella ( PIs. 1 5:2, 3, 6) up to the entire ventraI morphology with the full series of appendages ( Pls. 16:3;
22:2; 29:4; 30: 7) . There is no apparent subsequent diagene
tic transformation of the phosphatized cuticula, and even delicate details such as bristles, surface texture, and pores down to a size of below 1 !lm are preserved. On the other hand, there seems to be an upper size limit for phosphatiza
tion and preservation of chitinous soft parts in the orsten at 1 .5-2 mm (Muller 1 985) .
The extraordinary mode of preservation also allows realis
tic assumptions about the life conditions of A. pisiformis. For example, the preservation of entirely enrolled individuals with their appendages still within the testal cavity provides information on the position and articulation of the limbs, and thus for their possible range of movement and locomo
tion. Pl. 3 1 : l shows a collapsed and wrinkled specimen of meraspid stage 2, the dorsal cuticle of which was obviously pliable at the time of fossilisation. Similar preservation has been observed also in other growth stages (e.g. Pl. 32:4;
stage h l ) but is very rare. I t is possible that such individuals died immediately prior to moulting or after moulting at a stage when the cuticle was softer.
In general, the phosphatized orsten arthropods lack inter
nal structures referable to soft tissue, organs or muscles . In some cases, however, the structures are toa regular to be simply regarded as artifacts. In a number of specimens of Skaracarida internal fillings of the trunk may represent the gut ( Muller & Walossek 1 985a) . Also in specimens of A . pisiformis internal structures have been observed, sometimes even in a corresponding position. In most cases, however, their interpretation remains equivocal, and thus insufficient for a detail ed description.
A wide variety of modes of deposition can be observed in the material ( Pl. 3 1 :2-9; see also explanations of figures) . The preservation also may have been influenced by different degrees of decay. This has to be taken into account when reconstructing the fossils. On the other hand, the different modes of preservation may give useful information about the environmental conditions at the time of precipitation of phosphatic matter.
FOSSILS AND STRATA 19 (1987)
Table l. List of illustrated speeimens, with plate and text figure referenees. Abbreviations, see p. 124.
no. ST loe UB stage plates/figures l. 4612 6787 806 mlb LI, 2; 7:2 2. 4143 6761 807 la 1:3-5 3. 4626 6780 808 lb 1:6; 9:5 4. 4610 6787 809 lb 1:7; 28:5 5. 4137 6761 810 lb 1:8; 28:7 6. 4628 6783 811 le 2: I; 8:5--7 7. 3741 6757 812 le 2:2, 3; 9:4 8. 1834 6417 813 2a 2:4-8 9. 4616 6750 814 2a 3:1,2
10. 4632 6760 815 2a 3:3-5; 10:7; 13:3; 15:3 Il. 4372 6776 816 2a 3:6, 7; 7:8; 9:3 12. 4625 6784 817 2b 4: I; 15:5; 30:2 13. 2450 6417 818 2c 4:2-4; 8:8, 9; 9:6 14. 4956 6758 819 2b 4:5
15. 3738 6787 820 2d 4:6, 7; 19:2, 6; 25:6; 29:4 16. 4063 6757 821 2d 4:8; 13:5; 19:1,7; 21:1-3;22:4, 5;
26:3; 30: I , 3, 6
17. 4624 6765 822 hl 5:1-5; 8:4; 9:1; 10:9; 13:7, 8;
20:4, 7; 28:9; 31:2; Fig. 28D
18. 4716 823 late 6:1, 2
holaspid
19. 4719 824 late 6:3
holaspid
20. 4715 825 late 6:4,6
holaspid
21. 4720 826 late 6:5
holaspid 22. 4308 6776 827 mlb 7:1; 14:6 23. 4619 6783 828 la 7:3; 8: 1-3 24. 2435 6414 829 la 7:4
25. 3991 6755 830 2c 7:5--7; 22:3; 25:1-4; 26:7,10, Il;
Fig.28B 26. 4939 6750 831 lb 9:2 27. 4297 6750 832 le 10:1; 18:1 28. 2250 6414 833 la 10:2 29. 2451 6417 834 la 10:3, 4; 23: I 30. 4600 6783 835 lb 10:5
31. 4028 6749 836 2d 10:6; 15:6; 26:2; 30:4; 31:3 32. 4029 6749 837 2c 10:8; Fig. 28C
33. 2247 6414 838 lb 11:1; 12:2; 17:2; 23:2; 29:2; 31:9 34. 4135 6761 839 le 11:2; 13:1; 14:3,4
35. 4168 6783 840 2a 11:3,4
36. 2042 6409 841 lb 11:5,6; 12:1; 28:6; 33:3
Mc;tterial and methods
Material
In all, 2 1 5 kg of rock material was available from those nodules which yielded phosphatized Agnostus with preserved ventrai organs. A total of 1 3 3 individuals of different growth stages, the majority of them enrolled, were sufficiently well preserved fqr examination ( 76 figured individuals are listed in T4ble I ) . The specific determination, based on compari
son with the calcified, rock-forming adult stages, and the homogeneity of the material on hand, proved that all speei
mens belong to the single speeies A. pisiformis. Furthermore, based on analysis of the allometric and morphological ehanges of the dorsal and ventrai side, the individuals were grouped into eight growth stages. The numbers of speeimens for each of the stages is given in the chapter on ontogeny.
