Biol. Rev. (1964), 39, p p . 232-258

THE PHYLUM ARCHAEOCYATHA

I. Introduction . .

BY DOROTHY HILL University of Queensland

(Received 7 May 1963)

CONTENTS

232 VII. Trends of development . . 244

11. Distribution in space and time . 233 ~ 1 1 1 . Intraspecific variability . . . 247

II1* Palaeoecology . . . . ‘ 235 IX. The soft parts . . . . 247

IV. Skeletal morphology . . . 236 X. Classification . . . . 250

and origin of the skeleton . , 242 XI. Summary . . . . . 252

VI. Ontogeny . . . . . 244 XII. References . . . . . 253

V. Mineral nature, microscopic texture

I. INTRODUCTION

The Archaeocyatha were, geologically speaking, a very short-lived group that inhabited the carbonate-shelf and reef environments of the Lower Cambrian and early Middle Cambrian seas. They are the only phylum of animals to have become extinct. They were amongst the first phyla to develop mineral skeletons and used calcium carbonate for this purpose. From the beginning of their study they have excited interest in their systematic position and in the nature of their soft parts, and both these are subjects of lively controversy in current literature, which, because of the rich development of archaeocyatha in Siberia, is mostly in Russian.

I t was at first thought that they might be sponges or corals (Billings, 1861), protozoa (Billings, I 865), foraminifera (Dawson, I 865), corals or foraminifera (Meek, 1868), coelenterates (Bornemann, 1886), calcareous algae (von Toll, I 899) or sponges (Walcott, 1886; Taylor, 1910). Bedford & Bedford (1936) and Vologdin (1937b) thought of them as a subphylum of the Porifera, but since 1953 (Okulitch & de Lauben- fels) they have been regarded by all who have worked on them as a separate phylum.

The basic form of the skeleton or ‘cup’ is an inverted cone (Fig. I ) , erect or curved, slowly or rapidly expanding, and, in most reef forms, with holdfasts. Compound skeletons, catenulate or dendroid, are not common. Some cups are one-walled; most have two walls and a normally empty central cavity. The walls are perforate and are connected across the intervallum by perforate radial longitudinal plates (septa or taeniae), by radial rods, by perforate tabulae, by imperforate dissepiments, or by radial or inclined hexagonal tubules with perforate walls. The walls are normally perforate sheets, in which the pores are in opposite or alternate longitudinal rows. The pores may be simple rounded or hexagonal holes when the wall is thin, and straight or geniculate pore-canals when it is thick; they may be complexly ‘protected’

The phylum Archaeocyatha 23 3 by tumular, porous roofs, or scoop-like or eaves-like or spiny projections from their rims, or ‘screened’ by hair-like rods growing across their mouths. Some walls are formed of louvre-like plates set obliquely between the edges of the septa or taeniae so as to bound pore-tubes; others are formed by shelf-like rings (annuli) applied to the edges of the septa. Other (inner) walls are formed into pore-tubes by fluting of the inner edges of the septa.

d

Fig. I . Reconstruction of archaeocyathan skeleton after Vologdin. u, Outer wall of cup with small pores; b, inner wall with large pores; c, intervallum; d, central cavity; e, septum; j, interseptal loculus; g, anchoring outgrowths. (Text-figs. 1-4 by courtesy of the Trans- Antarctic Expedition Committee.)

Septa are plane, with longitudinal rows of pores diverging from their mid-lines ; taeniae are waved or plane, with the pore-rows directed outwards and upwards from the inner wall. Pores in both types of radial plates are always simple. Tabulae are either simply porous or pectinate (formed of horizontal outgrowths from the septa and walls like the teeth of a comb, not meeting in the mid-line).

The diameter of the great majority of cups ranges between 10 and 20 to 25 mm.

11. DISTRIBUTION IN TIME AND SPACE

Archaeocyatha are known from all the continents except South America, whence the only record has been shown to be false (Feruglio, 1949), They have recently been found in situ in Antarctica (Laird & Waterhouse, 1962), having previously been described from that continent only from morainic material.

Authentic occurrences range through the Lower Cambrian (but not the Eocambrian), with a maximum in the Lena Stage of the south Siberian fold-belt. Only a few genera are known from the early Middle Cambrian Paradoxides oelundicus zone, in Australia

15 Biol. Rev. 39

234 DOROTHY HILL Opik, 1956, 1961), Czechoslavakia (Orlowski, 1959, 1960) and the Altai Mountains of Siberia (Khomentovskii, Zhuravleva, Repina & Rozanov, 1962) where they include Archaeocyathus, Erbocyathus, Tegerocyathus, Ethmophyllum, Vologdinocyathus and Nochoroicyathus. However, Vologdin (1957 a) considers the Elanka faunal horizon at the top of the Lena Stage on the Siberian Platform to be Middle Cambrian, and Opik (1956, 1961) suggests that the greater, upper part of the Lena Stage is of the P. oelandi- cus zone. If either of these contentions is correct, the number of genera ranging into the Middle Cambrian becomes quite large.

The views of Russian specialists on the stratigraphy and sequence of the archaeo- cyathan, algal and trilobite assemblages are conflicting, owing to the disconnected nature of the outcrops and to facies variations. In the classical descriptions of archaeo- cyathan assemblages by Vologdin (1931, 1932, 1937a, b, 194oa, b, 1956b, 1957a, 1962b) many of the assemblages were given ages that have been contested by later workers." Other difficulties have arisen because of lack of good international correla- tion of the Lower Middle Cambrian boundary (apik, 1956,1961). I have outlined the successive and varied published views in another publication (Hill, 1964).

Table I shows some of these classical faunas in a currently acceptable scheme of correlation. The ranges of the faunas in the different stratigraphic profiles are still under intensive study and modifications may be expected.

Table I. Correlation of some classical archaeocyathan faunal horizons of the Siberian fold-belt and platform

horizons

stage and substage fold-belt p 1 at f o rm stage and substage

M. Camb. Amga stage - - M. Camb. Lena stage

L. Camb. Lena stage Obruchev = Elanka L. Camb. Lena stage Angara substage Solontsov = Ketema Angara substage

I >

Kameshki correlation Tolbachan Botoma substage I Botoma substage Sanashtikgol = Olekma

uncertain - Bazaikha Sinyaya I

L. Camb. Aldan stage Zhurin substage

Tolba substage

Atdaban Kenyada Sunnagin -

References to archaeocyatha older than Lower Cambrian and younger than early Middle Cambrian occur in the literature but in my opinion they relate to organisms other than archaeocyatha. Thus Mithracyathus vindhianus Vologdin ( I 957 d ) refers to small calcareous fossils found in the Upper Proterozoic Rohtas horizon of the Semi Series of the Lower Vindyan of India, previously and possibly better regarded as

* Anon., 1959; Arkhangel'skaya, Grigor'ev & Zelenov, 1960; Khomentovskii et al., 1962; Krasno- peeva, 1955, 1958; Repina & Khomentovskii, 1961 ; Repina, Semichatov & Khomentovskil, 1956; Sivov, 1955; Sivov & Tomashpol'skaya, 1958; Zhuravleva, 19606; Zhuravleva & Repina, 1959; Zhura- vleva, Repina & Khomentovskii, 1959a, b, 1960.

The phylum Archaeocyatha 23 5 sporangia of dasycladacean algae. The scale (diameter 0.2 mm.) is a good deal smaller than that normal in archaeocyatha, but Vologdin argued that this is only to be expected in ancestral forms. A small archaeocyathan fauna from the River Serlig, Tuva, that Vologdin ( 1 9 5 9 ~ ) referred to the Upper Cambrian, was subsequently considered by Menner, Pokrovskaya & Rozanov (1960) to be typical of the Lower Cambrian (Middle Lena Stage). Records of Ordovician archaeocyatha from Austria (SchouppC, 1950; Kahr, 1951) refer to fossils too poorly preserved for safe identification.

