Classification and Phylogeny of Hexactinellida (Porifera)1

195

REVIEW / SYNTHÈSE

Classification and phylogeny of Hexactinellida (Porifera)1

H.M. Reiswig

Abstract: Development of the present classification scheme of the class Hexactinellida was delayed because authors of the early species descriptions did not figure their specimens and few of them had access to compound microscopes necessary to visualize characters important in determining relationships. When microscopic information on spicule symmetry became available, Wyville Thomson in 1868 recognized the distinctive triaxial symmetry of the group and named it Vitrea. Schmidt’s 1870 name, Hexactinellidae, was without the contamination associated with Thomson’s Vitrea, and gained widespread support. Through a series of proposals and refinements, the present scheme recognizes two main lineages, Amphidiscophora and Hexasterophora, differing in shape and type of their microscleres. Since these structures are generally not retained in fossil material, paleontologists have long had an independent classification for hexactinellids, but the trend to unify the systems is now nearly complete. Hypotheses on phylogeny of the group remain virtually dependent upon scarce paleontological information, pending data from nucleic acid and protein sequence analyses that are not yet available for unravelling relationships among hexactinellids. Résumé : L’élaboration du schéma actuel de classification de la classe des Hexactinellida a été retardée parce que les descripteurs des espèces plus anciennes n’ont pas fourni d’illustrations de leurs spécimens et parce que peu d’entre eux avaient accès aux microscopes nécessaires pour voir les caractères importants dans l’établissement des relations. En 1868 au moment où ilyaeudesrenseignements sur la structure microscopique de la symétrie des spicules, Wyville Thomson a reconnu la symétrie triaxiale caractéristique du groupe qu’il a nommé Vitrea. Le nom d’Hexactinellidae proposé par Schmidt en 1870 était exempt de la contamination associée au nom Vitrea de Thomson et a été largement adopté. Après une série de propositions et de raffinements, le schéma actuel reconnaît deux lignées principales, les Amphidiscophora et les Hexasterophora, qui se distinguent par la forme et le type de leurs microsclères. Comme ces structures ne persistent généralement pas chez les fossiles, les paléontologues utilisent depuis longtemps une classification indépendante pour les hexactinelles; cependant, la tendance visant à unifier les deux systèmes est pratiquement ar- rivée à terme. Le débrouillement des relations chez les hexactinelles est basé sur des hypothèses phylogénétiques qui dépendent des rares informations paléontologiques en attendant les données non encore disponibles sur les séquences d’acides nucléiques et de protéines. [Traduit par la Rédaction] Reiswig 204

Classification: zoological versus torically and still work mainly with dredged specimens, of- paleontological ten mud-filled, with loose spicules displaced by crushing and scouring during collection, but most or all of their skele- The group of 500–600 Recent species of sponges now tal components are usually retained and can be analyzed. grouped as the class Hexactinellida originated in the Protero- Long hoped for, but rarely collected, immaculate, minimally zoic and has left a long, if not extensive, fossil record. Zool- disturbed specimens are increasingly becoming available to ogists and paleontologists working with hexactinellid these workers from submersible surveys. Paleontologists, in sponges each have access to specimens that usually differ contrast, still work with partial and extensively altered speci- strikingly in quality. Taxonomic zoologists (this author) his- mens. Soft hexactinellids with skeletons of loose (unfused)

Received 15 August 2005. Accepted 21 November 2005. Published on the NRC Research Press Web site at http://cjz.nrc.ca on 3 March 2006. H.M. Reiswig.2 Natural History Section, Royal British Columbia Museum, P.O. Box 9815, Stn. Prov. Govt., Victoria, BC V8W 9W2, Canada. 1This review is one of a series dealing with aspects of the biology of the phylum Porifera. This series is one of several virtual symposia on the biology of neglected groups that will be published in the Journal from time to time. 2Present address: Department of Biology, University of Victoria, P.O. Box 3020, Stn. CSC, Victoria, BC V8W 3N5, Canada (e-mail: [email protected]).

Fig. 1. Basic outline of the present classification of scopic inspection of skeletal components. Most specimens Hexactinellida with thumbnail figures of typical body forms. have some hexactine spicules, but some have only reduced forms with one or more rays undeveloped; needle-shaped diactine spicules, where four rays have been suppressed, are sometimes the only large spicules present. Evidence of deri- vation can be found in the axial cross near the center of each spicule where remnants of reduced rays can be seen at mod- erate magnification of a compound microscope. In those fossil forms where only impressions of spicules remain, a level of doubt accompanies assignment of such specimens to Hexactinellida.