ACNOSTUS PISIFORMIS 5
no. ST, loe UB stage plates/figures 37. 3917 6781 842 le 11:7; 13:2 38. 4596 6750 843 lb 12:3 39. 4071 6417 844 le 12:4
40. 3572 6761 845 lb 12:5; 14:1,2; 16:1 41. 2985 6416 846 2b 13:4
42. 4950 6763 847 2b 13:6 43. 4140 6761 848 2a 14:4; 28:4 44. 4958 6763 849 2c 15:1; 33:6 45. 4960 6759 850 lb 15:2 46. 3930 6743 851 2a 15:4
47. 4286 6771 852 lb 16:2; 17:9; 19:3,4; 20:1, 5; 25:5;
26:8,9
48. 4307 6763 853 le 16:3; 17:6; 18:4; 20:3; 2L5; 23:4, 5; Fig. 28A
49. 4373 6760 854 le 16:4; 30:5
50. 4615 6764 855 2a 16:5; 17:8; 22:2; 30:7 51. 2579 6409 856 lb 17:1,4,5
52. 4293 6763 857 le 17:3; 18:5 53. 4597 6763 858 lb 17:7; 20:2; 21:4
54. 4304 6776 859 2b 18:2; 20:6; 22:1; 25:7; 26:1; 29:3 55. 4306 6763 860 2a 18:3; 19:5
56. 4565 6763 861 l? 19:8 57. 3929 6780 862 lb 23:3, 33:7 58. 4305 6776 863 2a 24: 1-4; 26:4-6 59. 4134 6761 864 la 27:1,2 60. 2041 6409 865 lb 27:3
61. 3230 6409 866 27:4
62. 4609 6787 867 lb 27:5 63. 4627 6784 868 2a 27:6; 29: I 64. 4136 6761 869 le 28:1,2 65. 4613 6784 870 2a 28:3,8 66. 4062 6757 871 2 31:1 67. 4608 6784 872 lb 31 :4 68. 4614 6784 873 lb 31 :5 69. 2248 6414 874 lb 31:6 70. 4611 6787 875 le 31:7 71. 4309 6760 876 le 31:8 72. 4927 6776 877 hl 32:1-3 73. 4948 6732 878 hl 32:4-8 74. 4933 6784 879 hl 33:1, 2, 4 75. 4961 6760 880 m2b 33:5 76. 4936 6784 881 late 33:8
holaspid
For comparisons, some calcitic immature speeimens, most of them enrolled, were also examined. These are from Pleis
toeene drift boulders of Northern Germany, collected and kindly supplied by Kurt Eichbaum, Hamburg, and Max Giessier, Flensburg (see also Giessier 1 969) . All calcitic individuals wcre assigned to A. pisiformis by the finders. As there are some minor morphological differences between these and the phosphatized material, it is, however, possible that some of the calcitic individuals belong to speeies c!osely related to A. pisiformis but not to this partieular speeies.