The Asiatic Siberian archaeocyathan faunas, which are very rich, have been des- cribed by Vologdin, Zhuravleva, Krasnopeeva, Maslov, Rozanov, Fonin and Yaro- shevich in the works listed in the References. European archaeocyatha are rare, but have been described from Spain, France and Sardinia by Debrenne (1958, 1959b, c), from the South Urals by Vologdin (1939) and from Czechsolovakia by Orlowski (1959). African archaeocyatha have been described by Debrenne (1959a, d, 1960, 1961), largely in relation to the Lower Cambrian trilobite faunas established for Morocco by HupC (1960). Apart from these from north-west Africa, the only other African record is a doubtful one from the Kuibis quartzite of the Nama System of South-west Africa (Haughton, 1959). North American archaeocyatha, all of them apparently Lower Cambrian, have been described by Okulitch (1943, 1946, 1956), Okulitch & Roots (1947) and Okulitch & Greggs (1958). Australian faunas were worked by Taylor ( I ~ I O ) , Bedford & Bedford (1934, 1936, I939), Ting (1937) and Debrenne & Debrenne (1961) and Antarctic faunas by Gordon (1920) and myself

111. PALAEOECOLOGY (1964).

Archaeocyatha were marine benthonic organisms, mainly sessile with adherent out- growths, but some were possibly shifting in the bottom layers of the water. Their predilection for carbonate sedimentary facies has long been noted and they have been considered the reef builders of the Lower Cambrian. However, it would seem that they were less important in this respect than algae. The limestones in which they occur include biostromal and biohermal* types (Debrenne, 1959 e ; Zhuravleva & Zelenov, 1955 ; Vologdin, 1959b). Ecological variation is notable between the biohermal and interbiohermal faunas. Some species are confined to the bioherms or to one or other of the different types of bioherms ; others are interbiohermal only (Zhuravleva, 1960b). Latticed walls were considered by Rozanov (1961) to be characteristic of forms living in volcano-terrigenous facies.

The most favourable depth, as indicated by the studies of Zhuravleva & Zelenov (1955), Zelenov (1957) and Zhuravleva (1960b), was from 20 to 30 and down to 50 m. ; within this range they were able to construct bioherms and were associated with the blue-green alga Rendcis. Above and below these optimum depths they were smaller and did not construct bioherms. From 50 to IOO m. they were commonly associated with the red alga Epiphyton, were thin-walled with a narrow intervallum and not fragmented by wave action. They are not known in sedimentspresumed to have been deposited below IOO m.

* A biostrome is a stratiform deposit constructed by sedentary animals; a bioherm is a lenticular or mound-like reef constructed by sedentary organisms whose skeletons fonn its framework.

15-2

236 DOROTHY HILL Associates were algae and the benthonic gastropods, brachiopods and trilobites, but

hyolithids and tentaculitids (small conoidal shells of uncertain affinities) are also found with them in interbiohermal strata. When sponges were common, archaeocyatha tended to be rare.

The surface temperature of the waters in which they lived has been assumed to be between 25 and 30' C., because they formed bioherms, though this assumption is not completely logical. Zhuravleva & Zelenov (1955) distinguished five types of algal, algal-archaeocyathan and archaeocyathan bioherms in the Lower Cambrian vari- coloured suite of the upper Aldan Stage which Zelenov (1957) and others considered to have formed off lands with a hot, humid climate.

Their salinity tolerances were thought to be related to the characters of the sedi- ments in which they occur. Zhuravleva (1960b) found that they are commonest, and constructed bioherms, in rocks with 46-50y0 calcium oxide (= 78-91 yo calcium carbonate), but that they may be quite rich in rocks with a calcium oxide content of only 7%. They flourished best when the content of magnesium oxide in the deposit did not exceed 0-2-0.5 yo. Percentages of 5-8 yo magnesium oxide were endured by very few species, and with a still greater concentration, as in lagoonal deposits, the archaeocyatha disappeared completely.

They were quite tolerant of terrigenous matter on the Siberian platform, and actually flourished best in sediments that contained 19.34 yo of insoluble residues.

A planktonic existence has been postulated for the larval stages by both Vologdin (1932, 1957c, 1962a, b) and Zhuravleva (1960b). Zhuravleva considers that the larvae were without skeletons, like coral planulae, and that they settled ready to grow into the normal cups before forming any calcareous matter; she points out that the youngest stages of the archaeocyatha cups have a diameter of 0-3-0-5 mm. Vologdin, on the other hand, suggests that certain small calcareous fossils 0.7-1.8 mm. in diameter represented the skeletons of planktonic larval and young stages of archaeocyatha, since some of the larger types appear to be two-walled but with a central cavity sealed and of equal diameter above and below. Zhuravleva thinks these small calcareous fossils might be fragments of the epiphyton flora.

IV. SKELETAL MORPHOLOGY

Our knowledge of the morphology of the skeleton has been greatly enriched in the last decade. A very recent paper (Vlasov, 1962) defines the terms in use in Russia.

Outer-wall morphology is more varied than was previously known (Fig. 2). Missar- zhevskir & Rozanov (1962) have devoted a special paper to it. Walls are described as having simple porosity, when the pores, arranged nearly always in longitudinal rows, with the pores of neighbouring rows opposite, sub-opposite or alternate, and round, oval, polygonal, chink-like or crescentic in shape, are simple openings in the wall, without secondary structures round their rims ; such pores are called pore-canals when the wall is thick, and these canals may be straight, or curved, or geniculate. Walls with secondary structures round the mouth of the pore are quite common. The secondary structures include scoop-like processes extending from the lower half of the pore margin, tumuli with one opening over the pores and tumuli with multiple openings ;

The phylum Archaeocyatha 237 longitudinally overlapping scales sometimes form the wall which might then be called Zouvred; in the Russian literature the spaces between the scales and the bounding septa are called pore-canals, but I think it advisable to call them pore-tubes, leaving the word canal for elongate passages in a thick wall. Walls with beaked pore-canals occur

Fig. 2. Types of outer wall. I . Part of perforate wall with two alternating longitudinal rows of simple rounded pores per intersept (diagrammatic). 2. Part of retiform wall with hexagonal pores: Dokidocyathus regularis Zhuravleva, x 15, tangential section. 3. Part of perforate wall with fine longitudinal rows of opposite rounded pores per intersept (diagrammatic). 4. Part of one intersept in wall with pores like chinks: Svetlunocyathus primus Missarzhivskii & Rozanov (diagrammatic). 5. Part of wall with alternating longitudinal rows of rounded pores, one of which is of stirrup pores affecting the septum at its junction with the wall (diagrammatic). 6. Radial section showing septum in Tumulocyathellus unicum Zhuravleva, x 20; stirrup pores are seen at outer (left) and inner walls, each outer wall pore being protected by a tumulus. 7. Geniculate pore-canals in alternating longitudinal rows in part of a thick wall (diagrammatic). 8. Louvred wall showing three rows of overlapping slats to an intersept (diagrammatic). 9. Longitudinal section through the slats of a louvred wall, showing slightly resupinate pore- tubes (diagrammatic). 10. Louvred wall with one row of curved slats per intersept, as in Annulocyathus sp. of Missarzhevskii & Rozanov (diagrammatic). I I . Wall with scoops projecting perpendicularly from lower rims of pores : Robustocyathus spinosus Zhuravleva, x 14. 12. Part of one intersept showing these scoops (diagrammatic). 13. Wall with geniculate pore-tubes whose inner parts are formed by oblique channels through the thick wall, and outer parts are formed by the spaces between successive hemi-dome scoops projecting from the lower rim of the oblique channel as in Porocyathus sp. of Missarzhevskii & Rozanov (diagrammatic). 14. Wall with simple tumuli: Kotuyicyathus kotuyikensis Zhuravleva, x I . 15 . One such tumulus, enlarged x f 5 o (diagrammatic). 16. Part of wall with compound tumuli ; Lenocyathus Zenuicus Zhuravleva (diagrammatic). 17. Compound tumulus (as in 2, 16) seen in radial longitudinal section. 18. Wall with thin outer sheath over bell-mouthed pores, seen in part of transverse section of Piamaecyuthus sp. 19. Part of tangential section of Piamaecyuthus sp. (diagrammatic). 20. Clathrate wall of part of cup of Botomocyathus zelenovi Zhuravleva. 21. Finely porous outer sheath seen in tangential section of Laduecyathus sp. nov. Hill, showing method of formation (semi-diagrammatic). 22. Wall with thin outer sheath, seen in part of transverse section of Erbocyathus (diagrammatic). 23. Outer sheath and tabular wall of Clathricoscinus infirmus (Vologdin), seen in part of transverse section, diagram- matic, a, outer sheath; b, vertical plates; c, tabula. 24. Tangential view of outer sheath and outer edge of tabula, of C. infirmus. 25. Outer sheath and outer edge of tabulae seen in longitudinal section, in C. infirmus. 26. Wall formed by outer edges of tabulae, as in Coscino- cyathidae, cutaway diagrams. (2, 6, X I , 14-17, 20, 23-25 after Zhuravleva; 4,7-9, 13, 18, 19, 26 after Missarzhevskii & Rozanov.)

