Taphonomy of the Irregular Echinoid Clypeaster humilis from the Red Sea: Implications for taxonomic...

Figure 6.1. Location of study area.

114 Nebelsick

TAPHONOMY OF THE IRREGULAR

ECHINOID CLYPEASTER HUMILIS

FROM THE RED SEA: IMPLICATIONS

FOR TAXONOMIC RESOLUTION

ALONG TAPHONOMIC GRADES

James H. Nebelsick

Taphonomy desc r ibes t he t rans i t ion o f skele ta l r e m a i n s f rom o n c e l iv ing

organisms to t he preserved r e m a i n s tha t pa leon to log i s t s u l t ima te ly r ecove r

a t the ou tc rop and store in the i r c o l l e c t i o n s . U n d e r s t a n d i n g t a p h o n o m i c

gradients is of essen t ia l i m p o r t a n c e for pa leon to log i s t s w h o use fossil re

mains to m a k e s t a t emen t s c o n c e r n i n g s u c h th ings a s a spec t s o f eco logy ,

diversity, and evolu t ion . A c t u a l i s t i c ( observa t ions c a n m a k e f u n d a m e n t a l

contr ibut ions to unde r s t and ing t a p h o n o m i c gradients for t he s imple reason

that the c o m p l e x e c o l o g i c a l i n t e rac t ions tha t af fect t a p h o n o m i c p rocesses

and features c a n be d i rec t ly obse rved and m e a s u r e d .

E c h i n o d e r m s no t on ly have an i l lus t r ious past bu t a lso f lour i sh today

in diverse m a r i n e e n v i r o n m e n t s . T h u s , we c a n c o n d u c t ac tua l i s t i c observa

tions o f e c h i n o d e r m s and d i rec t ly invest igate those e c o l o g i c a l factors tha t

affect no t on ly the i r d is t r ibut ion bu t a lso t he preserva t ion o f the i r r e m a i n s

after death . A l t h o u g h e c h i n o i d s c o m m o n l y d isar t icu la te rapidly d e p e n d i n g

on a n u m b e r o f factors (see Kie r , 1977 ; Al l i son , 1 9 9 0 ; Kidwel l and B a u -

mil ler , 1 9 9 0 ; D o n o v a n , 1 9 9 1 ; G r e e n s t e i n , 1 9 9 3 ) , c lypeas te ro ids are c o m

paratively robus t and stay c o m p l e t e l ong e n o u g h for t a p h o n o m i c factors to

be obse rved on t he sur face of t he test. E c h i n o d e r m t a p h o n o m y is a b road

subjec t and has b e e n t h e s u b j e c t o f a n u m b e r o f reviews by L e w i s ( 1 9 8 0 ) ,

D o n o v a n ( 1 9 9 1 ) , B r e t t e t al . ( 1 9 9 7 ) , A u s i c h ( 2 0 0 1 ) , and m o s t r e c e n t l y N e b

elsick ( 2 0 0 4 ) .

C lypeas t e ro ids b e l o n g to t he younges t g roup o f e c h i n o d e r m s to have

evolved, o r i g i n a t i n g in t h e P a l e o g e n e f rom cass idu lo id a n c e s t o r s (see

S m i t h , 2 0 0 1 ) . T h e y have a n u m b e r o f s p e c i a l i z e d fea tures , i n c l u d i n g the i r

cha rac te r i s t i c f l a t t ened shape , sp ine m i n i a t u r i z a t i o n and d i f fe rent ia t ion ,

c o n c e n t r a t i o n o f respiratory pod ia i n t he pe ta l s on t he abora l sur face , s e c

ondary un ipo res no t res t r ic ted to a m b u l a c r a , food grooves on t he oral sur

face , l unu l e s in t h e test of m a n y sand dol lars , and a s p e c i a l i z e d Aris tot le 's

l an te rn ( D u r h a m , 1955 , 1 9 6 6 ; M o o i , 1 9 8 9 ) . S tab i l i ty to t he test i s g iven by

in te rna l supports i n m o s t t axa , w h i c h c o n n e c t t he oral and abora l sides o f

the test. I n t e r l o c k i n g plates fu r ther s t r e n g t h e n t h e test. T h e s e fea tures

m a k e c lypeas te ro ids a m o n g t he m o s t robus t o f al l e c h i n o d e r m s , a l l o w i n g

t h e m to survive even in h i g h - e n e r g y sho re face e n v i r o n m e n t s . As of ten

Taphonomy of the Irregular Echinoid Clypeaster humulis 115

Introduction

6

Figure 6.2. Bottom to

pography of the study

area with main collection

sites of complete

Clypeaster humilis as

well as the location of

bulk sediment samples

containing Clypeaster

remains.

no ted , t h e s e s t ab i l i z ing skele ta l fea tures c o r r e s p o n d i n g l y i n c r e a s e their

p rese rva t ion po t en t i a l ( S e i l a c h e r , 1 9 7 9 ; N e b e l s i c k a n d K a m p f e r , 1994;

B r e t t e t al . , 1997 ; Mof fa t and B o t t j e r , 1 9 9 9 ; N e b e l s i c k , 1 9 9 9 ) .

T h e genus Clypeaster L a m a r c k , 1 8 0 1 , l ives today in m a n y tropical

m a r i n e e n v i r o n m e n t s ( G h i o l d and H o f f m a n n , 1 9 8 4 , 1 9 8 6 ) . T h e y have a

wide m o r p h o l o g i c a l var ie ty a n d i n c l u d e f l a t t ened i n f auna l a nd domed

ep i fauna l forms. T h e la t ter i s r ep resen ted by t he type spec ies Clypeaster

rosaceous ( L i n n a e u s , 1758 ) , wel l k n o w n f rom sha l low C a r i b b e a n waters.

