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J. Paleont., 81(4), 2007, pp. 754–759Copyright � 2007, The Paleontological Society0022-3360/07/0081-754$03.00

THE OLDEST HINDIID DEMOSPONGE FROM THE DARRIWILIAN(MIDDLE ORDOVICIAN) OF THE ARGENTINE PRECORDILLERA:EVOLUTIONARY IMPLICATIONS FOR THE TRICRANOCLADINES

MARCELO G. CARRERACentro de Investigaciones Paleobiologicas (CIPAL) Facultad de Ciencias Exactas, Fısicas y Naturales, Universidad Nacional de Cordoba,

Av. Velez Sarsfield 299 (5000) Cordoba, Argentina �mcarrera@com.uncor.edu�

ABSTRACT—The tricranocladine sponges (now assigned to the suborder Eutaxicladina) are a conservative group consisting of the single familyHindiidae. The genus Hindia Duncan is considered the base of the tricranoclad evolutionary record. A new sponge discovered from theOrdovician limestones of the San Juan Formation in the Argentine Precordillera allows a reexamination of the Hindiid lineage and extends theearly history of the tricranocladines back to the Darriwilian. Eoscheiella concava n. gen. and sp. is described and illustrated. It is known froma single chertified, spherical specimen with a central hollow core, a typical character found in several genera of the family. Tricranoclads,megarhizoclones, and radial monaxonic spicules are the main skeletal components of the new genus. The presence of a band of subparalelltangential monaxons on the surface of Eoscheiella can be related to an inferred, small, root tuft that attached the spherical sponge to thesubstrate. The discovery of Eoscheiella modifies the evolutionary view of the family, for it is the oldest genus now known with simple dipodaland tripodal tricranoclads without a brachyome, radial monaxons, and megarhizoclones. Eoscheiella displaces the genus Hindia from the baseof tricranocladine evolutionary history. Variability in spicule form, arrangement, and clasping possibilities observed in Permian representativesare clearly more accentuated in the Ordovician Eoscheiella. All these features appeared earlier than expected in previously proposed evolutionarytrends.

INTRODUCTION

THE TRICRANOCLADINE sponges (now assigned to the suborderEutaxicladina Rauff, 1894, in Finks and Rigby 2004) are a

conservative group composed of the single Family Hindiidae thatbegan in the Ordovician and lasted until near the end of the Perm-ian. Finks (1971), Rigby (1986, 1991), and Rigby and Webby(1988) successively summarized the tricranocladine record anddiscussed the possible mosaic evolution of the hindiid family andtheir relationship with the Mesozoic dicranocladines.

Hindia Duncan, 1879 (see Rigby, 2004 for name validity) isone of the most characteristic genera of Ordovician lithistids, notonly for its wide stratigraphic and paleogeographic distributionbut also because it has been considered the base of the Tricran-oclad evolutionary record. Finks (1971) and Rigby (1986, 1991)visualized slightly different lineages among tricranocladinesponges. However, both authors agreed in identifying Hindia asthe basal genus of the tricranocladines and of the subsequent evo-lution of the tricranocladines into the dicranocladines (Finks,2003; Finks and Rigby, 2004), although this last relationship wasquestioned by Reid (2003).

A new sponge discovered from Ordovician limestones of theSan Juan Formation in the Argentine Precordillera allows a re-examination of the hindiid lineage and extends the early historyof the tricranocladines back to the Darriwilian.

The sponge fauna of the Early Ordovician San Juan Formationhas been the subject of several taxonomic studies (Beresi andRigby, 1993; Carrera, 1994, 1996a, 1996b, 2003), from whichvarious analyses focused on paleoecology and paleobiogeographyhave been carried out (Carrera, 1997, 2000; Carrera and Rigby,1999, 2004). In spite of these numerous studies, the taxonomicanalysis of the San Juan sponges is far from complete. New fieldwork frequently provides new specimens for study and new formsto be described. Such is the case for the new genus documentedin this contribution, which was recovered from the upper levelsof the San Juan Formation in the Cerro Viejo section near Jachal,San Juan Province (Fig. 1).