Methods
Because it is impossible to test the produetivity of the lime
stone during field work, the phosphatized mierofossils ean
6 Klaus J. Muller and Dieter Walossek
.�---csw---'.�
B Å I csh
I
FOSSILS AND STRATA 19 (1987)
,--��---��,
psh
t
,---=---I
c
...,-.- fol
+
�---:;---,
I:.,--- psw---..... �
on ly be recognized after etching in the laboratory. The standard preparation techniques for phosphatic microfossils, in particular conodonts, proved to be unsuitable for such extremely fragile, secondarily phosphatized material. Thus it was necessary to apply special etching techniques, de
scribed in detail by Miiller ( 1 979, 1 985) .
For routine etching, 1 5% acetic acid was used without a buffer. Jeppsson, Fredholm & Mattiasson ( 1 985) have re c
ommended the addition of filtered liquid of previously pro
cess ed samples, termed 'acetate soup'. Without buffering, a small amount of apatitie matter exposed to the acid at the beginning of the reaction is lost, until sufficient buffer solu
tion has developed. In our view, this is tolerable if enough rock material is available for preparation, as was generally the case here. Buffering (e.g. by powdery apatite) may be of value only if very small rock samples are available, such as from small lenses or museum collections.
The specimens sorted were processed as described for the Skaracarida (Miiller & Walossek 1 985a) . The majority of enrolled specimens were opened under a stereolens by appli
cation of a micromanipulator. The speeimens were mounted on the stubs, preferably in a position suitable to show the more interesting ventrai side but often concealing the dorsal side. This led to some difficulty to correlate the exoskeletal segmentation with the ventrai morphology. As all specimens are more or less fragmentary, reconstruction of the morpho
logy was based on observations of different individuals.
In some cases fragments were lost from the extremely brittle speeimens even during scanning. Thus to avoid fur
ther breakage, the lengths of various parts of the exoskeleton and soft body were measured excJusively from SEM photo-
Fig. l. Measurements of tergal details. DA.
Dorsal view, tergum outstretched. DB. Lat
eral view of enrolled test. D C . Foramen. (For abbreviations in this and following figures see p. 124.)
graphs, rounded off to the nearest 5 Ilm. However, as partial preservation, distortion and the systematic error of the SEM magnification cause some inaccuracy, the dimensions given . can be tak en only as approximations, serving to support evidence derived from the morphology. The mode of mea
suring is given in Fig. l .
The material at hand was grouped into different ontoge
netic stages, following the terminology of Whittington ( 1 959) , Robison ( 1 964) , and Hunt ( 1 967) . It consists of specimens ranging from the earliest larval stage, meraspid degree I, to the first stage of the holaspid period (h I ) . Closer examination of the morphology, supported by the length measurements reve al ed a further subdivision of the meras
pid period: three successive instar stages have been recog
nized for the first meraspid degree ( m l a-c) , and four for the second one (m2a-d) .
Description of the morphology
General remarks
The morphological terminology follows in general Harring
ton ( 1 959) , Whittington ( 1 959, 1 963, 1 965, 1975), Opik ( 1 967, 1 979) , and Robison ( 1 964, 1 972a, 1 982) . Accessory information has been taken from Kobayashi ( 1 939) , Jell ( 1 975) , and Shergold (personal communication) .
Severai terms in particular of the ventrai morphology have been adopted from the general arthropod terminology.
They differ somewhat from the current trilobite and agnos
tid terminology but may lead to the improvement of the
FOSSILS AND STRATA 1 9 ( 1 987) ACNOSTUS PlSIFORMIS 7
A B
2
Fig. 2. Tergal morphology of Agnostus pisiformis, reconstructed for the first holaspid stage (h I ) . D A. Dorsal view, tergum fully outstretched . D B . View from posterior of enrolled test. O C. Lateral view. O D . Anterior view.
comparability of agnostids with other arthropods. As the two rami of the appendages are considered to be homologous in all arthropod groups, we prefer to use the terms 'endopo
dite' and 'exopodite' for the two rami, arising from the coxa, rather than 'telopodite', 'epipodite' , 'inner and outer ram us' or even 'preepipodite'. Furthermore, in contrast to the ter
minology of severai authors (e.g. Whittington 1 975; Seiden 1 98 1 ) , the coxa is regarded as limb base from which the rami arise and thus not counted as the proximal podomere of the endopodite.