Clypeaster i s an i m p o r t a n t o r g a n i s m in t h e s e e n v i r o n m e n t s . F o r example ,

116 Nebelsick

they can i n f l u e n c e s e d i m e n t pa r t i c l e s ize w h e n f eed ing by c r u s h i n g sand-

sized par t ic les and p r o d u c i n g f ine r s e d i m e n t s ( K a m p f e r and T e r t s c h n i g ,

1992).

Clypeaster i s also o n e of the mos t c o m m o n ech ino ids in C e n o z o i c shal

low-water sed iments . Hundreds of n o m i n a l fossil species have b e e n desc r ibed

(Durham, 1 9 6 6 ) , and n u m e r o u s s p e c i m e n s are k n o w n from fossil e c h i n o i d

assemblages. C o m m o n l y , different spec ies of Clypeaster o c c u p y different

habitats showing co r respond ing variat ions in size and m o r p h o l o g y (i.e., B o g -

gild and Rose , 1 9 8 4 ; Poddubiuk and R o s e , 1 9 8 4 ; Ne raudeau e t a l , 2 0 0 1 ;

Kroh and Nebe l s i ck , 2 0 0 3 ) .

C L Y P E A S T E R H U M I L I S I N T H E S T U D Y A R E A . Clypeaster humilis ( L e s k e ,

1778) is a m e d i u m - s i z e d spec ies tha t is d is t r ibuted t h r o u g h o u t t h e I n d o -

West Paci f ic r e a l m (C la rk and R o w e , 1971) . T h e ske le tons o f C . humilis

examined he r e o r ig ina te f rom the N o r t h e r n B a y o f S a f a g a , R e d S e a , E g y p t

(Figs. 6 . 1 , 6 . 2 ) . T h i s is a c o m p l e x a rea abou t 10 by 8 km l o n g wi th coas t a l

indentat ions, is lands, and bas ins separa ted f rom o n e a n o t h e r and f rom the

open R e d S e a by s u b m a r i n e r idges (P i l l e r and Pervesler , 1 9 8 9 ; P i l l e r a n d

Mansour , 1 9 9 0 ) . T h e N o r t h e r n B a y o f Safaga i s a shal low-water , c a r b o n a t e

env i ronmen t and i s c h a r a c t e r i z e d by a wide r ange of b o t t o m fac ies . S h a l

lower areas o f t he bay are d o m i n a t e d by c o a r s e sands , sea grass m e a d o w s ,

sands with sea grass, and sands wi th co ra l pa t ches . C o r a l reefs are a lso

present and are a c c o m p a n i e d by r e e f f la t s . However , m o s t co ra l cove r i s

represented by a co ra l c a r p e t tha t o c c u p i e s ex tens ive areas of t h e seafloor.

D e e p e r bas ins are o c c u p i e d by c a r b o n a c e o u s m u d d y and sandy m u d d y

sediments (F ig . 6 . 2 ) . B o t h regu la r and i r regula r e c h i n o i d s are c o m m o n

macrofauna l e l e m e n t s o f th is shal low-water c a r b o n a t e sys tem a n d have

been the s u b j e c t o f a n u m b e r o f ac tua l i s t i c s tudies c o n c e r n i n g the i r e c o l

ogy and t a p h o n o m y (e.g., N e b e l s i c k , 1 9 9 2 a , 1 9 9 2 b , 1 9 9 9 ; N e b e l s i c k and

Kampfer , 1 9 9 4 ; N e b e l s i c k and Kowalewsk i , 1 9 9 9 ) .

Clypeaster i s a m o n g t he m o s t c o m m o n e c h i n o i d s in t he N o r t h e r n B a y

o f Safaga and i s found as l iv ing s p e c i m e n s , dead tests, a n d f r agmen t s . Of

the four sympa t r i c spec ies in the study area , Clypeaster humilis is by far t he

most c o m m o n and total ly d o m i n a t e s t he c o l l e c t e d s p e c i m e n s . I n a l l , 4 7

living s p e c i m e n s and 86 dead s p e c i m e n s were c o l l e c t e d . Clypeaster fervens

Koeh le r , 1 9 2 2 , Clypeaster reticulatus ( L i n n a e u s , 1 7 5 8 ) , a n d Clypeaster

rarispinus de M e i j e r e , 1 9 0 2 , are m u c h less c o m m o n , and on ly a few spec i

mens o f e a c h spec i e s were c o l l e c t e d ( N e b e l s i c k , 1 9 9 2 b , 1 9 9 9 ) . S p e c i e s dif

ferent iat ion i s based on shape and size re la t ionships of test s ize , p e t a l o d i u m

length , a m b i t u s shape , and t he form o f t he frontal a m b u l a c r u m (C la rk and

Rowe, 1971) .

C o m p l e t e Clypeaster humilis s p e c i m e n s were found in shal low-water

areas of t h e bay wi th sandy substrates (F ig . 6 .1) . L i v i n g Clypeaster were

found bu r i ed just u n d e r n e a t h the s e d i m e n t sur face , rarely b e i n g just visible

as an o u t l i n e b e l o w t h e sur face , a t m a x i m u m dens i t ies o f two individuals

per square m e t e r ( N e b e l s i c k , 1 9 9 2 a ) . D e a d e c h i n o i d tests were d i scovered

on the s e d i m e n t sur face , par t ia l ly bu r i ed , o r u n d e r n e a t h t he sur face . S o m e

s p e c i m e n s were ex t r ac t ed by rak ing t he s e d i m e n t wi th a s tout rake p e n e

trat ing to a dep th of approx ima te ly 5 cm into the s e d i m e n t .


Figure 6.3. Clypeaster

humilis from the North

ern Bay of Safaga. Egypt.

1-3, 5-6, Aboral view. 4,

Oral view. 1, Test with

spines; 2, very well-pre

served, denuded test; 3,

test showing abrasion of

surface characters, apical

system depressed; 4,

abraded test with encrus

tation (serpulids and

bryozoans); note how

some serpulids follow

food grooves; 5, abraded

test with abrasion, frag

mentation, and bioero-

sion; apical system is

missing; 6, totally

abraded and corroded

test. Scale bars = 1 cm.