238 DOROTHY HILL rarely ; in the Russian literature these are termed geniculate pore-canals, but I think beaked pore-canals is better, and avoids confusion with those canals which are bent within the thickness of the wall, and not bent at the junction of a canal with the base of a beak. Walls with supplementary, thinly porous sheaths are moderately common. In the earlier Russian literature some of these walls, such as those in Erbocyathus, were described as having branching pore-canals; but the sheath is formed by the growth of thin spines radially across the outer mouths of the pores, and Mizzarzhevskil & Rozanov rightly consider that the term ‘branching pore-canal ’ should be restricted to those rare thick-walled canals which branch anywhere throughout their length. Latticed walls form where two systems of plates or bar-like elements are developed one outside the other, at right angles; usually the longitudinal elements are applied outside the horizontal elements. Tabular walls are those formed by the down-bent outer edges of successive tabulae.

Inner walls (Fig. 3 ) are more varied even than outer walls, though the tumulose (with a pierced tumule over each pore) and latticed types are rare to absent. Walls with simple pores and pore-canals are common. Walls with secondary structures (scoops, beaks, spines) around the mouths of the pores are common (Zhuravleva, 1960b), so that many types of beaked pore-tubes are formed. Ethmophylloid pore-tubes formed by the opposed waving of the inner edges of neighbouring septa are not seen in outer walls, but fairly commonly form inner walls. Annulate shelves applied to the inner edges of the septa form the inner wall in many genera, but rarely form outer walls. Composite inner walls, with a screen suspended from secondary structures around the pore, are also known.

Septa, taeniae and rods (Fig. 4, 1-13). The most notable advance in knowledge of the radial longitudinal elements has been the demonstration that the attitude of the longitudinal pore-rows in septa and in taeniae (the radial longitudinal plates of Archaeocyatha Regularia) is always at right angles to the curvature of the tabulae, when tabulae are present. This is the same relation seen between the septal trabeculae and the tabulae in corals. In septa the longitudinal rows diverge from an axis near the middle of the septal face, in taeniae they are curved upwards and outwards from the inner wall. Standard taeniae are waved, the crests being parallel to the longitudinal rows of pores, but plane taeniae are known that look straight in transverse section; they can then only be distinguished from septa by the attitude of their longitudinal pore-rows (Bedford & Bedford, 1936 ; Zhuravleva, 1960b).

Tabulae (Fig. 4, 18-21). In addition to tabulae with simple rounded pores, tabulae with chink-like pores and pectinate tabulae are now known. Chink-like pores occur in two radial rows, with a median bar between the rows, in Retecoscinus (Zhuravleva, 1960b). Pectinate tubulae, first described by Bedford & Bedford (1936), have now been shown by Zhuravleva (1951b, 1960b) to be characteristic of the suborder Nochoroicyathina. They are formed by slender spines growing from the septa like the teeth of a comb, towards but not meeting at the median lines of the interseptal loculi, normally at the same level in neighbouring loculi. The septa are thickened where the spines issue, and in many cups or part of cups only these thickenings, nodulose in tangential section of the septa, are seen. The pectinate tabular floors are straight or

The phylum Archaeocyatha 239 only slightly curved; in some individuals they are very rare and can easily be missed unless special search is made for them. Pectinate tabulae are not known in Irregularia.

Tubuli (Fig. 4, 14-16). Hexagonal tubuli radially arranged, fill the intervallum in one order, the Syringocnemida.

Fig. 3. Types of inner wall. I . Simply porous inner wall, with 2-3 alternating longitudinal rows per intersept, including stirrup-pores ; two intersepts viewed from central cavity (dia- grammatic). 2. Wall formed of thin transverse bars, in obliquely tangential section, Pycnoi- docoscinus pycnoideum Bedford & Bedford, x 4. 3. Scaly inner wall, in obliquely tangential section, Cadniacyathus asperatus Bedford & Bedford, x 8. 4. The same, radial section of cup, inner wall scales shown to right, x 8. 5. Louvred inner wall, viewed from central cavity, in Beltanacyathus conicus Bedford & Bedford, x4. 6. The same in radial section of cup, x4. 7. The same, in transverse section of cup, x4. 8. Spinose inner wall, the spines partially screening the pores, in Archueofungia nivorovae Zhuravleva, ~ 4 0 . 9. a-d, Four types of spines developed from the lower rims of inner wall pores (diagrammatic). 10. Inner wall pore with protective spine, and hair-like projections into the pore cavity, Robustocyathus spinosoporus (diagrammatic). I I . Scaly inner wall in Leptosocyathus polyseptus (Latin), with each scale extending in front of three intersepts, x 60. 12. a-d, Tumulose and tubulose inner wall pores, with protective hairs and screens (diagrammatic). 13. Ethmophylloid inner wall, formed by waving of the inner edges of the septa, opposed waves of neighbouring septa being contiguous, and the crest of the waves being directed steeply upwards and inwards to the central cavity in radial section of Ethmophyllum, x7) approximately. 14. The same, in transverse section of cup, x 4 approximately. 15. Branching geniculate pore-canals in inner wall of Heckericyathus heckeri (Zhuravleva), x 50. 16. The same, in radial section of cup, x 50. 17. Annulate inner wall, the annular shelves being V-shaped in section, in Botomo-

cyathus zelenovi (Zhuravleva), x 2). 18. Composite inner wall in Compositocyathus muchatensis (Zhuravleva); the wall consists of annual shelves wedge-shaped in section, from which project radial rods in line with the septa, a fmely porous screen being suspended from the inner ends of the rods; x 50. ( 2 1 after Bedford & Bedford; 8-12,15-18 after Zhuravleva).

Dissepiments (Fig. 4, 26). The imperforate condition of these plates, in contrast to the perforate nature of all other elements in archaeocyathan skeletons, is emphasized in recent works. Their development in the tips of the cups of Irregularia before any inner wall or radial longitudinal elements occur has been used to distinguish this class from the Regularia (Vologdin, 1937b; Zhuravleva, 1960b). One may cross the

240 DOROTHY HILL

Fig. 4. Structures of the Intervallum. I. Growth of a new septum in Ajacicyathus sp., lateral view, outer wall (o.w.) to the right, x 20. 2. Decreasing size of pores with growth of septum in Ajacicyathus sunnaginicus Zhuravleva, lateral view of septum, x 40. 3. Decrease in number of longitudinal pore-rows with growth of septum in Erbocyathus hetmmallum Vologdin, lateral view of septum, x 40. 4. Septum with stirrup-pores at outer (at left) and inner walls (i.w.) of Tumulocyathellus unicw lateral view, x 40. 5. Pore-arrangement characteristic of septa, i.e. in diverging longitudinal rows (diagrammatic). 6. Pore-arrangement characteristic of taeniae, i.e. in longitudinal rows curving upwards and outwards from the inner wall (at right); dotted lines are projections of the growth lines into the central cavity (diagrammatic). 7. Taenia with longitudinal, angulate waves curving upwards and outwards from the central cavity (at left); synapticulae project from crest of the waves; lateral view. 8. Taenia of Fig. 4, 7, in oblique transverse section (a-b) nearly parallel to the longitudinal waves and pore-rows. 9. Taenia of Fig. 4, 7, in oblique transverse section (c-d) nearly normal to the longitudinal waves. 10. Taeniae in part of transverse section of Pycnoidocyathus synapticulosus Bedford & Bedford, showing the opposed angulate waves of neighbouring plates joined by synapticulae, x 4. I I . Intervallar rectangular framework of vertical pillars, radial links and tangential rods in Dictyocyathus sp., x 100. 12. Spines growing from the framework rods of Spinosocyathus sp., x 100. 13. Radial rods in the intervallum of Dokidocyathus regularis, x 5. 14. Syringo- cnema faous Taylor, external view, x approximately. 15. The hexagonal tubules of the intervallum of S.favus; transverse fracture of the cup, x 2. 16. The same, in radial longitu- dinal fracture of the intervallum, x4. 17. A single hexagonal tubule of S. faous, in serial sections a-g from central cavity to outer wall, x 8. 18. A coscinocyathid tabula, showing arrangement of simple pores. 19. Tabula with chinky pores, diagrammatic. 20. Pectinate tabula in Nochoroicyathus mirabilis Zhuravleva, x 60. 21. Nodular swellings of septa at the level of a pectinate tabula in Nochoroicyathus sp., x 100. 22. Hairy cap at the growing edge of the cup in Ajacicyathus sunnaginicus, in radial section, x 5 . 23. Upper edge of cup and curved coscinocyathid tabulae in radial section (diagrammatic). 24. Upper edge of cup and pectinate tabulae in radial section (diagrammatic). 25. Hairy cap at growing edge of atabulate cup in radial section (diagrammatic). 26. Dissepiments and upper edge of cup in radial section (diagrammatic). 27. a-e, Secondary thickening in Ajacicyathus sp., diagram; secondary layers over outer wall outwards; b-c, over outer wall inwards, enveloping septa; d-e, over inner wall outwards and inwards, filling all spaces. 28. Layers of secondary thickening in Ajacicyathus anabarensis (Vologdin), over part of outer wall and septum (black), ~ 4 0 . 29. Granular texture of skeletal material in thin radial section of septum of Ajacicyathus gigantoporus Zhuravleva, showing both dense and sparse granularity, x 25. (1-6, 11-13 and 18-29 after Zhuravleva, 1960; 7-10 and 14-17 after Bedford & Bedford, 1936.)