A single specimen of a chertified spherical sponge has beenrecovered from the top of the San Juan Formation in the CerroViejo section (Fig. 2). This interval is mainly composed of nod-ular mudstones and wackestones and minor intercalations of skel-etal wackestones. The upper part of the unit belongs to the Ah-tiella brachiopod Biozone (Benedetto, 2003) and the Lenodusvariabilis conodont Biozone (Hunicken and Ortega, 1987), whichconfirms a Darriwilian age (base of the Early Llanvirn).

The Cerro Viejo section bears an important sponge associationmainly represented by a high percentage of elongate sponges in-dicative of low-energy conditions (Carrera, 1997, 2000). Rocksof this locality were deposited in a middle to distal ramp settingrarely affected by storm waves (Canas, 1999).

The holotype is housed at the Centro de Investigaciones Paleo-biologicas (CIPAL) Universidad Nacional de Cordoba, under theprefix CEGH-UNC.

SYSTEMATIC PALEONTOLOGY

Class DEMOSPONGEA Sollas, 1875Subclass TETRACTINOMORPHA Levi, 1953

Order STREPTOSCLEROPHORIDA Dendy, 1924Suborder EUTAXICLADINA Rauff, 1894

[�TRICRANOCLADINA Reid, 1968]Family HINDIIDAE Rauff, 1893

Type genus.⎯Hindia Duncan, 1879 (name conservation: Rig-by, 2004)

EOSCHEIELLA new genus

Type species.⎯Eoscheiella concava n. sp.Diagnosis.⎯Spherical to subspherical sponge 30 mm in max-

imum diameter. Skeleton of radially stacked series of tricranoclo-nes with two or three, rarely four, cladomes and no brachyome.Dipodal and tripodal tricranoclones slightly arched outward, withaxes parallel to the radiating canal system. Spicule surfacessmooth with minute tubercles irregularly distributed, but mainlygrouped at bulbous extremities of the cladomes, and at the distalarched center. Smooth megarhizoclones also occur as monoclon-id-based desmas. Radiating large oxeas occur parallel to radialcanals and protrude from the external surface. Small tangentialoxeas, all subparallel and vertically oriented, occur on the outeror dermal surface, apparently restricted to a circular band coin-cident with the equatorial zone of the slightly ellipsoidal sponge.

Etymology.⎯Eoscheiella: old Scheiella. The name refers to similarities ofthis new form to the Permian genus Scheiella Finks, 1971.

Discussion.⎯The Ordovician representatives of the FamilyHindiidae (Rauff, 1893) include the type genus Hindia (Ordovi-cian to Devonian), Cotylahindia (Rigby and Bayer, 1976), and aremarkably diversified association from the Late Ordovician ofAustralia (Rigby and Webby, 1988) consisting of the genera Be-lubulaspongia Rigby and Webby, 1988, Palmatohindia Rigby andWebby, 1988, Arborohindia Rigby and Webby, 1988, Mamelo-hindia Rigby and Webby, 1988, and Fenestrospongia Rigby and

755CARRERA—OLDEST HINDIID DEMOSPONGE FROM THE MIDDLE ORDOVICIAN

FIGURE 1—Geologic Map of the Cerro Viejo area and location of the col-lection site in Los Gatos Creek (map based on Furque, 1979).

FIGURE 2—Stratigraphic section of the San Juan Formation in Los Gatoscreek and position of the sponge level indicated by an asterisk.

Webby, 1988. The Devonian representatives include ScheielloidesRigby, 1986 and Sadleria Rigby, 1986, while the Upper Paleozoichindiids are represented by the Lower Permian Scheiia Tscher-nyschew and Stepanov, 1916, and the Upper Permian Scheiella.

