Evolution of the Embryonic Development in Lingulid Brachiopods

Evolution of the embryonic development in lingulid brachiopods

ANDRZEJBALINSKI

Baliiski, A. 1997.Evolution of the embryonic developmentin lingulid brachiopods.- Acta P alaeontolo gica P olonica 42. 1. 45-56.

The style of embryonic development in the lingulids has changed through time; that of Recent lingulids is not primitive for the group. The shell of Devonian lingulids consists of two valves already atthe embryonic stage, whereas in Recent lingulids the protegulum originates as a single plate, subsequentlyfolded in two. The protegulum ofthe Devonian lingulids is a cup-shaped, subcircular plate, usually with a characteristic radial sculpture suggesting the presence of marginal setae, similar to those occurrilg in early juvenile stagesof Recent discinids. Devonian protegula are 81 to 100 pm il width and thus are three times smaller than protegula of the Rec ent Lingula utd Glonidia, and twice as small as those of the Late Cretaceous Lingula sp. The embryonic development of lingulids underwent important modification during last 370 Ma.

Key word s: Lingulidae, protegulum, ontogeneticdevelopment, Late Devonian.

Andrzej Balir4ski,Instytut Paleobiologii PAN, ul. Twarda 51/55, PL-00-114 Warsryrwa, Pobnd.

Introduction

Our knowledge of the embryonic developmentof lingulid brachiopodsis based exclusivelyon extantspecies. Although somebrachiopods secrete their first exoskele- ton asearly as the embryonicstage, findings of truly embryonicfossil shellsare usually questionable.Most recordsactually correspond to the postembryonicstages of growth (Popovet al. 1982:pp. 102-103;Rowell 1986:p. 1056;Holmer 1989:p.52). The supposedprotegulum, in a strict sense,was recorded in the Ordovician discinid Trem.atiselliptopora Cooper, 1956 (Chlang 1971b) and in the Middle Ordovician acrotretidEoconulus cf. semiregularisBiernat, 1973 (Holmer 1989). On the other hand, fossilized larval (postembryonic)shell has been frequently recordedin different stocks of inarticulatebrachiopods. Several papers have been published describing detailsand peculiarities of the larval shells(e.g., Biernat & Williams 1970;Ludwigsen 1974;Popov et al.1982; Holmer 1989). 46 Embrtoni c development in lins.ulids: BALINSKI

The larval shell is easilyrecognizablein the studiedLate Devonianlingulids (Figs 1A, B, 2A, B, D). It is clearly delineatedfrom the rest of the valve (postlarval) especially at its posterior margin, where conspicuousgrowth disturbanceis seen, causedby an intemrptionof the shell deposition(e.g., Figs lB-F, 2E,F). This change in the secretionalregime undoubtedly reflects a changefrom free-swimming(larval) to sedentary(postlarval) modes of life. The lengthof thelarval shellin studiedlingulids rangesfrom 320 to 560 pm and closelycorresponds to the dimensionsof larval shells in many fossil lingulaceans(Holmer 1989:p. 59). Most intriguing,however, is the apicaipart of the larval shellwhere atiny, uniquely structured,cup-shaped subcircular plate developed(Figs 1-3). The plateis interpreted hereas the embryonicshell, i.e. protegulum,as defined by Beecher(1891) and Chuang (197Ia, b). Most probablyit wassecreted by the embryojust prior to hatchingfrom the vitelline membrane.The major difference in topographybetween the protegulum and larval shell may reflect a changefrom ernbryonic lecitotrophic to larval planktotrophic modeof life. This is the first recordof embrvonicshell in fossil lineulids.