The phylum Archaeocyatha 241 intervallum in several interseptal or intertaenial loculi, but normally they form separately in individual loculi. They may also be developed in the central cavity.

The growing end of the cups (Fig. 4, 22-6). Some interesting structures at this end have been described. One of the simpler conditions is an incurving of the wall in simple-walled cups, so that the orifice is constricted (Hill, 1964). Maslov (1960, 1961) has described several types of opercula or pelta in single-walled cups. One type has a similar porosity to the wall, another is more coarsely porous, a third aporose. Some have a central orifice; in others, none has been observed. Some appear to have been formed by one wall (or both walls) growing over to meet the other in a curve or at an angle.

Zhuravleva (1960 b) has described several different end-structures to the intervalla of two-walled cups. She concluded that all tabulate archaeocyatha end their develop- ment and their life with the formation of the top tabulae, whether these are porous or pectinate. Krasnopeeva ( I 960) regarded the tabulae as ceilings, concluding definite growth stages. In dissepimented forms, such as Loculicyathus, dissepiments close off the intervallum at the top. In at least one non-tabulate and non-dissepimented species, Ajacicyathus sunnaginious Zh., the cup ends not with an open intervallum but with its intervallum tapered off by the coming together of the two walls, the outer arching over and growing more rapidly than the inner, forming a sort of cap, from which small spines may project. Zhuravleva found traces of such spiny caps attached to the inner wall in the central cavity of over 50 individuals of A . sunnaginicus or Ladaecyathus limbatus, as if they had been pushed aside by upward growth.

Extravallar growths, including holdfasts (Figs. I and 5). Skeletal growths are some- times seen outside the outer wall and sometimes also outside the inner wall in the central cavity. Some are clearly holdfasts, of which Zhuravleva (1960 b) has distin- guished several types. The simplest is probably the dense and aporose material in which the tips of many coscinocyathids may be embedded. Canaliculate dense material is seen in some, the canals opening into the wall pores. Tubular processes with or without dissepiments are common.

Other structures, called protuberances of the intervallum by Zhuravleva, and which possibly also have a holdfast function, have a considerable literature on their interpre- tation. Most of them contain elements that are or resemble septa, taeniae, tabulae or dissepiments and are seen in contact with archaeocyatha of similar or different inter- vallar structure. Zhuravelva (19593, 1960b) and Okulitch (1946) consider them to be protuberances from the cups with similar intervallar structure, holding fast to other cups which may have different intervallar structure. Vologdin (1959c, 1962a) holds that most of these are encrusting archaeocyatha of irregular growth form, single- walled and hence without central cavity, and has given separate generic names to the several types (Tersia, Labyrinthomorpha, Exocyathus, etc.). Maslov (1958) suggested that facultative parasitism was involved in the association of ‘ Twsa ’ with Mikhnocyathus.

Zhuravleva suggests that these ‘protuberances ’ were formed from protoplasmic extrusions from the intervallum, rather like those in foraminifera.

Skeletal tissue in the central cavity (Fig. 5 , I). In addition to dissepiments, canalicu- late or irregular calcareous tissue is sometimes seen in the central cavity. Vologdin

242 DOROTHY HILL (see below, Section VI) interprets this as calcified assimilative organs with or without calcareous supporting plates. Zhuravleva considers it due to extravallar protuberance and Maslov (1958) suggests facultative parasitism for some.

Secondary thickening versus cakification of soft parts (Fig. 4, 27a-e). As will be discussed in Section VI, calcareous material sheathing walls, septa and taeniae in many cups is regarded by Zhuravleva as secondary thickening of skeletal elements and by Vologdin as accidentally calcified soft parts.

1 2 3 Fig. 5. Extra-vallar growths, including holdfasts. I . Tubulose elements of central cavity in Leecyathus mikhnoi Vologdin; a, median longitudinal section, b, transverse section, x 3). Vologdin regards them as calcified assimilative organs ; Zhuravleva considers them formed by extravallar protoplasmic protrusions. 2. Tersioid processes (a-n), associated with Bicyathus crassimurus (0) x 24. 3. Labyrinthomorph processes (a), associated with Coscinocyathus dim- thus Bornemann (c ) , and Profopharetra Zaxa Bornemann, (b); x s, all figures (1-3) after Vologdin.

V. MINERAL NATURE, MICROSCOPIC TEXTURE AND ORIGIN OF T H E SKELETON

I t is now clear that, as Hinde (1889) pointed out, the skeleton was originally formed of calcium carbonate, though Ting (1937) and Simon (1939, 1941) thought it was of silica. Subsequent replacement of the skeletons by silica is a common feature in many archaeocyathan limestones.

In the least-altered specimens that I have examined from Antarctica, the septa and walls consist of sub-equidimensional contiguous, microgranular crystals of calcite, arranged with their c-axes in random directions. The finest average granuIarity that I have observed is in Ladaecyathus sp.nov., where the septa have granules of average diameter about zp. This texture resembles that described for the wall of living porcellanous foraminifera by Wood (1949). Slightly coarser textures have been observed, but in these considerable recrystallization is suspected, or can be proved,

The phylum Archaeocyatha 243 and frequently has also caused considerable inequality as well as enlargement in grain size. In a few places in a few species, acicular crystals appear to be present.

Fine equigranularity has also been observed as typical for the Siberian archaeocyatha, but Zhuravleva (1960b) cites 0.01-0.02 mm., that is, five to ten times the grain size recorded above. Clearly, more measurements of grain size are desirable, to establish whether this varies from species to species or only with degrees of recrystallization.

Growth lamellation is visible in some skeletal elements as variation in amount of light transmitted. Some layers appear dark, others light, by transmitted light, but by reflected light show more densely or less densely white. Some authors consider the dense parts to have contained originally, or still to contain, a higher percentage of organic matter than the lighter, but so far as I know no geochemical or electron microscpe studies have been applied to the problem. The septum of corals shows by transmitted light a median dark line where the needles of calcium carbonate of the two halves of the septum meet, but no such line is visible in the middle of the septa or the walls of archaeocyatha.

There is never any ‘dark line’ at the junction of one skeletal element with another, such as forms when plates having differently oriented fibres are in contact or where one plate is formed later than another. All appearances in thin section suggest com- plete continuity of the skeletal elements.

In comparing this microstructure with that of other phyla, we may note that it resembles that of the porcellanous foraminifera. Considerable variation is known in the microstructure of foraminifera, ‘chitin’, aragonite and calcite may be found; aragonite may occur as elongate needles or ‘fibres’, calcite occurs as either granules or fibres, or as granules arranged in rows, mimicking fibres-‘ pseudofibres ’ ; the ‘chitin’ may form the entire test, or be interlayered or intermixed with the carbonate; some ‘bilamellid’ walls are formed of two lamellae deposited at different times, by different protoplasmic secreting surfaces (Todd & Blackmon, 1956; Cummings, 1956; Reiss, 1958). So far no trace of such variation has been seen in archaeocyatha, which are rather

strikingly uniform. Although they resemble the porcellanous foraminifera in their grain size, they are characteristically finely perforate, whereas the porcellanous fora- minifera are imperforate.

