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UNIVERSITYj 



Miisemim of Comparative Zoology 



Cambridge, Mass. 



15 May 1970 



Number 350 



FIMBRIA AND ITS LUCINOID AFFINITIES (MOLLUSCA; 

BIVALVIA) 



Kenneth Jay Boss 



Abstract. Fimbria is the only surviving genus of the Fimbriidae, a 
family of bivalve moUusks that flourished during the Mesozoic. The con- 
chological and anatomical features of Fimbria show that the family be- 
longs to the superfamily Lucinacea and that it is closely allied to the 



Lucinidae. The systematic position of the Fimbriidae 



relation to other 



lucinoid families is discussed, and a possible phylogeny for the superfamily 
is re-constructed. 



NTRODUCTION 

The relationship of the Fimbriidae with other lucinoid bivalves 
has been questioned by McAlester (1966), who suggested that 
additional anatomical and morphological data would be necessary 
before certain families, the Fimbriidae included, could be unequiv- 
ocally assigned to the Lucinacea. It has been usual to include 
Fimbria and its fossil relatives in the Lucinacea (Dall, 1895; 
Thiele, 1935; Franc in Grasse, 1960; Eberzin in Orlov, 1960; 
Newell, 1965; Moore, 1969). Dall in Zittel (1900) considered 
the Fimbriidae an offshoot of the Lucinidae. The reason for this 
assignation was conchological, based on the structure of the dental 
and ligamental elements of the hinge of Fimbria, Anatomical 
characteristics supplementing the features of the shell allow the 
definite inclusion of the Fimbriidae in the Lucinacea. 

The species of Recent Fimbria were reviewed by Lamy ( 1 92 1 ) 
and Nicol (1950), and according to the latter there are only two 
living species, both distributed in the Indo-Pacific area. During 
the Mesozoic, particularly the Jurassic and Cretaceous, the genus 
Fimbria was rich in number of species and was widely distributed 
throughout warm and temperate seas (Pictet, 1855; StoHczka, 
1871). Both species-number and distribution dwindled during 
the Tertiary, 



2 



BREVIORA 



No. 350 



THE ANATOMY OF FIMBRIA FIMBRIATA 

Mantle. The mantle is thin, translucent over much of its sur- 
face and open ventrally in an extensive pedal gape. Its edge is 
fringed with two rows of tentacles. It is fused anteriorly above 
the anterodorsal surface of the anterior adductor muscle (Fig. 1 ) 
and posteriorly, ventral to the incurrent aperture (Fig. 2). The 
fusion of the mantle lobes anteriorly is much like that in Myrtea 
spinifera (Montagu) as figured by Allen (1958, fig. 7c) and pro- 
vides a broad anterior incurrent pathway by which water and sus- 
pended matter are drawn into the mantle cavity. 

Muscles. The adductor muscles are large and are differentiated 
into 'quick' and 'catch' fibers. The posterior adductor muscle is 
irregularly oblong and rounded, whereas the anterior adductor is 
larger and subsemilunate, with the ventral portion wider or more 
extensive. 

Siphons. Incurrent and excurrent apertures are developed pos- 
teriorly (Fig, 2). The incurrent opening consists of a simple 
rounded aperture, laterally flanked by two ridges of tentacles; ven- 
trally, the muscular lobes of the mantle are fused to form a narrow 








Figure 1. Diagrammatic view of anterior portion of Fimbria, 1. Point 
of anterior mantle fusion, 2. Anterior pedal retractor muscle. 3. Ante- 
rior adductor muscle. 4. Inner mantle fold. 5. Outer mantle 




1 




THE LUCINOID FIMBRIA 



3 



isthmus, and dorsally they arc joined in a broad union, which sepa- 
rates the excurrent siphon. The excurrent siphon consists of a 
hirge muscular tube which is capable of being invaginated and 
lies, when turned inside out, in the suprabranchial chamber. The 
siphonal retractor muscles are not developed, and the orifice of 












Figure 2. Diagrammatic view of posterior portion of Fimbria. 1. Point 
of fusion of outer mantle fold. 2. Secondary papillate fold. 3. Extent 
of tissue grade mantle fusion. 4. Inner mantle fold. 5. Posterior adduc- 
tor muscle. 6. Excurrent invaginable siphon. 7. Tncurrent aperture, 
8. Outer mantle fold. 



