shell structures of the recent vetigastropoda

J. Moll Stud. (1997), 63, 369-377 © The Malacological Society of London 1997

SHELL STRUCTURES OF THE RECENT VETIGASTROPODA

CLAUS HEDEGAARDBiologisk Instttut, Afdeling for Genetik & Okologi, Aarhus Universitet, Ny Munkegade, 8000 Aarhus C,

Denmark and Museum of Paleontology, Valley Life Sciences Building, University of California, Berkeley,California 94720, USA

ABSTRACT

A cladistic analysis of the shell microstructures ofRecent Vetigastropoda demonstrates, it is moreparsimonious to assume crossed lamellar shell struc-tures, rather than nacreous, are plesiomorphic. Theclade Vetigastropoda is characterized by havingintersected crossed platy shell structure. The analysissuggests resolution of the internal phylogeny of theVetigastropoda, particularly separation of a crowngroup with nacreous shells, composed of Haliotidae,Pleurotomariidae, Seguenziidae, Stomatellidae,Trochidae, and Turbinidae, separated from anunresolved grade with cross lamellar structures,Fissurellidae, Osteopeltidae, and Pseudococculin-idae. It is also suggested, that Phasianellidae isneither part of nor sister taxon to Turbinidae.

INTRODUCTION

The purpose of this paper is to establish achronicle of events (sensu O'Hara, 1988), thesequence of evolution of shell ultrastnictures inthe Recent Vetigastropoda, to identify theirplesiomorphic shell structures, and to re-evaluate the plesiomorphic condition forgastropod shell structures in general. This isaccomplished by mapping shell structures onphylogenetic hypotheses, based on other char-acters, and by a cladistic analysis of vetigastro-pod shell structures alone.

In 1902 Johannes Thiele in his paper 'Die sys-tematische Stellung der Solenogastren und diePhylogenie der Mollusken' presented a view ofgastropod evolution, which with slight modifi-cations still pervades our text books and thebackbone feeling of many maJacologists. With-out elaboration he stated that 'the zeugo-branchs . . . Pleurotomaria and Haliotis are byfar the most primitive gastropods . . .'. Thiele(1931), Wenz (1938-1944), and Wenz (1940)outlined a scenario with a monoplacophoran asthe ancestor of gastropods: a monoplacophoran(untorted, partly segmented, bilaterally sym-metrical) evolved into a primitive gastropod

(torted, unsegmented, bilaterally symmetrical),ancestor of other gastropods (torted, unseg-mented, partly asymmetric).

Pleurotomariidae—Thiele's (1902) 'by farmost primitive' gastropod—and some bivalves,cephalopods, monoplacophorans, and othergastropods—have interior layers with nacreousshell structures. Consequently, it did not seemunreasonable to conclude, a nacreous shell wasindeed plesiomorphic. Recent phylogeneticanalyses (Haszprunar, 1988; Ponder & Lind-berg, 1995, in press) have shown Pleuroto-tomariidae is several nodes removed from thebase of the gastropod phylogeny, and neither ofthe more plesiomorphic clades have nacreousshells, necessitating a reconsideration of theassumed plesiomorphic state and chronicle ofevolutionary events.

MATERIALS AND METHODS

The data and illustrations are from Hedegaard(1990). Unless otherwise noted, the illustrations ofshell structures are of fractured sections of shells,with the exterior towards the top. Specimens werecovered with approximately 50 nm gold-palladium.SEM micrographs were made with a Jeol JSM-40Scanning Electron Microscope.

Vetigastropoda was diagnosed by Salvini-Plawen(1980), and redefined by Salvini-Plawen & Hasz-prunar (1987) to be characterized by their ctenidialsense organs (bursicles), epipodial tentacles, andoesophageal structure. The families Fissurellidae,Haliotidae, Lepetodrillidae, Phasianellidae, Pleuro-tomariidae, Scissurellidae, Stomatellidae, Trochidae,and Turbinidae were originally included. In accord-ance with Ponder & Lindberg (1995, in press),Osteopeltidae, Pseudococculinidae, and Seguenzi-idae are added in this analysis. Muricidae, Neritidae,and Patellidae are chosen as out-groups. Each familyis represented by an exemplar taxon. Each exemplartaxon has all the shell structures observed in thatfamily, but that is not necessarily the case for allmembers of each family. The exemplar taxa werechosen because they have all the shell structures,

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370 C. HEDEGAARD

observed within their respective families (Hede-gaard, 1990).