No comparison of microstructure is possible with the sponges, since archaeocyathan skeletons are not spiculate. No comparison with the coelenterates is possible either: there are few if any of the acicular crystals and no spherulitic crystallization such as are together characteristic of the corals (Bryan & Hill, 1941). No similarity to the microstructure of the skeletons in higher phyla is noted.

Opposed views are held by Zhuravleva (1959, 1960b) on the one hand and by Vologdin (1962a) and Krasnopeeva (1960) on the other on whether the skeleton is internal or external.

Zhuravleva considers that the skeleton was secreted externally to the protoplasm as in protozoa (foraminifera), as a continuous sheath. She says that, judging from the initial stages of the skeleton and from the bilaminate nature of the septa and taeniae, all the main skeletal elements (the porous walls, septa, taeniae and tabulae), the

244 DOROTHY HILL dissepiments and the walls of protuberances or ‘ tersioid ’ and other outgrowths, may be regarded as external skeleton. Only secondary skeletal elements, such as disconnected rods in the intervallum of some Irregularia, have a doubtful origin. Krasnopeeva (1960) regards the tabulae as the ceilings, concluding the growth stages.

Vologdin, however, basing his conclusion on his view that the ‘secondary thickening’ of Zhuravleva is calcified soft tissue, and noting that this may sheath all types of skeletal elements, considers that the latter are laid down internally to the sheathing soft tissue.

VI. ONTOGENY

Archaeocyatha are considered by Bedford & Bedford (1939), Okulitch (1943), Vologdin (1957d, 1959c), Zhuravleva ( I ~ ~ I U , 1960b) and Debrenne (1959f) to recapitulate their phylogeny in their ontogeny. The two main divisions of the phylum, the Regularia and the Irregularia, differ not only in the nature of their radial plates but also in their earliest stages; in the two-walled Regularia the inner wall and radial elements appear before any dissepiments but in the Irregularia dissepiments are the first elements to appear in the cup.

Zhuravleva’s classification of the phylum (1960b) makes use also of the later onto- genetic stages, as well as the adult characters of the cups. Her diagrams are adapted in my Fig. 6. She concluded that post-larval development began with the formation of an aporose curved sheet that became the very tip of the cup; the edge of this calcareous sheet grew upwards and outwards to form the outer wall of the cup. At a cup diameter of 0-15-0-2 mm., simple pores appear, and this stage is common to all archaeocyatha. Subsequent development differs in the Regularia and Irregularia.

In the Regularia so far studied ontogentically, the inner wall, with small, simple pores, appears at a cup diameter of about 0.2 mm. and a height of not more than 0.8 mm., and is commonly connected to the outer wall by radial rods; no dissepiments are known. At this point there is usually a noticeable widening of the cup. In the Irregularia at this diameter, on the other hand, the whole internal cavity is filled with dissepiments and rods without orderly orientation, and the inner wall has not yet appeared.

In the Regularia septa and tabulae (either porous or pectinate) appear at a cup diameter between 0.22 and 0.45 mm. Zhuravleva’s classification gives ordinal value to the presence or absence of septa and subordinal value to the presence or absence of tabulae and the type of tabulae, so that ordinal and subordinal characters are present in Regularia at a diameter of 0.45 mm. In the Irregularia no new types of skeletal elements appear between 0.2 and 0.45 mm. diameter, but dissepiments are obvious.

Zhuravleva’s classification, while professedly ontogenetically based, has much of the character and usefulness of a key and regards the types of outer wall pores in Regularia as characteristic of superfamilies ; diagnostic types appear at an average cup diameter of 0-5-0.7 mm. Thus in Lenocyathidae complex tumular pores begin to develop from the earlier, simpler outer wall pores at this diameter of cup. The inner wall pores still remain simple. Rozanov & Missarzhevskil (1961) agree that

cup diam.

&!&

>I4

- 1 -7

to

1 *1

- 1 s o

to

0.8

- 0.7

to

0.5

- 0.45

to

0.22

0.20 to 0.1 3

to

-

-012 a

The phylum Archaeocyatha

4 Metacyathus type U Alaclcyathus type

Fig. 6. Ontogeny in Archaeocyatha. The cup diameters at which the different stages appear are shown in the left column; in the middle column are representatives of the Irregulars, and in the right column of the Regulares. A, The embryonic stage, the aporose tip, is common to both classes. B,-G,, Stages of development in representative Irregulares : x, form with teniae; y, form with teniae and tabulae; z, form with radial tubuli (Syringocnemida). B,-G,, Stages of development in representative Regulares : a form with pectinate tabulae, (Nochoroi- cyathina) ; p, atabulate form (Ajacicyathina) ; y , form with porous tabulae (Coscinocyathina). a, outer wall; b, inner wall; c, radial rods in intervallum; d, septa; e, pectinate tabula;f, porous tabula; g, pore-tubes of outer wall; h, composite tumulus of outer wall; i, rudimentary pore- tube of inner wall;j, pore-tube of inner wall; k, hairs at end of pore-tube of inner wall; I , rod in intervallum; m, dissepiments, n, tenia; o, tenial spine; p , convex tabula; q, hexagonal loculu s . (After Zhuravleva.)

245

246 DOROTHY HILL complexity of the outer wall pores appears ontogenetically earlier than complexity of the inner wall pores.

The inner wall of Irregularia first appears at a diameter between 0.5 and 0.7 mm., and then has simple pores. Between 0.8 and I mm. diameter, in the Regularia, development of complex outer wall pores is completed and complex inner wall pores may replace simple pores. At this diameter in the Irregularia the rods of the inter- vallum begin to change into taeniae and the various family characteristics appear.

Between 1.1 and 1-7 mm. diameter, according to Zhuravleva (1960b), all archaeo- cyatha attain the characters that distinguish one genus from another within the family, and finally, between 1.8 and 3 mm., appear the specific characters, such as numbers of rows of pores in septa and tabulae, ‘protective’ processes round the wall pores, taenial spines, etc. All later changes are proportional to the growth of the cup-increase in diameter, in number of septa, and change in diameter of pores, etc.

Zhuravleva’s views on the use of ontogentic stages in classification may be sum- marized as follows : The class characters (presence or absence of dissepiments before the inner wall appears) are established at a cup diameter 0. I 3-02 mm. ; then the inner wall appears between 0-5 and 0.7 mm. in two-walled Irregulares, and between 0.13 and 0.2 in Regulares. She regards the regularan subordinal characters (type of tabulae) as established between 0.22 and 0.45, whereas the subordinal characters of Irregulares do not appear until 0.5-0.7 mm. The family (outer wall) characters of Regularia develop between 0.5 and 0.7, but in Irregularia not till 0.8-1.0 mm.; at which diameters in Regularia subfamily characters develop. Generic characters appear in both classes between 1.1 and 1-7 mm. cup diameter, and specific characters enter thereafter.

Vologdin ( I ~ s ~ c ) , using a combination of comparative morphology of adult and young stages, has indicated the derivation of several genera from others, and has shown that some genera previously regarded as Regularia are Irregularia.

McKee (1963) has studied the ontogeny of Ethmophyllum whitneyi Meek and given details of the formation of its complex inner wall, He has described the one-walled stage at the tip of the cup as perforate, in contrast to Zhuravleva’s view that it is always imperforate.

VII. TRENDS OF DEVELOPMENT

It can be seen that in Zhuravleva’s or Vologdin’s views of phylogeny more complex forms develop from less complex. While noting the general occurrence of recapitula- tion of phylogeny in the ontogeny of archaeocyatha, Rozanov (1961) indicated that in this phylum, as in the graptolites and foraminifera, heterochronous and convergent evolution may occur as a result of the operation of trends of development. Rozanov’s lineages are deduced in part by morphological comparison of bioseries in successive strata and in part by ontogenetic observations. Some of the trends of development he recognized are trends in reduction of the number of a character, such as the number of longitudinal pore-rows in the intersepts in both inner and outer walls. This trends acts in different families at different times and rates. Another such reduction trend is seen in Nochoroicyathus, where the younger species have fewer pectinate tabulae than the

The phylum Archaeocyatha 247 older. Trends in colony development may act at the same time in different lineages. Trends in complication, such as the production of tumuli over the pores of the outer wall, are noted in several assumed lineages.