The specimen recovered from the San Juan limestones in theArgentine Precordillera is distinctly different from the Ordoviciangenera, although it possesses the hindiid spicular pattern: a tri-cranoclad-based skeleton, radially disposed large monaxons andsmall tangential oxeas. Distinctive features of the Argentinian ma-terial are knobby, irregular dipodal or tripodal tricranoclones asprincipal spicules, marked by tiny tubercles, and absence of abrachyome. It has megarhizoclones as secondary elements.

Hindia is also a spherical form with a hollow center, as in theArgentinian specimen, but it has a regularly arranged skeleton oftricranoclad spicules with three cladomes and a distal brachyome.Almost the same features are found in the Late Ordovician bowl-shaped form Cotylahindia, which also separates this genus fromthe Argentinian material.

The Late Ordovician genera from Australia have irregularforms and tricranoclads with a differentiated brachyome. The onlyexception is one species of the genus Arborohindia (A. parvaRigby and Webby, 1988), which lacks a brachyome, at least inthe dermal layer. However, it is a branching form with a differentappearance and arrangement of the tricranoclones.

The Devonian genera Scheielloides and Sadleria both are cy-lindrical to funnel-shaped sponges with knobby-sculptured tri-cranoclads bearing small lateral spines and having rhizoclones.

As in the Argentinian form, some dipodal tricranoclads are pre-sent.

The new Argentinian genus resembles the Permian represen-tatives Scheiia and Scheiella. Bulbous tuberculate tricranocladsare present in both Permian forms; however, simple megarhizo-clones occur only in Scheiella (Finks 1971) and are somewhatsimilar to those observed in the Argentinian new genus Eos-cheiella. As in the Permian genera, Eoscheiella possesses a hol-low core with isolated remnant spicules, radial and tangential ox-eas, and tricranoclads without a brachyome. It is obvious,however, that Eoscheiella has a different and more irregular skel-etal arrangement; its tricranoclads are also more irregular in shapewith very small tubercules. In addition, Eoscheiella has a signif-icant proportion of dipodal tricranoclads similar to those found inthe Devonian Sadleria or Scheielloides and related to those ap-pearing in the Mesozoic dicranocladines.

EOSCHEIELLA CONCAVA new speciesFigures 3.1–3.6, 4.1–4.6, 5

Diagnosis.⎯As for the genus.Description.⎯Spherical to subspherical sponge 30 mm in maximum di-

ameter. A hollow core, 16 mm in diameter, is exposed in the sectioned spec-imen, occupying half of the sponge diameter. This feature previously notedin Hindia and Scheiia has been related to resorption of early secreted spicules.This hollow center contains disarticulated spicules of the skeletal net, sug-gesting the presence of original sponge tissue in this central part.

756 JOURNAL OF PALEONTOLOGY, V. 81, NO. 4, 2007

FIGURE 3—Eoscheiella concava n. gen and sp. Scale bars 1 mm (except scale bar in 1 � 1 cm). Holotype CEGH-UNC 22136 from the San Juan Formationin Los Gatos Creek, San Juan Province, Argentina. 1, longitudinal section through the holotype showing wide hollow core and dense skeletal mesh of thewall; 2, enlarged view of the wall and the hollow core showing disarticulated spicules of the original secreted skeletal net; 3–5, different views of the tangentialsmall oxeas on the external surface; 6, detail of the external wall with protruding radial monaxons; note the light irregular line at the outer extremity of theprotruding monaxons possibly related to the exopinacoderm.

The canal system consists of a main series of radiating openings that are1.3–2.0 mm in diameter. Canals are straight, gradually expanding toward thesponge exterior, and are separated 3–6 mm apart. Walls between adjacentcanals consist of two or three series of spicules and large radiating monaxons.Canals are uninterrupted from the center to the exterior, although they aremore irregular and thinner towards the hollow interior. Skeletal pores at theexterior are of essentially the same dimensions as the internal canals, withthe exception of those appearing among intraserial spicules of the major canalwalls. A smaller series of skeletal canals lie between individual tricranoclones.They are irregular, forming a radial series between the cladomes of the spic-ules, ranging from 0.05 to 0.20 mm in diameter. Short tangential canals, 0.10–0.25 mm in diameter, occur across the arched cladomes of dipodal tricrano-clads. They are irregularly distributed and perpendicularly connected to thelarge radial canals.