As the ability to enroll proved to be an important charac
ter of the life strategy of A. pisiformis, a functional terminol
ogy is preferred when necessary. In this, the cephalic shield forms the upper (dorsal) cover of the animal, its spines pointing posteriorly, while the pygidial shield, forming the lower (ventrai) counterpart of the cephalic shield, is anter
iorly flexed, with its ventrai surfaee upwards oriented. Ac
cordingly, the posterior end of the pygidial shield and its spines point forwards while the truneate anterior end is at
the re ar of the dosed test (see Fig. 2B, C ) .
A num ber o f terms of the trilobite and agnostid terminol
ogy refer to both the tagmosis and the description of tergal components. For example the 'cephalon' describes the 'ce
phalic shield' as well as the tagma 'head' . In polymerid trilobites the postcephalic region, the trunk, may be subdi
vid ed into a thoracic region, the segments of whieh have tergites, and a postthoracic region, generally covered by the uniform 'pygidium'. This portion is, however, no true tagma. Its nature as an 'interkalare Bildung' and product of the fusion of a varying num ber of postthoraeic segments has been discussed by Lauterbach ( 1 980) in great detail.
In A. pisiformis the trunk tergum consists of two tergites and the large pygidial shield, but on the ventrai side, the re is no distinctive subdivision into a 'thorax' and a 'pygidium'.
Again, in A . pisiformis not only the number of segments of the 'pygidium' changes during ontogeny, as it releases its two anteriormost segments suecessively, but also the ventrai trunk body merges into the ventrai eutide in about one half
8 Klaus J. Muller and Dieter Walossek
taxf mgl pgl
+
ind gina
o
g� ----
> '
, , , ,
h�'�
__ ��/ ����atl
iaxf
el4 es
tl1 tg1
axr
tg2
to two thirds of the shield length. The segmentation of the abdominal region is unknown. In consequence we prefer to use separate terms for components of the dorsal exoskeleton, such as 'cephalic shield', 'tergites', 'pygidial shield', and of the tagmosis, such as 'cephalon' (=head) , and 'trunk', the latter including thoracic and postthoracic, pygidial seg
ments. The term 'doublure', describing inwards flexed rims of the dorsal exoskeleton, is well-established in the trilobite literature and, although serving the same purpose, has not been transforrned to 'duplicature', commonly us ed in crusta
cean terminology.
Most of the relevant morphological terms of the dorsal exoskeleton are shown in Figs. 2 and 3; those of the ventrai side are labelled in Fig. 4. The abbreviations are listed at the end of the paper, and a separate card is enclosed addition
ally.
Due to incomplete preservation of most specimens, break
age after opening of enrolled tests, and the fairly small number of speeimens of later developmental stages, the ventrai morphology has been reconstructed for a late mera
spid instar while the exoskeletal morphology has been recon
structed for an earl y holaspis. Again, details of the morpho
logy have also been derived from younger stages when neces
sary. This method may be j ustified because the ontogeny of A . pisiformis proved to be very regular and anameric (see chapter on ontogeny) .
Dorsal exoskeleton
The tergum is divided into a cephalic shield (cs ) , two ter
gites (tg I, 2) forming a short thoracic region, and a pygidial shield (ps; Fig. 2A) . Both shields are horseshoe-shaped and of about the same outline, eonvexity, and height. The ce
phalic shield is truncate posteriorly, while the pygidial shield is truncate anteriorly. The surface relief is also similar on both shields. It consists of an arched area (axis) medially, the 'giabeIla' (gl) on the cephalie shield and the 'axis' (ax) on the pygidial shield, ample moderate ly convex pleural are as lateral to the axes, the 'genal fields' (gf) on the cephal
ie shield and the 'pleurai fields' (pf) on the pygidial shield, and a marginal 'border' (bo) (except for the truncate ends of the shields) .
I n the course of fus ion of the cephalic and pygidial seg
ments and their tergal cuticle, most of the external segmenta
tion has been reduced during evolution. In partieular the pleural areas and borders are almost devoid of the original
ax
pax
tlS
ps
an
FOSSILS AND STRATA 1 9 ( 1 987)
Fig. 3. Axial region of the dorsal exo
skeleton, with insertions of the appen
dages (dashed areas) and correspond
ing marks on the dorsal surface (dashed circles ) .
segmental surface relief (effaced) . Only the posteriormost eephalie and the anteriormost pygidial segments have re
tained parts of the original segmental surfaee struetures.