VIII. INTRASPECIFIC VARIABILITY

Zhuravleva (1960b) has included in her descriptions of species of the Siberian plat- form much information on variability. She has demonstrated geographical variation, in that individuals of a species found in the north-west of the platform may differ in size of cup, or in the ranges of size of septa1 or taenial pores, or in the frequency of septa, or in the number of spines on the inner wall, from individuals of the same species found in the south-east of the platform. She showed, by a block diagram, variation in time and space in Ajacicyathus anabarensis. The tendency to form colonies in a species may increase in a given geographical direction.

Dimensional differences are also associated with depth of floor, character of bottom, and possibly rate of flow of currents, temperature, gas rCgime and faunistic composi- tion. Thus outer wall tumuli may be larger in individuals from deeper seas.

Individuals of one species may have different growth forms in biohermal and inter- biohermal facies. Thus OKuZitchicyathus discoformis (Zhuravleva) may be discoid in the sediment between the bioherms, owing probably to frequent overturning, but is often of irregular conical form in the bioherms. Biohermal individuals usually have external adherent processes; interbiohermal individuals usually lack them. Many species have smaller individuals in the bioherms, contrary to what one might have expected. Rozanov (1961) drew attention to the occurrence of archaeocyatha with latticed walls in volcano-terrigenous facies, and considered this to be evidence that such a wall is an adaptive character.

Perhaps the most notable example of intraspecific variability is seen in species with pectinate tabulae. They may be so rare in an individual as to be missed unless special, very careful search is made for them; in another individual they may be immediately apparent and common (Bedford & Bedford, 1936, 1939).

Dimorphism is suggested by some fairly sharply divorced end-points of variation series (Zhuravleva, 1960b). Thus, perhaps half the members of a species may have a narrow intervallum that remains almost constant in width as the diameter of the cup increases; in the other half the intervallum expands as the cup grows. The develop- ment of pectinate tabulae is another, similarly contrasting phenomenon.

IX. THE SOFT PARTS

No impressions of the soft parts in mud or fine sand, like those of Eocambrian jelly- fish, have been found, unless the problematica tentatively referred to the Archaeo- cyatha by Haughton (1959) from the Kuibis beds of the Nama quartzite of south-west Africa may be such.

Some Russian authors (Vologdin, 1931, 1948, 1957a, 1962a; Maslov, 1960) hold that accidentally calcified soft parts are found on or within the skeletons of some Siberian individuals. These ' calcifications ', which they suggest occurred because of

248 DOROTHY HILL the sterility of the water (absence of putrefactive micro-organisms) or because of thc rapid fossilization of the dead bodies in water which was quite rich in dissolved bicarbonates of calcium and magnesium, are of two kinds: (I) dense enveloping material, sheathing both the septa and the outer and inner surfaces of the walls (Fig. 4, 27), and (2) material surrounding canal-like spaces within the central cavity, the canals being continuous with the pores of the inner wall (Fig. 5 , I). Vologdin ( 1 9 6 2 ~ ) considers that in some instances true skeletal supporting plates exist at the angles between three such surrounded spaces.

Zhuravleva (1959), however, regards the sheathing tissue of the first type as secondary thickening of the skeletal elements. She points out (1960b) that it is often banded in such a way as to indicate periodic accretion parallel to its surface. This explanation seems more satisfying than the one of accidental calcification. The material has something of the appearance of the intercameral deposits of the nauti- loids.

The question of the tubulose structure in the central cavity of some archaeocyatha is of greater complexity. It is bound up with the interpretation of the nature of the archaeocyathan individual. Vologdin has interpreted the structure as due to accidental calcification of a ramifying digestive system ; this would suppose an organization higher than that of sponges. Zhuravleva (1959, 1960b) regards it as formed from an extrusion into the previously empty central cavity of soft tissue from the intervallum, this soft tissue then proceeding to secrete septa1 or taenial or dissepimental or tabular skeletal elements, and this is the explanation I prefer. Fonin (1961) objected to both interpretations. He though some effects were purely petrological and some others resulted from the breaking of a cup under pressure. He was unconvinced of the existence of an ‘internal digestive organ’ in the central cavity, but said that it was impossible to agree that the central cavity was empty, since without the pressure of tissue the formation of such complex skeletal elements as tubules (in Prismocyuthus) was impossible. Another explanation offered for certain of the calcareous elements in the central cavity is that they were deposited by a second but single-walled archaeo- cyathan living in symbiosis with the first (Maslov, 1960).

The pores of the outer and inner walls are frequently considered to have served to draw food-bearing currents into the body. Vologdin considers that the currents were drawn into the central cavity from above, then passed through the outer wall. Zhurav- leva considers the currents flowed in the opposite direction, and in this she is sup- ported by Fonin (1961). Maslov (1961), basing his conclusion on his observation that either perforate or aporose pelta or opercula may develop at the growing end of the cup, considers that currents must have been able to flow either way through the pores of one-walled cups.

This brings us to a consideration of where the vital activities of an archaeocyathan proceeded, whether in the central cavity or in the intervallum. Vologdin, as seen above, is convinced that the digestive processes occurred in the central cavity and that each cup is a single individual, but Zuhravleva (1959, 1960b) agrees with Taylor (1907, I ~ I O ) , Okulitch (1955) and Fonin (1961) that the intervallum was the site of the main life processes.

The phylum Archaeocyatha 249 Zhuravleva based her interesting conclusions on her view that the skeleton of the

archaeocyathan is an external, continuous sheet, and on her comparative study of the archaeocyatha with their closest morphological types-protozoa, sponges and coelen- terates. She noted that although the protoplasm of neighbouring interradial loculi was continuous through pores in the septa or taeniae, yet a certain amount of inde- pendent vital activity between neighbouring chambers was suggested by differing development of dissepiments, of skeletal thickening and sometimes of tabulae within them. She considered the view of Taylor and of Okulitch that the living matter in the loculi could be explained by analogy with the mesogloea of sponges and she thought, like Taylor, that the sponge ‘individuum’ should be compared with the ‘individuum’ of the interseptal or intertaenial loculus rather than with any occupant of the central cavity. Taylor expressed this view by saying that he thought the archaeocyathan cups were compound, like a complex sponge. Zhuravleva considered this arrangement to indicate that digestion took place not in a distinct layer of epithelial cells, as in coelenterates, but intracellularly, and also that special secretory organs were absent. Her view of the living matter of the archaeocyathan is that it consists of uniform, undifferentiated cells (except for some sex cells), filling the intervallar loculi, the basic life processes taking place intracellularly, digestion and secretion being analagous to those in protozoa and sponges. Vologdin (1962~) as a corollary to his views on the ‘ calcification’ of the soft tissues, concluded that the latter were cellular, with func- tional and morphological differentiation, and that there was a system of capillary vessels connecting the various parts of the body in a single organism.

Zhuravleva considered that the formation of the skeleton was similar to that in protozoa, but the connexion between the groups of cells in neighbouring loculi was ‘incommensurably higher’ than that between the cells even in such complex colonial unicellular organisms as the Volvocidae. This connexion is shown by the presence of pores in the septa, taeniae and tabulae, by the continuity of the several skeletal elements and by the fact that after injury to some loculi of the living cup, the archaeo- cyathan could heal the injuries and continue to build the cup. She thought it possible that single-layered forms as well as single-layered forms with undifferentiated and polar cells, and two-layered forms, might have been represented amongst the Archaeo- cyatha, the last two in the more specialized forms.

Zhuravleva (1959) gave a short discussion of the relevance of her hypothesis on the nature of the soft parts of archaeocyatha to hypotheses on the origin of multicellular organisms. She, with the majority of Russian scientists, recognizes as the ancestor of the multicellular organisms a creature with a blastula-like (one-layered) structure, rather than one with a gastrula-like structure. Her view is based in part on the obser- vation that the gastrula stage is not characteristic of the sponges, and in part on the observation that the gastrula stage is seen quite late in the ontogeny of the coelenterates, and then, in the majority, not as a result of the invagination of half of the blastula into the interior, but due to the immigration of some of the cells from the surface. From these considerations, she stated, Mechnikov (1953-55) developed the theories of ‘ parenchymellae ’ and ‘ phagocytellids ’. She also said that investigators of recent Metazoa assert that scarcely any analogies of the hypothetical blastea can be found

16 Biol. Rev. 39

250 DOROTHY HILL among present living forms. All acknowledge that if the blastea existed (and Zhurav- leva considers that it must certainly have existed), it would be necessary to search for it in very ancient strata.

