Page 428 - Introduction to Paleobiology and The Fossil Record

DEUTEROSTOMES: ECHINODERMS AND HEMICHORDATES 415

and is characteristic of the suborder Virgel-
Dendroidea
lina. The thecae grew out from the sicula and
The Dendroidea is the older of the two main subsequent thecae grew in sequence as the
groups with important geological records, fi rst rhabdosome developed.
appearing in the Middle Cambrian and disap-
pearing during the Late Carboniferous. The Graptoloid taxa The architecture of the grap-
dendroid rhabdosome was multibranched, like toloid skeleton depended on three sets of
a bush, with its many stipes connected laterally structures: the number of stipes or branches,
by struts or dissepiments. Two types of theca, their mutual attitudes and the shape of the
of different sizes, the autotheca and bitheca, thecae. Morphology in this order is thus based
grew along the stipes. The earlier genera were on permutations of these structures; the
benthic, attached to the seafloor by a short following genera illustrate this variation

stalk and basal disk. Probably during the latest (Fig. 15.23).
Cambrian a few genera, including Rhabdino- Tetragraptus, common during the Floian
pora, detached themselves to evolve a new (later Early Ordovician), typically had four
lifestyle in the plankton; together with minute stipes arranged in horizontal, pendent or
brachiopods and the occasional trilobite, they reclined attitudes with simple, overlapping
probably formed a major part of the preserved thecae. Didymograptus was twin-stiped or
Early Paleozoic plankton. biramous, commonly with the branches in
horizontal, pendent or reclined orientations;
Dendroid taxa Dendrograptus was a benthic thecae were simple. Isograptus, however, had
genus, bush-like, erect and attached to the sea- two relatively wide stipes, reclined with a

floor by a rooting structure or holdfast. Dicty- long, thread-like sicula. Nemagraptus had a
onema was also benthic and ranged in age very distinctive rhabdosome consisting of two
from the Late Cambrian to the Late Carbonif- sigmoidal stipes, initially diverging from the
erous. The rhabdosome was conical to cylin- sicula at about 180˚, with additional stipes,
drical in shape. Planktonic dendroids similar curved, and arising at intervals along the main
to Dictyonema are placed in Rhabdinopora. branches. Thecae were long, thin and diverged
The following anisograptid genera are in at small angles from the stipes. Dicellograptus
some ways intermediate between the typical had a pair of stipes that adopted reclined atti-
dendroids and graptoloids and may be classi- tudes but often the branches were curved or
fi ed with either group. Here they are included even coiled; the thecae were characterized by
with the dendroids. Radiograptus, for extravagant sigmoidal shapes and incurved
example, developed large spreading colonies. apertures. Monograptus was a uniserial scan-
Both Kiaerograptus and some early species of dent form with a straight or curved rhabdo-
Bryograptus had both auto- and bithecae, and some and a nema embedded in the dorsal wall
the latter had triradiate rhabdosomes with, that projected distally. Rastrites possessed
initially, three primary stipes. Clonograptus long, straight, widely separated thecae, often
had a horizontal, biserially symmetric rhab- with hooked ends. Cyrtograptus had a spi-
dosome with stipes generated by dichoto- rally coiled rhabdosome with secondary
mous branching from an initial biradiate branches or cladia oriented like the arms of a
confi guration. spiral galaxy. Corynoides was minute, con-
sisting of a sicula and three to four thecae.
Graptoloidea
Retiolitids
Compared with the dendroids, the graptoloid
rhabdosome is superficially simpler and con- The retiolitids are a spectacular group of

sists of an initial sicula, divided into an upper apparently scandent, diplograptid biserials
prosicula and a lower metasicula, with at its with a reduced, minimalist periderm consist-
apex, distally, a long thin, spine, the nema. ing of a network of bars or lists probably
The metasicula, like the rest of the rhabdo- surrounded by a net-like structure, termed the
some, was composed of fusellar tissue, bundles ancora sleeve in Silurian forms (Fig. 15.24).
of short, branching fi brils. The virgella pro- The group appeared in the Mid Ordovician
jected below the secular aperture, proximally and continued successfully, for almost 50 myr,