Page 369 - Introduction to Paleobiology and The Fossil Record

356 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Box 13.8 Gradualistic evolution of belemnites

There are relatively few long fossil lineages that can be used to demonstrate either phyletic gradual-
ism or punctuated equilibria. Most of the best case studies (see p. 124) are based on mobile or sessile
benthic organisms. The Cretaceous (Campanian) belemnite faunas, particularly the genus Belem-
nitella, of North Germany are abundant, well preserved and known in great detail and provide an
unequalled opportunity to test these models using a pelagic group of organisms (Christensen 2000).
There is not much variation in lithology throughout the succession in Lower Saxony – they are
mainly rather boring, monotonous marly limestones. There are thus limited opportunities for facies

shifts to influence the morphological record of Belemnitella by the migration of more exotic mor-
photypes in and out of the basin. Although samples of Belemnitella through the section are superfi -
cially similar, several measurements show gradualistic trends when treated quantitatively (Fig. 13.24).
Not all changes are unidirectional, some exhibit reversals. It seems most likely that the Campanian
belemnites of northern Germany conformed to continuous, gradual phyletic evolution in narrowly

fluctuating, slowly changing environments.

LAP in mm BI SD in mm FA in degrees AA in degrees
TV (30) (30) (28) (28) (27) B.mis- burgensis
“Vulgaris”/
Stolleyi Zone TP (43) (43) (36) (37) (36)

(46) (46) (38) (38) (36)
TM
Lower Upper Campanian Stobaei/ GR (89) (88) (77) Belemnitella mucronata
“Vulgaris”/
Basiplana Zone

Basiplana Zone

(78) (78) (77) (85)
GM (67) (63) (69)
(108)
GL (108) (84) (80) (87)
Conica/
Mucronata Zone
(38)
GE (22) (12) (38) (7) (34) (4) (30) (12) (6) (17)
(33)
Gracilis/
Upper Lower Campanian Conica/ 20 30 40 50 60 2.5 3.0 3.5 4.0 4.5 4 6 8 10 12 14 10 20 30 40 50 18 20 22 24
(22)
GC
(15)
(12)
GA
Mucronata Zone
Papillosa Zone
20 m
Papillosa Zone
mean value,
Senonensis Zone ± 1 standard deviation
and observed range
Pilula/
Senonensis Zone
(38) = sample size
Figure 13.24 Gradualistic evolution of Cretaceous belemnites from North Germany. Summary of
changes of the length from the apex to the protoconch (LAP), Birkelund index (BI), Schatzky
distance (SD), fissure angle (FA) and alveolar angle (AA) of nine samples of Belemnitella.

Successive mean values are different at the 5% level (one arrow), 1% level (two arrows) and
0.1% level (three arrows). (Courtesy of the late Walter Kegel Christensen.)