Material

The protegular stageof the shell has beenstudied in lingutid brachiopodsfrom several horizons of the Upper Devonian of the DEbnik anticline (southernPoland) and Holy CrossMountains (central Poland). Thirteen well-preserved dorsal protegrla have been revealedamong those specimens(Tables 1, 2). Additional23 dorsal and 10 ventral protegula, although not so well preserved,have provided an important contribution to the presentresearch. All specimensare from samplesdissolved in acetic acid. The specificor evengeneric identification of the studiedspecimens has not beenpossible becauseall arecrushed. Fortunately, the posteriorparts of both valves,which arethick and resistantare most frequent in samples.Several specimensreveal a distinctly punctateshell substance- the featurecharacteristic of the genusLingulipora (Baliflski 1985; Table 1). Non-punctatespecimens were identified as Lingula s.1.,and are referredto 'Lingula' below (Thble2). Sevenspecimens of Lingula sp. from the Late Cretaceousof Mielnik (eastemPoland) have been studiedfor comparativepurposes (Table3). The describedspecimens are housed in the Instituteof Palaeobiologyof the Polish Academyof Sciencesin Warsaw,for which the abbreviationZPN, is used.

Description of embryonic shell 'Lingula' Dorsal protegulum. - Dorsal protegulumof the Late Devonian and Lingulipora is a subcircular structure with slightly blunt and expandedposterior margin(Figs 1,2,5). The width of protegulumrangesfrom 81 to 100pm. Its maximum length is lessthan its width and rangesfrom 64 to 89 pm. Thus,the index of lengthis 79 to 977o(mean = 86Vo:,see Tables 1-3). The protegulumis a convex structure,but its centralregion is frequentlyconcave or flattened(Figs 1B-E, 2B-E). This regionis usuallyseen as a circularcentral depression bordered at its peripheryby a roundedand very weak rim (Fig. lC-F). The height of the protegulumreaches 25 trtm.The most ACTAPALAEONTOLOGICA POLONICA (42) (I) 47

Fig. 1. A-F. Brachialvalve of Lingulipora sp.,Late Devonian,southern Poland, Debnik, ttenchZ-6,ZPAL Bp MsV25-6. A. General exterior view of posterior part of the valve showing protegular,larval and post-larvalshell; periphery of the larval shellmarked with white triangles.B. More detailedview showing iocationof protegulum.C-8. Lateral,posterior, and postero-lateralview of the protegulumshowing radial ribs, centraldepression, and marginalrim; note also a distinct boundarybetween posterior margin of the larval shell and the post-larvalshell (markedwith white trianglesin D). F. Detailedview of posteriorpart - of the protegulumshowing the thickenedmarginal rim. ls - larval shell,pr - protegulum,ps post-larval shell. striking characterofthese dorsalprotegula is their distinct radial sculpture.It consists of thick, roundedradial ribs which startat the rim of the centraldepression and extend almost to the margin of the plotegulum,where they reacha thickenedmarginal rim (Figs lC-F, 2B-E). The marginalrim is especiallywell exposedat the posteriorpart of the protegulum.In the anteriorpart, the marginal rim is gradually embeddedin 48 Embryonicdevelopment in lingulids:BALINSKI post-embryonicshell. The radial ribs are distributedmore or less symmetricallyon both sidesof the protegulum. They are best delineatedin lateral and posterior parts of the protegulum. Those in anterior part are very weak and usually difficult to trace. In exceptionalcases, well preservedprotegula do not revealany radial ribs (Fig. zG,H). Their number varies from five to six pairs.

Table 1. Dimensions of dorsal pr otegria of Lingulipora sp. from the Late Devonian of the Dgbnik anticline (southernPoland) and Holy Cross Mountains (central Poland). SD - standarddevation.

CaL No ZPAL Bp XLUI-2 XLV6-IO MsV25-6 xt u6-1 xLv6-2 Y,V4-20 mean SD

Length (pm) & 65 ta 80 88 89 76.5 4.9 Width 1pm; 8t 8l 83 94 100 92 88.5 3.6 Length : Width (7o) 79 81 88 85 88 97 86.3 3.1

'Lingula' Table 2. Dimensions of dorsal protegula in fromLate Devonian of the Dgbnik anticline (southem Poland) and Holy CrossMountains (central Poland).