Tricranoclones are the main spicules in the skeleton, ranging from 0.22–0.50mm long (Figs. 4.1–4.6, 5.1–5.6). They have varied forms, commonly with twoor three, exceptionally four, clonomes or proximal rays, but all forms lack abrachyome or distal ray. Spicules are arched, locally strongly convex, and faintlytuberculate with clonomes directed towards the interior of the sponge, so that themaximun dimension of the arched spicule is parallel to the sponge surface. Thesespicules form layers cross-connected by the perpendicular radial oxeas. Successivetricranoclads articulate in what has been generally termed the ‘‘shoulder’’ pattern,in which tricranoclads are attached to the convex shoulders of subjacent spicules.In our specimen this pattern is more irregular mainly due to the strongly convexforms of some tricranoclads, to the absence of a brachyome, and to the frequencyof zygome to zygome junctions.

Clonomes of the tricranoclones are short, variably spaced, and asymmet-rical in length or thickness, and in general are slightly thinner than the spiculecenter. They are not straight but are bowed convexly outward with variableangles and are terminally expanded to form an articulatory surface. Clonomeshave expanded zygomes 0.10–0.25 mm wide. They vary from a simple clasp-ing articulatory surface, to a concave tongue-like or cup-like articulatory face,and rarely to a digitate or nodose termination.

Surfaces of tricranoclads are comparatively smooth but some are finelymarked by tubercules, producing a faintly rugose appearance. The tuberculesare concentrated in the convex-arched center of the spicules and ray termi-nations.

Megarhizoclones, in the sense of Finks (1971, 2003), are commonly foundin Eoscheiella n. gen. This type of desma, superficially similar to rhizoclones,was described by Finks as antlerlike spicules with curving branches withoutfinely arborescent terminations. They appear less sculptured than the typicalrhizoclones found in the rhizomorinids, or even the rhizoclones found in theDevonian hindiids Scheielloides and Sadleria. Eoscheiella’s megarhizoclonesare commonly present throughout the sponge skeleton, and are monoclonid-based desmas with small zygomes and knobs over their entire surface. Theyare 0.20–0.35 mm long, rarely branched with one order of short branches(Fig. 5.7–5.10).

Other spicules rarely found in Eoscheiella resemble heloclones with semi-circular clasping facets (Figs. 4.3, 5.11). They look transitional to megarhi-zoclones for they are monoclonid-based and have similar clasping surfaces.Zygomes and branches characterize the megarhizoclone aspect.

Monaxons are represented by radially disposed oxeas 2–3 mm long andcommon fragmented oxeas are 1–1.5 mm long. Externalmost oxeas of thesponge body protrude 2 or 3 millimeters beyond the sponge surface. Stylesof the same size as oxeas are also present. They have a rounded end and tapertowards the sponge surface. Relative proportions of oxeas and styles are dif-ficult to establish due to the broken tips of most monaxons in the skeleton.

Tangential monaxons 1–1.5 mm long occur on the sponge exterior (Fig.3.3–3.5). There is neither distinct dermal layer nor zones of spicule variation,with the sole exception of this dermal band of small, subparallel tangentialmonaxons. These oxeas are all parallel and vertically oriented, forming a‘‘fence’’ layer along the equatorial zone of the oblately spheroidal sponge.There is no other part of the sponge surface with such a concentration oftangential monaxons. Even isolated oxeas are apparently absent. Only theradially disposed, large oxeas are noted as nodes in the external surface.

Etymology.⎯Concava (Lat.): concave, referring to the big hollow centerfound in the specimen.