Lepetodrillidae were excluded from the analysisbecause it is unknown whether they possess inter-sected crossed platy structure. Hunt (1992), whodescribed their shell structures, does not indicate anyintersected crossed platy structure. Fissurellidaehave intersected crossed platy structure in theirlarvaJ shells (Batten, 1975), which may also be thecase for Lepetodrillidae, but it is currently unknown.

Haszprunar's (1988) analysis of the strepto-neurous gastropods (Figure 8) is used to evaluate theevolution of shell structure characters. The cladisticanalysis of vetigastropod shell structures (data fromTable 1) is made as a branch-and-bound search withthe program PAUP 3.1.1 (Swofford & Begle, 1993).All characters are treated as unordered, unweightedand unsealed in the analysis. The 'steepest descent'was not used.

SHELL STRUCTURE DESCRIPTIONS

Shell structure diagnoses generally followCarter and Clark (1985). 'First order' and'second order' describe increasingly fine ele-ments of the structures. Transverse is vertical(ranging from inner to outer shell surface) andparallel to the shell margin. Radial is verticaland perpendicular to the shell margin. Thesedesignations refer to the shell layer, which maybe inclined relative to the surface of the shell.

Simple prismatic and homogeneous struc-tures are almost ubiquitous in the Vetigastro-poda and the outgroups and have not beenincluded in Table 1 or in the analysis. Both ofthese structures show differences in mineralogyand gradual transitions into other structures,and may be homoplastic. Several apomorphicshell structures in the out-groups have alsobeen excluded from consideration (B0ggild,1930; MacClintock, 1967; Hedegaard, 1990).

Sphenilitic prismatic structure (Figure 1) hascoarse first order prisms, where the secondorder prisms originate in sphenilitic sectors atthe depositional surface. These appear as fansof radial laths in vertical sections. This structureis composed of aragonite and found only inVetigastropoda—references to sphenilitic andsphenilitic prismatic structures in other groupsgenerally refer to the organisation of crystals inthe outermost, incipient layers.

Calcitic composite prismatic structure (Fig-ure 2) is composed of calcite scales or platelets,forming irregular prisms with an axis parallel tothe shell exterior. This structure only occurs inHaliotidae.

Nacreous structures are 'aragonitic laminarstructures consisting of polygonal to rounded

tablets arranged in broad, regularly formed,parallel sheets' (Carter & Clark, 1985, p. 55).Columnar nacre (Figure 3) is composed oftablets of rather uniform size, stacked in a moreor less regular net, with coinciding centres,containing calcified organic material that deter-mines the nucleation site of the overlyingtablet (Mutvei, 1980). Columnar nacre is foundin some Vetigastropoda and Cephalopoda.Columnar nacre differs from the sheet nacre,found in Bivalvia and Monoplacophora.Columnar nacre is deposited in a narrow zoneat the margin of the shell, sheet nacre isdeposited over most of the inner surface. Thecentre of each tablet in columnar nacre rests onthe centre of the underlying tablet The centreof each tablet in sheet nacre rests on the inter-face between underlying tablets like bricks ina wall.

Lamello-fibrillar structure (Figure 4) is a'laminar structure consisting of sheets of moreor less parallel, horizontal rods, with the rodsoriented in different directions in adjacentsheets' (Carter & Clark, 1985, p. 58), super-ficially similar to crossed lamellar structure (seebelow), except the first order sheets of thelamello-fibrillar structure are thin, composed ofa few thin rods, and horizontal, and the firstorder lamellae of crossed lamellar structure arevertical. The name was introduced for a struc-ture in the Spirula septum, but was applied to astructure with similar morphology in Stoma-tellidae by Hedegaard (1990), though almostcertainly not homologous.

Simple crossed lamellar structures (Figure 5)have first order lamellae, composed of thin,mutually parallel laths or rods, which show twonon-horizontal dip directions, alternating inadjacent lamellae. The structure is designatedcomarginal, if the first order lamellae are paral-lel to the shell margin (i.e., parallel to the trans-verse section), radial, if perpendicular. Simplecrossed lamellar structures are widely distri-buted in bivalves, gastropods, scaphopods, andin other molluscs (B0ggild, 1930; Taylor,Kennedy & Hall, 1969,1973; Runnegar, Pojeta,Morris, Taylor, Taylor & McClung, 1975;Carter, 1990; Hedegaard, 1990).