Furthermore, the axes are more or less divided into a num
ber of lobes by 'transverse axial furrows' (taxf; see also Fig.
3) , still indieating their segmental nature.
The axes are separated from the pleurai areas by 'axial furrows' (axf). On the cephalic shield a furrow, the 'pregla
bellar median furrow' (pmf) expands between the axial furrow (anterior margin of giabeIla) and the 'border furrow' (bof), whieh marks off the border from the genal fields. The margins of the two shields are strongly flexed inwards, forming the 'doublures' (du) . On the shields the originally tripartite nature of the 'axial lobes' or 'rings' (axr) is less defined (see the posterior glabellar lo be) or absent; on the two tergites the axial rings are, however, well developed and tripartite. The serial homology of all axial lobes is shown in Fig. 3 .
Both shields have pairs o f posteriorly directed spines pro
jeeting from the posterolateral margins at an angle of about 20-30°. The eephalie spines (esp) project from the posterola
teral edges of the eephalic shield. They are eireular in eross
seetion and slightly eurved ventrally. The pygidial spines (ps p ) proj eet from the posterolateral part of the pygidial border. They are subtriangular in cross-seetion and slightly reeurved. The doublure of the shield eontinues also onto the flat ventrai side of the spines (PIs. 7 : 7 , 8; 1 0:2, 3; Figs. 1 7- 1 9, 22) .
Cephalic shield
In dorsal view the shield is broadly rounded anteriorly and widest in its first third (Fig. 2A) . The lateral margins con
verge slightly towards the spines at the posterolateral edges of the shield. The posterior margin between the spines is almost straight. The giabeIla is divided into three portions.
The 'anterior glabellar lobe' (agl) is semi-eircular, anteriorly slightly aeute, and only m'oderately eonvex as compared to the more posterior glabellar lobations. It is separated from the following lobe by a distinct transverse furrow. The 'me
dian glabellar lo be' (mgl) has sides almost paralleI to one another. Two shallow i�dentations or furrows at the flanks indieate that this portion belongs to more than one cephalic segment (Fig. 3) . The posterior portion of the median glabeI
lar lo be is very swollen and eulminates in an axial 'glabellar node' (gin) .
The last postion, the 'posterior glabellar lobe' (pgl) , is U-
FOSSILS AND STRATA 1 9 ( 1 987)
shaped and marked off from the bulging rear of the median lobe by a curved transverse axial furrow. This lobe, as part of the last cephalic segment, is clearly serially homologous to the subsequent axial rings. A pair of shallow intraaxial furrows, indicating the originally tri parti te nature of this lobe, is present in all developmental stages (PIs. 1 :2; 2:7; 3: 1 ; 5 : I , 3 ; 6:2; 8:5; Figs. 2A, B , 3 ) . These furrows separate the narrow median part from the subtriangular lateral knobs, the so called 'basal lobes' (a misleading term, as these are no axial lobes in the strict sense) . During ontogeny the median portion, which is easily recognizable in the early larva l stages, becomes progressively smaller and shorter, until only a small convex bridge or band remains (PIs. 1 :2; 2:4; 3:4;
5 : 1 , 3; 6:2) . The serial homology between this last glabellar portion and the subsequent axial rings can be seen in Pl. 5:3, 4 as well as in Figs. 3 and 1 3 .
The reniform genal fields are gently swollen, sloping an
teriorly and laterally, and somewhat steeper towards the posterior end (Fig. 2B-D) . At their posterior end they are separated from the raised posterior margins, the 'posterior pleurai bands' (ppb) of the last cephalic segment, by a short furrow, which is homologous to the 'pleurai furrow' (pif) of the subsequent segments. As the cephalic spines arise from these bands, they may be regarded as elongated 'fulcral prongs' or fulcral spines. Thus they are not homologous to the pygidial spines, which are outgrowths of the pygidial border (Pl. 1 : 7; 2:7; 28:5; Fig. 2A, C ) .
