The discovery of a radula in a Dentimargo species and its taxonomic implications

2005·3.9 MB·English

by  Franck Boyer

© SociedadEspañola deMalacología Iberus, 23 (2): 47-52, 2005
The discovery of a radula in a Dentimargo species and its
taxonomic implications
Descubrimiento de la rádula en una especie de Dentimargo y sus
implicaciones taxonómicas
Franck BOYER*
Recibidoel28-IV-2005.Aceptadoel6-X-2005
ABSTRACT
The discovery of a comb-like radula in specimens of Dentimargo cf. oureocinctum (Stearns,
1872) is recorded. Theoccurrenceofsuch a radula in a presumed non-radulategenus suggests
a verycióse relationship between thegenus Dentimargoand thegenus VolvarinaHinds, 1844.
This matter invalidatesthetaxonomic organisation currentlyaccepted within theMarginellinae,
and especiallythatbetween theTribesMarginellini and Prunini. Theconchological differenciation
of Dentimargo from the Volvarina-Dentimargo common stem is shown to have arisen before
the loss ofthe radula, and this loss is considered to have little taxonomic valuéwithin the mar-
ginellid gastropods. Due to its high conchological similaritywith the type species D. dentifera
(Lamarck, 1803), D. cf. oureocinctum isconserved in thegenus Dentimargo, which is provisionally
considered as being composed both ofradulate and of non-radulate species.
RESUMEN
Se reseña el descubrimiento de una rádula en forma de peine en ejemplares de Dentimargo
cf. oureocinctum (Stearns, 1872). La presencia de tal rádula en un género que se considera
carecer de la misma indica una relación estrecha entre el género Dentimargo y el género
Volvarina Hinds, 1844. Ello invalida la ordenación taxonómica actualmente aceptada entre
los Marginellinae y particularmente entre las Tribus Marginellini y Prunini. Se muestra que la
diferenciación conquiológica de Dentimargo a partir del estirpe común Volvarina-Denti-
margo se ha producido antes de la pérdida de la rádula y que esta pérdida es de escaso
valor taxonómico en gasterópodos marginéllidos. Considerando su gran semejanza con-
quiológica con la especie tipo D. dentifera (Lamarck, 1803), D. cf. oureocinctum se man-
tiene en el género Dentimargo, considerando en este tanto especies provistas de rádula
cómo careciendo de la misma.
PALABRAS CLAVE: Marginellidae, Dentimargo, Volvarina, radula, clasificaciónsupraespecífica, Caribe.
KEYWORDS: Marginellidae, Dentimargo, Volvarina, radula, supraspecificorganisation, Caribbean.
INTRODUCTION
The generic ñame Dentimargo Coss- monly attributed to a series ofmarginel-
mann,1899,haseáonMarginelladentifera lid species widely distñbuted in the
Lamarck, 1803 from the Mid-Eocene Recent throughout the tropical and the
(Lutetian) of the Paris Basin, is com- subtropicalzonesfromlittoraltobathyal
* 110CheminduMaraisduSouci, 93270 Sevran, France.
47
Iberus, 23 (2), 2005
levéis. The generic placement in Denti- Prunini because of its light cylindrical
margoiscurrentlyusedforspecieswitha shell similar to that found in many
tall spired, rather biconic, tiny shell, speciesofVolvarinaHinds, 1844).
whitish shaded, generally not decorated BOYER (2001: 160) underlined that
or poorly banded, and presenting more "the presumed lack of a radula in the
or less developed labial denticles, the genus Dentimargo was controlled only
uppermostonebeingthelargest. abouta restrictednumberofspeciesand
Defined on this basis, the genus has the type species D. dentifera Lamarck,
been considered to be non-radulate 1803, a fossil species apparently repre-
because a radula could not be found in sented from the Eocene to the Miocene,
individual species checked respectively was naturally not checked for this char-
by Barnard (1969), Ponder (1970) and acter". Boyer (2001: 160) also explained
CoovERT (1987). Commenting upon the that "numerous marginelliform species
record oí "9 species known or strongly from the European Eocene... constitute
suspected tobe non-radulate", Coovert a poorly differentiated Volvarina-Denti-
AND Coovert (1995) considered the margo complex, in which the "comb-
whole genus Dentimargo as non-radu- like" radulae (typical of the radulate
late, analogous with the genera Mar- Prunini species) or their derived forms
ginella Lamarck, 1799 and Glabella might be often represented". Boyer
Swainson, 1840, also claimed to be non- (2001: 160) observed that "theloss ofthe
radulate and considered to be closely radula in a uniserial rachiglossan group
allied with Dentimargo. Marginella and is probably contracted easily, and this
Glabella are defined on the basis oí derived character seems to have been
having larger and thicker shells, gener- formed on several occasions in the radi-
ally not bearing a stronger upper labial ation of the Marginellidae. From that, it
denticle and exhibiting a richer axial, is inferred that some Dentimargo lin-
spiralorocellatedecoration. eages may have conserved their radula,
Coovert and Coovert (1995) like some Volvarina species may have
groupedtogetherDentimargo,Marginella, losttheirs".
