Page 434 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 421

Leptograptus Climacograptus Dicranograptus Glyptograptus

with lappets
triangulate hooked lobate isolate (Cucullograptus)
(M. triangulatus) (M. priodon) (M. lobiferus) (Rastrites)

Figure 15.28 Evolution of thecae. M, Monograptus.

45
I II III
40
35
30
Number of genera 25
20
15
fauna
10 Dichograptid Diplograptid
fauna
5 Anisograptid
fauna
0
1a 1b 1c 1d 2a 2b 2c 3a 3b 4a 4b 4c 5a 5b 5c 5d 6a 6b 6c
Lower Ordovician Middle Ordovician Upper Ordovician
Ordovician
Figure 15.29 Graptolite biostratigraphy and graptolite evolutionary faunas. I–III indicate the three
main radiations: anisograptid, dichograptid and diplograptid; 1a–6c represent 19 time slices through
the Ordovician Period. (Based on Chen et al. 2006.)

peared during the Early Devonian (Pragian) lence and aided the motion of the graptolites
in China, Eurasia and North America. Nev- through the water column on feeding
ertheless this uniserial morphology had sur- forays. It may also have prevented the inter-
vived for over 30 million years and may have ference between thecae on adjacent stipes,
continued after the Early Devonian in lineages providing a simpler, more effi cient colony
that did not secrete a preservable skeleton. structure.
Why should a trend towards a reduction in Graptolite morphology and stratigraphy
stipes be such an advantage? Perhaps the have formed the basis for the defi nition of
simpler stipe confi guration was hydrodynami- evolutionary faunas within Ordovician assem-
cally more stable, better adapted to turbu- blages that include anisograptid, dichograptid