Development of the concept: Hexactinellida The process of recognition of Recent hexactinellids as a natural group of animals required awareness of shared unique characters among a reasonable number of known specimens and species. For this group, the pre-recognition period before its erection by Schmidt in 1870 was rather extended in time since deep-sea collecting was limited, micro- glass spicules are occasionally found as full body fossils, scopes were not widely available, and many of the first usually as impressions with spicule indications more-or-less specimens consisted of single fragments or incomplete spec- in place, but more commonly as only skeletal fragments of imens lacking loose spicules. The first specimen described parts of the body wall, or as individual spicules dispersed in and later recognized as a hexactinellid was the “extraordi- sediments and replaced with pyrite or other minerals. Hard nary Glass Rope” from Japan in the collections of the Brit- hexactinellids, those with skeletons of siliceous spicules ish Museum that Gray (1832) named as Hyalonema, without fused into a rigid network, are more readily preserved as species designation; he completed its binomen as H. sieboldi fossils, but even here, the complement of associated loose in 1835 (Fig. 2). He placed it among the sea pens in spicules is generally lost and details of even the fused skele- Pennatulidae and not with other sponges. A long convoluted ton can be destroyed by mineral replacement. Zoologists literature dealing with whether or not the glass rope have historically based classification of Recent Hexactinel- Hyalonema was actually part of the soft sponge located at its lida upon loose spicules, items not commonly available to base extended through the 1860s. Most correspondents ac- paleontologists. It is thus understandable that two partly in- cepted the glass rope as being part of the associated sponge, dependent classification systems have been developed for while Gray died in 1874 steadfastly refusing to accept that Recent and fossil members of this group, with various de- interpretation. grees of overlap in categories above the family level. Indeed The second hexactinellid-to-be was described, figured, many of the ordinal-level grouping now accepted in zoologi- and named Alcyoncellum speciosum by Quoy and Gaimard cal classification were developed first by paleontologists in 1833 as an amorphous zoophyte. Unfortunately the speci- (Fig. 1). Happily, there is now very strong movement by pa- men, obtained by Quoy and Gaimard on their voyage of dis- leontologists to rearrange their classification scheme to con- covery of the “Astrolabe” as a gift from the Governor of the form to that used by zoologists (Rigby 2003). Mollucas, consisted only of a rigid sac of fused siliceous fil- Classification schemes developed by both groups of work- aments (spicules) with all loose spicules washed out (Fig. 2). ers are now, and have always been, based upon details of However, the specimen had, and still has, a series of taxo- skeletal organization of these sponges. Soft tissue characters, nomic problems that remain incompletely corrected. Blain- e.g., cytological details and biochemical data such as amino ville (1830) described and named another sponge from Quoy acid and nucleotide sequences, have been available from and Gaimard’s collection, clearly what we now know as a only a few species, adequate for testing relationships be- calcareous sponge, as A. speciosum. He apparently applied tween hexactinellids and other classes of Porifera, but not Quoy and Gaimard’s manuscript name to the wrong specimen yet from a wide enough range of specimens to enable testing and attempted to rectify it soon after (Blainville 1834) by of hypotheses of relationships between groups of hexactinel- renaming the calcarean as A. gelatinosum. The genus lids formed from comparisons of the more readily preserv- homonymy was eventually corrected by Gray (1867) forming able physical elements — the shape, size, distribution, etc., the new name Corbitella for it, but the species homonymy re- of their siliceous skeletal spicules. The single diagnostic fea- mains unresolved. The third hexactinellid-to-be, eventually ture of the spicules, besides being siliceous, is that either known as “Venus’s Flower-basket” was described without fig- they exhibit overt six-rayed (hexactine) symmetry, two rays ure and named Euplectella aspergillum by Owen in 1841 in each of three axes emanating perpendicular to one another (Fig. 2). The specimen from the Philippines was probably from the spicule center (alternately considered cubic sym- complete, but loose spicules were not included in Owen’s de- metry since the rays intersect the center of the six faces of a scription. He placed it close to the alcyonoid (horny) sponges. bounding cube) or they show clear evidence of being derived In the same year Stutchbury (1841) described in text-only from such a form. That evidence and even overt determina- Dactylocalyx pumiceus, the fourth of the eventual members tion of expressed symmetry of small spicules require micro- of the hexactinellids. This specimen from Barbados was a