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

Clypeaster humilis s p e c i m e n s r a n g e d in s ize f rom ~ 2 0 to 1 2 0 mm in test

l e n g t h bu t are m o s t c o m m o n a t app rox ima te ly 75 mm in l e n g t h with a

c o r r e s p o n d i n g h e i g h t o f 15 m m . T h e abora l su r face o f t he Clypeaster skel

e ton is c h a r a c t e r i z e d by a d i s t inc t s l ight ly ra ised p e t a l o d i u m (F ig . 6.3.1)

c o n t a i n i n g t he respiratory pod ia , typ ica l ly wi th up to 60 pore pairs along

e a c h a m b u l a c r u m o f t he pe ta l . T h e ap ica l sys tem cons is t s o f f i v e distinct

gonopores su r round ing t he madrepor i t e . T h e a m b i t u s i s rounded . T h e oral

sur face c o n t a i n s the c e n t r a l p e r i s t o m e , t he vent ra l ly pos i t i oned periproct

n e a r t he pos ter ior e n d of t he ske le ton , a n d s t ra ight food grooves l ead ing to

t he p e r i s t o m e (F ig . 6 .3 .4 ) . T h e p e r i s t o m e i s f lush wi th t h e oral surface o f

t he test. T h e r e is a u n i f o r m d is t r ibu t ion of s m a l l , s u n k e n , perforated tu

be r c l e s on b o t h t he oral and abora l sur faces . T h e s e t u b e r c l e s are generally

less t h a n 0.5 mm in d i a m e t e r and suppor t t h e m i n u t e spines tha t cover the

test dur ing l ife. An i m p o r t a n t m o r p h o l o g i c a l fea ture for t he preservation

o f t he test i s t he p r e s e n c e o f t h i c k in t e rna l suppor ts tha t i n t e r c o n n e c t the

oral and abora l sur faces . T h e s p e c i a l i z e d c lypeas te ro id jaws c o m p l e t e the

( larger) skele ta l features o f t h e test (see M o o i , 1 9 8 9 ) .

T r a n s p o r t is no t regarded as a m a j o r fac tor in red i s t r ibu t ing fragments

i n t he study area ( N e b e l s i c k , 1 9 9 2 a , 1 9 9 2 b , 1 9 9 9 ) . T h e R e d S e a i s generally

no t a f fec ted by h i g h - e n e r g y s to rms , and t h e study a rea i t se l f is a relatively

118 Nebelsick

protected e n v i r o n m e n t separa ted f rom the o p e n sea by is lands and under

water swells . T h e h i g h d i f ferent ia t ion o f e c h i n o i d f r agmen t s w i th in b u l k

samples and the i r c lose co r re l a t ion to t he hab i ta t o f c o r r e s p o n d i n g l iv ing

echinoids also suggest tha t t ranspor t i s no t an i m p o r t a n t t a p h o n o m i c factor

a t the baywide sca l e of inves t iga t ion (F ig . 6 . 2 ) . S m a l l - s c a l e t ranspor t i s dis

cussed in N e b e l s i c k ( 1 9 9 2 b ) and inc ludes t ranspor t f rom co ra l p a t c h e s a n d

coral c a r p e t to t h e su r round ing s e d i m e n t as well as f rom t h e reefs across

the r e e f f l a t s a n d to t h e r e e f s lopes . T h e s e pe r t a in mos t ly t o r egu la r e c h i

noids l iv ing in m o r e e x p o s e d e n v i r o n m e n t s .

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

H U M I L I S . T w o previous inves t iga t ions involving Clypeaster humilis f rom

the study a rea are o f i m p o r t a n c e in t he in te rpre ta t ion o f t a p h o n o m i c gra

dients p resen ted he re . N e b e l s i c k and K a m p f e r ( 1 9 9 4 ) s tudied shor t - te rm

t a p h o n o m i c p rocesses a f fec t ing test p reserva t ion of Clypeaster humilis a n d

Echinodiscus auritus L e s k e , 1 7 7 8 , by us ing an e x p e r i m e n t a l array of under

water c a g e d dead s p e c i m e n s over a t i m e pe r iod of 1.5 weeks . S p i n e disar

t i cu la t ion in b o t h spec i e s c o m m e n c e d wi th in 18 hours and was f in i shed

by 91 hours after t h e start o f t h e e x p e r i m e n t . After sp ine d i sa r t i cu la t ion ,

mos t tests r e m a i n e d s table , bu t o n c e plate d i saggrega t ion was in i t i a ted (in

th ree o f 20 cases ) , t h e tests rapidly d i sa r t i cu la ted in to larger f r agmen t s and

individual plates .

N e b e l s i c k ( 1 9 9 9 ) s tud ied t h e t a p h o n o m y o f Clypeaster f r a g m e n t s

t h r o u g h o u t t h e study area . F r a g m e n t s c o u l d on ly be ident i f ied to g e n u s

level b e c a u s e o f t h e l ack o f cha rac t e r i s t i c test fea tures n e e d e d for spec i e s

ident i f ica t ion . Mul t i va r i a t e s ta t is t ical analysis o f sur face preserva t ion fea

tures ( i nc lud ing abras ion , e n c r u s t a t i o n , and sur faces marks) of Clypeaster

f ragments led to t h e des igna t ion of four different t aphofac ies in t h e study

area ( N e b e l s i c k , 1 9 9 9 ) . T h e s e t aphofac ies were re la ted to dif ferent ia l e x p o

sure a n d s e d i m e n t a t i o n rates, e c o l o g i c a l factors tha t are no t readi ly dis

c e r n e d by t h e analysis o f diversity and m o r p h o l o g i c a l fea tures a l o n e .

F ive different p reserva t ion states a l o n g a t a p h o n o m i c g rad ien t were differ

en t i a t ed by us ing t he qual i ta t ive analysis o f t he sur face c h a r a c t e r s (F ig s .