Zhuravleva points to the Archaeocyatha as fulfilling the requirements of the hypothetical blastea. Their position in the chronological scale (Lower Cambrian) is right, and the unilamellar, multicellular organisms she deduced for the simple types of archaeocyatha correspond to the characteristics of the polyspecific blastea of Mech- nikov. The later stage in the development of the (more complex) archaeocyatha-a possible acquisition of a two-layered form-can, she states, by no means be correlated with the gastrea or with the gastrula stage.

She concluded that the archaeocyatha were the first attempt of nature to create a single multicellular organism, and that as soon as a broad proliferation of similar ecological forms began (sponges and later coelenterates), the archaeocyatha were quickly extinguished. She therefore placed the archaeocyatha in a multicellular phylum, arising independently from the unicellular organisms, with a degree of organization higher than that of the protozoa but with less differentiation than the sponges.

X. CLASSIFICATION There are almost as many classifications as there are workers on the group. The

latest authoritative system in English (Okulitch, 1955 in Volume E of the Treatise of invertebrate palaeontology (ed. R. C. Moore)) is now inadequate, in view of the great amount of later Russian work. The three major modern classifications are those of Krasnopeeva (1955, 1960), Vologdin (1956c, 1957a, 1962b) and Zhuravleva (1960b).

All are now agreed that the Archaeocyatha are an independent phylum, somewhere near the sponges in the animal kingdom. Zhuravleva (19593) has given a detailed comparison of the phylum with the Protozoal the Porifera and the Coelenterata. Rejecting, as seems reasonable to me, the possibility of accidental calcification of the soft tissue, and interpreting such sheaths as secondary thickening of the sheathed skeletal elements, she regards the archaeocyatha as a primitive phylum of single multicellular animals, with a level of organization lying between that of the Protozoa and that of the Porifera, and with this I concur. Krasnopeeva (1960) and Vologdin ( 1 9 6 2 ~ ) seem to prefer a position nearer the Coelenterata.

Fundamentally, all classifications of the Archaeocyatha recognize two main divisions in the phylum, Regularia and Irregularia. Two small groups, the Silurian Aphrosal- pingidae Myagkova 1955 and the Carboniferous to Cretaceous Sphinctozoa Stein- mann 1882 (= Thalamida), which are homeomorphic with some families of archaeo- cyatha, the first with the Syringocnemida and the second with the Archaeosyconidae, are sometimes added (Vologdin, 1957 a, as Classes Aphrosalpingoidea and Tabuloidea?) or doubtfully included (Zhuravleva, 1960b), but in my view both are better excluded. The first might possibly be algal, and the second seems best left in the Porifera, as calcareous sponges.

Vologdin (1962b) has named two classes. One of these, Cribrocyathea, is for two- walled cups in which the outer and sometimes the inner wall is clathrate, of marked

The phylum Archaeocyatha horizontal elements applied outside parallel longitudinal elements. As no published descriptions of the members of the class exist, it is not considered further herein, except that, from the illustrations in a French translation” of an unpublished MS. by Volog- din, it appears to me that its members might be algal. They are on a smaller scale than all but the so-called ‘planktonic’ or ‘larval’ archaeocyatha. The second class named by Vologdin is the Monocyathea, in which he placed all one-walled cups, though some of them appear to have the early first development of dissepiments characteristic of Irregularia.

I prefer to consider the phylum as consisting of two classes: (I) the Regularia, of one- or commonly two-walled cups in which the radial elements and the inner wall of the skeleton appear in ontogeny earlier than dissepiments, and consist either of longitudinal rods, or septa, the septa having divergent longitudinal rows of pores, and (2) the Irregularia in which dissepiments appear in ontogeny earlier than the inner wall and the radial skeletal elements, which consist of rods, or taeniae in which the longitudinal rows of pores are directed upwards and outwards from the inner wall, or of radiating hexagonal tubuli (Hill, 1964).

In the classifications that are most easily applied, the presence or absence of an inner wall serves to divide both Regularia and Irregularia into orders. The main characters of value in the subdivision of the two-walled orders are the nature of the radial skeletal elements, whether rods, plates or tubes, and the absence of tabulae or the presence of different types of tabulae.

Krasnopeeva (1955) adopted an interesting family classification in which the presence or absence of tabulae was regarded as of generic value only. But experience seems to show that tabulae are of subordinal value. Raymond (1931) suggested that absence of tabulae might mean that they had been resolved by the protoplasm.

Class Regularia : one- or two-walled, radial skeletal elements rods or septa, tabulae

Order Monocyathida : one-walled cups, without dissepiments or radial skeletal

Order Ajacicyathida: two-walled.

To my mind the phylum Archaeocyatha is best divided as follows:

present or absent, inner wall developed before dissepiments.

elements.

Suborder Dokidocyathina : radial skeletal elements rods, tabulae absent. Suborder Putapacyathina: without septa but with porous tabulae. Suborder Ajacicyathina : with septa, without tabulae. Suborder Nochoroicyathina: with septa and pectinate tabulae. Suborder Coscinocyathina: with septa and porous tabulae.

Class Irregularia : one- or two-walled, radial skeletal elements rods or taeniae or radral tubules, tabulae present or absent, dissepiments developed before inner wall.

Order Rhizacyathida : one-walled. Order Archaeocyathida: two-walled.

Suborder Archaeocyathina : radial skeletal elements rods or taeniae, without tabulae.

* Service d’information gbologique, Bureau de recherches gbologiques et minibres, No. 2605. 16-2

252 DOROTHY HILL Suborder Archaeosyconina : radial skeletal elements rods or taeniae, with porous

Order Syringocnemida: two-walled, radial skeletal elements hexagonal tubuli,

In this system the characters of the walls are diagnostic of the different families within the suborders. It is the system of Zhuravleva but with the Aphrosalpingoidae and Sphinctozoa excluded and the class name Euarchaeocyatha excised, but its sub- classes, Regularia and Irregularia, restored to class status.

The classification, which has many of the virtues of a key, is easy to use; the chief difficulty is the technical difficulty in obtaining thin longitudinal sections through the tips of the cups, to discover the presence or absence of dissepiments, or alternatively, thin longitudinal sections along the septa or taeniae to disclose the arrangement of the longitudinal pore rows. Once the class character has been established, the assignment to orders and families is straightforward.

tabulae.

without tabulae.

XI. SUMMARY

I. The Archaeocyatha are, geologically speaking, a short-lived group, characteristic of the carbonate-shelf and reef environments of the Lower Cambrian and early Middle Cambrian. They are found in all continents, excepting South America but including Antarctica. They are the only animal phylum that has become extinct.

2. They were marine, benthonic organisms, probably with planktonic larval stages. Suggestions that larvae had skeletons and that certain small calcareous fossils were planktonic young stages of archaeocyatha are discounted.

3. They existed in depths down to ~ o o m . , could construct bioherms at depths between 20 and 50 m. and flourished best between 20 and 30 m. Surface temperature of the water is assumed to have been that of hot regions. They are not found in sedi- ments with a primary magnesium oxide content of more than 5-8%, and are com- monest in sediments with less than 0-2-0.5 % of magnesium oxide. 4. The skeleton or cup is basically a two-walled cone with the central cavity

normally empty. The overwhelming majority of cups attain a diameter of 10-20 or 25 mm. All plates are perforate except the dissepiments. The pores of the walls, but not those of the septa, taeniae and tabulae, may be complex canals or tubes bounded by variously shaped and arranged skeletal structures, and the different types of wall so distinguished are consistent within genera and species.

5. Pectinate tabulae are characteristic of the suborder Nochoroicyathina, but within a single species individuals may occur in which they are scarcely perceptible.

6. Cap-like structures at the growing end of the cone in some atabulate species raise unsolved problems on the manner of growth of the cup.

