gravity of the colony matched that of the surrounding seawater.
Computer models and physical models, including exposure to wind-tunnel conditions that mimic the effects of water currents, have emphasized the importance of harvesting strategies for the success of the colony. These probably exerted an important influence on the evolutionary pathways that the grapto-loids followed.
Evolution: graptolite stipes and thecae
Graptolite evolution has been described in terms of four main stages of morphological development:
1 The transition from sessile to planktonic strategies in the dendroids during the Late Cambrian and Early Ordovician.
2 At the end of the Tremadocian (early Early Ordovician), the appearance of the singletype thecae of the graptoloids.
3 The development of the biserial rhabdo-some in the Floian (late Early Ordoviaian).
4 Finally, the origin of the uniserial monograptids.
The small, stick-like benthic organisms reported from Middle Cambrian rocks on the Siberian platform and ascribed to the graptolites may be better assigned to the
rhabdopleurids. The first undoubted graptolites include the dendroids Callograptus, Den-drograptus and Dictyonema occurring in Middle Cambrian rocks of North America. But by the Late Cambrian, the diversity of the dendroid fauna had markedly increased. The fauna included genera such as Aspidograptus and Dictyonema, which resembled small shrubs and were attached to the substrate by holdfasts or more complex root-like structures. During the Late Cambrian and Early Ordovician, some dendroids made the jump from the sessile benthos to the plankton; attachment disks continuous with the nema suggest these genera may have hung suspended in the surface waters and pursued an epi-planktonic life strategy. Both Radiograptus and Dictyonema have been cited as possible ancestors for the planktonic graptolites, and perhaps Staurograptus was in fact the first planktonic graptolite. The Tremadocian seas witnessed the radiation of the anisograptids.
The explosion of dichograptid genera during the Floian introduced a variety of symmetric graptolites with from about eight to two stipes oriented in declined, pendent and scandent attitudes (Fig. 15.27). A twin-stiped dichograptid was probably ancestral to the next wave of graptolites, the diplograptids, which radiated in the Mid Darriwilian (Middle Ordovician).
The single-stiped monograptids dominated Silurian graptolite faunas and, despite their apparent simplicity, the group developed a huge variety of forms (Fig. 15.28). The last graptolites, species of Monograptus, disap-
hooked (M. priodon)
lobate (M. lobiferus)
with lappets (Cucullograptus)
Figure 15.28 Evolution of thecae. M, Monograptus.
peared during the Early Devonian (Pragian) in China, Eurasia and North America. Nevertheless this uniserial morphology had survived for over 30 million years and may have continued after the Early Devonian in lineages that did not secrete a preservable skeleton. Why should a trend towards a reduction in stipes be such an advantage? Perhaps the simpler stipe configuration was hydrodynami-cally more stable, better adapted to turbu lence and aided the motion of the graptolites through the water column on feeding forays. It may also have prevented the interference between thecae on adjacent stipes, providing a simpler, more efficient colony structure.
Graptolite morphology and stratigraphy have formed the basis for the definition of evolutionary faunas within Ordovician assemblages that include anisograptid, dichograptid and diplograptid evolutionary faunas (Chen et al. 2006). These faunas not only help us understand better the Ordovician radiation of the graptolites but add an additional dimension to the zonal framework of the Ordovician System.
Biostratigraphy: graptolites and time
Graptolites are among the best zone fossils (see p. 28) and are excellent for biostratigraphic correlation. Traditionally, four sequential graptolite faunas have been recognized through the Early Ordovician to Early Devonian interval (Fig. 15.29). The anisograp-tid fauna, with Rhabdinopora and allied genera, characterizes the Tremadocian; although Upper Tremadocian graptolite faunas are rare, the genera Bryograptus, Kiaerograptus and Aorograptus are important and have been described in detail from western Newfoundland (Williams & Stevens 1991). The appearance of the Floian dichograp-tid fauna is signaled by Tetragraptus, associated with didymograptids and some relict anisograptids. The later diplograptid fauna contains four smaller units: the Glyptograptus-Amplexograptus (Darriwil-ian), Nemagraptus-Dicellograptus (Sand-bian), Orthograptus-Dicellograptus (Katian) and Orthograptus-Climacograptus (Hirnan-tian) subfaunas. The monograptid fauna contains a variety of evolving single-stiped forms. The last graptoloids disappeared in the Pragian (Lower Devonian).