6 . 3 - 6 . 5 ) . T h e basis o f this g rad ien t i s t he preserva t ion o f t h e fo l lowing sur

face cha rac t e r s : sp ines , t he ap ica l sys tem, gonopore s , m a d r e p o r i t e , p e t a l o -

d i u m , pore pairs , plate b o u n d a r i e s , t u b e r c l e s , and t he a m b i t u s (F ig . 6 . 5 ) .

T h i s s tage co r r e sponds to those a n i m a l s tha t have just b e e n k i l l ed , and a

few dead s p e c i m e n s were r ecove red wi th spines still a t t a c h e d by e p i t h e

l i u m ( F i g . 6 .3 .1 ) . T h e c o l o r o f t h e a n i m a l s , r a n g i n g f rom l igh t t o dark

b r o w n , i s sti l l r e c o g n i z a b l e . E v e n p e d i c e l l a r i a are ne s t ed b e t w e e n t h e

spines . T h e i n t e g u m e n t covers t he per iproc t . T h e t ee th are i n p l a c e wi th

t h e jaws still a r t i cu la ted . I t i s c l e a r tha t dead s p e c i m e n s of th is s tage c a n

k e e p the i r sp ines for on ly a few days as a resul t of t he d e c a y of soft t issue

and d i sa r t i cu la t ion o f t he spines ( N e b e l s i c k and K a m p f e r , 1 9 9 4 ) .

Taphonomic

Gradient of

Clypeaster humilis

Tests

Stage 1: Specimens

with Spines


Figure 6.4. Fragment

preservation of

Clypeaster sp. 1, 3, 5,

Successive enlargements

of a single, well-preserved

plate. 2, 4, 6, Successive

enlargements of two

joined plates showing

high rates of abrasion and

encrustation of a serpulid

worm tube and encrust

ing foraminifera. Note

that the serpulid worm

tube has been broken

along the plate boundary,

suggesting encrustation

of a complete test and

subsequent fragmenta

tion. Scale bars: 1, 2=1

mm; 3,4 = 400 µm; 5, 6

= 100 µm

C H A R A C T E R P R E S E R V A T I O N . T h e ap ica l sys tem is c o m p l e t e (F ig . 6.3.2) .

G o n o p o r e s are o p e n and t h e madrepo r i t e i s r e c o g n i z a b l e . T h e pe ta lod ium

i s d is t inc t , and pore pairs are free of s e d i m e n t . A m b u l a c r a l a n d in te rambu

lac ra ! plate b o u n d a r i e s c a n be readi ly d i s c e r n e d . T h e a m b i t u s i s perfectly

preserved. P r i s t ine sur face c h a r a c t e r s are p resen t a t t h e m i c r o s c o p i c scale

wi th c o n s p i c u o u s , s u n k e n , per fora ted t u b e r c l e s , a n d we l l -deve loped are-

oles (F ig . 6 .4 .1) . S e c o n d a r y t u b e r c l e s a n d sur face k n o b s c a n a lso be distin

gu i shed (F ig . 6 .4 .3 ) . D i f f e ren t i a l s t e r e o m is readi ly ev iden t (F ig . 6 .4 .5 ) .

T A P H O N O M I C P R O C E S S E S . D e c a y o f c o n n e c t i v e t issue has led to the

d i sa r t i cu la t ion o f sp ines f rom the sur face o f t h e test. T h e d e c a y o f ambu la

cra l t u b e feet resul t i n o p e n pores o f t h e p e t a l o d i u m and s e c o n d a r y uni-

pores . Jaws are n o w loose w i th in t he test. T h e ba re test i s n o w whi te . M o s t

s p e c i m e n s are still free o f in t e rna l s e d i m e n t .

T A X O N O M I C R E S O L U T I O N . S p e c i m e n s c a n readi ly b e ident i f ied at the

spec ies level . C o l o r var ia t ions are , however , no t d i s ce rn ib l e .

C H A R A C T E R P R E S E R V A T I O N . T h e ap i ca l sys tem c a n b e d a m a g e d (F ig .

6 .3 .3 , 6 .3 .4 ) . G o n o p o r e s , i f p resent , are f i l l e d wi th s e d i m e n t . T h e pe ta lo

d i u m is less d i s t inc t , and m a n y pore pairs are p lugged . A m b u l a c r a l and in

t e r a m b u l a c r a ! plate b o u n d a r i e s are hardly d i sce rned . T u b e r c l e s are abraded

wi th d a m a g e d c r e n u l a t i o n s and m a m m a l o n s . S t e r e o m di f ferent ia t ion i s

120 Nebelsick

Stage 2: Very Well-

preserved,

Denuded Tests

Stage 3a:

Abraded Tests

b e c o m i n g diff icul t to r e c o g n i z e a t t he sur face . T h e a m b i t u s i s still wel l

preserved.

T A P H O N O M I C P R O C E S S E S . Abras ion has led to loss o f sur face c h a r a c t e r

resolut ion. F i r s t s igns o f e n c r u s t a t i o n and b ioe ros ion are apparent . T h e test

has t aken a dul l co lor . T h e test c a n be f i l l ed wi th s e d i m e n t .

T A X O N O M I C R E S O L U T I O N . S p e c i m e n s c a n still b e ident i f ied at t he spe

cies level b e c a u s e shape and size re la t ionships o f t he test and p e t a l o d i u m

leng th , a m b i t u s shape , and frontal a m b u l a c r u m are still r e c o g n i z a b l e .