Fig. 2. The three first Hexactinellida described, only one of spongia for sponges with armed or hooked spicules (as- which (Corbitella speciosa Quoy and Gaimard, 1833/1834) was ters) — and still refused to include the glass plant Hyalonema figured in its original description. The figure of Euplectella as- proper, which he persisted in considering to be a barked pergillum is from Owen (1843) and Hyalonema sieboldi from coral (near pennatulaceans), among the sponges. Although Schultze (1860). Figures are not to scale. Gray’s attempt to organize all known sponges failed to win respect or support, it had great value in stimulating others to develop a more acceptable classification for sponges. Early in the following year, Thomson (1868) presented a preliminary replacement of Gray’s classification, based on the much earlier but regionally restricted classification (without any prospective hexactinellids) of Schmidt (1862). He developed the first concept of a group of siliceous sponges he grouped as the order Vitrea, with spicules which “whether of the skeleton or of the sarcode, may all be re- ferred to the hexradiate stellate type” — essentially the basis of Hexactinellida. The following year (Thomson 1869), he presented his full proposal and classification of the entire class Porifera, including his order Vitrea with eight genera without family grouping under subclass Silicea. Thomson was clearly the first worker to publish the concept of the group we now accept as hexactinellid sponges. The following year, Schmidt (1870) published his synopsis of sponges of the Atlantic region in which he proposed the group name Hexactinellidae (to become class Hexactinellida) for those sponges with siliceous spicules of three-axis (six-actine) type, with first examples as Hyalonema and Euplectella, and total named species now expanded to 31 species. He noted his new taxon was similar to Vitrea of Thomson (1868) but differed in that Thomson’s Vitrea included at least two species, Dactylocalyx pratti and Dactylocalyx azorica, that lacked triaxial spicules and were indeed members of the Lithistida, a group of demosponges Schmidt had here defined. Schmidt’s criticism of Thomson’s minor failure was rigid stony mass consisting of a network of siliceous tubules. upheld by the influential figure Zittel (1877 and subsequent Stutchbury designated it as a sponge and made no other publications) and the scientific community awarded author- statement of its resemblance or possible affinities with other ity for the concept to Schmidt for Hexactinellida, rather than sponges. Of these first four sponges which eventually to Thomson for Vitrea, not because Schmidt developed the formed part of the founding group of Hexactinellida, only concept and named the group of sponges with six-rayed one was figured and that only as the entire body. spicules first, but that he made the first clear distinction be- By the time Gray (1867) published his arrangement of the tween hexactinellids and lithistids. known sponges, eight additional good species and three Perhaps in an attempt to side-step this controversy, Claus questionable synonyms of prospective hexactinellid species (1872) proposed the group name Hyalospongiae (glass had been described, bringing the total to 15 species. Figures sponges) for those having silica scaffolds and stratified free of whole bodies or parts of 13 and spicules of 7 of them had silica bodies cementing fiber networks of siliceous material. been published in the original descriptions or later surveys Claus included only hexactinellids and one lithistid in the (first ever hexactin figured from Euplectella cucumer by group. As Reid (1957) pointed out, Hyalospongiae, occa- Owen 1857; spicule set from Hyalonema sieboldi by sionally used in place of Hexactinellida, cannot, as Claus Schultze 1860; spicules from several species by Bowerbank defined it, be linked to any modern taxon, and is not a syn- 1858). Hyalonema sieboldi was the only species for which onym of Hexactinellida. Likewise Schulze, who used the the complete set of spicules, including their internal axial ca- name Hexactinellida in his 1885 preliminary note on H.M.S. nal system, was known through the elegant figures produced Challenger specimens (Schulze 1885), decided to use in the by Schultze (1860) (Fig. 3). Paleontologists had by now final report a new name, Triaxonia, a clear junior synonym been producing excellent whole-body and spicule illustra- of Hexactinellida. tions of fossil sponges (e.g., Smith 1847–1848) (Fig. 4). Gray, still without a microscope at his disposal and having lit- Development of Hexactinellida tle experience with microscopy, ignored most of the available classification evidence of spicule symmetry in his arrangement of sponges (Gray 1867). He distributed the known prospective hexactin- While both Thomson and Schmidt resisted grouping genera ellids among the other siliceous and horny sponges (demo- into families within their Vitrea and Hexactinellidae, respec- sponges) in two of his seven orders of Poriphera — the tively, others quickly proposed such subdivisions. Kent (1870) Coralliospongia for hard, coral-like forms and the Acantho- divided the hexactinellids into two suborders: those with hard,

Fig. 3. Part of the first set of complete spicule drawings from microscopy of a hexactinellid sponge, Hyalonema sieboldi, from Schultze (1860).