Complex crossed lamellar structures (Figure6) have first order lamellae with more than twodip directions, and may be perceived as irregu-lar aggregates of intergrown pieces of simplecrossed lamellar structure. Complex crossedlamellar structures, always composed of aragon-ite, are found in many gastropods and bivalves(B0ggild, 1930; Taylor et aL, 1969,1973; Carter,1990; Hedegaard, 1990).


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372 C. HEDEGAARD


VETIGASTROPOD SHELL STRUCTURES 373

Figure L Spherulitic prismatic structure from Gibbula cineraria (Linnaeus, 1758). Scale bar is 10 \un.Figure 2. Calcitic composite prismatic structure from a polished and etched section parallel to the exteriorshell of Haliotis midae Linnaeus, 1758. Scale bar is 10 ujn.Figure 3. Columnar nacre from Trochus maculatus Linnaeus, 1758. Scale bar is 10 (im.Figure 4. Lamello-fibrillar structure from Stomatella planulata Lamarck, 1816. Scale bar is 5 u.m.Figure 5. Simple crossed lamellar structure from Tugali elegans Gray, 1843. Scale bar is 10 ujn.Figure 6. Complex crossed lamellar structure from Tugali elegans Gray, 1843. Scale bar is 10 u.m.

Intersected crossed platy structure (Figure 7)is 'a crossed structure with two predominantdip directions consisting of intersected platycrystallites' (Carter & Clark, 1985, p. 62), origi-nally described as 'Uberkreutzt plattige Struk-tur' from the inner layer of the early whorls ofMikadotrochus beyrichi (Hilgendorf, 1877) byErben & Krampitz (1972), and is identical to'type II crossed lamellar structure' (Batten,1975; Hedegaard, 1990).

RESULTS

Table 1 lists the shell structures of exemplartaxa of the investigated families. Figure 9 showsthe single most parsimonious tree from thebranch-and-bound analysis of the shell struc-ture data in Table 1, excluding the characterssimple prismatic and homogeneous structures.Table 2 lists the inferred character transforma-tions. ACCTRAN and DELTRAN assump-tions give the same inferred charactertransformations.

DISCUSSION

The tree in Figure 9 is the result of an analysisof shell structure characters only, and not acomprehensive set of characters. It obviouslydemonstrates shell structures may be usefulin phylogenetic analysis, but also has a coupleof interesting features. Haszprunar's (1988)

Figure 7. Intersected crossed platy structure fromLiotia granulosa (Dunker). Scale bar is 10 p,m.

analyses provided little resolution of the cladeVetigastropoda. Subsequent analyses byPonder & Lindberg (1995, 1997) are moreresolved, but only consider a few taxa. Thetopology of the tree in Figure 9 is only weaklysupported, being based on few characters. Itdoes, however tentatively, suggest a resolutionand a sequence of evolution.

In Haszprunar's (1988) analysis (Figure 8),all nacreous gastropods (some Vetigastropoda,

Table 2. The character transformations inferred to have taken place in thetree in Figure 11. Inferences are identical under ACCTRAN and DELTRANassumptions.

Outgroups —> Node ANode A-» Node BNode B -> Node CNodeC-» NodeDNode D —* Haliotidae

NodeD-* Node ENode D -> StomatellidaeNode B —> Phasianellidae

7. Intersected crossed platy6. Complex crossed lamellar5. Simple crossed lamellar2. Columnar nacre3. Calcitic composite prismatic7. Intersected crossed platy1. Spherulitic Prismatic4. Lamello-fibrillar1. Spherulitic Prismatic


374 C. HEDEGAARD

PatelloideaNacclloideaNeolepetopsidaeCocculinoideaLepetelloideaNeritimofphaMelanodrymiaNeomphalusFissurelloideaLepetodriloideaScissurelloidcaHaliotoideaPleurotomaroideaTrocboidcaSegue nzoideaCydophoroidcaAmpullaroidcaOther gastropods

Figure 8. ,The phylogeny of Gastropoda of Haszprunar (1988), 'other gastropods' are the remaining taxa,without relevance to this discussion.

