FOSSIL INVERTEBRATES/Graptolites 357 Graptolites R B Rickards, University of Cambridge, Cambridge, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction The graptolites were a group of colonial animals that inhabited the Palaeozoic seas, often in very great abundance Their remains can be found as small elongate (3 mm–1 m) skeletons of thin collagenous periderm forming simple linear or complexly branched colonies Each branch is composed of a linear succession of interconnecting tubes (thecae) originating from a single conical individual called a sicula (Figure 1) Graptolites are of value in the study of the stratigraphy of many Ordovician and Silurian deposits the world over, because of their abundance, wide geographical distribution, and rapid evolution Benthic graptolites appeared in the Middle Cambrian, becoming extinct in the Upper Carboniferous; planktonic graptolites ranged from the earliest Ordovician to the Middle Devonian The Chequered History of Graptolite Studies Although graptolites were originally observed and illustrated as long ago as the late eighteenth century, it was not until the mid-nineteenth century that much attention was paid to them as fossils and their animal nature was fully appreciated Even then there was considerable doubt about their zoological affinities, and they were variously allied to the plants, molluscs, corals, protozoa, and bryozoans Modern interpretation of their morphology places them in the Hemichordata based on comparisons with living forms Thus, their zoological position has been elevated over the years from plants, through the lower invertebrates, to the lower vertebrates Understanding of their morphology has progressed greatly, aided by technical developments in microscopy, especially since the late 1950s with the use of scanning electron microscopes Consequently, there is now a tremendous amount of detailed information available on this extinct group of strange little animals that might at first appear to be, as their name (graptos, written or marked; lithos, stone) implies, little more than markings on rock Classification, Broad Evolution, Stratigraphy, and Mode of Life The ancestors Eorhabdopleura and Eocephalodiscus of the living Hemichordata probably arose with the order Dendroidea from a common ancestor in the early Middle Cambrian (Figure 2) The common ancestor may have been a phoronid-like worm that evolved through one or more of the following stages of evolution: infaunal, in dense association; infaunal with loose coloniality; and finally sessile with several stolons inside one upright peridermal tube This last stage would have been equivalent to a primitive Mastigograptus-like graptolite The evolutionary relationships of the orders other than dendroids and graptoloids are unknown, and the stratigraphic record of these other orders is poor The evolution within the Order Graptoloidea is discussed in further detail later in this article, and the distinctions between dendroids and graptoloids are discussed below The Sicula (or Larval) Stage Eggs and embryos are known but are extremely rare in graptolites However, the peridermal sheath of the larval stage, called the sicula, is known in almost all graptolites and is very common in the rock record Eggs, embryos, and larval stages were produced by sexual reproduction, but the remainder of the colony grew by asexual budding from the sicula In dendroid siculae (Figure 3A) a resorption foramen (or hole) formed in the prosicula, and from this grew the first asexual bud of the colony In most graptoloids (Figure 3B) the resorption foramina were in the metasicula, but in some, the Monograptina (Figure 3C), the growth lines construct a notch, then a hole (lacuna), through which the first theca grew The dendroid sicula shown here (Figure 3A) is of a benthonic type: a planktonic type would have a nema instead of a basal disc and may have longitudinal threads The Stolon System In dendroids and tuboids the thecal tubes are attached at their bases to a black (sclerotized) stolon system; in dendroids (Figure 4) each node divides into three stolons, but in tuboids (Figure 5) the division is always into two stolons and some species have only partly sclerotized stolons (i.e only partly hardened) In graptoloids (Figure 6) there are no sclerotized stolons,