7. Archaeocyathan skeletal encrustations on normal cups are interpreted by Zhuravleva as protuberances from the intervallum of the encrusted or some other normal cup. Vologdin, however, interprets them as encrusting species and genera of archaeocyatha in which the inner wall and central cavity are not developed, and Maslov regards them as symbiotic.

The phylum Archaeocyatha 25 3 8. The skeleton consists of subequigranular, microcrystalline grains of calcium

carbonate about 2 p in diameter, which Russian authors consider to have an appre- ciable organic content. It is considered to be external in origin by Zhuravleva and internal by Vologdin. Neither spicules nor acicular crystals have been conclusively demonstrated.

9. Ontogeny is consistent within genera and species and is of value in classification. 10. Trends of development, both in reduction and in complication, have been

distinguished. I I . Skeletal carbonate sheathing walls, septa and taeniae are regarded by some,

including myself, as secondary skeletal thickening, and by others as calcifications in situ of the soft parts that deposited the element sheathed. 12. Canaliculate or tubulose skeletal material found in the central cavity of some

individuals is considered by Zhuravleva to be formed by protuberances from the intervallum but by Vologdin to represent calcification in situ of soft assimilative organs.

13. Vologdin, on the basis of his above-mentioned view that calcified soft parts occur, regards the soft body as cellular, with differentiated assimilative organs located in the central cavity, drawing food currents through these organs and out through the wall pores of the intervallum and depositing the skeletal elements internally. 14. Zhuravleva on the other hand, in an interpretation which I prefer, considers

that assimilation and some production of sexual cells took place in the intervallum and that the central cavity was not the site of any vital activity. She thinks of the food currents as drawn into the intervallum through the outer wall pores and expelled into the central cavity through the inner wall pores. The skeleton was external, formed as in foraminifera. Digestion was analogous to that in Protozoa and took place intra- cellularly in uniform, undifferentiated cells, and not in a distinct layer of epithelial cells as in coelenterates. Thus the phylum was primarily of single-layered animals, with the possibility that some single-layered forms with undifferentiated and polar cells, and some two-layered forms might have been present in the more specialized members. She suggested that the archaeocyatha fulfil the requirements of the hypo- thetical blastea.

I 5 . Following Zhuravleva’s analysis, it seems probable that the Archaeocyatha are a phylum of single, multicellular organisms, with a degree of organization higher than that of the Protozoa but with less differentiation than the Porifera.

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A0x.n. A i d . Hayx CCCP.

xypasnesa I?. T. (ZHURAVLEVA, I. T.) (1950). Apxeoqma~a c KOnOHHanbHbIM CKeJIeTOM. Aom. Axad . Hayx CCCP. Dokl. Akad. Nauk U:S.S.R. 75, 855-8.

XvoasneBa kI. T. (ZHURAVLEVA, I. T.) (1951~). 0 6 IIHAllBMHYaJIbHOM Pa3BIITEIII KY6KOB nDa- ;knbHMx ApxeoqiaT m ApXeOqUaTOBbIX-~HYHHKaX. goxfi." Axad . H a y z CCCP. Dokl. Aiad. Nauk U.S.S.R. 80, 97-100.

Kypannesa M[. T . (ZHURAVLEVA, I . T.) (1951b). 0 HOBOM pone ApxeoqnaT c rpe6eHYaTaMn aHHIqaMA B K ~ M ~ ~ M B C K H X H3BeCTHRKaX C M ~ I I P H . AoKfi. A s a d . Hayx: CCCP. Dokl. Akad. Nauk U.S.S.R. 81, 77-80.

KypaBnesa H. T. (ZHURAVLEVA, I. T.) (1959). 0 no.nomeHmi ApxeoqkiaT B @moreHeTmecKo$i CMcTeMe. ITaseonm. X y p n . Palaeont. Zh. 1959 (4), 3-40.

Xypasnesa H. T. (ZHURAVLEVA, I. T.) (1960~). HosbIe gaHHbIe 06 ApxeoquaTax CaHa- wTbIKroabCKOl'0 rOpII30HTa. reOa. reO$Ua. Geol. Geophys. 1960 (2), 42-6.

XypaBnesa H. T. (ZHURAVLEVA, I. T.) (1960b). Apxeoyuambb cu6upcxoil nnam$opabc. MocKBa. Xypasnesa H. T. H Penma JI. H. (ZHURAVLEVA, I.T. & REPINA, L.N.) (1959). PoAolebIe

KOMnneKCbI TpMJIO6MTOB II ApxeoqmaT H H X H W O IEeM6pMR AJITae-CaRHCKOfi O ~ J I ~ C T H . f l o ~ f i . Axad . Hayx CCCP. Dokl. Akad. Nauk U.S.S.R. 129, 181-3.

Xypannesa H. T., P e n m a JI. H., XOMeHTOBCKHR B. B. (ZHURAVLEVA, I . T., REPINA, L. N. & KHOMENTOVSKI'~, V. V.) (1959~). EmocTpaTxrpa@fi HumHero ~ e ~ b p m s cKJrapaToro

Mosk. Obshch. Ispyt. Prirody (Geol.), 34 (2), 67-90. Xqypasnesa PI. T., P e n m a JI. H., XOMeHTOBCKMfi B. B. (ZHURAVLEVA, I. T., REPINA, L. N. &

KHOMENTOVSKII, V. v.) (1959 6). HMmHeKeM6pXfiCKlle rOpHBOHTh7 rOpHOfi ~ O P E I H . &ox&. A x a d , Hayx CCCP. Dokl. Akad. Nauk U.S.S.R. 128, 1030-3.

Nypasnesa H. T., Penma JI. H. , XOMeHTOBCKHB B. B. (ZHURAVLEVA, I. T., REPINA, L. N. & KHOMENTOVSKI~,~. v . ) (1960). HerIpepbIBHbIB Kap6OHaTHbIfi pa3pe3 JIeHCKOI'O Rpyca HmXHerO KeM6pMR AJITae-CaRHCKOfi ropof i cTpaHbr II ero rraneoHTonormecKaR xapawepmcmma. A0x.n. Axad . Hayx: CCCP. Dokl. Akad. Nauk U.S.S.R. 132, 116-2.

Zypasnesa H. T. M 3 e n e ~ o ~ K . Ic. (ZHURAVLEVA, I. T. & ZELENOV, K. K.) (1955). BMorepMN neCTp0~BeTHOfi CBHTbI peKH JIeHbI. Tpyabs naaeonm. Hucm. AEaa. H a y x CCCP. Tr.palaeont. Inst. Akad. Nauk U.S.S.R. 66, 57-77.

JIMTOJIOI'HR KeM6pIIfiCKLIX OTJIOXeHMfi CeBepHoro CKnoHa Annaiicmro Maccasa. T p . 2eos. Mucm. Axaa . H a p CCCP. Tr. geol. Inst. Akad. Nauk U.S.S.R. 8, 1-123.

06paMneHMR MHHYCPIHCKO~~ BIIaAMHbI. Glonfi. M O C K . 0 6 q . HCnbCm. npUp0dbZ (zeofi.). Byull.

3 e n e ~ o e K . K. (ZELENOV, K. K.) (1957).

DOROTHY HILL

EXPLANATION OF PLATE 1

Photographs by courtesy British Museum (Natural History).

Fig. I . 1 ‘phacyathus annularis Bedford & Bedford; a, etched oblique transverse section, x 10; b, etchei oblique ngitudinal section, X 10.

Fig. 2 . Cadniacyathus asperatus Bedford & Bedford; etched oblique section, scaly inner wall above, x 8. Fig. 3. Ethmocyathus lineatus Bedford & Bedford; part of somewhat damaged inner wall, x 14. Fig. 4. Ethmocoscinus papillipora Bedford & Bedford; x 4. Fig. 5 . Putapacyathus regularis Bedford & Bedford; etched transverse section showing part of tabula from above, x 8. Fig. 6 . Flindersicyathus decipiens Bedford & Bedford; etched median longitudinal section showing taeniae and synapticulae, x 6. Fig. 7. Metafirngia reticulata Bedford & Bedford; part of etched outer wall, x 6. Fig. 8. ‘Exocyathus awtm2is’ Bedford & Bedford; etched transverse section, x 4.

Biological Reviews, Vol. 39, No. 2

DOROTHY HILL

Plate I

(Facing p . 258)