Fig. 4. Drawings of the lychniscose framework (left) and a single lychnisc spicule (right) from microscopic examination of fossil Ventriculites simplex by Smith (1847).

reticulate (fused) skeletons in Coralliospongiae Gray, and those tinellids now became an annual event, with authors compet- with non-reticulate skeletons as his new suborder Callici- ing for acceptance of their schemes (Carter 1873; Gray 1874; spongiae. This gained no support, as it retained lithistids and Carter 1875, Marshall 1876; Zittel 1877). Most of these failed incorrectly partitioned the known hexactinellid genera. Gray to gain support, but Zittel’s (1877) division of the fossil (1872a, 1872b) proposed including all of the known hexactinellids into suborders Lysakina for soft sponges and hexactinellids except for Hyalonema (which he still retained Dictyonina for rigid sponges with a framework of fused in the Actiniaria) in one order, Coralliospongia, with two un- hexactins (dictyonine skeleton) was accepted and widely used named sections subdivided into 4 subsections and 13 families. by many zoologists and is the authority of the presently used Several of Gray’s families remain in present use, but the basis order Lyssacinosa, the scope of which has been modified. The of section and subsection definitions were not acceptable to group Dictyonina is still used by some paleontologists, but other workers. Proposal of new arrangements for hexac- most have abandoned it as they shifted to the zoological clas-

Fig. 5. The two basic microsclere types of Hexactinellida: (A) the amphidisc characteristic of Amphidiscophora (from Hyalonema populiferum) and (B) the hexaster characteristic of Hexasterophora (from Farrea sp.). Scale bars are both 25 µm.

sification system. In his monumental and extraordinarly influ- nomic work on the class. Reid (1958) joined many disparate ential report on the H.M.S. Challenger hexactinellids, Schulze families of fossil hexactinellids together in the order Reticul- (1886, 1887) retained Zittel’s primary subdivisions as sub- osa, which he was unable to place in the scheme at that time. orders and added new taxa to accommodate the now expanded It is now tentatively included in Amphidiscophora (Finks variety of extant genera. Schulze divided the Dictyonina into 2003). Tabachnick and Reiswig (2000) recognized the basic two tribes (now orders), Uncinateria and Inermia, on the pres- distinction in skeletal structure of a small group, family ence and absence of uncinate and sceptrule spicule types, re- Aulocalycidae, of Hexactinosa in which hexactins are joined spectively. These groups have not survived. His division of the in a very loose organization. They raised the group to order Lyssacina into the tribes (now orders) Amphidiscophora with status as Aulocalycoida, completing the Hexactinellida ar- amphidisc microscleres (Fig. 5A) and Hexasterophora with rangement as summarized in the modern synthesis as 28 hexaster microscleres (Fig. 5B) has survived and is now the ba- chapters of Systema Porifera: A Guide to the Classification sis of the primary division of the class Hexactinellida. of Sponges (Hooper and van Soest 2002). Since that monu- Many other proposals for modification of Schulze’s ar- mental work, intended as a starting point and incitement for rangement have been made, but only those contributing new taxonomic endeavors, Tabachnick and Janussen (2004), to the modern classification of Hexactinellida are reviewed through review of very old specimens and study of newly here. Schulze (1899) changed his arrangement toward the collected material, have raised the small genus Fieldingia, modern system in elevating microsclere type as the basis of considered of uncertain placement within Hexactinosa, to the primary division into the subclasses Amphidiscophora full order status equivalent to that group within Hexastero- and Hexasterophora. He abandoned the orders Uncinateria phora. and Inermia, since distinction between them could no longer The present classification scheme (Table 1) has proved ad- be maintained. Schrammen (1902) recognized the distinction equate for finding an appropriate location for newly discov- of the long known group of both Recent and fossil lychni- ered species and genera. It is not, however, satisfactory in scose hexactinellids, those with nodes of their dictyonal reflecting probable phylogenetic relationships between skeletons being perforated or each bearing 12 symmetrical groups. Likewise, many fossil genera and families cannot be struts between adjacent rays (lychnisc nodes), and raised the placed with confidence even to order within the scheme. group from an insignificant level in Schulze’s Inermia to Much work remains in gaining an understanding of how the suborder (equivalent now to subclass) Lychniscophora. The presently known species build their skeletons, the basis of following year (Schrammen 1903), he lowered the group to this classification system, and many new patterns no doubt tribe, modern equivalent of order, and brought all of the non- remain to be discovered in this still poorly known group of lychniscose families of Hexasterophora together in the simple animals. equivalent tribe (now order) Hexactinosa; these persist in the modern system as orders Lychniscosa (-ida) and Hexactinosa Phylogeny within Hexactinellida (-ida). Schrammen (1924) added another group, tribe Hemidiscaria, known only as fossils with unequal-ended Although all classification schemes imply phylogenetic amphidiscs, to Amphidiscophora; it is retained in modern relationships among subgroups of hexactinellids, few state- paleontological classification as the order Hemidiscaria. ments or hypotheses have been proposed. There has been no Ijima (1927) brought these modifications together into the doubt that the Hexactinellida is a natural monophyletic arrangement that formed the basis for all following taxo- group since its recognition. The point of concern within the

