PhD thesis

Introduction
9
Chapter I
Introduction
Brachiopoda
The phylogenetic relationship of Brachiopoda is intensely debated among
biologists and paleontologists alike. Brachiopods were already known by Linné,
and 370 extant and more than 12.000 described fossil species are known (Linné
1758; Ax 2003; Logan 2007). Brachiopods were significant members of the early
Cambrian marine fauna and thus are one of the few phyla which are represented
throughout the 550 million years of the Phanerozoic era, which extends from
the first widespread appearance of organisms with mineralized skeletons until
modern times (James et al. 1992). Historically, brachiopods have been assigned
to different invertebrate groups, including molluscs (Lamarck 1801; Cuvier
1805), bryozoans (Huxley 1853; Hancock 1858), bryozoans and phoronids
(Hatschek 1888 ‘Tentaculata’; Hyman 1959 ‘Lophophorata’), or annelids (Morse
1871). The three lophophorate groups or Brachiopoda alone have subsequently
sometimes been regarded as deuterostomes (Brusca and Brusca 1990; Schram
1991; Eernisse et al. 1992; Nielsen 1995). Since the appearance of molecular
research tools, brachiopods have commonly been accepted to be protostomes
(Field et al. 1988; Lake 1990; Halanych 1995; Hejnol et al. 2009). Brachiopod
internal phylogeny distinguishes three clades; the inarticulate Linguliformea
and Craniiformea and the articulate Rhynchonelliformea (Williams et al. 1996).
Members of Linguliformea live buried in mud and have swimming juveniles
instead of a true larval stage (Yatsu 1902). Members of craniiformea live with
their ventral valve attached to stones and have two-lobed lecithotrophic larvae
(Rowell 1960). Members of Rhynchonelliformea have a pedicle with which they
attach themselves to rocks or other hard substrates (Williams et al. 1997). Their
larvae have three lobes and are lecithotrophic (Freeman 2003). Traditionally,
Linguliformea and Craniiformea have been grouped together as Inarticulata,
while Rhynchonelliformea have been named Articulata because their valves
are connected by a hinge (James et al. 1992).
Brachiopods are certainly a comparatively minor phylum when only the number
of recent species is considered. Nevertheless, they are present in all of the
world’s oceans within all depth zones and the approximately 12.000 fossils
species represent a rich source of paleontological information (Logan 2007).