Hemichordates

What was the character of the vegetation that clothed this earliest prototype of Europe is a question to which at present no definite answer is possible. We know, however, that the shallow sea which spread from the Atlantic southward and eastward over most of Europe was tenanted by an abundant and characteristic series of invertebrate animals - trilobites, graptolites, cystideans, brachiopods, and cephalopods, strangely unlike, on the whole, to anything living in our waters now, but which then migrated freely along the shores of the arctic land between what are now America and Europe.

Sir Archibald Geikie from a lecture delivered to the Royal Geographical Society (1897)

The hemichordates form a small phylum of only a few hundred species and are unfamiliar to most people, but their importance for the study of vertebrate evolution cannot be underestimated. Their most common fossil representatives, the graptolites, were abundant in anus

Figure 15.18 Reconstruction of a living carpoid: the Devonian Rhenocystis moving across and through the sediment from left to right. (From Sutcliffe et al. 2000.)

Box 15.7 The oldest stylophorans: echinoderms with a locomotory organ

How much further can we go with the debate on the affinities of the carpoids? New material and new investigative techniques will always help. The oldest stylophoran carpoid is from the Middle Cambrian rocks of Morocco. Sebastien Clausen and Andrew Smith (2005) have analyzed the morphology of the animal in great detail, particularly its microstructure with the scanning electron microscope. Ceratocystis in fact has a stereom microstructure typical of most echinoderms, but its appendage was covered by articulating plates and filled with muscle tissue and ligaments (Fig. 15.19). It seems that this bizarre asymmetric animal has an echinoderm skeletal structure but also possessed a muscular locomotory appendage rather similar to its sister taxon, the pterobranchs. This group has all the features of a stem-group echinoderm prior to acquiring five-fold symmetry and presumably a water vascular system, and it rather conclusively disproves a close phylogenetic association between carpoids and chordates.

Hemichordata Devonian

Figure 15.19 Ceratocystis from North Africa. (a) Basic anatomic features. (b-d) Three current interpretations of the soft-tissue anatomy of the stylophoran appendage in proximal longitudinal (left) and distal transverse (right) section: (b) primitive echinoderm model, (c) calcichordate model and (d) crinozoan model. (Based on Clausen & Smith 2005.)

Figure 15.19 Ceratocystis from North Africa. (a) Basic anatomic features. (b-d) Three current interpretations of the soft-tissue anatomy of the stylophoran appendage in proximal longitudinal (left) and distal transverse (right) section: (b) primitive echinoderm model, (c) calcichordate model and (d) crinozoan model. (Based on Clausen & Smith 2005.)

the ancient seas of the Early Paleozoic, in communities and environments quite different from those of today. Graptolites are widely used for correlation because of their abundance, widespread distribution and rapid evolution. Although graptolites are extinct, and their life styles are difficult to interpret, they were hemichordates - a phylum containing about 100 living species characterized by a rod-like structure, the notochord. They were small, soft-bodied animals with bilateral symmetry and a lack of segmentation. The phylum contains two very different classes: first, the tiny, mainly colonial, pterobranchs that lived in the sessile benthos and, second, the larger infaunal acorn or tongue worms, the entero-

pneusts that lived in burrows mainly in sub-tidal environments.

The hemichordates have a mixture of characters suggesting links with the lophopho-rates, the echinoderms and the chordates. They have been closely related to the cepha-lochordates and urochordates or tunicates, but molecular and other data suggest that the latter two groups are more closely related to the chordates than the hemichordates. Although the notochord is now known to be unrelated to a true backbone, the hemichor-dates have, nevertheless, gill slits and a nerve cord.

Modern hemichordate analogs_

Pterobranchs superficially resemble the bryo-zoans - both are colonial animals and the individual zooids feed with tentaculate, ciliated arms (Fig. 15.20). The group has a long geological history with early records such as

Rhabdotubus from the Middle Cambrian and Graptovermis from the Tremadocian. The living genera Cephalodiscus and Rhabdo-pleura (Fig. 15.20) have been used as analogs for many aspects of graptolite morphology, ontogeny and paleoecology. These living genera and the graptolites both have a periderm, or skin, with fusellar tissue, while the dendroid stolon, a tube that connects the thecae to each other, may be related to the pterobranch pectocaulus. Rhabdopleura is known first from the Middle Cambrian and occurs in oceans today mainly at depths of a few thousand meters. The genus is minute with a creeping colony hosting a series of exoskeletal tubes, each containing a zooid with its own lophophore-like feeding organ comprising a pair of arms. The zooids are budded from a stolon and interconnected by a contractile stalk, the pectocaulus.