C H A R A C T E R PRESERVATION.The ap ica l sys tem i n c l u d i n g t h e g o n o p o r e s

and m a d r e p o r i t e c a n b e c o m p l e t e l y e roded (F ig . 6 . 3 .5 ) . T h e p e t a l o d i u m i s

less d i s t inc t , and m a n y pore pairs are p lugged . A m b u l a c r a l a n d i n t e r a m

bu lac ra l plate b o u n d a r i e s are hardly d i s c e r n e d . T h e a m b i t u s i s stil l wel l

preserved. A t t h e m i c r o s c o p i c s ca l e , e n c r u s t a t i o n cover sur face c h a r a c t e r s

(Fig. 6 . 4 . 2 ) , t ube rc l e s are abraded (F ig . 6 .4 .4 ) , and s t e r e o m di f fe rent ia t ion

b e c o m e s diff icul t (F ig . 6 . 4 . 6 ) .

T A P H O N O M I C P R O C E S S E S . E n c r u s t a t i o n is by u n i l a m i n a r b r y o z o a n s ,

serpulids, and e n c r u s t i n g fo ramin i fe ra . C o n s p i c u o u s ho les on t h e test sur

face m a y b e t h e resul t o f b o r i n g act ivi ty. T h e test c a n b e f i l l e d wi th

sed imen t .

T A X O N O M I C R E S O L U T I O N . T h e gross m o r p h o l o g y is stil l r e c o g n i z a b l e

s o spec i e s iden t i f i ca t ion c o u l d b e poss ib le . T h e d e g r e e o f e n c r u s t a t i o n

dictates i f o t h e r spec i e s -d i agnos t i c features , such as t he shape o f t he frontal

a m b u l a c r u m and pe ta l pores , are still r e c o g n i z a b l e .

C H A R A C T E R P R E S E R V A T I O N . C o r r o d e d test (F ig . 6 . 3 . 6 ) . Al l su r face c h a r a c

ters are c o m p l e t e l y miss ing . T h e a m b i t u s i s co r roded . T h e tests are n o t i c e

ably heav ie r and are f i l led wi th s e d i m e n t .

T A P H O N O M I C P R O C E S S E S . T h e s e tests are heavi ly co r roded . E a r l y dia-

genes is has l ead t o t h e f i l l i n g o f s t e r e o m and l i t h i f i ca t ion o f in f i l l ing

sed imen t .

T A X O N O M I C R E S O L U T I O N . S p e c i e s and genus iden t i f i ca t ion , as s u c h ,

i s no l o n g e r poss ib le wi thou t de ta i l ed k n o w l e d g e of t he c lypeas te ro id pres

ent in t he study area . G e n e r a l fo rm and h e i g h t are still d i s c e r n i b l e , bu t t he

co r roded a m b i t u s c h a n g e s t h e l e n g t h and width pa rame te r s o f t h e spec i

m e n . Al l deta i ls c o n c e r n i n g t h e p e t a l o d i u m and pore pairs are des t royed.

Sp ines d i sappear soon after dea th after t he d e c a y o f c o n n e c t i v e t issues a n d

musc l e s tha t c o n n e c t t h e spines t o t he test. T h e m i n u t e spines ( < 2 m m

long) are t h e n c o m m i t t e d t o t he s e d i m e n t . B e c a u s e t he analysis o f b u l k

s ed imen t samples was res t r ic ted to gra in sizes larger t h a n 2 mm ( N e b e l s i c k ,

1 9 9 2 a , 1 9 9 2 b ) , t he f r e q u e n c y o f d is t r ibut ion o f spines in t h e s e d i m e n t i s no t

recorded .

T h e apica l system with the gonopores and madrepor i t e shows a steady

degradat ion from stage 1 to 4. A l though perfect ly preserved in stage 2 (F ig .

Preservation of

Surface Features

along the

Taphonomic

Gradient


Stage 3b:

Encrusted—

Bioeroded Tests

Stage 4:

Corroded Tests

Taphonomic gradient >

1 2 3a 3b 4

Character preservation Apical system 0 0 1 2 2 Gonopores 0 0 1 2 2 Madreporite 0 0 1 2 2 Petalodium 0 0 1 0-1 2 Pore pairs 0 0 1 0-1 2 Plate boundaries 0 0 2 1 2 Tubercles 0 0 1 1 2 Ambitus 0 0 0 0 1

Taphonomic processes Denudation 0 2 2 2 2 Abrasion 0 0 1 1 2 Encrustation 0 0 1 2 0 Bioerosion 0 0 1 2 0 Corrosion 0 0 0 0 2 Diagenesis 0 0 1 1 2

Taxonomic resolution

species species species species/ genus

genus/ family

Characters: Taphonomic features: 0 = complete/well preserved/free 0 = not present 1 = fragmented/partially preserved/partially plugged 1 = present 2 = destroyed/missing/completely plugged 2 = dominant

Figure 6.5. Taphonomic

gradient character pres

ervation, taphonomic

features, and taxonomic

resolution for Clypeaster

humilis test from the

Northern Bay of Safaga,

Red Sea, Egypt.

6 .3 .2 ) , i t c a n comple te ly disappear by stage 3 and 4 . T h e gonopores are open

in stage 2 but b e c o m e plugged by s e d i m e n t or c e m e n t by stage 3 (F ig . 6.3.3).

T h e apical system is obviously not as stable as the rest of t he test as observed

in the c o n t i n u o u s degradat ion from F igu re s 6 .3 .2 to 6.3.3 to 6.3.5 to 6 .3 .6 .

T h e p e t a l o d i u m is d i s t inc t in s tage 1 a n d e spec i a l l y in s tage 2 spec i

m e n s , as t he pore pairs are free of s e d i m e n t and thus resul t in dark holes

in a stark con t ra s t to t he b r igh t pla te sur faces on t h e abora l sur face of the

Clypeaster ske le ton . By s tage 3 , however , t h e p e t a l o d i u m b e c o m e s less

p r o m i n e n t as t he a m b u l a c r a l pore pairs are p lugged by s e d i m e n t o r c e m e n t

(F ig . 6 . 2 . 3 ) . T h e m i n u t e a m b u l a c r a l s m a l l pores a t t he base o f the peta ls

n e x t to t he ap ica l sys tem also b e c o m e progressively ind i s t inc t (F ig . 6 .3 .3) .

