Pterobranchs the living relatives of graptolites

There are only two living genera ofpterobranchs, Rhabdopleura and Cephalodiscus (Fig. 10.2). They have a fossil record that extends back to the Middle Cambrian, and it was probably then that both they and graptolites evolved from a common ancestor. Living pterobranchs are known worldwide, but are usually small and are easily overlooked. They are found from intertidal areas to abyssal depths, in water of normal salinity. Their preference is for areas with a rapid flow of water, and they often stick to boulders or dead shells in current-winnowed channels.

Both forms are colonial filter feeders, but they are quite dissimilar to one another. Cephalodiscus grows a transparent, collagenous colony within which zooids move freely. One or several groups of zooids can be found in a single colony, with each group including between two and 20 zooids in various stages of development. The mature zooids are about 1 mm in size and are attached to the rest of the group by a contractile stalk. They have a simple body and an unusual head from which sprouts a ring of filtering tentacles, called a lophophore, that forms a spherical filtering array. The head also includes the cephalic shield, a highly evolved and unusual organ used to secrete the skeleton and to hold the zooid in place.

Rhabdopleura grows transparent to brown colonies in which each zooid has its own "theca" and is connected to all the other zooids in the colony by a thin tube of tissue. Each zooid has a cephalic shield and a pair of filter-feeding arms. The colony form is variable, from single strands to complicated bushes. The most common type is a series of tubes that always grow in contact with another tube, forming a complicated two-dimensional maze on the surface of a shell or boulder.

Lophophore

Feeding zooid

Tissue connection to rest of colony

Lophophore

Feeding zooid

Colony hard parts

Cephalic shield

Connected group of zooids of different ages

Fig. 10.2 Drawings of living pterobranchs: (a) Rhabdopleura, and (b) Cephalodiscus. The zooids are approximately 1 mm long.

Colony hard parts

Soft tissue connection to rest of group

Cephalic shield

Connected group of zooids of different ages

Fig. 10.2 Drawings of living pterobranchs: (a) Rhabdopleura, and (b) Cephalodiscus. The zooids are approximately 1 mm long.

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