Page 140 - Introduction to Paleobiology and The Fossil Record

MACROEVOLUTION AND THE TREE OF LIFE 127

Box 5.4 Punctuated speciation in bryozoans

Metrarabdotos is an ascophoran cheilostome bryozoan (see p. 320) that is represented today in the
Caribbean by three species. Coastal rocks on Dominica and other islands document the past 10 myr
of sedimentation in shallow seas, and they yield abundant fossils of this bryozoan. The fossils show
that Metrarabdotos radiated dramatically from 8 to 4 Ma, splitting into some 12 species, most of
which then died out by the Quaternary. Studies by Cheetham and Jackson have established a variety
of protocols for distinguishing species within Metrarabdotos, taking into account the genetics of
related extant species, and their amount of morphological differentiation, and then extending com-
parable statistical tests of morphological differentiation to the fossil forms (they demonstrated highly

significant correlations between genetic and morphometric differences among the modern forms).
Based on 46 morphometric characters, the authors established a mechanism for distinguishing lin-
eages among the fossils (Jackson & Cheetham 1999).
Lineage splitting in Metrarabdotos seems to have been rapid and punctuational in character (Fig.
5.6). Speciation was especially rapid in the interval from 8 to 7 Ma, with nine new species appearing.
There is some question about sampling quality here, since sampling is poor in the preceding interval,
and so some of these nine new species might have appeared earlier. However, the interval from 8 to
4 Ma, represented largely by information from Dominica, has been intensely sampled (DSI, Dominican
sampling interval). So, although there are questions over the origins of the nine basal species within
this interval, the origins of the remainder (tenue, n. sp. 10 and n. sp. 8) are more confi dently docu-
mented as punctuated. The same kind of punctuated pattern of speciation has been found also in virtu-
ally all other studies on fossil marine invertebrates that have been carried out.
Read more about speciation and punctuated equilibrium at http://www.blackwellpublishing.
com/paleobiology/.

tenue group unguiculatum group
tenue pacificum unguiculatum
0
auriculatum
n.sp. 10 lacrymosum
n.sp. 9 n.sp. 8 n.sp. 3
colligatum n.sp. 6 n.sp. 4
5
n.sp. 7
n.sp. 5 DSI
Time (Ma) 10 kugleri

n.sp. 1 n.sp. 2
15
chipolanum

20

micropora

Figure 5.6 Punctuated evolution and speciation in the bryozoan Metrarabdotos in the Caribbean.
Today, there are three species of this genus, but there have been many more in the past. Careful
collecting throughout the Caribbean has shown how the lineages exhibited stasis for long
intervals, and then underwent phases of rapid species splitting, especially in the time from 8 to
4 Ma, the Dominican sampling interval (DSI), where records are particularly good. (Courtesy of
Alan Cheetham.)