Ultrastructure And Chemistry Of The Graptolite Periderm

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

Dendroidea

The Dendroidea is the older of the two main groups with important geological records, first appearing in the Middle Cambrian and disappearing during the Late Carboniferous. The dendroid rhabdosome was multibranched, like a bush, with its many stipes connected laterally by struts or dissepiments. Two types of theca, of different sizes, the autotheca and bitheca, grew along the stipes. The earlier genera were benthic, attached to the seafloor by a short stalk and basal disk. Probably during the latest Cambrian a few genera, including Rhabdino-pora, detached themselves to evolve a new lifestyle in the plankton; together with minute brachiopods and the occasional trilobite, they probably formed a major part of the preserved Early Paleozoic plankton.

Dendroid taxa Dendrograptus was a benthic genus, bush-like, erect and attached to the sea-floor by a rooting structure or holdfast. Dicty-onema was also benthic and ranged in age from the Late Cambrian to the Late Carboniferous. The rhabdosome was conical to cylindrical in shape. Planktonic dendroids similar to Dictyonema are placed in Rhabdinopora.

The following anisograptid genera are in some ways intermediate between the typical dendroids and graptoloids and may be classified with either group. Here they are included with the dendroids. Radiograptus, for example, developed large spreading colonies. Both Kiaerograptus and some early species of Bryograptus had both auto- and bithecae, and the latter had triradiate rhabdosomes with, initially, three primary stipes. Clonograptus had a horizontal, biserially symmetric rhab-dosome with stipes generated by dichoto-mous branching from an initial biradiate configuration.

Graptoloidea

Compared with the dendroids, the graptoloid rhabdosome is superficially simpler and consists of an initial sicula, divided into an upper prosicula and a lower metasicula, with at its apex, distally, a long thin, spine, the nema. The metasicula, like the rest of the rhabdo-some, was composed of fusellar tissue, bundles of short, branching fibrils. The virgella projected below the secular aperture, proximally and is characteristic of the suborder Virgellina. The thecae grew out from the sicula and subsequent thecae grew in sequence as the rhabdosome developed.

Graptoloid taxa The architecture of the grap-toloid skeleton depended on three sets of structures: the number of stipes or branches, their mutual attitudes and the shape of the thecae. Morphology in this order is thus based on permutations of these structures; the following genera illustrate this variation (Fig. 15.23).

Tetragraptus, common during the Floian (later Early Ordovician), typically had four stipes arranged in horizontal, pendent or reclined attitudes with simple, overlapping thecae. Didymograptus was twin-stiped or biramous, commonly with the branches in horizontal, pendent or reclined orientations; thecae were simple. Isograptus, however, had two relatively wide stipes, reclined with a long, thread-like sicula. Nemagraptus had a very distinctive rhabdosome consisting of two sigmoidal stipes, initially diverging from the sicula at about 180°, with additional stipes, curved, and arising at intervals along the main branches. Thecae were long, thin and diverged at small angles from the stipes. Dicellograptus had a pair of stipes that adopted reclined attitudes but often the branches were curved or even coiled; the thecae were characterized by extravagant sigmoidal shapes and incurved apertures. Monograptus was a uniserial scandent form with a straight or curved rhabdo-some and a nema embedded in the dorsal wall that projected distally. Rastrites possessed long, straight, widely separated thecae, often with hooked ends. Cyrtograptus had a spirally coiled rhabdosome with secondary branches or cladia oriented like the arms of a spiral galaxy. Corynoides was minute, consisting of a sicula and three to four thecae.

Retiolitids

The retiolitids are a spectacular group of apparently scandent, diplograptid biserials with a reduced, minimalist periderm consisting of a network of bars or lists probably surrounded by a net-like structure, termed the ancora sleeve in Silurian forms (Fig. 15.24). The group appeared in the Mid Ordovician and continued successfully, for almost 50 myr,

Isograptus Bilder

Figure 15.23 Some graptolite genera: (a) Rhabdinopora (x2), (b) Tetragraptus (x2), (c) Tetragraptus, proximal end (x20), (d) Isograptus, proximal end (x20), (e) Xiphograptus (x20), (f) Isograptus (x10), (g) Appendispinograptus (x2), (h) Dicranograptus (x2), (i) Dicellograptus (x2), (j) Orthograptus (x2), (k) Undulograptus (2), (l) Nemagraptus (x2), (m) Didymograptus (Expansograptus) (x20) and (n) Atavograptus (x2). (a) An Early Ordovician dendroid, (b-f, k, m) Early Ordovician graptoloids; (g-j, l) Late Ordovician graptoloids; and (n) a Silurian monograptid. (Courtesy of Henry Williams.)