© 2006 NRC Canada 200 Can. J. Zool. Vol. 84, 2006 discohexasters composed of series ofapproximately dictyonalia same spaced distance along as strands mesh at width or paraulocalycoid of fused stauractins aspidoscopule, or absent absent polyradial and false more orders of quadules Skeletal type* Diagnostic characters No. of fossil genera No. of Recent genera Schrammen, 1924 1 Ln Microscleres are hemidiscs † Reid, 1958 118 Ln? Linear series of dermalia tangential rays circumscibe one or † Classification of Recent Hexactinellia to family level, with number of Recent genera, number of fossil genera, and main skeletal types indicated. Family 1/3Family 2/3Family 3/3 Pheronematidae Gray, 1870 Monorhaphididae Ijima, 1927 Hyalonematidae Gray, 1857Family 1/7 6 1Family 2/7 5 Farreidae Gray, 1872Family 3/7 Euretidae Zittel, 1877Family 4/7 DactylocalycidaeFamily Gray, 5/7 1867Family 6/7 TretodictyidaeFamily Schulze, 7/7 1886 Aphrocallistidae Gray, Ln 1867 Ln Craticulariidae Rauff, 1893 CribrospongiidaeFamily Roemer, 1/2 Ln 1864Family Major 2/2 choanosomal Major 6 spicules choanosomal are spicules pentactins are tauactins Aulocalycidae Major 3 16 Ijima, choanosomal 1927 spicules UncinateridaeFamily are Reiswig, 1/1 diactins 2002 8Family 1/2 2 FieldingidaeFamily Tabachnick 1 2/2 and Janussen, 1 2004 Aulocystidae Sollas, 1887 Diapleuridae Ijima, 1 1927 D 6 2 D D Farreiod D framework; sceptrule 1 as clavule, Framework Euretoid D sarule, neither framework; lonchiole, farreoid sceptrules D or as euretoid; narrow-headed D scopule dictyonal or nodes Euretoid mainly 2 framework; with schizorhyses Euretoid 1 framework; D with Euretoid diarhyses framework; Euretoid with framework; diplorhyses; with aporhyses diplorhyses; in D aporhyses quincunx D in quadrunx (As for order) Longitudinal Longitudinal strands strands are are single multiaxial rays, with unlimited overlap in of length rays D D Walls/pillars several dictyonalia thick; lychniscs Walls in 1–3 ranks dictyonalia thick; lychniscs not in ranks Table 1. RankPhylumSubphylum 1/2Class 1/1 Symplasma ReiswigSubclass and 1/2 Taxon Mackie name 1983 PoriferaOrder Grant, 1/1(3) 1836 Amphidiscophora Hexactinellida Schulze, Schmidt, 1886 1870 Amphidiscosida Schrammen, 1924Order 119 2/(3)Order 3/(3) Reticulosida Subclass 2/2 432 Hemidiscosida 12Order 12 1/5 Hexasterophora 119 Schulze, 1886 ~680 Hexactinosida L/D Schrammen, 1903 160 432 41 Syncytial porifera with enucleate choanocytes = “choanomeres” 107 Ln L/D Ln 37 L/D With Siliceous amphidiscs; spicules without Amphidiscs of hexasters; are 272 triaxonic, without equal-ended hexactinic spiculeOrder (cubic) fusion 2/5 symmetry Sessile metazoa with choanocytes generating 129 water currents Aulocalycoida Tabachnick L/D andOrder Reiswig, 3/5 2000 Microscleres D are hexasters;Order rare 8 4/5 amphidiscs Fieldingida occur Tabachnick and but Janussen, as 2004 variant Framework of fused simple Lychniscosida Schrammen, dictyonalia; 1903 longitudinal strands 1 4 D 1 3 Framework of fused simple dictyonalia; framework D aulocalycoid 81 Framework without strands, with Weltner bodies; surface crust D Rigid framework of fused lychnisc dictyonalia

