Asgaard & Stentoft_1984. Recent Micro Morph Brachiopods

8/4/2019 Asgaard & Stentoft_1984. Recent Micro Morph Brachiopods

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R E C E N T M I C R O M O R P H B R A C H I O P O D S F R O M B A R B A D O S :P A L A E O E C O L O G I C A L A N D E V O L U T I O N A R Y I M P L I C A T I O N S

b yU L LA A S G A A R D * & N IE LS S T E N T O F T *

A B S T R A C T R I ~ S U M I ~B r a c h i o p o d s w e r e c o ll e c te d f r o m 1 6 t o 3 2 5 m d e p t hf r o m r e e f a n d o f f - r e e f e n v i r o n m e n t s, o f f t h e w e s tc o a s t o f B a r b a d o s ( W e s t I n d i e s ) . T h e 1 5 s p e c i e s c o l -l e c t e d s h o w e d d i s t i n c t i v e d e p t h d i s t r i b u t i o n s , w h i c ha r e d e p e n d e n t i n p a r t o n t h e d i s tr i b u t i o n o f p r e f e r r e ds u b s t r a t e s a n d i n p a r t o n t h e l a r v a l b i o l o g y . C o m p a r i -s o n w i th U p p e r T e r t ia r y a n d e a r ly Q u a t e r n a r y m i c r o -m o r p h b r a c h i o p o d f a u n a s f r o m t h e M e d i t e r r a n e a na r e a a n d s i m i l a r f o r m s f r o m t h e u p p e r m o s t C r e t a -

c e o u s a n d l o w e r m o s t T e r t i a r y w h i t e c h a l k a n d c a l c a -r e n i t e s f r o m N W E u r o p e i n d i c a t e s t h a t t h e m i c r o -m o r p h b r a c h i o p o d f a u n a s w e k n o w t o d a y a lr e a d yw e r e e s t a b li s h e d b y t h e e n d o f t h e C r e t a c e o u s .

C e s b r a c h i o p o d e s o n t 6 t6 r 6 co l t6 s e n t r e 1 6 e t 3 2 5 md e p r o f o n d e u r , e n d o m a i n e r 6 c i f a l e t p 6 r i r 6 c i f a l , a ula rge des cb tes occ identa les des Barbades (Ant i l l e s ) .L e s 1 5 es p ~ ce s m o n t r e n t u n e r 6 p a r t it i o n b a t h y m 6 t r i -q u e d i s t i n c te , d 6 p e n d a n t e n p a r t i e d e l a r6 p a r t i ti o n d e ss u b s t r a t s f a v o r a b l e s e t d e l a b i o l o g i e l a r v a i r e . U n ec o m p a r a i s o n a v e c l es fa u n e s d e b r a c h i o p o d e s m i c r o -m o r p h e s d u T e r t i a i r e s u p ~ r i e u r e t d u Q u a t e r n a i r ea n c i e n d e M 6 d i t e rr a n 6 e e t a v e c d e s f o r m e s s i m i la i re sde l a c ra ie b lanche e t des ca lca ra r6ni t es du Cr6tac6t e r m i n a l e t d u T e r t i a i r e b a s a l d u N W d e l ' E u r o p e ,m o n t r e q u e l e s f a u n e s d e b r a c h i o p o d e s m i c r o m o r p h e sm o d e r n e s r e m o n t e n t / ~ l a f i n d u C r 6 t a c 6 .

* G e o l o g i sk C e n t r a l i n s t i t u t , O s t e r v o l d g a d e 1 0 , D K - 1 3 5 0 K o b e n h a v n K . D a n e m a r k .