Cat. No ZPAL Bp n u2-4 MsVl-4 xLv2-2 K,U4-10 n-u4-5 K-A+8 )(Lv6-8 mean SD Length (pm) 70 72 75 78 80 80 88 77.6 6.7 Width (pm) 86 85 85 90 90 100 100 90.1 5.6 Length: Width 17o) 8l 85 88 87 89 80 88 85.4 3.3

Table 3. Dimensionsof dorsalprotegulainLingulasp. fromtheLateCretaceous ofMielnik(easternPoland).

Cat. No ZPALBp n-v5-21 n v6-r5 )0v5-13 )0y5-18 n v5-17 mean SD Length (pm) 130 130 (130) 140 160 138 6.5 Width (pm.1 170 200 2lo 2lo 200 198 8.2 Length: Width (7o) 76 65 62 67 80 70 3.8

Apart from the sculpture described above, the extemal surface of the studied protegulais usually smooth.Some of exceptionallywell-preserved specimens show original microtopographic features, such as densely packed depressionsor pits up to 3.5 pm in diameter(Fig.2G, H). Thesecan be interpretedas an imprint of a disinte- gratedperiostracum and may reflectfraces of intercellularboundaries. Some protegula show characteristicarched drapes (Fig. 2F). Similar structureswere describedon the shell surface of the CarboniferousLingula squamifurmisPhillips, 1841 and were interpreted as deformation formed by stresscouples between periostracum,primary layer andunderlying outer mantlelobe (Cusack& Williams 1996). Ventral protegulum. - The ventral protegulaof the studiedspecimens are less well preservedthan the dorsal ones. Apparently during life of the animal the ventral protegulumwas exposedto more intensemechanical wear. Whereas the dorsalprote- gulum could be well protectedby the thickened and frequently elevatedmargin of the valve (Fig. 2G), the ventral protegulumoccupied the most apical position and was gradually worn out. In effect, posterior margins of the ventral protegulaare never preservedin pediclevalves studied; instead, they show distinct tracesof mechanical abrasion(Fig. 3A-D). This may be explained,at leastin part, by the infaunalmode of life of Devonianlingulids. ACTA PALAEONTOLOGICAPOLONICA (42) (I) ,40

'Lingula', Fig. 2. A-E. Brachial valve of Late Devonian, southern Poland, Dgbnik, locality PG (see galifisk1995),ZPALBpMsV12-4.A C.General viewandtwoclose-upsoftheposteriorpartofthevalve showing location of protegulum and its morphological details. D, E. Postero-lateral views of the same specimen showing well-developed radial ribbing and central depression of the protegulum; note also a-distinct boundary between the larval and post-larval shells. F. Dorsal protegulum of Lingtilipora sp., Late Devonian, Holy Cross Mts, Jablonna, ZPN-BpXLI/4-20; note a weakly-developed radial ribbing and strong concentrically disposed arched drapes, the most probably lbrmed by stress couples between periostracum. primary layer, and underlying outer mantle lobe. G, H. General and detailed view of the protegulum of lingulipora sp., age and locality as in F, ZPAL MsV21-8. Note a thickened posterior margin of the posllan al shell protecting protegulum from mechanical abrasion and well-preserved sudace of the protegular shell with imprints of the intercellular boundanes. 50 Embryonicdevelopment in lingulids:BALINSKI

Fig. 3. A-D. Postero-ventralview of the ventralprotegul a of Lingulipora sp. showingdiferent degreeof their mechanicalabrasion, ZPAL BpXLIl3-2,5-23,5-24, and4-14.E, F. Generaland more detailed ventral views of the ventral protegulumof Lingulipora sp. (sameas in C) showingpoorly preserved,supposed tracesof imprints of the intercellularboundaries. Late Devonian,Holy CrossMts, Jablonna.pr - protegulum.