4 BREVIORA No. 350 



the aperture is flanked by two rows of papillae. The posterior 
aspect of Fimbria (Fig. 2) is similar to that of Codakia orbicu- 
laris (Linnaeus) as is illustrated by Allen (1958, fig. 9b). 

Foot. The foot in Lucina has been studied by Barrois (1885). 
In Fimbria, it is large, subtrigonal, somewhat laterally compressed, 
longitudinally grooved or creased, heeled posteriorly, and pointed 
anteriorly (Fig. 3). In addition to its strong intrinsic longitudinal 
and circular muscles which surround a central sinusoidal lumen, 
the foot has a pair of anterior pedal retractors that attach to the 



shell just posterodorsal to the anterior adductor muscle and a 
pair of posterior pedal retractors that insert just anterodorsal to 
the posterior adductor muscle. 

Gills and palps. The true gills or ctenidia consist of single, 
bihiteral, inner demibranchs that are fused to the visceral mass 
dorsally and to each other posteriorly. They also attach to the 



muscular portion of the edge of the mantle between the ventral 
incurrent and dorsal excurrent apertures, thus effectively forming 
a septum that creates the dorsal suprabranchial chamber, into 
which empty the reproductive, excretory and faecal products. 

Although longitudinally wrinkled, the gills are non-plicate, 
smooth or flat and apparently homorhabdic. They are thus similar 
to other luncinoids (Ridewood, 1903). In ^ the specimens 
dissected, the demibranchs were retracted and thin, not thick and 
turgescent as are the gills in Lucina (Mencgaux, 1889). The 
filaments are interconnected, and the inner ascending lamella is 
shorter than the outer descending lamella, so that the latter ap- 
pears to be a supra-axial extension of the demibranch. Unlike 
certain other lucinoids (Read, 1962; Hartman and Boss, 1966), 
the gills are not distinctively colored by hemoglobin. 

As in other lucinoids (Thiele, 1886), the labial palps of Fimbria 
are very much reduced and consist of narrow flaplike lips border- 
ing the mouth that accept, at their distal ends, the anteriormost 
portion of the demibranchs (Fig. 4), The reduction of labial 
palps is probably related to the increased efficiency of the ciliary 
mechanisms of the anterior incurrent canal and permits the in- 
gestion of relatively large food particles (Allen, 1958), 

The relation between the labial palps and ctenidia has been 
studied by Stasek (1963). Lucinoids, Fimbria included, exhibit 
a pattern, the category III of Stasek, in which the ventral tips of 
the anterior filaments of the inner demibranch are not inserted 
into a distal oral groove. Further, the inner demibranch may be 
fused to the inner palp lamella. Although the demibranch inserts 



1970 THE LUCINOID FIMBRIA 5 



between the palp lamellae in Fimbria, there is no fusion (Fig. 4). 

Mantle gills. The reduction of the true gills to only a single 
demibranch and the existence of the anterior incurrent canal 
probably were preadaptive anatomical features that aided in the 
development of another unusual character in lucinoids, namely, 
the mantle gill, noted by Duvernoy (1854: 1 15, pi. 5, f. 3) and 
Semper (1880). 

Pelseneer (1911) discerned three different types of mantle 
gills and these have been discussed by Allen (1958). Fimbria is 
somewhat unusual in the development of two large flaps on the 
posteroventral surface of the anterior adductor muscle (Fig. 3). 



A large pallial vessel courses through the mantle diagonally from 



the mantle gill to the auricles. The flaplike structures of the 



mantle gills lie on either side of the channel of the anterior in- 
current canal on the ventral surface of the anterior adductor 

muscle. 

Intestine. The morphology of the alimentary canal was not 
studied in detail. A short esophagus enters the rather large 



stomach anteroventrally and the combined style-sac and midgut 



leaves it posteroventrally; the stomach is of the Type IV category 
of Purchon (1958). The digestive glands surround the stomach 
and are not developed into external visceral pouches. The in- 
testine forms a simple loop in the visceral mass and enters the 
pericardium slightly to the right of the midline; the rectum passes 
over the dorsal surface of the posterior adductor muscle and emp- 
ties into the suprabranchial chamber near the nephroprocts. 