The cephalic border i s narrower than that of the pygidial shield and of almost even width throughout ( Pl. I :2; Fig.
2A) . In side view, the lateral margin is slightly concave, whereas that of the pygidium is correspondingly convex (PIs.
1 :3, 6; 2:6; 4:3; 32:3; Fig. 2C ) . Also the doublure is narrower than on the pygidial shield (Pl. 1 8: 1 ) , terminating below the cephalic spines ( PIs . 1 0 : l ; 27 :4; 28:6) .
Foramen (in particular Pl. 28: 7-9)
The cephalic shield and the trunk tergum are connected to one another by a simple hinge joint (hi; PIs. 1 :4; 2:8; 3: I ; 5 :3-4; 6:3) . Its narrow arthrodial membrane (am) is visible when pressed out of the joint (Pl. 30: 1 , 2) . In dorsal view the adjacent margins are almost straight (Fig. 2A) . However, when looking at the enrolled test from the posterior, they are excavated medially, forming a spindle-shaped gap, the 'fora
men' (fo) or 'cephalothoracic aperture' (PIs. 2: 7-8; 30: I ; Figs. 2B, 20) .
Representing the median part of the joint, the foramen is clearly serially homologous to the median parts of the articu
lations between the subsequent tergal components. Accord
ingly, during the life of the animal it is covered by the joint membrane, which is finely folded lengthwise (Pl. 5:3) . It is noteworthy that due to construction the foramen is visible only when the animal is enrolled, i.e. when the trunk tergum is strongly flexed against the cephalic shield (Fig. 20) . When the test was unrolled prior to embedding, however, the anterior margin of the first tergite pressed the arthrodial membrane into the foramen, destroying the whole structure ( Pl. 33:3; see also Fig. 1 6A-F) . As in all enrolled specimens the foramen is similarily preserved, it is most likely that the foraminal membrane was expanded permanently.
Slightly below the midd le of the foraminal membrane a pair of openings (fop) are located which point somewhat
ACNOSTUS PISIFORMIS 9
ventrally (PIs. 1 :4; 28: 7-9; Figs. 2B, 1 3, 20) . They obviously represent exites of ducts from the interior (Pl. 28:8) . The function of these ducts is unknown, but from their position it appears rather unlikely that they led to secretory glands .
Thoracic pleurotergites
When the animal is enrolled, the two tergites form the rear of the test like the spine of a book, fitting perfectly into the gap between the shields (Fig. 2B, C; see also PIs. 5:4; 6:3) . The tergites are released successively from the anterior end of the larval 'incipient pygidial shield' (i ps) during ontogeny (Fig. 1 2A-C ) , and their characteristic surface relief can be already observed before liberation. Furthermore, due to their derivation, the pleural ends are forrned by short bor
ders, with short doublures ventrally (PIs . 3 : 3 , 5; 5:2; 20: 7;
30:3, 4; Figs. 1 2, 1 3 ) . The axial rings are distinctly arched and tripartite, divided by posteriorly diverging 'intraaxial furrows' (iaxf) into median and lateral knobs (mkr, Ikr; Pl.
5:3, 4; Figs. 3, 1 3) .
The anterior margin of the first tergite is slightly broader but longer and narrower than that of the second. The me
dian parts of the anterior tergitic margins are differently developed. In the second tergite it is crescentic, forming a well-defined 'articulating half-ring' (ahr) , which projects beyond the median knob of the preceding axial ring (PIs.
4: I ; 5:4, 5; 30:2) . In the first tergite, the median part of the margin, the lower rim of the foramen (Fig. 2A, B), is dis
tinctly convex as in the subsequent segments ( PIs. 1 :4; 2:3, 8;
3: 1 ; 5:3, 4; 6:3; 32: 1-3 ) , but only slightly extended (Pl. 9:2) or even straight in dorsal view ( PIs . 1 :8; 2:8; 3:2; 5:2) . The crescentic outgrowth is apparently reduced. However, since this structure is clearly serially homologous to the articulat
ing half-rings of the subsequent terga l parts in detail as well as in position and function, it is term ed 'reduced articulating half-ring' (r ahr; see also Pl. 5:4, 5; Figs . 2A, 3, 1 3) .