Ge obio s , M 6m . sp6c ia l n ~ 8 p . 29-33 , 2 p l Ly on , 1984


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1 - DEPTH DISTRIBUTION

S u b s t r a t e s a n d s e d i m e n t s w e r e c o l l e c te d w i t hS C U B A o n t h e f r i n g e r e e f o f f S p e i g h t s t o w n ( - 1 2 t oe 4 2 m ) a n d f r o m a t r a n se c t o u t f r o m H o l e t o w n a n d

P a y n e s B a y : b a r r i e r r e e f ( S C U B A , - 1 2 t o - 4 2 m ) ,o u t e r r e e f sl o p e ( P e t e r s e n g r a b , - 5 5 t o -9 2 m ) a n d o f f -r e e f e n v i r o n m e n t ( d r e d g e , - 1 1 0 t o - 3 2 5 m ) . T h e f o l l o -w i n g b r a c h i o p o d s w e r e f o u n d :

Crania pourtalesi DAL LCrania s m o o t h s p .Crania c o s t a t e s p .Cryptopora rect imarginata COOPERGryphus bar t le t t i ( D a l l )Terebratulina cailleti CROSSEA r g yr o t h eca c f . b er mu d a n a DALEA r g yr o t h eca s h r a mmi ( C r o s s e & F i s c h e r )Argyrotheca rubrot incta (Da l l )Argyrotheca barret t iana ( D a v i d s o n )Argyrotheca crassa COOPERA r g yr o t h eca sp .Meger l ia c f . echinata ( F i s c h e r & O e h l e r 0Plat id ia anomioides ( S c a c c h i & P h i l i p p i )Platidia clepsydra COOPER

D e p t h a li v e D e p t h d e a d P I . : F i g.5 5 - 3 2 5 m 1 : 8

1 j u v . 3 2 5 m2 4 0 - 3 2 5 m2 0 1 - 2 4 8 m

9 0 - 2 0 1 m2 5 - 4 5 m

(16)55 - 140m9 2 - 1 4 0 m92 - 140m9 2 - ? 2 8 5 m

1 9 0 - 2 1 9 m2 1 9 - 2 5 0 m2 4 0 - 3 2 5 m

5 5 - 3 2 5 m6 0 - 3 2 5 m

1 1 0 - 3 2 5 m2 4 0 - 3 2 5 m1 4 6 - 3 2 5 m

6 0 - 3 2 5 m2 5 - 7 0 m5 5 - 1 5 0 m9 2 - 1 5 0 m6 0 - 1 7 2 m6 0 , 7 2 1 9 m

? - 7 2 8 5 m2 2 0 - 3 2 0 m?2 4 0 - 3 2 5 m

1 : 91 : 42 : 6 ,71 : 6 ,71 : 12 : 4 , 51 : 5

1 : 2 ,32 : 82 : 1 , 21 : 10,112 : 3

T h e b r a c h i o p o d s f a l l i n t o 6 g r o u p s a c c o r d i n g t od e p t h . 1 ) A r g yr o t h eca c f . b e r m u d a n a o c c u r re d f r o m -2 5 t o - 4 2 m . 2 ) A r g yr o t h eca s ch r a mmi o c c u r r e d f r o m -5 5 t o - 1 4 0 m . 3 ) Argyrotheca rubrot incta a n d A. bar-ret t iana o c c u r r e d f r o m - 9 2 t o - 1 4 0 m . 4 ) O n e ( o r m a yb e t w o ) A r g yr o t h eca s p p . a n d Plat id ia anomioideso c c u r r e d i n t h e in t e r v a l b e t w e e n g r o u p 3 a n d t h e s h a l-l o w e r l im i t o f g r o u p 5 . 5 ) Cry ptop ora rectimarginata,Megerl ia c f . echinata a n d Plat id ia c lepsydra w e r ef o u n d f r o m a b o u t - 2 2 0 t o - 3 2 5 m ( a n d d e e pe r ) . G r o u p6 : Crania s p p . , Gry phu s bart let ti , Terebratul ina cail-let i a n d Argyrotheca crassa s e e m t o h a v e a w i d e rr a n g e o f d e p t h .

2 - SUBSTRATESF r o m - 25 t o - 42 m A r g yr o t h eca c f . b er mu d a n a w a s

f o u n d o n t h e u n d e r s i d e s o f l i v i ng , f o l i a c e o u s s c l er a c ti -n i a n s , m a i n l y Aga ricia agarici tes , b u t a l s o o n Myce-tophyl l ia s p p . a n d Montas t rea annular i s . L e s s c o m -m o n l y , l i v e a n d d e a d , b r a n c h e d s c l e ra c t in i a n s a l sos e r v e d a s s u b s t r a t e s , e . g . Porites astreoides, Mussaangulosa a n d Mad racis m irabil i s.