class, the primary branch between Amphidiscophora and Hexasterophora, has been consistently upheld since its origin by Schulze (1887). These groups probably diverged early in the Paleozoic and have had long independent histories (Salomon 1989; Mostler and Mehl 1990). Tabachnick and Lévi (1997) discovered amphidisc-like microscleres in a small group of hexasterophorans, but they concluded that this was a case of convergence and that it had no phylogenetic significance. Within Amphidiscophora, Tabachnick and Menshenina (1999) analysed body form and distribution of inhalant–exhalent surfaces in Recent members. They hypothe- sized that the ancestral form was a cup-shaped sponge rooted in sediments, which gave rise to two radiating lineages, a Pheronematidae–Monorhaphididae line and a Hyalonematidae line.

of usually separate megascleres, which may be Hexasterophora has been subject to more phylogenetic speculation, partly because of its more extensive fossil record.

ork of fused hexactins), or L/D (i.e., both lyssacine and Schmidt (1880), even before the development of acceptable classification schemes, warned that dictyonal skeletons could be formed and lost in independent lineages of hexactinellids, and Zittel’s (1877) grouping of all hexactinellids with skele- never involve regular 6’s diactins tons of fused hexactins as Dictyonina should not be adopted without critical review of supporting evidence. Schulze (1887) nonetheless used Zittel’s Dictyonina but soon abandoned it with the realization that all forms with dictyonal skeletons

Skeletal type* Diagnostic characters were not necessarily closely related. Ijima (1927) reinforced Schmidt’s view and stated clearly, from evidence of spicule development, that Lychniscosa and Hexactinosa must have had independent origins from a protohexasterophoran ances- tor. Mehl (1992), in her monumental reconstruction of the history of Hexactinellida from both cladistic character analy- No. of fossil genera sis of Recent forms and evidence from the fossil record, agreed that dictyonal skeletons arose twice, once early in the Hexactinosa lineage and again more recently in a Graphio- comida lineage, the grouping of Euplectellidae and Lychni- scosa. She also concluded that Lyssacinosa cannot be considered a monophyletic group, since part of its contents No. of Recent genera group more closely with the dictyonine Lychniscosa. Her suggestions have not (yet) been incorporated in the modern classification scheme. Two contradictory reconstructions of hexactinellid phylogeny from the fossil record have recently been developed by Mehl (1996) and Finks (2003). The earliest fossils interpreted as hexactinellids are body impressions and isolated spicules from the Neoproterozoic Ediacara beds of Australia (Gehling and Rigby 1996), equivalent age beds of the Yang- tze Gorge of China (Steiner et al. 1993), Neoproterozoic of India (Tiwari et al. 2000), and Upper Precambrian of northern Iran (Brasier 1992). The first hexactinellid body fossils occur in the Lower Cambrian of China (Steiner et al. 1993), along with representatives of other sponge groups, demosponges and heteractinids. Early hexactinellids, e.g., Proto- spongia, were thin-wall sacs with spiculation consisting of a single layer of tangential stauractins (“+” like forms with ). Hexasterophora incertae sedis 5 21four rays in L/D a plane), (Unable to assign to recognized family) like those of larvae of modern forms. The stauractins occurred in several discrete orders by size and were arranged rectangularly to form a pattern of surface concluded

( quadrules. This dermal arrangement pattern is basic to the Position of order Recticulosa here follows Finks (2003), but others would place it in Hexasterophora or as incertae sedis within Hexactinellida. extinct Paleozoic order Reticulosa of which the Protospongi- Taxon is only known as fossils; it is either extinct or surviving Recent members have not yet been discovered.