The preservedantero-lateral margin of ventralprotegula is regularly arched.Char- acteristicradial ribs, similar to thosefrom dorsalprotegula are seen in onefragmentar- ily preservedventral protegulumof Lingulipora sp. Other featurescharacteristic for the dorsal protegulum(central depression and pitted surface)have been sporadically observedin ventralprotegula (Fig. 3E, F). The maximum width (80 to 98 pm) as well as convexity (25 pm) of the available venfralprotegula precisely corresponds to the measurementsof the dorsalprotegula.

Discussion

It is generallyaccepted that lingulid brachiopodsare extremeevolutionary conserva- 'living tives and the genusLingula is traditionally quotedas an exampleof a fossil'. ACTA PALAEONTOLOGICAPOLONICA (42) (I) 51

Fig. 4. A-D. Lingula sp.,Late Cretaceous,eastern Poland, Mielnik. A, B. Generaland more detailedvie*'s of a fragmentof the brachialvalve showingwell delimitedlarval shell (markedwith white triangles)and the protegulum, ZPN,Bp )(LV5- 13. C. Posteriorview of a fragmentof the brachialvalve with presen-ed protegulum,ZPALBp XLUs-ll . D. Dorsolateral view of a small fragment of the brachiai valve with protegulum.ZPALBpXLU5-21.is-larvalshell,pr-protegulum,ps post-larvalshell.

One might thus expect,that the structureof protegulumof Devonianlingulids would not differ from thoseof Recenttaxa. This, however,is clearly not the case. The protegulaof the Late Devonian 'Lingula' andLingulipora show some fun- damentaldifferences in comparisonto the embryonic shell of living Lingula and Glottidia.In his statisticalstudy, Chuang (1962) found that the width of protegulumin Lingula unquisfrom the coastsof SingaporeIsland ranges from232 to 300 pm. Yatsu (1902: p. 36) measuredthe maximum width of the protegulumof Lingula anatina along its posteriorstraight edge and obtained280 pm on average.These data demon- stratethat the embryonicshell in the studiedLate Devonianlingulids was as much as threetimes smaller than that in the extantrelatives. The LateCretaceous lingulids from Mielnik (easternPoland; see Bitner & Pisera1979) reveal intermediate dimensions of protegulum;they are illustratedhere for comparisonin Fig. 4. In thesebrachiopods, the protegulumranges from 170to 210 pm in width, i.e. it is twice as largeas that in the Late Devonianlingulids. Even more important is the differencebetween the studiedLate Devonian and Recentlingulid protegulain the detailsof their morphology.As notedby Yatsu(1902: pp.32,33), the first embryonicshell in Lingula anatina is secretedas a singlecircular lamella which is folded over along one of its diametersand is then divided to form a dorsal and a ventral hatf. In consequence,the dorsal and ventral protegula are semicircularin outlineand have straight posterior edge (Fig. 5; Yatsu1902: pI. 5:73, 77; Ashworth1915: pl. 4: l-6; Paine1963: fig. 5). In contrast,the dorsalprotegula of 52 Embryonic development in lingulids: BALINSKI

Age (Ma) 0

ul]a$u!

Lingula

400

0 50 100 150 2ffi 250 300 350 Widtlr(pm)

Fig. 5. Comparisonofthe generalmorphology of theprotegula ofthe Late Devonian,Late Cretaceous,and Recentlingulid brachiopods.Morphology of Recentprotegulum mainly after Yatsu(1902), Paine (1963), andChuang (1983).

the Late Devonianlingulids aresubcircular, with broadlyarched posterior margins. As notedabove, the studiedvenffal protegula lack the posteriorregion because it hasbeen progresivelyabraded during life of the animals.There is no reasonto believethat they were of basically different shape than dorsal protegula. Thus, immediately after hatching,the larvaeof Devonianlingulids possessedcertainly two separate,ventral and dorsal,protegula. This basicdifference between fossil and Recentprotegula suggests that during 370 Ma sincethe DevonianPeriod, the embryonicdevelopment of these brachiopodshas undergone an importantgradual change (Fig. 5). The subcircularoutline and very well developedposterior and marginalrim in the Late Devonianprotegula suggest that therewere two separatesecretional areas of the ACTA PALAEONTOLOGICAPOLONICA (42) (I) "7 53

main body POSTERIOR

Fig. 6. Hypothetical reconstruction of the main anatomical features of the embryo of the Late Devonian lingulids; dorsalview.