Type.⎯Holotype CEGH-UNC 22136.Occurrence.⎯A single specimen, the holotype, came from upper levels of

the San Juan Formation (Darriwilian, base of the Early Llanvirn), Los GatosCreek in the Cerro Viejo area, San Juan Province, Argentina. These levelsare included in the Ahtiella brachiopod Biozone (Benedetto, 2003) and theLenodus variabilis conodont Biozone (Hunicken and Ortega, 1987).

757CARRERA—OLDEST HINDIID DEMOSPONGE FROM THE MIDDLE ORDOVICIAN

FIGURE 4—Eoscheiella concava n. gen and sp. Scale bars 1 mm. Holotype CEGH-UNC 22136 from the San Juan Formation in Los Gatos Creek, San JuanProvince, Argentina. 1, general view of the skeletal mesh showing parallel series of tricranoclads bowed convexly outward; 2, enlarged view of the previousfigure, showing tricranoclads articulated in a ‘‘shoulder’’ pattern and radial monaxons flanking radial canals; 3, finely tuberculate tricranoclad (centered), amonaxon with axial filament (down-left), and a faintly preserved heloclone-like spicule (centered-left); 4, partially resorpted skeleton located between thehollow core and the entirely welded skeleton of the wall including properly disposed radial monaxons; 5, same area of the previous figure with moredisarticulated spicules; 6, still well organized partially dissolved skeleton with monaxons and dipodal tricranoclads

FIGURE 5—Camera lucida drawings of spicules of Eoscheiella concava n.gen and sp. 1–2, tripodal tricranoclads; 3–6, dipodal tricranoclads; 7–10, me-garhizoclones; 11, heloclone-like spicule with semicircular clasping facet.

MODE OF LIFE

Finks (1971) gave a convincing explanation for the possiblemeaning of the small tangential oxeas found throughout the der-mal layer of Scheiia or Scheiella, based on structural features seenin living sponges. He suggested that the protruding large radialoxeas served as support for an exopinacoderm partially reinforcedby the small tangential oxeas (Finks, 1971, p. 983, text-fig. 2).This is a possible explanation for those genera that have smalltangential oxeas irregularly distributed on their surfaces, but theArgentinian form has a band of small tangential oxeas, verticallyoriented, all parallel like a ‘‘fence’’ along the equatorial zone ofthe spheroidal sponge (Fig. 3.3–3.5). Both upper and lower sur-faces of the sponge lack tangential monaxons. This particular fea-ture needs further explanation and could be related to the partic-ular mode of life of this form.

Hindia and the related spherical genera of the Hindiidae havecentripetal skeletons and canal systems. Their skeletons are built

of concentric layers of spicules, and the sponges grew by periph-erally adding new layers. They have pores equally developed onall sides and have no sign of attachment. Finks (2003) suggestedthat these forms could have rolled on the bottom. The worldwidedistribution of these genera allowed Rigby and Webby (1988) andCarrera and Rigby (2004) to interpret these forms as probablyfloaters with a planktonic mode of life. Although these spongeswere probably too heavy to float, as Finks (2003) stated, the hol-low interior and its small size open several possibilities for dis-persion, as for example, an epiplanktonic life attached to sea-weeds or as true floaters with the interior filled with gas.

The discovery of this ‘‘crown’’ of small oxeas in Eoscheiellapermits another possible explanation for their mode of life, at leastfor this new form. This band of parallel oxeas could be the ex-tremity of a small root-tuft that attached the spheroidal sponge tothe substrate (Fig. 6). A basal attachment of spongine fibres wassuggested for Hindia (Mehl-Janussen, 1999). Root-tufts are fairlycommon in the limestones where Eoscheiella was found. Al-though previously described root-tufts in the Argentine Precor-dillera are associated with hexactinellid sponges, there are re-markable variations in form and size (Beresi and Rigby, 1993;Carrera, 1994) and some of the smallest root-tufts could be relatedto Eoscheiella.