Family 1/3Family 2/3 LeucopsacidaeFamily Ijima, 3/3 1903 Euplectellidae Gray, 1867 Rossellidae Schulze, 1885 3 27 23idae is considered Ls Lsthe Ls No hypodermalia; principalia No as hypodermalia; hexactins basal principalia as stauractins, tauactins, and With pentactinegroup. hypodermalia; principalia as diactins The scheme proposed by Note: *Skeletal types are designated by Ln (i.e., loose lyssacine† network of separate megascleres without any kind of fusion), Ls (i.e., lyssacine network Table 1 RankOrder 5/5 Lyssacinosida Taxon Zittel, name 1877 53 36 Ls Principalia as 2’s, 3’s, 4’s, and (or) 6’s; fusion may occur but partly rigidified by silicadictyonine fusion members at included). spicule contacts or by synapticulae but never involving fusion of hexactins), D (i.e., rigid dictyonine framew Finks (2003) is summarized here and in Fig. 6. The

Fig. 6. Graphic representation of the extent and development of major Hexactinellida taxa constructed from text of Finks (2003), which is a scheme that is not widely supported by other paleontologists. Vertical scale is approximate for Paleozoic (lower section), but the post-Paleozoic interval is seriously compressed.

reticulosans radiated into several lineages, retaining the ba- primitive and developed into more regular dictyonal forms sic dermal pattern but with stauractins replaced with five- or later. A surprising conclusion from Finks’ reconstruction is six-rayed spicules. The various groups derived from the that Amphidiscophora is likely to be paraphyletic. protospongiids are characterized by different forms of inter- Finks’ (2003) scheme conflicts on many points with those nal spicule layers added under the dermal layer (Finks proposed by Mehl (1996) and others, particularly in interpre- 2003). Some reticulosans had paraclavules (amphidisc-like tations of affinities of Reticulosa, which Mehl assigned to spicules with disc at only one end) and Finks considered Hexasterophora, and positioning of Protospongiidae as basal these homologes of the amphidiscs to be strong evidence for forms in hexactinellid phylogeny. Other workers (e.g., inclusion of Reticulosa and its basic constituent Proto- Krautter 2002) consider many fossil families still unassign- spongiidae in the subclass Amphidiscophora. A group of able to firm positions within the present classification scheme reticulosans, the Hintzespongiidae with paraclavules, gave of Recent forms. Although the literature of fossil hexactinel- rise in the Ordovician to forms with equal-ended amphi- lids has a long and rich history, perusal of the publication discs, the Pelicaspongiidae, and thereby initiating the or- dates of major papers documenting the early fossil record of der Amphidiscophora. Another group of reticulosans, the this class suggests that important new discoveries are to be Dictyospongioidea, gave rise to a group bearing only a small expected for some time. disc on the basal end of the paraclavule, thus forming a hemidisc spicule known in Microhemidiscia and defining Phylogeny of Hexactinellida within Porifera the new order Hemidiscosa, a group now extinct and consisting perhaps of only a single species. The origin of The unique position of Hexactinellida within Porifera has Hexasterophora is more difficult to determine from the fossil been widely supported in comparisons between the groups record, since many members of the reticulosan group using morphological characters (Reid 1958; Reiswig and Dictyospongioidea had both paraclavules and hexasters. Mackie 1983; Mehl 1992; Reitner and Mehl 1996; Mehl- Finks considered it likely that the two main hexactinellid Janussen 1999). Suggestions that Hexactinellida might be lineages, Amphidiscophora and Hexasterophora, were not accorded phylum status separate from the other classes of differentiated in the main Early Paleozoic lineages, but cer- sponges by Bidder (1929) and Bergquist (1985) reinforce tainly by the Permian a branch from the dictyosponges, the this distinction. In recent molecular sequence analyses and Brachiospongioidea, had taken on new body organization in those where resolution between classes is available, the that can be associated with that of lyssacine hexastero- hexactinellids fall out as the basal group within Porifera and phorans. Mere presence of hexasters may not be an adequate within Animalia (Mehl et al. 1998; Müller 1998; Müller et indicator of hexasterophoran assignment in fossil forms. The al. 1998). Paraphyly of Porifera itself, supported by most of Permian genus Pileolites is interpreted to be a true Hexactin- the recent molecular analyses, has not yet offered a chal- osan and indication that the aulocalycoid framework was lenge to this position of Hexactinellida.

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