A la rge , l i v ing Agar ic ia f ragi l i s c o l o n y w a s o b t a i n e df r o m - 5 5 m , c a r r y i n g a sm a l l p o p u l a t i o n o f A r g y r o -theca schrammi a n d Crania pourtalesi . O t h e r g r a b -s a m p l e s f r o m - 5 5 t o - 7 3 m c o n t a i n e d b i o c l a s t i c s a n da n d g r a v e l a n d l a r g e r c o r a i - f r a g m e n t s , b u t n o l i v i n gb r a c h i o p o d s w e r e f o u n d .

F r o m - 7 3 t o - 1 7 2 m , t w o d e m o s p o n g e s u b s t r a t e so c c u r r e d : t h e o r a n g e , i r r e g u l a r ly b r a n c h e d A g e l a ss ch mi d t i a n d a w h i t i s h , f o l i a c e o u s Plakor t i s n . s p .t h a t c o n t a i n e d a s y m b i o t i c $il iquaria s p . T h e t u b u l a rb r a n c h e s o f A . s ch mi d t i a r e a b o u t 1 c m i n d i a m e t e ra n d t h e s k e l e t o n i n c o r p o r a t e s g r a in s o f t h e s e d i m e n tt h e s p o n g e h a p p e n s t o r e s t o n . B o t h t o t h e s e g r a i n sa n d t o t h e Sil iquaria s h e l l s w i t h i n t h e Piakor t i s sp. ar i c h e p i f a u n a i s a t t a c h e d , i n c l u d i n g A r g yr o t h ecaschram mi, A. rub rot incta, A . barret tiana, A. crassaa n d Terebratulina cailleti. In a few ca s e s a l l 5 s pec ie sw e r e f o u n d t o g e t h e r o n t h e s a m e s p o n g e .

A t - 7 3 , a n d d o w n t o - 3 2 5 m ( a n d d e e p e r ) s y n s e d i -m e n t a r y c e m e n t a t i o n h a s t a k e n p l a c e . T h i s h a s r e s u l -t e d i n n o d u l a r l u m p s o f b i o c la s t i c m a t e r i a l u p t o 1 0 c mt h i c k i n t h e s h a l l o w e r p a r t o f t h e t r a n s e c t s , w h i l e i nt h e d e e p e r p a r t r e g u l a r c r u s tl i k e h a r d g r o u n d s s e e m to


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be present (Stentof t, 1982). The lumps and crustsboth bore a rich epifauna including brachiopods fromgroups 3, 4, 5 and 6.

3 - ECOLOGY AND POPULATION STRUCTURE

The micromorph Argyrotheca spp. are fastened tothe substrate by an extremely short and stout pedicle.The commissure is thus perpendicular to the substrateand the valves gape widely when the animal feeds.When observed in the laboratory A. cf. bermudanaand A, s ch r a mmi were seen to swivel violently whendisturbed by swimming crustaceans and ridd ing them-selves of sediment. Argyrotheca spp. are known tobrood their larvae, which results in a patchy distribu-tion. In the case of A. cf. bermudana many m2 ofapparently suitable substrates were found to be bar-ren, and we were unable to confirm the distributiongiven by Logan (1975) ; no specimens were found onthe dives a t the fringe reefs off Holetown in -6 to -10mdepth. The populations collected in late April andearly May 1980 showed a bimodal distribution ofmature specimens and recently settled individuals.Argycotheca cf. bermudana, A. schrammi and A.rubrotincta quickly attain full size and have a shortlifespan. In the accompanying sediments very fewshells of immature specimens were found, whichwould indfcate a low juvenile mortality. The deeperwater Argyrotheca spp. have occasionally clear andcrowded growth-lines near the commissure, indicatinga longer lifespan.