Heart and kidney. The complex of the heart and kidney is 
located posterodorsally and does not vary significantly from the 
typical eulamellibranch pattern (Menegaux, 1890; Odhner, 1912). 
The heart is simple and lies in an expansive, somewhat triangular 



pericardium; two extremely large, thin-walled auricles receive 
blood from the large paUial vessel and the ctenidia. The ventricle 
is thick-walled and does not completely encircle the rectum; a 
dorsal aorta extends anteriorly. Pericardial glands (White, 1942) 
were not detected. 

The kidney lies between the heart and the posterior adductor 
muscle and consists of two bilateral portions. The anteroventral 
reno-pericardial funnels collect fluid from the pericardial cavity; 
the posterior saclike parts of the kidney are broadly connected 
medially. The posterior portion of the kidney encircles the pos- 
terior retractor muscles. The kidneys open via bilateral nephro- 
procts to the suprabranchial chamber posterior to the gonadial 
aperture near the excurrent invaginable siphon. 



6 



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THE LUCINOID FIMBRIA 



7 



Nervous system. The general pattern of the nervous system 
of Fimbria is not significantly different from that of other lucinoids 
or eulamellibranchs (Duvernoy, 1854). There are three main 





ic centers: cerebropleural, pedal, and visceral. The cere- 
bropleural ganglia are located on the inner surface of the anterior 
adductor muscle, just over the labial palps on either side of the 
mouth; they are united via a medial cerebropleural commissure 
(Fig. 4). Anteriorly, the ganglia give rise to nerves which supply 









Figure 4. Diagrammatic view of the buccal area. 1. Outer labial palp. 
2. Inner labial palp. 3. Ctenidium. 4. Mouth. 5. Cerebropleural gan- 
glion with cerebropleural commissure. 6. Esophagus. 7. Cerebropleural- 
visceral connective. 




the anterior adductor muscle and the anterior pallial musculature. 
Anterolaterally are branches to the labial palps, while extending 
ventrally from the ganglia are the cerebropleural-pedal connectives 
and posteriorly the cerebropleural-visceral connectives. Both 

and visceral ganglia are closely juxtaposed and fused; the 
pedal ganglion gives rise to branches that innervate the intrinsic 
pedal musculature as well as portions of the viscera. The visceral 
ganglion is located on the inner surface of the posterior adductor 
muscle. Rather large branchial nerves arise laterally from the 
ganglion. The posterodorsal surface of the ganglion gives rise to 
small, short renal nerves, whereas the large posterior pallial nerve 
splays out over the ventral surface of the adductor muscle. 



Specimens examined 



(All Fimbria fimbriata (Linnaeus 



1758)): Ifaluk Atoll, Caroline Ids. (USNM 616497); Bikini 



8 



BREVIORA No. 350 



Lagoon, Bikini Atoll, Marshall Ids. (USNM 5847 11 ); NE side of 
Ngaloa Ids., Fiji Ids. (USNM 674578); E of Mathuvata, N coast 
of Vanua Levu, Fiji Ids. (USNM 674577); Yanutha Id., N Coast 
of Viti Levu, Fiji Ids. (USNM 674576). 



DISCUSSION 

The unusual anatomical features in the Lucinacea were noted 
long ago by the famous naturalist Poli (1791), who figured the 
Mediterranean Tellina [Lucina] lactea with its elongate, cylin- 
drical foot and single demibranch. Some of the anatomical 
characters of Fimbria were described by Valenciennes (1845a and 
b), who noted the reduction of the ctenidia to single demibranchs 
and the smallness of the labial palps. He showed that the foot 
of Fimbria differed in shape from that of other lucinoids. Thiele 
(1935), following Dall, remarked that in Fimbria, the mantle was 
fringed by two rows of papillae and the excurrent siphon was 
retractile. Allen (1958) studied a number of species of lucinoids 
and distinguished the Ungulinidae [Diplodontidae], Thyasiridac, 
and Lucinidae anatomically. 

