Page 427 - Introduction to Paleobiology and The Fossil Record

414 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

marine snow, a bonding material composed ognized in the class Graptolithina, but only
of organic debris and mucus; this seems two, the Dendroidea and Graptoloidea have
less likely because the synrhabdosomes are important geological records. The patterns of
remarkably symmetric, which suggests they evolution linking these groups are uncertain
grew that way. About six orders are now rec- (Box 15.9).

Box 15.9 The fi rst graptolites: a cryptic Cambrian dimension?

By the Ordovician, the graptolites were represented by a number of well-defi ned groups including
the familiar dendroids and graptoloids and the less well-known camaroids, crustoids, dithecoids and
tuboids. It has long been a mystery where these diverse groups came from because the Cambrian
record was virtually non-existent. Barrie Rickards and Peter Durman (2006) have reassessed all the
possible ancestors, Cambrian specimens that have been variably assigned to graptolites, hydroids or
algae from the Middle and Upper Cambrian. They reassigned some of these cryptic Cambrian speci-
mens to the rhabdopleurids and excluded a number of them from the graptolites. The graptolites
and rhabdopleurids therefore probably shared a common ancestor in the Early Cambrian (Fig.
15.22). The rhabdopleurids are remarkable animals; Cambrian forms are virtually identical to
modern rhabdopleurids, making them true living fossils. The common ancestor to the graptolites
and rhabdopleurids was probably a solitary, worm-like animal, equipped with a lophophore, and

living in pseudocolonial filter-feeding clumps on the seafloor. Thus the graptolites, which dominated
the Early Paleozoic water column, started out as rather anonymous benthic filter feeders in the

shadow of the more obvious early arthropods, crinoids and mollusks of the Cambrian evolutionary
fauna.

Carboniferous Rhabdopleurids Dendroids

Devonian Graptoloids

Tuboids

Silurian Crustoids Dithecoids

Ordovician Camaroids

Cambrian

Figure 15.22 Generalized phylogenetic model for rhabdopleurid and graptolite evolution. (From
Rickards & Durman 2006.)