The brachiopods down to -73m were found incryptic habitats on the undersides of foliaceous scle-ractinians or between branches of branched forms.Below -73m the brachiopods came out into the openfastened to sponges and cemented lumps and crusts.No caves were situated on the chosen reef transectsand this might explain why no thecideans were found.Thecideans have been reported from reef-caves else-where in the Caribbean (Jackson & alii , 1971, Logan ,1977).The almost ubiquitous Terebratulina cailleti mayalso be considered a micromorph species since noneof the specimens found exceeded 1 cm in length. It ispossible that this species broods its larvae as does T.septentrionalis, but the material did not show a parti-cularly low juvenile mortality as was the case for thesmaller Argyrotheca spp. T. cailleti is furnished withj long, distally split pedicle. Specimens in the labora-tory gave lively demonstrations of their ability to flaptheir valves and whip round on their pedicles whendisturbed.

Gryphus bartletti also was very small for the genus.The only two living specimens found were attached topebbles smaller than their shells with short distallysplit pedicles. Dead shells o f all sizes were common inthe sediment samples.Few live Crania spp. were fo und, and no dead ven-tral valves were observed on apparent ly suitable subs-trates. Dorsal valves were common in the sedimentsamples. They were all irregular in shape ; many ofthem show xenomorphic sculpture that indicates thatthe ventral valves were cemented with the entire sur-face to the substrates.Only two populations of Platidia anomioides werefound. The close adpression to the substrate in thisspecies is clearly to be seen from the irregularities ofthe figured specimens. The Argyrotheca spp. from

group 4 are less brightly coloured than the shallowwater species and tend to be much wider than longand have large endopunctae compared to the shallowwater Argyrotheca spp.Cryptopora rectimarginata and Platidia clepsydrawere the dominant living brachiopods found in thedeeper dredges. They do not exceed 3ram in lengthand owing to the transparency of their shells they areeasily overlooked. These species do not bore into thesubstrates as do the other brachiopods and are there-fore easily dislodged. P. clepsydra does not live withits dorsal valve pressed against the substrate.The quantitative analysis of living populations and

death assemblages is not yet completed, but theresults so far do not seem to deviate from similar stu-dies in other regions (e.g. Noble & alii , 1976, Nobleand Logan, 1981 and Steward, 1981). Constituentgrain analyses made on selected samples show thatwhere the brachiopods are common they do notexceed 2% of the grains and in many cases they cons-titute only 0.5% (Stentoft, 1982). Similar results werereached elsewhere by Caulet (1967) and Loga n (1977).

4 - OTHE R RECENT MICROMORPH BRA CHIOPOD FAU-NA S

The Caribbean Sea exhibits the greatest diversity ofmicromorph brachiopods (Cooper, 1977) closely fol-lowed by the Mediterranean and tropical to subtropi-ca~, Eastern Atlant ic (Logan, 1979). Scattered recordsexist from the Red Sea, Persian Gulf, Indian Oceanand the Pacific (Thomson, 1927, Cooper, 1973a,1973b, 1981, 1982), Gulf o f California (Hertlein andGrant, 1944) and Bermuda (Logan, 1975).


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n 32 m

All these faunas show a surprisingly similar compo-sition to the Barbados fauna, having a dominance ofArgyrotheca spp. in the sublittoral areas and Kraussi-nidae, Platidiidae and Cryptopora spp. tak ing over atthe shallow end of the bathyal range. The similarityalso extends to the morphology of the species. TheArgyrotheca spp. of group 1 in most cases are smoothor have few and nearly obsolete costae, the greatestwidth being anterior to the hinge line. Argyrothecaspp. of groups 2 and 3 are variably costate and someforms have their greates t width at the hinge line. Mostof the Argyrotheca spp. of group 4 are alate and fur-nished with large endopunctae.

This parallelism in the circumtropical belt wouldseem to indicate a long history of shallow micro-morph brachiopod faunas.