B e c a u s e they represen t depress ion on t h e uppe r side o f t h e test , t he pore

122 Nebelsick

2 2 2 2 2 2 2 1

2 2 0 0 2 2

genus/ family |

ic features: ssent t

int

>nopores are open

tage 3 (Fig. 6.3.3).

ie test as observed

to 6.3.5 to 6.3.6.

in stage 2 speci -

iult in dark holes

ral surface of the

m becomes less

iment or c e m e n t

ase of the petals

tinct (Fig. 6 .3 .3) .

he test, the pore

pair rows are c o m m o n l y preferred sites o f e n c r u s t a t i o n , w h i c h t h e n m a s k

the pore pairs (F ig . 6 . 3 .5 ) . T h e s l ight ou t l i ne o f t he pe ta l s i s bare ly r ecog

n izab le in t h e s tage 4 s p e c i m e n s (F ig . 6 . 3 . 6 ) .

A l t h o u g h plate b o u n d a r i e s are readi ly vis ible in l iv ing a n d s tage 1

s p e c i m e n s , t h e y b e c o m e even m o r e d i s t inc t i n t h e d e n u d e d s tage 2 e x a m

ples. P la te b o u n d a r i e s are n o t i c e a b l y l ighter in co lor , whereas t h e in ter ior

of t he plates i s darker, l e ad ing to a d i s t inc t pa t te rn on t h e sur face of t he

skele ton (F ig . 6 . 3 . 2 ) . P la te b o u n d a r i e s b e c o m e ind i s t inc t by s tage 3 , al

t h o u g h they c a n just b e r e c o g n i z e d . F o r e x a m p l e , i n F i g u r e 6 .3 .5 , t he plate

b o u n d a r i e s are s l ight ly depressed c o m p a r e d to t he plate cen te r s . T h e pro

gressive degrada t ion o f t he test sur face by abras ion a n d co r ros ion p rec lude

any r e c o g n i t i o n of pla te b o u n d a r i e s in s tage 4 (F ig . 6 . 3 . 6 ) .

T h e fate o f t h e t ube rc l e s c a n bare ly be d i s c e r n e d a t a m a c r o s c o p i c

sca le . T h e y are still o c c u p i e d by spines in s tage 1 . In s tage 2 , t h e t u b e r c l e s

are wel l def ined and d is t inc t ; by s tage 3 , t hey start b e c o m i n g ind i s t inc t ,

and they d i sappear total ly by s tage 4 . T h e s e c h a n g e s c a n bes t be s e e n a t a

m i c r o s c o p i c level in F i g u r e s 6 .4 .1 , 6 .4 .3 , and 6 .4 .5 . We l l -p re se rved test sur

faces have d i s t i nc t s u n k e n t u b e r c l e s wi th d e e p e n e d a r eo l e s , a d i s t i nc t

c r e n u l a t e d boss , a n d p r o m i n e n t , pe r fora ted m a m m e l o n s . T h e s e t h r e e

s t ruc tures are d i s t inc t b e c a u s e t hey are c o n s t r u c t e d o f different s t e r e o m

types , i n c l u d i n g c o a r s e l a b y r i n t h i c s t e r eom o f t he s u n k e n a reo le a n d gal -

le r ied s t e r e o m c o r r e s p o n d i n g to t h e m u s c l e a t t a c h m e n t areas . As i l lus

trated in F i g u r e 6 .4 .5 , t he m a m m e l o n i s c o n s t r u c t e d of a sparsely per fo

rated d e n s e s t e reom. T h e a rea b e t w e e n t he t u b e r c l e s i s c h a r a c t e r i z e d by

s e c o n d a r y t ube rc l e s a nd p r o t u b e r a n c e s . T h e s t e r e o m i s c o m p l e t e l y free o f

s e d i m e n t a n d c e m e n t a t i o n .

An abraded and e n c r u s t e d s p e c i m e n i s dep ic t ed in F i g u r e s 6 . 4 . 2 , 6 .4 .4 ,

and 6 . 4 . 6 . T h e t u b e r c l e s are no t on ly abraded bu t a lso cove red by an en

c rus t ing serpul id w o r m t u b e ( i t se l f ab raded) a n d e n c r u s t i n g fo ramin i fe ra .

A l t h o u g h p r i m a r y t ube rc l e s are abraded , p r o t u b e r a n c e s b e t w e e n t h e tu

be rc l e s c a n still be r e c o g n i z e d (F ig . 6 . 3 . 2 ) . As s e e n in F i g u r e 6 .4 .4 , t h e pri

m a r y t u b e r c l e s are less d i s t inc t , and t h e a reo les are par t ia l ly f i l l ed wi th

s e d i m e n t par t i c les . Abras ion o f t he t u b e r c l e i s typ ica l ly a c c o m p a n i e d by

the s h e a r i n g o f f o f t h e m a m m e l o n . T h e pore space b e t w e e n t he s t e r e o m

b e c o m e s r e d u c e d due probably t o d i a g e n e t i c a c c r e t i o n o f c e m e n t o n t h e

struts o f t h e s t e r e o m (F ig . 6 . 3 . 6 ) .

O t h e r features i nc lude the a m b i t u s o f t he c lypeas te ro id skele ta l , w h i c h

r e m a i n s s table t h r o u g h o u t t he t a p h o n o m i c grad ien t unt i l s tage 5 , w h e n t he

p e r i m e t e r o f t he ske le ton starts t o b e c o m e co r roded . T h e oral su r face has

the s a m e gene ra l t e n d e n c i e s a s t h e abora l sur face . In teres t ingly , t h e serpu

lid w o r m t u b e s fol low t h e food grooves in s tage 3a ( F i g . 6 .3 .4 ) . By s tage 4 ,

t he food grooves b e c o m e i n d i s t i n g u i s h a b l e , a n d t h e b o u n d a r i e s o f t h e

pe r ip roc t and p e r i s t o m e also b e c o m e ind i s t inc t .