Cephalodiscus, however, is rather different, being constructed from clusters of stalked lophophore lophophore contractile stalk retracted zooid

Cephalodiscus Chordata

Figure 15.20 Rhabdopleurid morphology: (a, b) Rhabdopleura and (c) Cephalodiscus. (Based on Treatise on Invertebrate Paleontology, Part V. Geol. Soc. Am. and Univ. Kansas Press.)

contractile stalk retracted zooid

Figure 15.20 Rhabdopleurid morphology: (a, b) Rhabdopleura and (c) Cephalodiscus. (Based on Treatise on Invertebrate Paleontology, Part V. Geol. Soc. Am. and Univ. Kansas Press.)

tubes budded from a basal disk. Moreover, in further contrast to Rhabdopleura, species of Cephalodiscus usually have five pairs of ciliated feeding arms. Individual zooids in the Cephalodiscus colony can actually crawl outside the colony along its exterior and often farther afield onto adjacent surfaces. The zooids of living Cephalodiscus, with their considerable freedom of mobility, can even construct external spines from outside the skeleton.

Graptolites_

The graptolites, or Graptolithina, are generally stick-like fossils, very common in many Lower Paleozoic black shales. In fact the group is so prevalent that it has proved to be of key importance in correlating Lower Paleozoic strata. The majority of Ordovician and Silurian biozones are based on graptolite species or assemblages. Graptolites, from the Greek "stone writing", usually occur in black shales as flattened carbonized films resembling hieroglyphics. Graptolite fossils often show evidence of having been transported by currents, although fortunately complete, unflattened specimens have been extracted from cherts and limestone by acid-etching techniques. The affinities of the group were largely unknown until the 1940s, when the Polish paleontologist Roman Kozlowski identified a notochord in three-dimensional material isolated from limestones. There are several groups of graptolites and graptolite-like animals (Box 15.8).

Morphology: the graptolite colony

The basic graptolite architecture consists of a probably collagenous skeleton characterized by a growth pattern of half rings of periderm interfaced by zigzag sutures, similar to the construction of the pterobranchs (Fig. 15.21). Each colony or rhabdosome grew from a small cone, the sicula, as one or a series of branches or stipes. The stipes may be isolated or linked together by lateral struts to resemble a reticulate lattice. A series of variably cylindrical tubes are developed along the stipes; these thecae house the individual zooids of the colony. Aggregates of rhabdosomes, synrhab-dosomes, have been documented for some species. These complex structures have generally been explained by asexual budding or common attachment to a single float or patch of substrate. A more recent taphonomic explanation, however, suggests they formed by entrapment of clusters of rhabdosomes using sicula dissepiment stipe sicula dissepiment

Graptolite Morphology

dissepiment

distal dissepiment growth lines v al prosicula sicula sicula aperture growth lines prosicula sicula sicula aperture nema nema

Graptolite Morphology

theca thecal aperture initial bud virgula proximal theca thecal aperture initial bud

Figure 15.21 Graptolite morphology: (a) dendroid morphology with a detail of the thecae (b), and (c) graptoloid morphology.

nema nema

Box 15.8 Graptolite classification

A complete classification of the group is presented here although in practice it is only the dendroids and graptoloids that have good fossil records. (The stolonoids, an encrusting or sessile group, restricted to Poland, may be Pterobranchia.)

Class GRAPTOLITHINA

Order DENDROIDEA

• Multibranched colonies; stipes, commonly supported by dissepiments, have autothecae, bithecae and a stolotheca. Anisograptids, intermediate between the dendroids and graptoloids, are retained here

• Cambrian (Middle) to Carboniferous (Namurian) Order TUBOIDEA

• Similar to dendroids but characterized by irregular branching and reduced stolothecae. Autothe-cae and bithecae commonly form clusters

• Ordovician (Tremadocian) to Silurian (Wenlock)

Order CAMAROIDEA

• Encrusting life mode; endemic to Poland. Autothecae with expanded, sack-like bases. Bithecae small and irregularly spaced. Stolotheca black and hard

• Ordovician (Tremadocian-Darriwilian)

Order CRUSTOIDEA

• Encrusting life mode; endemic to Poland. Autothecae with complex apertures

• Ordovician (Floian-Darriwilian)

Order DITHECOIDEA

• Sister group to the dendroids and graptoloids; central axis with a holdfast

• Cambrian (Middle) to Silurian (Lower)

Order GRAPTOLOIDEA

• The jury is still out on the detailed classification of this group. The position of the anisograptids (included here with the dendroids) is uncertain, as is the status of the dichograptids; the retiolitids are aberrant diplograptids. Colonies with few stipes (one to eight), a nema and sicula and a single type of theca

• Ordovician (Tremadocian) to Devonian (Pragian) Suborder DICHOGRAPTINA

• Basal graptoloids lacking both bithecae and virgellae

• Ordovician (Tremadocian-Katian)

Suborder VIRGELLINA

• Virgella always present

• Ordovician (Floian) to Devonian (Pragian)

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

Box 15.9 The first graptolites: a cryptic Cambrian dimension?

By the Ordovician, the graptolites were represented by a number of well-defined 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 specimens 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.

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