Muricidac

Neritidae

Patellidae

Fissurcllidae

Haliotidae

Pleurotomariidae

Trochidac

Turbinidae

Seguenziidae

Stomatellidae

Scissurcllidae

Phasianellidae

' Osteopeltidae

Pseudococculinidae

Figure 9. The resulting most parsimonious tree found by a 'branch-and-bound' search of the data in Table 1.Tree length = 0; Consistency index (CI) = 0.778; Homoplasy index (HI) = 0.222; CI excluding uninformauvecharacters = 0.714; HI excluding uninformative characters = 0.286; Retention index (RI) = 0.909; Rescaledconsistency index (RC) = 0.707.

o

o ffl

represented by Haliotoidea, Pleurotomaroidea,and Trochoidea, and the Seguenzioidea) arelocated several nodes from the base of the tree.Furthermore, all the respective sister taxa, i.e.,Patellogastropoda, Neolepetopsidae (Hasz-prunar's 'Hont Vent C ) , Neritimorpha, Coccu-linoidea/Lepetelloidea, and Melanodrymia,have simple crossed lamellar shell structures, as

do the remaining Vetigastropoda and the morederived Gastropoda. Consequently, simplecrossed lamellar structures are independentlyderived at least seven times within the Gastro-poda, if we are to believe the gastropod orconchiferan ancestor had a nacreous shell.

The alternative explanation, that simplecrossed lamellar structure evolved once and is


VETIGASTROPOD SHELL STRUCTURES 375

plesiomorphic, and that nacre is apomorphicand evolved once or twice, is more parsi-monious. The most parsimonious chronicle ofevents (sensu O'Hara, 1988) is that simplecrossed lamellar structure is a plesiomorphicgastropod character, and that nacre evolvedlate during evolution of the Vetigastropoda.'Late' is in the Palaeozoic, as Batten (1972)finds well-preserved columnar nacre inCarboniferous Pleurotomariidae, but aftersegregation of the lineages leading to the otherextant clades.

The character transformations are polarizedby the choice of out-groups, suggests thatneither the vetigastropod ancestor, nor themore plesiomorphic Vetigastropoda had nacre-ous shells. Two of the out-groups (Patellidaeand Neritidae) are closer to the root ofHaszprunar's (1988) tree than the Vetigastro-poda, and one of the out-groups (Muricidae) isfurther from the root. All of the out-groupshave aragonitic crossed lamellar shell struc-tures. Furthermore, no alternative gastropodoutgroup will change this polarization. Withingastropods, the shell structures columnar nacreand insersected crossed platy structure onlyoccur within the Vetigastropoda.

In Figure 9, node A designates the cladeVetigastropoda, characterized by the synapo-morphy intersected crossed platy structure, asimplied by Hedegaard (1990), and identified byPonder & Lindberg (1995, 1997), and by thesymplesiomorphies simple crossed lamellar andcomplex crossed lamellar structures.

The base of the Vetigastropoda includes thefamilies with crossed lamellar structures,including Phasianellidae, traditionally includedin or considered a sister taxon of Turbinidae(Hickman & McLean, 1990; Thiele, 1931).Phasianellidae retain the plesiomorphic crossedlamellar shell structures, and do not have apo-morphic nacreous structure of the Turbinidae.Phasianellidae also have other supposedlyplesiomorphic characters, like paired shellmuscles (Haszprunar, 1985), and their phylo-genetic position should be examined by meansof a comprehensive phylogenetic analysis.

Node D designates the nacreous Vetigastro-poda—Thiele's (1931) basic stock of primitive'Archaeogastropoda'—which are interpretedas a crown group. These are diagnosed by 1)vetigastropod intersected crossed platy struc-ture, 2) absence of crossed lamellar structures,3) presence of the distinctive columnar nacre,and 4) presence of spherulitic prismatic struc-ture. These data urge a reconsideration of thepaired pallial organs and other supposedly

plesiomorphic characters of Pleurotomariidae.If Pleurotomariidae, contrary to Thiele (1902)is not ' . . . by far the most primitive gastropods. . .,' and furthermore if few of the potentialsister taxa and none of the more distant gastro-pod clades have paired pallial organs or shellmuscles (Haszprunar, 1988; Ponder & Lind-berg, 1995,1997), the symmetry of members ofPleurotomariidae must be secondarily derived.Pleurotomariidae could tentatively be thoughtof as paedomorphic Trochidae, rather than thearchetypal ancestral gastropods. This hypothe-sis may be tested by means of a comprehensivephylogenetic analysis and ontogenetic studies.