Glabella and several Dentimargo-lookm^ The present article is devoted to
genera inthetribeMarginelliniFleming, reporting on the discovery of a comb-
1828, considered to be distinct from the like radula inaCaribbeanspecies attrib-
other Marginellinae tribes (Aus- utable to Dentimargo, and to the first
troginellini CoovertandCoovert 1995 generaltaxonomic inferencesthatcanbe
and Prunini Coovert and Coovert, issuedfromthisdiscovery.
1995), principallyby their ratherbiconic
shelloutlineandbytheclaimedlackofa Abbreviations:
radula (the exception being the genus
Hyalina Schumacher, 1817, which is said ERC: E. RolanCollection.
to have lost its radula but placed in the FBC: Collectionoftheauthor.
RESULTS
FamilyMarginellidae Fleming, 1828
Subfamily Marginellinae Fleming, 1828
Genus Dentimargo Cossmann, 1899
Typespeciesbyoriginaldesignation:MarginelladentiferaLamarck,1803.
Dentimargo cf. aureocinctum (Stearns, 1872) (Figs. 1-3)
Marginella(Glabella)aureocinctaStearns,1872:22[Typelocality:LongKey,Florida].
48
BOYER: The discoveryofa radula in aDentimargospecies and its taxonomic implications
mm
1
fmw^^^
lOiam
Figures 1-3. Dentimargo cf. aureocinctum (Stearns, 1872). 1: shell from Puerto Moreios, Yucatán,
height 3.50 mm; 2: radula píate from ajuvenile specimen of3.20 mm ofshell length; 3: position
oftheradulaseenbytransparency(samespecimenasin Figure 1).
Figuras 1-3. Dentimargo cf. aureocinctum (Stearns, 1872). 1: concha de Puerto Moreios, Yucatán,
altura3,50 mm;2: diente radularde unjuvenilde3,20 mm de longituddeconcha;3:posición dela
radula vistaportransparencia (mismoespécimenquelaFigura 1).
Typematerial:Holotype(livecollected)intheUnitedStatesNationalMuseum.Notexamined.
Othermaterialexamined:Bandedform.Florida:4adultspecimens,2juvenileshells,screeningin
mud and grass, lowtide,2 feet,Tampa Bay (FBC); 2adultspecimens,hand dredged, 1-2feet, St
AndrewsBay(FBC).
Whiteform.Florida:2adultshells.CrawlKey(FBC):thesquatterofbothshellsasD.cf.aureocinc-
tum.Yucatán:3adult(Figs.1,3)+1subadult+1juvenilespecimens,1juvenileshell.PuertoMoreios
M
(FBC,ex-ERC,lot57 1994):allasD.cfaureocinctum.
Description: Stearns (1872: 22). biconic profile, with 2 dark narrov^
COOVERT (1987: 35-37) provided a spiral bands on the body v^horl and 1
good figure of the type-2 live animal of band onthespirewhorls.
D. aureocinctum from southw^est Florida Radula: Coovert (1987: 37) did not
(p. 36, fig. 3) and an extensive descrip- "attempt to extract radula from this
tion ofthe animal external anatomy and species", but on the basis of the lack of
chromatism. Tw^o specimens were radula displayed by Ponder (1970) in
studied, with the adult and subadult Dentimargocairoma(Brookes,1924)andof
mm
specimens shell lengths being 3.88 the apparentlackofradula inthe Florid-
mm
and 4.36 respectively. The shell in ian Dentimargo eburneola (Conrad, 1834)
dorsal view is shown to have a slender. checked by himself, Coovert (1987: 37)
49
Iberus, 23 (2), 2005
considered thatitwas "quite likely that the 2 other adult shells show 2 faintly
D.aureocinctaisalsonon-radulate". distinct denticles below the pronounced
In the frame of this study, a radula upperone. Foralltheotherfeatures, our
has been extracted from an adult (shell specimens perfectly match the one pic-
length= 3.50 mm), a subadult (shell turedinVokesandVokes(1983).