embryonic shell. These areasreflect an early mantle differentiation into ventral and dorsallotes. In the courseof evolution,the shell secretionalareas haye 6ssa unified into a single area, as it is examplified by RecentLingula. Later in ontogeny,this area is folded into two parts and a furrow develops sepamtingdorsal and ventral mantle lobes (Yatsu 1902: p. 3I-33, pl. 4: 60 6$. In other words, during the course of evolution, the formation of the mantle lobes underwent retardation in relation to the secretionof protegulum. The most peculiar feature of the Late Devonian protegula is their surface topo- graphy. The central depressionand radial ribs have no equivalentsin neither the Late Cretaceousnor Recentlingulid protegula.The differenttopography of the protegulain the Late Devonian lingulids most probably reflects some anatomicaldifferences of their embryos.Perhaps the radial sculptureof protegulumis relatedto dispositionof setaesimilar to thoseoccurring in embryonicand larval stagesof Recent Pelagodiscus andDiscinisca (Blochmann 1880; Ashworth 1915;Chuang 1977;Harnmond 1980; Long & Stricker 1991).The radial ribs may reflect internal grooveswhich accomo- dated the embryonic setae.Williams & Holmer (1992: p. 680, pl.1: 6) noted that periostracumof RecentDiscinisca striata (Schumacher)may be indentedon its inner surfaceby radial groovesaccomodating the outer surfacesof adpressedsetae, The radial ribs of the Late Devonianprotegula may also indicatea dispositionof radially arrangedmuscles serving the setal follicles. The arrangementof these musclesis well-known in post-embryonicstages of RecentLingula (Williams &Holmer 1992: pp. 687-689,frg.12).The moreor lessconstant number of radialribs on LateDevonian protegulasuggests that therewere five or six pairs of long embryonicsetae (Fig. 6). The first setaeand the protegulum were formed by the embryo probably at the end of the embryonicstage, just beforehatching from its vitelline membrane.The arrange- ment of setaeis reminiscentof the condition in Recentdiscinids. Juvenile stageof discinids is characterizedbya successionof severalkinds of more or less radially disposedlong setae(Ashworth 1915: p. 60, pl. 5: 11;Hammond 1980: pp.65V652, Embryonicdevelopment in lingulids:BALINSKI figs 1-3; Chuang1983: p. 231,figs 12, I3). Ifthe presenceofthe earlyjuvenilesetae in discinidsis a plesiomorphicfeature, then the proposedpresence of embryonicsetae in the Devonianlingulids suggeststhat in the Paleozoicthe earliestontogenetic stages of thesetwo brachiopod stockswere much more similar than they are now. In Recentlingulids, however,setae of this kind are absent.According to Yatsu (1902)and Paine (1963) the first setaein Lingula andGlottidia appearas late as at the end of larval stage,i.e. just prior to settlement.They appearalong the entire mantle margin. These very short and very densely distributed mantle setaecertainly cannot explain the radial sculptureof the embryonic shell in the Devonian lingulids. The function of hypothetical embryonic setaein the Devonian lingulids, as well as of those in Recentdiscinids, remainsunclear. It is possiblethat they may help in keeping the proper orientation of a newly hatched swimming larva, although theil sensoryfunction cannotbe excluded.