In some parts of the world graptolites have provided the basis for some high-resolution stratigraphy. The Upper Ordovician-Lower Silurian clastic succession in the Barrandian basin in the Czech Republic was deposited on an outer shelf, influenced by the end-Ordovi-cian Gondwanan glaciation and an aftermath that included a persistent post-glacial anoxia related to upwelling systems. High-resolution graptolite stratigraphy based on some 19 biozones has provided a framework to link sedimentary environments with graptoloid faunal dynamics and fluctuations in organic content (Storch 2006). The resulting analyses have provided an accurate time line through four major transgressive cycles. On these are superimposed glacial and interglacial events, intervals of upwelling and oceanic perturbations (Fig. 15.30). Moreover these new data suggest that, far from being a quiet period, the Silurian was punctuated by a number of significant extinction events associated with large climatic and environmental fluctuations. Critical is the accurate correlation between sections in the Barrandian basin with sections elsewhere, otherwise we cannot show these changes were indeed global. Higher in the Silurian, Lennart Jeppsson and Mikel Calner (2003) have reported that the Mulde Secundo-Secundo Event (Wenlock), first identified in the Silurian platform carbonates of the Swedish island of Gotland, includes three extinctions, widespread deposition of carbon-rich sediments, and wild sea-level fluctuations together with a glaciation event. These extinctions are related to a severe reduction in primary plankton productivity. Amazingly, such precision and the recognition of these important events was only made possible by the accurate graphic correlations of sections based on the rapid evolution of these small, beautiful creatures.
Biogeography: graptolites in space
Since the majority of graptolites lived either in the water column or within the plankton, quite different factors influenced their distribution in contrast to, say, that of the coeval benthos. Provinciality was most marked in the earlier Ordovician (Darriwilian) when two main provinces, the Atlantic and Pacific, were recognized. The Atlantic province, including the then high-latitude regions of Avalonia and Gondwana, was characterized by pendent Didymograptus species. The Pacific province, including low latitude, tropical regions such as the Laurentian margins, was more diverse with isograptids, cardiograptids and oncograptids. The isograptid biofacies, itself, was more pandemic, occupying deeper water and associated with the world's continental margins. During the end-Ordovician extinction events (see p. 169), the Pacific province graptolites suffered particularly badly, and although graptolites again diversified during the Silurian their provinciality developed a different if less obvious pattern. During the Early Silurian, the Gond-wanan province was characterized by endemic taxa while later, in the Mid Silurian, the equatorial region hosted taxa not known from elsewhere.
Graptolite biozones and subzones
Lithology and gaps in sedimentation
Relative sea level fall rise
Sequence -stratigraphic units and system tracts ivn w <
Upwelling possibly weakened
Upwelling possibly weakened
Anoxic turnover deglaciation
2nd glacial maximum interglacial
1st glacial maximum
Climatically warm period shale, mudstone black shale sandy/ silty-micaceous laminite storm sandstones, siltstones and shale interbeds graded sandstone and conglomerate glaciomarine limestone nodules and lenticular beds alternating pale mudstone and black shale diamictite
Figure 15.30 Graptolite biostratigraphy of the Upper Ordovician-Lower Silurian strata of the Barrandian basin. HST, highstand systems tract; TST, transgressive systems tract; LST, lowstand systems tract. (Based on Storch 2006.)
Box 15.10 The Vetulicolians: protostomes, deuterostomes or phylogenetic orphans?
Sometimes the fossil record throws out a weird animal that it is just impossible to classify. The material may be common, distinctive and well preserved but there are simply not enough key characters to link it with other groups. The vetulicolians have been characterized as unusual arthropods, stem-group deuterostomes and even tunicates (Aldridge et al. 2007). They have been reported from a number of Cambrian Lagerstätten and two classes have been recognized, the Vetulicolida and Banffozoa. They were probably active, nektobenthic animals with the facility to both deposit and filter feed. But what were they? In simple terms they lack limbs, making assignment to the arthropods difficult, whereas they have gills similar to those of the deuterostomes (Fig. 15.31). If they were, in fact, deuterostomes they probably lay close to the tunicates as stem vertebrates. But despite well-preserved material from the Chengjiang fauna and careful phylogenetic analyses, it remains impossible to classify them. Their unique combination of characters is thus still an enigma awaiting the discovery of new animals that could link the vetulicolians to a crown group.
1 The deuterostomes include two apparently morphologically different groups, the echinoderms and hemichordates. What sort of characters could be used to unite them?
2 Crinoids are most common in deep-water environments but probably exploited much shallower-water environments during the Paleozoic. When and why did they move to deeper water?
3 Echinoids have a long history. Why did it take over 250 myr to develop the buried (sand dollars) and burrowing (sea urchin) life strategies?
4 Graptolites evolved through time by reducing their numbers of stipes and developing more complex thecae. What were the ecological advantages of this more streamlined body plan with more elaborate zooid openings?
5 The vetulicolians highlight one of the difficulties of the fossil record, identifying definitive characters of phylogentic significance in bizarre taxa. Should new higher taxa, for example classes of phyla, be established to accommodate such material or should it be shoehorned into existing taxa?
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