Table 1 contrasts the living families of the Lucinacea. The 
Fimbriidae are most closely allied to the Lucinidae. Both families 
have in common: 1) an excurrent siphon consisting of a unique 
invaginable tube; 2) an anterior point of fusion of the mantle 
edges that is dorsal to the anterior adductor muscle; 3) the ctenidia, 
consisting of a single inner demibranch on each side of the animal; 
4) greatly reduced palps; 5) a tissue-grade fusion of the muscular 
lobes of the mantle ventral to the posterior incurrent aperture. 
Some species of the Lucinidae also develop accessory respiratory 
organs or mantle gills and have a foot which is differentiated 
posteriorly into a heel for digging. Fimbriids differ from lucinids 
in the development of a bilaterally compressed subtrigonal and 
pointed foot, thin demibranchs without large amounts of subfil- 
amentar tissue, a double row of papillae along the mantle margin, 
and the confluence of the ventral surface of the anterior adductor 
muscle with the pallial musculature. 

In addition, the shell of Fimbria is strong and heavy with thick- 
ened dentition, elongately-ovate in shape and cancellately sculp- 
tured (Fig. 5). The scar of the anterior adductor muscle is con- 
tiguous with the pallial line, a configuration unlike that of lucinids, 
in which the anterior adductor scar protrudes inwardly and is 
separated from the pallial line. 



1970 



THE LUCINOID riMHRIA 



y 




Figure 5. External view of left valve of Fuiihrui finihridta, length 
30.5 mm, from Yanutha Id., N Coast of Viti Levu (USNM 674576). 



Although the ecology of living Fimbria was not studied in the 
field, it is known that the species are reef-dwellers, preferring rela- 
tively shallow water in coralline sand (Nicol, 1950). The heavy, 
strongly ribbed shell is an adaptation to the coarse substrate which 
the species inhabits. Further, Fimbria probably lives near the 
sand-sea interface, for it possesses the anterior-posterior water- 
flow mechanisms of other lucinoids but lacks the vermiform foot 
to construct a long anterior incurrent mucus tube. 

In addition to the family-level characteristics, Fimbria may be 
included in the Lucinacea because of the following anatomical 
features: 1) an extensive pedal gape; 2) the enlargement of the 
ventral surface of the anterior adductor muscle to facilitate an 
anterior inhalant current; and, 3) the relatively poorly differen- 
tiated and simple posterior incurrent aperture. 



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1970 THE LUCINOID FIMBRIA II 



The Fimbriidae are more distantly related to other luclnoid fam- 
ilies. The Thyasiridae are unique in having a highly differentiated 
vermiform foot, external pouches for the digestive diverticula, both 
outer and inner demibranchs, a posterior incurrent slit without 
differentiated papillae, and a nontissue grade connection of the 
muscular folds of the mantle beneath the incurrent slit (Allen, 



1958; Nakazima, 1958; Kauffman, 1967). The Ungulinidae 
[^Diplodontidae] possess posterior sensory papillae, both inner 



and outer demibranchs (Duvernoy, 1842), moderately sized labial 
palps, and a fusion of the mantle musculature over one-half the 
surface of the anterior adductor muscle. 

Working with the Thyasiridae, Ungulinidae [=-Diplodontidae 
and Lucinidae, Allen (1958) suggested that these families were 
related in an evolutionary series, starting with the Ungulinidae 
[=Diplodontidae], which were considered the most 'primitive' 
family, and progressing through the Thyasiridae to the more 'spe- 
cialized' Lucinidae. McAIester (1966) postulated that the rela- 
tionship was actually reversed and that the Ungulinidae [=Dip- 
lodontidae] and Thyasiridae were convergent with 'typical heter- 
odont' bivalves. The single important heterodont feature referred 
to by McAIester is the presence of both inner and outer demi- 
branches in thyasirids and ungulinids; however, it is now known 
that this feature may not be as important as previously thought, 
since different species in the same genus (e.g., Astarte) may either 
have a full complement of demibranchs or have only a single demi- 
branch. Nevertheless, if the Paleozoic stock of lucinoids had only 
a single demibranch, then the Ungulinidae [^Diplodontidae] and 
Thyasiridae, which branched from them in the late Paleozoic or 
early Mesozoic, had to re-acquire this morphological feature. Thus, 
McAlester's interpretation of the phylogenetic relationship among 
these lucinoid families is essentially correct. However, the geo- 
logical record itself provides a better clue to phylogeny than do 
inferences based on supposed anatomical features of fossil lineages. 