Conclusions

The uniquely structuredfossil protegularevealed evident morphologicaldifferences between them and those in Recent Lingula and Glottidia. This suggestsfundamental anatomical difference between the embryonic stagesof Late Devonian and Recent lingulids. So basic evolutionary modification of their embryoscontradicts the common opinion that the lingulid brachiopodsare extremely conservativeevolutionarily. It thus supportsBassett (1986) and Biernat & Emig (1993) who questionedthe long strati- graphicrange of the genusLingula (but seeWilliams 1977). It seemsobvious that more paleontological data are neededto trace the processof gradual morphological modification of lingulid embryonic shell. Especially valuable would be information on the lingulid protegula from the post-Devonianto Late Cretaceousinterval.

Acknowledgements

I would like to sincerely thank Professors Gertruda Biernat ard lerzy Dzik (Warsaw) for critical review of the earlier versionsof the paper and anonymousreviewers for most useful comments.I am also indebted to Dr Lars Holmer (Uppsala) for discussion and for aid during my stay in Uppsala in 1995. Thanks are due to Dr Maria A. Bitner and Professor Jerzy Dzik (Warsaw) for making available their collections of Late Cretaceousand Late Devonian lingulids for studies; a few specimensof Recent lingulids were obtainedfrom Dr Christian C. Emig, Marseille. The specimenswere examined on Philips XL-20 scanning electron microscope at the Institute of Paleobiology in Warsaw.

References

Ashworth, J.H. 1915. On the larvae of Lingula and Pelagodiscus(Discinisca). - Royal Society of Edinburgh, Transactions 51, 45-69. Baliriski,A. 1988.Shell morphology and strucfiteinLinguliporaGW. -ActaPalaeontologica Polonica 33,123-133. Bassett,M.G. 1986.Brachiopodes inarticulds. 1n.' P.R. Racheboeuf(ed.), Le Groupede Lidviry.Pridoli- Lochkoviende I'Artois (N. France).Biostratigraphie de Paldoloique3,85-97,Lyon. ACTA PALAEONTOLOGICAPOLONICA (42) (I) 55

Beecher,C.E. 1891.Development of theBrachiopoda. Part I. Introdttction.-AmericanJournal of Science' Ser.3.41.343-357. - Biernat,G. & Emig, C.C. 1993.Anatomical distinctionsof the Mesozoiclingulide brachiopods. Acta PalaeontologicaPolonica 38, 1-20. - Biernat, G. & Williams , A. lgT}.llltrastructure of the protegulum of some acrotretide brachiopods. Palae ontolo gy 13, 491-502. Bitner,M.A. & Pisera,A. 1979.Brachiopods fromtheUpperCtetaceous chalkof Mielnik(EasternPoland). - Acta GeologicaPolonica 29, 67-88. Blochmann,F. 1898. Die Larve von Discinisca (Die MuellescheBrachiopodenlarve). - Zoologische Jahrbiiche r (Anat. ) li, 417426. - Chuang,S.H. 1962.Statistical study of variationsin the shell of Lingula unguis (L.). Videnskaplige Meddelelserfra Dansk Naturhistorisk Forening 124, 199-215. Chuang, S.H. l9?la. New interpretation of the morphology of Schizambonaustralis Ulrich and Cooper (Ordovician siphonotretid inarticulate brachiopod). - Journal ofPaleontology 45,824-832. Chuang, S.H. 1971b.The morphology and paleobiologyof Trematiselliptopora Cooper (Inarticulata, Brachiopoda).- SmithsonianContributions to Paleobiology3' 93-100. Chuang,S.H. 1977.Lmval developmentin Discinisca (inarticulatebrachiopod). - American Zoologist 17,39-54. Chuang, S.H. 1983. Brachiopoda.In: K.G. Adiyodi & R.G. Adiyodi (eds), ReproductiveBiology of Invertebrates,Volume II: Spermatogenesisand SpermFunction,517-530, John Wiley & SonsLtd' New York. - Cusack. M. & Williams , A. 1996. Chemico-structural degradation of Carboniferous lingulid shells' Philosophical Transactions of the Royal Society of I'ondon B 351, 3349. Hammond,L.S. 1980.The larvaeof discinid(Brachiopoda: Inarticulata) from inshorewaters near Towns- ville, Australia, with revised identification of previous records. - Journal of Natural History 14' 647-661. Holmer, L.E. 1989. Middle Ordovician phosphatic inarticulate brachiopods from Viistergiitland and Dalarna,Sweden. - Fossilsand Strata 26, l-l'72. Long, J.A. & Stricker, s.A. 1991. Brachiopoda.In: A.C. Giese, J.S. Pearse,& V.B. Pearse(eds), Reproductionof marine invertebrates,vol. 6: Echinodermsand lophophorates,4T-84.Blackwell Scientific,Boston. Ludvigsen, R. 1974. A new Devonian acrotretid (Brachiopoda, Inarticulata) with unique protegular ultrastructure.- NeuesJahrbuch fiir Geologieund Paltiontologie,Monatshefte 3,133-148. Paine, R.T. 1963. The ecology of the brachiopod Glottidia pyramidata. - Ecological Monographs 33' t87-2r3. Popov,L.E., Zezrna,O.N. & Nylvak, J. (Popov,L.E.,Zezina, o.N., & Nylvak, A.) 1982.Microstructure - of the apical part of the shell of Ecardhes and its ecological importance [in Russian]. Bfilleten' MoskovskogoOfresna IspytatelejPrirody, Otdel Biologiieskij 87'94-104. Rowell, A.J. 1986.The distributionand inferred larval dispersionof Rhondellinadorei: anew Cambrian brachiopod(Acrohetida). - Joumal of Paleontology60,1056-1065. - Williams, A. & Holmer, L.E. 1992. Ornamentation and shell structure of acrotretoid brachiopods' Palae ont olo gy 35, 657492. Williams,A.I.lgTT.InsightintolingulidevolutionfromtheLateDevonian. -Alcheringal,401406. Yatsu, N. 1902.On the developmentof Lingula dnatina.- Joumal of the Collegeof Science,Imperial Universityof Tolcyo17,1-112.