The presence or absence of shell structuresmay depend on size. In particular small speciesmay be subject to constraints, leading to theabsence of certain shell structures. First orderelements of many structures are in the range of10-20 u,m, making them unsuited for smallshells, which frequently are less than 50 \i,mthick. Homogeneous structure is prevalent inspecies with small shells (Hedegaard, 1990).Homogeneous structure is composed ofsmall sub-units, around 0.1 |xm in diameterand occurs within as well as outside Vetigas-tropoda.

Additional resolution of the tree could beprovided in a number of ways, the most import-ant of which would be including positionalinformation, aligning shell structures as sequen-tial arrangements. Apart from being morpho-logically different, shell structures occur indifferent places through an outside-insidesequence: Nacre is always interior, but inter-sected crossed platy and simple crossed lamel-lar structures may be interior as well asexterior, homogeneous structures are alwaysexterior. Further resolution may come fromdistinguishing between different complexcrossed lamellar structures (sensu Carter &Clark, 1988), and by resolving the homologiesof homogeneous structures.

Differences between nacreous structures inthe molluscan clades are rarely considered, andnacres are believed to be plesiomorphic (e.g.,Wingstrand, 1985; Runnegar, 1996; Salvini-Plawen & Steiner, 19%). The present analysissuggests crossed lamellar structures are plesio-morphic for the Gastropoda in general andVetigastropoda in particular. Wilmot, Barber,Taylor & Graham (1992) suggest crossed lamel-lar structures evolved several times. Theydemonstrate that crystals in crossed lamellarstructures with different orientation relative tothe shell are elongated along different crystallo-graphic directions, i.e., different crystal faces


376 C. HEDEGAARD

are developed. It is not surprising that crystalfaces vary between comarginal and radialcrossed lamellar structures—the two structuresmay be deposited with a common orientation ofcrystallographic axes, but form by developmentof different faces due to interaction with theorganic matrix of the shell. It is possible that wemay get more characters from crystallographicproperties of crossed lamellar structures, butBelda, Cuff & Yellowlees (1993) report modifi-cations of crossed lamellar shell structures as aresponse to dietary supplement in Tridacna,demonstrating that at least crystal morphologyalone can not resolve whether a given shellstructure evolved more than once.

The analysis suggests we have to adjust oursearch image of the plesiomorphic vetigastro-pod, and re-evaluate the assumed polarity ofshell structure transformations. We shouldnot assume plesiomorphic gastropods havenacreous-prismatic shells (e.g., Wingstrand,1985; Runnegar, 1996; Salvini-Plawen &Steiner, 1996). It is more parsimonious toconsider crossed lamellar structures to beplesiomorphic.

When describing properties of molluscanshells and their deposition, many text booksonly describe nacreous shells (e.g., Storer &Usinger, 1965; Villee, Walker & Barnes, 1984;Barth & Broshears, 1982; Buchsbaum, 1987;Hickman, 1988). This is probably due toThiele's paper from 1902, which did not addressshell structures, but introduced Pleutoromari-idae as the most primitive gastropod, combinedwith the subsequent discovery of nacreousstructures, albeit markedly different, in Mono-placophora (Schmidt, 1959). The main reasonfor the axiom of an ancestral nacreous gastro-pod may be an unfortunate trust in the fre-quency quoted. The most important data in thisanalysis, the distribution of nacre and crossedlamellar structures, has been available at leastsince B0ggild's (1930) monograph, but evensubsequent comprehensive compilations ofshell structure data (MacClintock, 1967; Tayloret al., 1969,1973; Lindberg, 1988; Carter, 1990)did not reconsider alternative interpretations ofthe plesiomorphic condition.

ACKNOWLEDGEMENTS

I am indebted to M.G. Harasewych and Ross Nehmand an anonymous reviewer for constructive criti-cism and assistance with the English language.

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shell structures of the recent vetigastropoda

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