Root tufts or basalia are common among hexactinellids, where-as demosponges often lack root structures. Demosponges tend tobe attached to the substrate by an enlarged or pedunculate basecomposed of nearly the same kind of spicules as the rest of thesponge skeleton. Exceptionally, monaxon clusters as root struc-tures have been described, for example, in the monaxonid de-mosponges Hazelia Walcott, 1920 and Pirania llanfawrensis Bot-ting, 2004, or the heteractinid calcareous Eiffelospongia Rigby

758 JOURNAL OF PALEONTOLOGY, V. 81, NO. 4, 2007

FIGURE 6—Sketch of Eoscheiella concava n. gen and sp. with reconstructedsuggested root-tuft.

and Collins, 2004. This is the first suggestive evidence amongOrdovician lithistids of their presence as fixation structure.

The protruding monaxons or marginalia found in Eoscheiellaare common in several genera of monaxonid and hexactinellidsponges, where they serve as protection or a fixation structure. Inthe hindiids they could be related, as postulated by Finsks (1971),to a supporting structure for an exopinacoderm. Figure 3.6 showsa particular feature possibly related to Finks’s suggestion: a smalland irregular light line, presumably part of the skeleton, at theextreme end of the protruding monaxons.

The Cerro Viejo Section bears a distinctive sponge associationcompared to other contemporaneous areas in the Argentine Pre-cordillera (Carrera, 1997, 2000). From a sedimentological pointof view the section also differs, with a predominance of mud-stones and nodular wackestones, suggesting a distal ramp setting(Canas, 1999). The sponge fauna is dominated by elongatedsponges, and the sediments contain numerous root-tuft fragments.The new genus Eoscheiella would have lived attached to this softsubstrate some centimeters above the sea bottom, anchored by asmall root-tuft (Fig. 6).

THE TRICRANOCLADINE MOSAIC EVOLUTION

Discovery of Eoscheiella n. gen. suggests revision of the evo-lutionary lineages envisaged for the tricranocladines (Finks, 1971,2003; Rigby, 1986, 1991). In these previous studies, Hindia wasconsidered as the stem genus of the evolutionary history of thesuborder. Finks (1971) suggested that the Permian Scheiiaevolved from the spherical Hindia and is differentiated from theformer largely by the loss of the brachyome of the tricranocladspicule. He proposed the evolutionary lineage of Hindia toScheiia, and to Scheiella and their connections with the Mesozoicdicranocladines. However, the structure of the Devonian Sadleriaand Scheielloides shows that the skeletal features that typifyScheiella (tricranoclads plus megharizoclones) are known to ex-tend back to the Devonian (Rigby, 1986).

According to these findings, Rigby (1991) suggested two lin-eages in tricranocladine evolution, one conservative line between

Hindia and Scheiia and the other line possibly from Scheielloidesor Sadleria to Scheiella and, ultimately, to the dicranocladines.

The presence of tricranoclads without a brachyome and me-garhizoclones in the Darriwilian Eoscheiella, the oldest genus ofthe family, displaces the genus Hindia from the base of the tri-cranocladine evolutionary history. Eoscheiella shows the simplesttricranoclad spicule, and its characteristics point to a connectionwith the Devonian and Permian forms. This evolutionary linewould include the basal Eoscheiella, the Devonian Scheielloides,and Sadleria (Rigby, 1986), and the Permian Scheiia and Scheiel-la (Finks, 1971).

Hindia has complex tricranoclads with a tuberculate brachyomeand a sculptured surfaces; it probably evolved as a separate linefrom Eoscheiella. Later it branched into the Ordovician NorthAmerican genus Cotylahindia and the Australian genera with welldeveloped brachyomes (Rigby and Webby, 1988).