Figure 15.23 Some graptolite genera: (a) Rhabdinopora (x2), (b) Tetragraptus (x2), (c) Tetragraptus, proximal end (x20), (d) Isograptus, proximal end (x20), (e) Xiphograptus (x20), (f) Isograptus (x10), (g) Appendispinograptus (x2), (h) Dicranograptus (x2), (i) Dicellograptus (x2), (j) Orthograptus (x2), (k) Undulograptus (2), (l) Nemagraptus (x2), (m) Didymograptus (Expansograptus) (x20) and (n) Atavograptus (x2). (a) An Early Ordovician dendroid, (b-f, k, m) Early Ordovician graptoloids; (g-j, l) Late Ordovician graptoloids; and (n) a Silurian monograptid. (Courtesy of Henry Williams.)

Figure 15.24 Retiolitid Phorograptus (Middle Ordovician) (x30). (Courtesy of Denis Bates.)

until the latest Silurian (Kozlowski-Dawid-ziuk 2004). The retiolitids probably represent a polyphyletic grade of organization where the rhabdosome of various groups may have functioned like a sponge, drawing in fluid and nutrients through the periderm and expelling waste upwards.

Growth and ultrastructure of the graptolites

Detailed studies on the ultrastructure of the graptolite using both scanning and transmission electron microscopes has identified two types of skeletal tissue. Fusellar tissue occurs together with cortical tissue in the form of longer parallel fibers. Fusellar material was secreted as a series of half rings with the cortical tissue overlapping the fusellar layer both inside and outside the rhabdosome (Fig. 15.25). The cortical tissue itself was secreted as "bandages", looking rather like multiple overlapping band-aids. Secretion may have been by mobile zooids, free to patrol the exterior of the colony while still attached by a flexible cord to the rest of the colony, rather like an astronaut maintaining a space station, or the entire rhabdosome may have been surrounded by soft tissue.

Although graptolites are abundant and important fossils in many Early Paleozoic assemblages, it is notoriously difficult to discover what they were actually made of. Most assemblages occur in black shales that have been compacted, diagenetically altered and often metamorphosed within or around oro-genic belts. Moreover, graptolite periderm, when actually preserved, consists mainly of an aliphatic polymer, immune to base hydrolysis. It lacks protein even though both the structure, as well as chemical analyses, of the periderm of living Rhabdopleura suggest that it was originally composed of collagen. Previous studies suggested that the collagen had been replaced by macromolecular material from the surrounding sediment. New analyses suggest that the aliphatic composition of graptolite periderm reflects direct incorporation of lipids from the organism itself by in situ polymerization (Gupta et al. 2006). A similar process may account for the preservation of many other groups of organic fossils (see p. 60).

Colonial growth of the graptoloids The growth of a colony lends itself to graphic and mathematical simulations. A few authors have devised computer models based on a set of simple rules that dictate such growth modes. These models are usually deterministic and static. For example, Andrew Swan (1990) generated a series of theoretical morphotypes based on a model of dichotomous branching at given stipe lengths; the orientation of the bifurcation together with the stipe length and width was varied. Additionally, soft tissue could be added to the computer reconstructions. Swan showed that the shapes of most graptolite colonies could be simulated using variations in only a few parameters, and he was able to test the efficiency of each colony for particular functions. Swan targeted the efficiency of the graptolite feeding strategy and tested the efficacy of nutrient capture for a sequence of computer-generated colonies, and he showed that known graptolite colonies pass the test as being the most efficient shapes

Figure 15.25 Graptolite ultrastructure: (a) collage of Geniculograptus rhabdosome showing banded fusellar tissue (x50); (b) detailed section through part of a rhabdosome showing relatively thin, parallel sheet fabric (top) and criss-cross fusellar fabric (below) (x1000); and (c) detail of aperture exterior of Geniculograptus showing the development of bandages (x500). (Courtesy of Denis Bates.)

Figure 15.25 Graptolite ultrastructure: (a) collage of Geniculograptus rhabdosome showing banded fusellar tissue (x50); (b) detailed section through part of a rhabdosome showing relatively thin, parallel sheet fabric (top) and criss-cross fusellar fabric (below) (x1000); and (c) detail of aperture exterior of Geniculograptus showing the development of bandages (x500). (Courtesy of Denis Bates.)

for capturing most food in the shortest time from a given water volume.

Ecology: modes of life and feeding strategy

There is little doubt that the earliest bush-like dendroids were attached to the seabed and functioned as part of the sessile benthos. Detachment in various benthic genera occurred at the beginning of the Ordovician with genera such as Rhabdinopora entering the plankton. More controversial is the mode of life of the various graptoloid groups (Fig. 15.26). Conventionally the graptoloids were considered to be passive drifters, their flotation being aided by fat and gas bubbles in their tissues or even by vane-like extensions to the nema. But they clearly occupied different levels in the water column (Underwood 1994).

The suggestion by Nancy Kirk (1969) that far from being passive members of the plankton, the graptolites were automobile, moving up and down in the water column, has stimulated considerable and continued interest and research on the life habits of these extinct organisms. During intervals of intense feeding, a reactive upward movement of the colony in the water column would have occurred. At night the colony could move vertically into the nutrient-rich photic zone and later, when replete, the rhabdosomes would sink to positions in the water column where the specific

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