length= 3.50 mm) and a juvenile (shell TheshellsfromFloridashowthesame
length= 3.20 mm) specimens originating generalmorphologyandthesameorgan-
from Yucatán (Puerto Morelos, FBC ex- isationofthecolumellarplaitsandofthe
ERC) andpreserved inalcohol. denticulatedlabrum (morecommonly3-
Adult specimen (Figs. 1, 3): radular 4tinydenticlesbelowtheproducedupper
extraction R-310, undetermined number one, occasionallyonly2tinydenticles or
of comb-like radular plates bearing 20- onlythelargerupperone),buttheypresent
21 cusps. The radular ribbon is very amoreslenderoutlinewithamorepointed
small and sub-translucent (see the size spireandanarroweraperture.Mostofthe
of the ribbon at the tip of the black shellsfromFloridabearonehoney-orange
arrow in Figure 3) and it was very diffi- narrow spiral band on the spire whorls
cult to find. The length of this ribbon and2bandsonthebodywhorl,onalight
mm
was 0,256 for a shell length of 32 honeyto deep whitebackground colour,
mm. The ratio ribbon length/shell butsomespecimensorpopulationsshow
lengthis ofaboutV125. a full-white shell (form immaculata Dalí,
Subadult specimen: radular extraction 1890). Intergrades between the "banded
R-609. The radula was observed by form"andthe"whiteform"arecurrently
transparency through the soft parts but found. The squat form represented in
was lost during the extraction process Yucatán(theshellpicturedinVokesand
by low dissolving, due to its minute Vokes, 1983 and our lot from Puerto
size. The radula was lying within a Morelos)isscarcelyfoundoffFlorida(FBC:
pouch situated at the distal tip of the 1 white shell of "squat form" collected
extended proboscis. This pouch is inter- togetherwithawhiteshellofthe"slender
pretedasbeingthebuccalpouch. form" at Crawl Key). No evident inter-
Juvenilespecimen:radularextractionR- gradesbetweenthe"squatform"andthe
308,54comb-likeradularplatesof20jam "slender form" are known to us, so the
ofwidthandbearing20-21 cusps(Fig.2). squat-shelled populations areprovision-
Distribution: The species is said to allynamed as D. cf. aureocinctum.
range from Florida to Yucatán and the The successful finding of a minute
GreaterAntilles, but the real identity of radulainD. cf.aureocinctumhasoccurred
the tiny littoral Dentimargo species 3timesoutofthe3checkingsmadeinthe
recordedfromtheGreaterAntülesremains limited material at hand. Examined by
tobeverified,duetothepossiblepresence transparencywithinthealcohol-preserved
ofseveralsimilarspeciesinthisárea. animal, the radula is verified tobe situ-
Remarks: Vokes and Vokes (1983) ated at the tip of the proboscis, as well
record our species from Yucatán as whentheproboscis is inextendedposi-
"Marginella (Dentimargo) aureocincta tion (observationin the subadult speci-
immaculata Dalí", and they picture (pl. men) than when the proboscis is in re-
mm
18,fig. 7) ashellof2.9 lengthresem- tractedposition (observationinanadult
blingourspecimensclosely (Fig. 1). This specimen,Fig. 3). In2 outofthe3 tenta-
shell is however thicker, with a strong tives, the radular extraction was per-
labrum bearing one produced upper formedwithsuccessandthesameminute
denticle and 4 smaller ones positioned comb-likeradulawas documented (Fig.
below the mid-part ofthe inner labrum. 2). These data allowtoleave outthehy-
Our 3 adult specimens have lighter pothesisgivingtheseradulaeasremains
shells with a thinner labrum, one of ofdigestedpreys. The data athandcon-
them bearing a singular, pronounced firmtheradulatestatusofthespeciesand
upperlabialdenticle,therestoftheinner thebelongingofthisradulatothe"Volva-
labrum being smooth (Fig. 1), whereas rina-Prunum comb-likepattern".
50
BOYER: The discoveryofa radula in aDentimargo species and its taxonomic implicatioi
DISCUSSION is demonstrated to be morphologically
very similar to the fossil type species D.