- FOSSIL MICROMORPH BRACHIOPOD FAUNAS

The Recent micromorph brachiopod faunas can betraced back to the Upper Tertiary, and at least theMediterranean species have existed since then." LowerTertiary shallow water brachiopods have been descri-bed from the Caribbean area, Western and EasternU.S.A. the Mediterranean area, N.W. Europe andfrom the Indopacific. In the offsh ore sublittoral sedi-ments of the Danian of Denmark and the nearshoresediments of Holl and and Belgium a variety of Argy-rotheca spp. and other micromorph brachiopods arefound (Asgaard, 1968 and unpublished observations ;M. Bagge Johansen pers. comm., 1983). By theDanian the different morphological patterns we canrecognize toda y in groups 1-4 already seem to be pre-sent. Steinich (1965) and Surlyk (1972) described theextremely diverse brachiopod fauna from the Maas-trichtian white chalk of northern Europe with its 32micromorph species. Surlyk (1972) divided the speciesin groups according to their morphology. 1) 20minute species adapted to a life on very small substra-tes e.g. slender, branched bryozoans. In this groupwere cancellothyridids, megathyridids, platidiids anddallinids. 2) Two cancellothyridids, one terebratellidand one thecidean were hemispherical and secondarilyfree-living. Finally 3) two craniids and 3 thecideanswere cemented by a small area to the substrates.However, the Maastrichtian nearshore sedimentshave not yet received a similar investigation. Surlyk(1982) described the brachiopods from the white

Chalk of the Campanian/Maastr ichtian boundary ofN.W. Germany and fou nd that the Lower Maastrich-tian species were present in the Upper Campanian. Incontrast, however, Campanian nearshore and coastalsediments of Sweden have an extremely diverse bra-chiopod fau na which seems totally devoid of micro-morph species (except thecideans). Further back inthe Cretaceous the information on micromorph bra-chiopods so far is scanty, apart from on thecideansfrom shallow water environments (see Pajaud, 1974for references). A rich fauna of minute cancellothyri-dids is fou nd in the Turonian white chalk of northernFrance (M. Bagge Johansen pers. comm. 1983).

6 - TH E ORIG IN OF MODERN MICROMORPH BRACHIO-POD FAUNAS

As mentioned above, shallow water micromorphbrachiopod faunas seem to have existed in the warmseas since the Eocene. By late Cretaceous, a diversemicromorph brachiopod fauna had evolved whichhad adapted to a life in the open, fastened to smalland rare substrates or free-lying on mud in a shallowoffshore environment. Cenomanian to Campaniannearshore environments that we know of are domina-ted by large brachiopods (and the small thecideans).The thecideans had a cryptic mode of life at least sincethe Upper Jurassic (Pajaud, 1974). We consider thatthe cryptic mode of life of the non-thecidean micro-morph brachiopods is a new adaptation by a faunaalready well established (( in the open >>below thephotic zone. Logan (1977, 1979) considered the fac-tors encouraging a cryptic mode o f life in brachiopodsto be the availability of suitable sttbstrates, competi-tion for space, larval photophobia, sedimentation andenergy level, while predation hardly played any role.We would consider another factor to be highly rele-vant in this matter : that of the almost ubiquitousarmies of regular echinoids grazing on all exposedsurfaces in the photic zone of warm seas (Samarco,1980, Hawkins, 1981). The active grazing on algae,particularly red calcareous encrusting algae, causesthe removal o f epilithic forms from the grazed surfa-ces.

The disappearance of large brachiopods from thephotic zone of the tropical seas by the end of the Cre-taceous, therefore, might be linked to the evolution ofthe camarodont Echinoida and the appearance ofDiadema at that time.


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R . G . B r o m l e y ( C o p e n h a g e n ) a s s i s t e d i n th e f i e l d w o r k a n do f f e r e d v a l u a b l e c r it i c is m o f t h e m a n u s c r i p t . T h e D a n i s h

N a t u r a l S c i e n c e R e s e a r c h C o u n c i l s u p p o r t e d t h e f i e l d w o r kf i na nc i a l l y .

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ST E N T O FT N . ( 1982) - E n s e d i m e n t o l og i s k og f a un i s t i s ku n d e r s o g e l s e a f H o l o c a e n e f o r - r e v s k a r b o n a t - s e d i m e n t e rv e d B a r b a d o s . Unpubl . mas ters thes i s , U n i v e r s i t y o fC o p e n h a g e n .