The pleurae of both tergites are crossed by a straight pleural furrow. This furrow separates the anterior and poste
rior 'pleurai bands' (apb, ppb) at the anterolateral and posterolateral margins from one another. The bands are slightly elevated and ventrally reflected (Figs . 2A, 1 3 ) . The anterior pleural bands are sloping steeper towards the pleu
rai furrow than the posterior ones. The bands derive from uniform 'lateral pleural lobes' (lpl) , which become divided into two by the newly developed terga I joints during on tog
eny (Fig. 1 5 ) . Three of these lateral lobes (lpl l-3 in Fig. 1 3) were present originally: between the cephalic shield and the first tergite ( PIs. 6:2; 30: I ) , between the two tergites, and between the second tergite and the pygidial shield (PIs. 6: 1- 3; 30: 1 ; 32: 1-3; Fig. 2A, C). The anteriormost one, however, is already divided in the earliest meraspid stage.
The posterior margins of the first tergite are anteriorly curved distally, corresponding to the curvature of the second tergite. This gives the pleural extremities a somewhat point
ed or falcated appearance (PIs. 4:5; 5:3; 32: 1 ; Figs. 2B, 1 3 ) . The pleurae of the second tergite are slightly longer than those of the anterior tergite. They are anteriorly curved, ventrally pointing, and slightly tapering towards their distal ends ( Pl. 32:2, 3) . When the test is enrolled, their short borders fill the short distance between those of the first tergite and the pygidial shield, j ust where the posterolateral margin of the cephalic shield curves dorsally (PIs. 5:4; 6:3;
l O Klaus J. Miiller and Dieter Walossek
Figs. 2C, 1 2C) .
All three tergal joints U 1-3) seem to terminate at the culminating points of the pleural bands (below the cephalic spine in the first joint) , while the pleurae are freed from one another distally ('free pleurae' according to Opik 1 979) . However, this is not true. All joints dearly terminate at the borders. The impression is mainly due to the shortening of the anterior band of the first tergite. By this, the culmination point and the lateral end of the joint are the same. Accord
ingly, the free posteriorly pointing anterolateral margin of the first tergite is forrned by the border and not by the pleurai band. This is also corroborated by the presenee of a short doublure on the ventrai side (PIs. 3:5; 5:2) . Due to this construction only in the anterior joint the adjacent margins are freed from one another distally. In the two posterior joints, recessed arthrodial membranes run distally to the borders, connecting the adj acent pleural bands during life of the animal ( Pl. 1 0: 7, 8; Fig. 1 3 ; see also chapter on articula
tion) .
With progressive elevation of the tergal lobes during on
toge ny (cf. Pls. 2:8; 5:5 and 6:3), the anterolateral margins of the second tergite and the pygidial shield become slightly inwards flexed, forming narrow 'articulating facets' (af).
These project slightly beyond the margins of the preceding segments ( Pl. 32: 1-3; Figs. 2A, 1 3 ) .
Pygidial shield
Corresponding to the second tergite, the anterior pygidial margin is almost straight, except for the broad crescentic articulating half-ring in the middle (Pl. 32:2, 3) . The shield widens rearwards to about the mid-Ievel and con verges again backwards to the spines. The posterior margin be
tween the spines has the same degree of convexity as the anterior margin of the cephalic shield (Fig. 2A) .
The anteriormost pygidial segment has retained parts of the original segmental surface structures, such as the broad, crescentic articulating half-ring, the axial ring (uniform) , anterior pleural bands, and pleurai furrows. The posterior bands are incorporated within the pleurai fields (Fig. 2A) . The latter are smooth and have a similar convexity as the genal fields (Fig. 2C, D) . Similar to the second tergite, the anterior pleurai band is bent inwards distally to form a narrow articulating facet, proj ecting underneath the adja
cent posterior band of the preceding tergite (Figs. 2A, 1 3 ) . The swollen axis i s lanceolate, being parallel-sided i n the first half and tapering towards its tipped end (Fig. 3 ) . Two axial rings can be recognized by shallow lateral deflexions of the axial furrows; distinct transverse axial and intraaxial furrows are not developed (Pl. 5:5) . The second ring carries a posteriorly pointing 'axial node' (axn; PIs. 6: 1 ; 32:3, 4; Fig.