Hindia’s skeletal regularity, spicule interlocking and attachmentare strongly related to occurrence and placement of the brachy-ome. In Scheiia and Scheiella, disappearance of the brachyomepermits more variable placing of the articulatory surface. Terminalexpansions of the distal rays may rest sometimes on the center ofthe underlying spicule, and sometimes farther down on the distalsurface of the ray. This feature permits a greater variation in an-gular relationships between spicules, and a resulting evolutionaryadvantage in the possible increase of size or change in form, whilemaintaining a relatively similar internal structure (Finks, 1971).

The evolutionary changes proposed by Finks, slightly modifiedlater by Rigby (1986), involve a transformation from regular andsymmetrically packed spicules of a spherical or hemisphericalbody (e.g., Hindia) to less regularly arranged spicules in a lessregular body (e.g., Scheiia or Scheiella). The changes in spiculeform can be related to these changes in the geometry of the net.Also the progressive increase in tubercle size and number wouldbe a consequence of this functional modification, as an improve-ment of clasping between spicules. Finks (1971) documentedfrom the data available at that time a phyletic size increase, thepresence of larger canals in forms with no brachyome and withvariable numbers of cladomes, and the absence of spherical shapein forms with megarhizoclones. He also documented a temporalsuccession of the three types: the first Hindia in the Ordovician;the second, Scheiia in the Early Permian; and the third, Scheiella,in the Late Permian. The discovery of hindiids with megarhizo-clones, tricranoclones lacking brachyomes, and a variable numberof cladomes in both the Devonian (Scheielloides and Sadleria,Rigby 1986) and the Middle Ordovician (Eoscheiella), indicatesthat the origin of these spicules was not a response to the phyleticsize increase. They do, however, seem to be associated with eitherlarge canals (Sadleria) or a non-spherical shape (Scheielloides,Sadleria). Other non-spherical forms with Hindia-like tricrano-clones possessing a brachyome and no megarhizoclones (Austra-lian hindiids, Rigby and Webby 1988) indicate that this spiculeform does not force a spherical form.

The discovery of Eoscheiella modifies the evolutionary viewof the family, for it is the oldest genus of the family now known,with an almost spherical form, irregularly arranged spicules with-out a brachyome, and interspersed megarhizoclones.

Variability in spicule form, arrangement, and clasping possi-bilities observed in the Permian representatives are clearly moreaccentuated in the Ordovician Eoscheiella. All these features ap-peared earlier than expected in the evolutionary trends visualizedby Finks (1971) and Rigby (1986), and represent a starting pointin the evolutionary lineage instead of the end of the line.

The new evolutionary trend proposed here for the Eoscheiella–Scheiella lineage (Fig. 7) implies that simple, asymmetrical, andfinely tuberculate tricranoclads without a brachyome are the basalspicule forms from which the rest of the tricranoclad spiculesapparently evolved. Hindia, which was previously considered asthe stem genus, can now be regarded as representing a separate

759CARRERA—OLDEST HINDIID DEMOSPONGE FROM THE MIDDLE ORDOVICIAN

FIGURE 7—Evolutionary lineages proposed for the tricranocladines.

and more specialized line. The presence of megarhizoclones isanother basal feature that appeared in the Middle Ordovician. Inthis sense, the Devonian genera Sadleria and Scheielloides sharesome characters found in both Scheiella and Eoscheiella and canbe considered either as intermediate forms, from which Scheielladerived, as suggested by Rigby (1991), or else as a divergent linefrom the Eoscheiella–Scheiella lineage.

ACKNOWLEDGMENTS

With gratitude, the author acknowledges financial support from CONICETGrant PIP 5599 and FONCYT Grant PICT 2002 (07-11741). Appreciation isextended to Drs. J. Keith Rigby (Brigham Young University, Provo, Utah)and Robert M. Finks (Department of Geosciences, University of Iowa) fortheir helpful reviews and supportive comments. The material was collected atthe time of a paleontological study by Leonor Sorrentino and Marcela Cicho-wolski, whose field work assistance is specially appreciated. This is a contri-bution for the IGCP Project 503 Early Paleozoic Palaeogeography and Paleo-climate.

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ACCEPTED 21 MARCH 2006