The Lucinidae and Fimbriidae are geologically older than the 
Thyasiridae and Ungulinidae [=Diplodontidae] (Moore, 1969) 
(Fig. 6). The lucinids may date from the Silurian; lUonia Billings, 
a Middle to Upper Silurian genus from Sweden and eastern Canada 
is the first known member of the Lucinidae. However, if Bahinka 
Barrande is considered a lucinoid (McAIester, 1965), the super- 
family is traceable to the Middle Ordovician. Fimbria von Miihl- 
feld appeared in the Bathonian of the Middle Jurassic and radiated 
in the Cretaceous while other fimbriids were present earlier in the 



12 



BREVIORA 



No. 350 



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1970 THE LUCINOH) FIMBRIA 13 



Figure 6. Proposed evolutionary relationships of four living families 
of the Lucinacea. Question marks indicate possible derivations of families. 
Solid lines denote known lineages: the Lucinidae, from the Silurian (Ilionia 
Billings); the Fimbriidae, from the Upper Triassic {SchafhaeiitUa Coss- 
mann); the Thyasiridae, from the Middle Triassic {Storthodon Giebel), 
and the Ungulinidae from the Upper Cretaceous (Brachymcris Conrad). 

The diagrams indicate anatomical features: in the fimbriid-lucinid line- 
age, invaginable excurrent siphon, rounded incurrent aperture and inner 
demibranch only; in the thyasirid-ungulinid lineage, no excurrent invagi- 
nable siphon; slitlike incurrent aperture and both outer and inner demi- 
branchs. 



1 



14 



BREVIORA No. 350 



Upper Triassic {Schajhaeiitlia Cossmann) and even in the Carbon- 
iferous (Scaklia de Ryckholt) (Eberzin in Orlov, 1960; Moore, 

1969).^ 

The Ungulinidae [=Diplodontidae] probably arose in the Up- 
per Cretaceous {Brachymeris Conrad and Felaniella Dall) and 
were certainly present in the Paleocene, whereas the Thyasiridae 
are known positively from the Cretaceous and probably from the 



Middle Triassic {Storthodon Giebel). Thus, the phylogenetic dia- 



gram presented by McAlester (1966) can be extended and aug- 
mented with the insertion of the Fimbriidae near to the Lucinidae, 
from which stem they diverged in the early Mesozoic or late Paleo- 



zoic (Fig. 6). 



ACKNOWLEDGMENTS 



Dr. J. Rosewater of the United States National Museum (USNM) 



loaned preserved specimens for dissection. The manuscript was 
critically read by Professor B. Kummel, Dr. R. D, Turner, and 
Mr. R. T. Johnson. 



LITERATURE CITED 

Allen, J. A. 1958. On the basic form and adaptations to habitat in 
the Lucinacea (Eulamellibranchia). Phil. Trans. Roy. Soc ser. B, 
241: 421-484. 

Barrois, T. 1885. Les glandes du pied et les pores aquifcres chez les 
lamellibranches. Lille, 166 pp. 

Dall, W. H. 1895. Tertiary Fauna of Florida. Part 3. A new classi- 
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1 Scaldia de Ryckholt from the Viscan and Tournaisian Dinantian of 
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by Chavan /// Moore (1969). Previously it was considered in the Edmon- 
diidae (Yokes, 1967). Ryckholl's original figures indicate that Scaldia 
has a pallial sinus, a characteristic which prevents the genus from being 
considered Lucinaccan and which removes it from the Fimbriidae. 



1970 THE LUCINOID FIMMRIA 15 



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I 



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16 



BREVIORA No. 350 



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(Received 16 March 1970.) 



ADDENDUM: While this paper was in press, J. A. Allen and J. F. Turner 
(1970. Pacific Science, 24(2): 147-154) published The morphology of 
Fimbria finihriata (Linnc) (Bivalvia; Lucinidae)' in which they proposed 
placing the genus Finihria in the family Lucinidae.