2A, C ) . The conical posterior half of the axis, the 'postero
axis' (pax) , ta pers towards the little elevated 'terminal node' (tn) and shows no further segmentation (Figs. 2A, 3 ) .
The doublure o f the trunk tergum (induding the short parts of the tergi tes) is broader than on the cephalic shield and continues also onto the spines ( PIs. 5: 1 ; 7:7, 8; 1 1 : 1 ) . The surface of the doublure is somewhat recessed between the slightly pronounced margin (Pl. 7:5-8) . In ventrai view it appears as if the doublure widens progressively posteriorly ( Pl. 9:2; Figs. 4, 1 7, 1 9) . However, the width increases only slightly towards the pygidial spines, while the slope changes
FOSSILS AND STRATA 19 ( 1987)
markedly: anteriorly it is very steep ( Pl. 32:3) and decreases progressively towards the spines (Pl. 1 :6; 2:6; 3:6; 5:2; 7:5-8;
1 1 : 1 ; 1 5:6) .
Surface structures
Cuticula and polygons (mainly Pls. 7; 8; 32:4-8) . - With the exception of the calcified specimens from drift boulders (Pl.
6) , the surface of the individuals investigated is full y permin
eralized by phosphatic matter. Thus, it cannot be ascer
ta in ed whether the cuticule was originally composed of a single or of severai layers . Again, the numerous calcified holaspids encountered in thin sections prepared for the study of the lithology were much too recrystallized. Howev
er, the dorsal cutide must have been fairly thin, and at least the uppermost layer may have been deformable (Pl. 32:7) .
In the phosphatized immature individuals the exoskeletal cutide appears only slightly more sderotized than that of the ventrai surface. There are no indications that the tergum was originally calcified (PIs. 1 0:9; 1 3 : 1 ; 1 7 : 1 ; 22:5; 27: 1 ; 28:2) . I n PIs. 3 1 :3 and 3 1 :9 the cutide is secondarily thick
ened by a layer of crystalline matter. Another kind of cuticu
lar preservation is shown in Pl. 33:5. In this speeimen the uppermost cuticular layer may not be preserved, leaving a void space. Due to partial breakage of the coating phosphate layer(s) the east of the inner surface of the cutide is exposed.
The tergal surface lacks any kind of setation. But depend
ing on the quality of preservation, a polygonal surface pat
tern (probably a specific character) can be seen, which appears to be made of shallow walls. The size of the poly
gons ranges from about 1 0 to 20 Ilm and appears constant throughout ontogeny. The shape differs on the various tergal parts. On elevated areas, the polygons are subcircular, while they become more elongated and outstreched within the furrows and along the margins (Pl. 7:2, 3) .
The polygon al pattern extends also onto the cephalic spines (not figured in detail) , the ventrai surface of the pygidial spines (Pl. 7 : 7 , 8) , the articulating half-rings (Pl.
4:4) , and the doublures of tergites and pygidial shield (Pl.
7:5, 6) . At the tergal margins the walls join a narrow band of the same height as the former (PIs . 7:2, 4; 8: 1, 2) . The interior of the polygons is slightly concave and may be smooth ( Pl. 7:2-8) or finely fold ed ( Pl. 32:4-7) . This may indicate that the surface pattern was restricted to the upper
most cuticular layer. However, it remains unknown whether it represented epicuticular compartments, comparable to the dadoceran crustacean Sida crystallina where the structure may have aided hydrodynamic advantage during escape movements (Giinzl, personal communication) , or whether it was of exocuticular origin, comparable to the surface pattern ofvarious other dadocerans where reticulation is assumed to strengthen the thin carapace (Dahm 1 976, 1 977) .
Pares (mainly Pls. 7-9) . - A large num ber of small pores (po) of a diameter between 2 and 3 Ilm are developed on the surface of the exoskeleton and on the doublures . There are different types of pores. Most of them are more or less circular ( Pl. 8:2, 3 ) , surrounded by the walls of the poly
gonal surface structure ( Pl. 8: 1-4) . Other pores may have been originally slit-like ( PIs. 7:6; 8: I , 6, 9) or divided into two, as can be seen in Pl. 7:4. The pores may be als o associated with foliaceous areas of unknown function (di-