Ewolucja rozwoju zarodkowegou lingulid6w

ANDRZEJBALINSKI

Streszczenie Na kilkudziesigciu okazach lingulid6w z p6i,nego dewonu antykliny Debnika i G6r Swigtokrzyskichstwierdzono wystepowanie okl4glawej, wypuklej tarczki o szeroko- 56 Embryonicdevelopment in lingulids:BALINSKI

Sci81-100 pm usytuowanejw najbardziejapikalnej czgsci obu skorup.Tarczka ta jest wyra1nieodgraniczona od muszli larwalnej. Brak jest na niej koncentrycznychlinii przyrostowychcharakterystycznych dla muszli larwalnej i postlarwalnej.Nie ulega w4tpliwoSci,2e tarczkata odpowiadamuszli zarodkowej,czyli protegulum.Jest to pierwszy przypadektak dobregozachowania protegulum u kopalnych lingulid6w. W por6wnaniuz protegulumwsp6lczesnej linguli i gtotidiijest onotrzy rzy filrriejsze, a ponadtocharakteryzuj e sigszczeg6ln4 morfologi4, nakt6r4skladaj4 sig promieniste faldki, cenfralnieusytuowana wklgslo6i oraz waleczkowatozgrubione brzegi. R6w- nieZprotegula p6Znokredowych lingul odbiegaj4znaczrieod muszli zarodkowej form p6znodewor4skich(Fig. 5). Mo2naprzypuszcza(, 2e r62nicew budowiemuszli zarodkowej odzwierciedlaj4 r62nice w szczeg6lachanatomicznychstadium embrionalnego p6Znodeworiskich,p6lnolredowych i wspdczesnych lingulid6w. Zapewne wigc w czasie ostatnich 370 milion6w lat rozw6j zarodkowy lingulid6w alegl znacznej modyfikacji, co podwaZa doSi powszechnie przyjmowany pogl4d o wyj4tkowym konserwatyzmieewolucyjnym tej grupy ramienionog6w.