T h e r e are obvious ly two e n d m e m b e r s o f t h e t a p h o n o m i c g rad ien t de

sc r ibed he re , wi th sp ine -cove red skele tons on t he o n e h a n d and tota l ly

c o r r o d e d s p e c i m e n s on t h e o ther . T h e s e a lso r ep resen t a t e m p o r a l se -

Discussion


q u e n c e wi th a c l e a r b e g i n n i n g and an end . S p i n e loss is ub iqu i tous in the

we l l -oxygena ted , shal low, ag i ta ted waters o f t h e study area . T h e loss o f sur

face cha rac t e r s t h r o u g h abras ion is obvious ly l inear . However , encrusted

and b i o e r o d e d s p e c i m e n s c a n have wel l -preserved sur face cha rac te r s , but

converse ly , ab raded s p e c i m e n s show no o r l i t t le e n c r u s t a t i o n . T h u s , there

i s a m o s a i c d e v e l o p m e n t of t a p h o n o m i c features p resen t on t he test surface

for s tage 3 (F ig . 6 . 5 ) . T o w a r d t he e n d of t h e t a p h o n o m i c pathway, all fea

tures p e r t a i n i n g to t he test su r face are c o m p l e t e l y des t royed, eradicating

al l e v i d e n c e o f e n c r u s t a t i o n a n d b i o e r o s i o n . T h e g e n e r a l s h a p e o f the

who le ske le ton as such , however , r e m a i n s r emarkab ly in tac t .

T h e t a p h o n o m i c g rad ien t c o n s t r u e d he r e represents a m i x t u r e o f de

s t ruct ive and cons t ruc t i ve t a p h o n o m i c p rocesses tha t are ac t ive a t different

s tages o f t he g rad ien t (F ig . 6 . 5 ) . D e c a y o c c u r s shor t ly after dea th . Abrasion

i s c o n t i n u o u s and b e c o m e s m o r e ev iden t a l o n g t he gradient . Encrus ta t ion

and b ioe ros ion a lso i n c r e a s e a l o n g t h e g rad ien t to s tage 3b bu t i s eroded

away by s tage 4 . D i a g e n e s i s c a n set in by s tage 3 , e s p e c i a l l y in th is tropical

m a r i n e e n v i r o n m e n t where p r i m a r y m a r i n e c e m e n t a t i o n i s prevalent and

c o n t i n u e s t h r o u g h s tage 4 . C o r r o s i o n , t h e last p rocess to a c t on the skele

tons , d e t e r m i n e s t he a p p e a r a n c e o f t he last s tage o f t h e gradient .

D e c a y i s destruct ive, caus ing the d isar t icu la t ion of spines, disaggrega

t ion of t he jaws, and a l oosen ing of plate boundar i e s . Abras ion is also obvi

ously destructive, as is b ioerosion. However, encrus ta t ion and early diagenesis

c a n serve to s t r eng then the test. For e x a m p l e , enc rus t i ng serpulids and bryo-

zoans cross plate boundar ies . Ea r ly diagenesis also f i l l s in the s te reom. Thus ,

stage 4 skeletons waste away and do no t readily b reak up into f ragments .

A l t h o u g h the re i s a t e m p o r a l s u c c e s s i o n of events , no s t a t emen t s can

b e m a d e a b o u t t h e absolu te t i m i n g . S tud i e s o n t h e ages o f differentially

preserved bivalves in shal low-water se t t ings have shown surpr is ingly larger

age d i sc repanc ie s a m o n g differently preserved s p e c i m e n s (e.g., Kowalewski

e t al . , 1 9 9 8 ) . T h i s shou ld a lso be e x p e c t e d for e c h i n o d e r m r e m a i n s , al

t h o u g h t o m y k n o w l e d g e , n o s u c h s tudies have b e e n m a d e o n ech ino -

de rms . Var ia t ions in t he in tensi ty of t a p h o n o m i c p rocesses such as different

rates o f abras ion , b ioe ros ion , a n d e n c r u s t a t i o n c a n be e x p e c t e d wi th in dif

ferent facies . T h e l e n g t h o f r e s i d e n c e t i m e o n t he s e d i m e n t sur face will

a lso a f fec t t he rate o f t a p h o n o m i c p rocesses a f f ec t ing t h e e c h i n o i d skele

ton. T h e s e d i f fe rences have b e e n s h o w n to o c c u r for Clypeaster f ragments

r ecove red f rom bu lk samples ( N e b e l s i c k , 1 9 9 9 ) s u c h tha t 1 ) low-energy

e n v i r o n m e n t s and low s e d i m e n t a t i o n rates (wh ich e q u a l l o n g sur face resi

d e n c e t imes ) lead to good sur face preserva t ion and h i g h e n c r u s t a t i o n rates;

and 2 ) h igh-ene rgy , shal low-water e n v i r o n m e n t s wi th h i g h e r sed imenta

t ion rates l ead to h igh ly abraded s p e c i m e n s wi th low e n c r u s t a t i o n rates.

However , m a n y m o r e s p e c i m e n s of Clypeaster humilis f rom different facies

would be n e e d e d in order to d i s ce rn s u c h pa t te rns a m o n g c o m p l e t e tests.