S T E W A R T I . R . ( 1 98 1) - P o p u l a t i o n s t r u c tu r e o f a r t i c u l a t eb r a c h i o p o d s p e c ie s f r o m s o f t a n d h a r d s u b s t r a te s . N e wZea l and Jour n . Zoo l , 8, 197-207.

SURLYK F. (1972) - M o r p h o l o g i c a l a d a p t a t i o n s a n d p o p u l a -t i o n s t r u c t u r e s o f t h e D a n i s h c h a l k b r a c h i o p o d s ( M a a s -t r i c h t i a n , U p p e r C r e t a c e o u s ) . Biol . Skr . kgl . Dan.Vidensk . Se l sk , C o pe n ha g e n , 19 ( 2 ) , 1- 57 .

S U R L Y K F . ( 1 9 8 2 ) - B r a c h i o p o d s f r o m t h e C a m p a n i a n -M a a s t r i c h t i a n b o u n d a r y s e q u e n c e , K r o n s m o o r ( N WG e r m a n y ) . Geol . Jahrb, H a n n o v e r , A 6 1 , 2 5 9 - 2 7 7 .

T H O M S O N J . A . ( 19 27 ) - B r a c h i o p o d m o r p h o l o g y a n dg e n e r a ( R e c e n t a n d T e r t i a r y ) . N ew Zea l and Boar d Sc i .Ar t . M anua l , W e l l i n g t o n N . Z . , 7 , 1 -3 38 .


6/9

P L A T E 1

F i g . 1 - - Argyrotheca c f . bermudana d o r s a l v i e w , x 3 2 .

F i g . 2 , 3 - - Argyrotheca crassa d o r s a l v i e w , x 8, c a r d i n a l i a a n d b r a c h i d i u m , X 1 4 .

F i g . 4 - - Cryptopora rectimarginata d o r s a l v i e w , x 1 8 .

F i g . 5 - - Argyrotheca barrdttiana d o r s a l v i e w , x 1 8 .

F i g . 6 , 7 - - Terebratulina cailleti d o r s a l v i e w s o f n a r r o w a n d w i d e f o r m s , x 6 .

F i g . 8 - - B r a c h i a l v a l v e o f Crania pourtalesi, x 7.

F i g . 9 - - B r a c h i a l v a l v e o f Crania c o s t a t e s p . , x 7 .

F i g . 1 0 , 1 1 - - Platidia anomioides c a r d i n a l i a a n d b r a c h i d i u m , x 4 2 : a n d b r a c h i a l v a lv e , x 2 0 .

N . B . N o t e v a r y i n g sc a le .


7/9

GeoblosM(~m. special 8 pl . 1U. Asgaard & N. Stentoft

.. ... _ ;:.~

~ ii i ,. :, ~;~.":i.~.".~.i.:..:...:.....~ c~.~;~L...:........~:~-., ....... ... ....,.%~', ~:..-~..,~~:~5.,~:'.._.: .:_ ..~.... ..~~ '. .~ ,~ -~ .. ., " - .:. . .-~ ~ # , ~ - .,~ - ,, ~ ,.' .. .. .~ ~ .~.. .... , .

:~: ' :,,_!i 'i.?!: "

~ 24

6


8/9

PLATE 2

F i g . 1 ,2 - - B r a c h i a l a n d p e d i c l e v a l v e o f Platidia anomioides, x 2 1 a n d x 1 6 .

F i g . 3 - - Platidia lepsydra d o r s a l v i e w , x 1 3 .

F i g . 4 , 5 - - Argyrotheca schrammi o n i t s s u b s t r a t e , s i d e v i e w a n d d o r s a l v i e w , x 3 5 .F i g . 6 , 7 - - B r a c h i a l a n d p e d i c l e v a l v e o f Gryphus bartletti, x 3 a n d x 2 . 5 .

F i g . 8 - - D e t a i l s o f w o r n b r a c h i a l v a l v e o f Megerlia echinata, x 10 .

N . B . N o t e v a r y i n g sc a le .


9/9

GeobiosM~m. special 8

pl. 2U. Asgaard & N. Stentoft

x\ ......., ~ " 2"".~' ~::., . ~. L -., ..: "

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