The centrally depressed outhne of dentifera. Forthis reasonitsplacement in
the plates and the smallest cusps Dentimargo sensu stricto is conserva-
placed in n:\edian position are unusual tively proposed as the most parsimo-
features compared to the comb-hke nioussolution.
radulae known to us from the Volva- However, it mustbe underlined that
rina-Prunum series (Coovert and other Dentimargo-sheWeá species closely
CoovERT, 1990), whichhave generally a matching with D. dentifera [like for
straight or faintly convex anterior instance the New Zealand D. cairoma
cusped edge, together with uniformly (Brookes, 1924) studied by Ponder
distributed sub-equal small cusps, or (1970), the Floridian D. eburneola
sub-equal small cusps with a larger (Conrad, 1834) checked by Coovert
central cusp, or series of sub-equal (1987) or the Mascarene D. pumita (Red-
small cusps separated by isolated field, 1870) checked by the author]
larger cusps. However the number of really do seem to be devoid of a radula.
plates like the number and the shape of The matter signifies that the occurrence
the cusps in D. cf. aureocinctum are of the radula is represented as a hetero-
similar to the pattern found in geneous characterwithinthe Dentimargo
numerous Volvarina-Prunum species. In series and that it cannot be used as a
summary, the radula of D. cf. aure- diagnosis feature for the genus. In other
ocinctum is coherent with the range of words, Dentimargo is provisionally con-
variability found in the Volvarina- sidered as being composed both of
Prunum series, more than with the radulateandofnon-radulatespecies.
radular patterns found in the Serrata As a direct consequence, the loss of
series (high number of subequal cusps) theradula islikelytobeoflow discrimi-
or in the Mesoginella complex (trian- nating valué in marginellid gastropods.
gular anterior cusped edge, with a Besides, it mustbe emphasized that the
large central cusp and few laterals). loss ofthe radula may have arisen inde-
Despite the minute size oftheradula pendantly in different lineages. This
found in D. cf. aureocinctum, there is no pointleads to a reconsiderationnotonly
reason, in the present state, to consider about the phyletic unity of Dentimargo,
it as vestigial. For instance, a minute butalso aboutthe degree ofrelationship
radula with single, narrow plates is also occurring between Dentimargo and the
found in the marginellid genus supposed non-radulate genera Mar-
Hydroginella known as ectoparasit ginella and Glabella.
feeding at night on sleeping fishes Onthe otherhand, the Volvarina-pat-
(BoucHET, 1989; Johnson, Johnson terned radula found in D. cf. aureocinc-
AND Jazwinski, 1995). As assumed by tum proves that the conchological dis-
Johnson etal. (1995) about the similar tinction between Dentimargo and Volva-
case found in Colubraria, such minute rina (notwell marked in the Eocene, but
plates seem to work as cutting out the more clearly displayed in the Recent)
fish's skin, as precondition of a feeding took place before the loss of the radula,
process by suction of the fish's blood. at least in one of the Dentimargo lin-
This point allows to infer thatthe comb- eages. Secondarily, the radula found in
like radula of D. cf. aureocinctum may as D. cf. aureocinctum suggests a very cióse
well be functional despite its minute relationship between Volvarina and Den-
size. Such a minute size of the radula timargo, without care about the order of
may also be considered, from an evolu- disbranching from a comb-like radulate
tionnary point of view, as an intergrad- ancestor.
ingstagetowards thelossoftheradula. The discoveryofa radula inthe sup-
Due to its biconical shell outline, its posed non-radulate group Dentimargo
produced upper labial denticle and its and the inferred phyletic proximity
faint lower denticles, D. cf. aureocinctum between Dentimargo and Volvarina lead
51
Iberus, 23 (2), 2005
to the assumption that the generic diag- tution of the disbranchings within the
nosis and the supraspecific distinctions subfamily.
currently accepted within the Marginel-
linae remain mainly non-operative, in
ACKNOWLEDGEMENTS
particular about the separation between
the Prunini and the MarginelUni, and
abouttheirrespectivecomposition. I am greatly indebted to Dr Emilio
The matter requires a general Rolan (Vigo University), for giving
reassessment of the organisation of the material from his 1994 collecting trip to
Marginellinae, based on a new docu- Yucatán, extracting the radulae and pro-
mentation provided by extensive com- vidingthepictures.
parisons concerning the morphologic Thanks are due also to Alain Robin
disparity occurring in fossil and Recent (Le Mesnil St Denis, France) for prepar-
shell material, and by correlative ing the digital píate, to Andrew Wake-
researches about the radula. Additional field (Buckhurst Hill, UK) for correcting
comparisons concerning internal the English form, and to Robert and
anatomy and DNApatterns may prove Nicole Hasselot (Jouques, France) for
to be more decisive for a clear reconsti- typingoutthemanuscript.
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52