T h i s study i s res t r ic ted to c o m p l e t e s p e c i m e n s , a l t h o u g h they are miss

ing the i r spines . O n c e t he s p e c i m e n s are f r a g m e n t e d , di f ferent patterns

e m e r g e (see N e b e l s i c k , 1 9 9 9 ) . F o r e x a m p l e , t he ba re e c h i n o i d test offers a

relat ively large sur face for e n c r u s t i n g o r g a n i s m s in o the rwi se uns tab le en

v i r o n m e n t s ( N e b e l s i c k e t al . , 1 9 9 7 ) . T h e r e f o r e , i t i s no t surpr is ing tha t en-

Nebelsick 124

c rus t a t ion c a n be relat ively h i g h in shal low-water e n v i r o n m e n t s . However ,

f r agmen t s in th is e n v i r o n m e n t are mos t ly h igh ly abraded a n d free o f e n

c rus ta t ion . T h i s m o s t l ikely has to do wi th t he different h y d r o d y n a m i c

proper t ies o f large tests on t he o n e h a n d and f r agmen t s on t he o ther . F rag

m e n t s are m o r e l ikely to be m o r e h igh ly abraded in h ighe r - ene rgy env i ron

m e n t s b e c a u s e o f the i r s m a l l e r s ize and the i r e n t r a i n m e n t i n wave m o v e

m e n t . P r i m a r y enc rus t a t ion on f ragments (as shown by encrus te rs o c c u r r i n g

d i rec t ly on t h e d i sa r t i cu la ted plate bounda r i e s ) a lso o c c u r s bu t i s largely

res t r ic ted to deeper , quie t -water se t t ings (in t he W e s t Area ; see F ig . 1) wi th

low s e d i m e n t a t i o n rates (see N e b e l s i c k , 1 9 9 9 ) . T h i s a l lows for l o n g sur face

r e s i d e n c e t i m e s wi th l i t t le d i s t u r b a n c e tha t wou ld be c o n d u c i v e to t h e set

t l e m e n t o f a n e n c r u s t i n g ep i fauna .

A n o t h e r factor tha t i s obvious ly i m p o r t a n t in t he t a p h o n o m y of t he

ske le tons and p r o d u c t i o n of f r agmen t s i s des t ruc t ive p reda t ion events (see

N e b e l s i c k , 1 9 9 9 ; Kowalewsk i and N e b e l s i c k , 2 0 0 3 ) . F i s h p reda t ion pro

duces a gapp ing w o u n d wi th j agged borders on t he oral sur face . T h e role

o f p reda t ion in t he t a p h o n o m i c s c e n a r i o de sc r i bed above i s t he s u b j e c t o f

c o n t i n u e d inves t iga t ion tha t shou ld add fur ther ins ights in to t he c o m p l e x

ity o f e c h i n o d e r m preserva t ion .

T h e differential stages o f preservat ion a long the t a p h o n o m i c gradient

will affect the t a x o n o m i c resolut ion o f ident if icat ion o f fossil s p e c i m e n s . T h e

level o f t a x o n o m i c ident i f icat ions depends on the loss o f d iagnos t ic t axo

n o m i c charac te rs a long the t a p h o n o m i c gradient . In stages 1 and 2 , spec ies

ident i f icat ion is readily identifiable. Ident i f icat ion at stage 3a and 3b depends

on w h i c h charac te rs h a p p e n to be preserved (i.e., no t destroyed by abrasion

or b ioeros ion) or exposed (i.e., no t covered by encrus ta t ion) . I t also depends

on h o w well the species from the study area are k n o w n with respec t to the i r

m o r p h o l o g i c features and pheno typ ic variat ions. T h e preservat ion o f stage

4 ech ino ids (F ig . 6 .3 .6 ) obviously precludes d i rec t ident i f icat ion at t he spe

cies and even genus level, bu t the order Clypeas te rero ida c a n still be identi

f ied. No o ther e c h i n o d e r m s have such a f la t tened form with in ternal sup

ports, so tha t they c a n be r e c o g n i z e d in even the mos t co r roded s p e c i m e n s .

However , i t will be difficult to m a k e any c loser de t e rmina t ion a t t he fami ly

and suborder levels. E v e n if the test is totally cor roded , the i r des igna t ion as

e c h i n o d e r m s i s possible b e c a u s e o f the u n i q u e s t ruc ture o f t he e c h i n o d e r m

s te reom at a m i c r o s c o p i c level ( S m i t h , 1 9 8 0 , 1 9 8 4 , 1 9 9 0 ) .

T h i s study demons t r a t e s tha t fossil ske le tons have to be r e m o v e d f rom

the t a p h o n o m i c c y c l e in order to be preserved. T h e o r e t i c a l l y , a l l t he s tud

ied s p e c i m e n s wou ld have e n d e d up as s tage 4 ske le tons i f t hey r e m a i n e d

o n t he sur face l ong e n o u g h , a n d even t h e n they c o u l d c o r r o d e c o m p l e t e l y

away. T h i s study a lso rei terates t h e c o m p l e x i t y and in terplay o f dif ferent

factors a f f ec t ing t he preserva t ion o f e c h i n o d e r m skele tons . A l t h o u g h the re

are def ini te e n d m e m b e r s , t h e t a p h o n o m i c p rocess i n b e t w e e n t h e s e e n d

m e m b e r s c a n dif ferent ia l ly af fec t t he preserva t ion o f t he ske le tons . I t has

b e e n a rgued tha t us ing f r agmen t s c a n i n c r e a s e ou r k n o w l e d g e o f t h e dis

t r ibu t ion o f e c h i n o d e r m s ( G o r d o n and D o n o v a n , 1 9 9 2 ; N e b e l s i c k , 1 9 9 2 a ,

1 9 9 2 b ; D o n o v a n , 2 0 0 1 , 2 0 0 3 ; K r o h , 2 0 0 5 ) : t he f r agment s c a n m o r e c lose ly

preserve t h e l ong - t e rm s e t t l e m e n t pa t te rns b e c a u s e l iv ing e c h i n o i d distri-


bu t ions are notor ious ly patchy. T h i s s tudy shows tha t b o t h c o m p l e t e skele

tons and f r agment s shou ld a lso be i n c l u d e d in t a p h o n o m i c analysis in or

der to ob t a in a m o r e c o m p l e t e p i c tu r e of t he e c o l o g i c a l factors affecting

preserva t ion a t different sca les o f obse rva t ion .

I t h a n k t he s taf f o f t he Ins t i tu te o f G e o s c i e n c e s Univers i ty o f T u b i n g e n for

the i r support , e spec i a l l y W. G e r b e r for t he pho tographs . I s ince re ly thank

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