Graptolites     Radiolaria     Stratigraphy

Graptolite Origins

The origin and evolution of coloniality in the hemichordates is a poorly understood process as fossil data are basically non-existent and the comparison of fossil Graptolithina with modern Pterobranchia is still difficult, especially as insufficient information exists on the skeletal development of living pterobranchs.

In the Graptolithina the proximal end development has been considered the most important indication for evolutionary and phylogenetic relationships. The taxonomy of the graptolites is, thus, based largely on the changes in the proximal development and the associated growth direction of the proximal (primordial) thecae. A comparison of the initial skeletal growth of modern pterobranchs and graptolites can show relationships and differences.

The initial parts of a graptolite colony are the prosicula and metasicula, while in modern – benthic – pterobranchs, the initial structure is termed a dome. The dome is formed by the secretion of the pterobranch larva and is structureless. The graptolite prosicula, however, bears  distinct structures like the spiral thread (Schraubenlinie of Kraft). It is differently constructed and does not resemble the dome of a modern pterobranch.  

Figure 1. Initial growth of the skeletons of Rhabdopleurida (modern Pterobranchia) and Graptolithina showing differences in development and structure (Maletz, Steiner & Fatka 2005). 

Pterobranchs are known from the Middle Cambrian to the recent, but have rarely been described from the fossil record. They are, however, common in the fossil record of certain regions, for example in the Middle Ordovician Limestones of Öland, Sweden and have been described from glacial erratic boulders in northern Germany and Poland. Numerous specimens have most likely been included in the diverse dendroid orders of the Graptolithina, while shale material may also have been identified as algae.

Dichograptid Evolution

The identification of proximal development types is essential for species identification and biostratigraphic interpretation of graptolite occurrences. Misidentification may cause considerable miscorrelations, as is known from the example of the pendent didymograptids in the Early to Middle Ordovician. Even though proximal developments tend to be conservative in dichograptids, characteristic changes can be observed and used for understanding the higher levels of taxonomy. Cladistic analysis of character associations supports the taxonomy and evolutionary studies.

The taxonomy and evolution of the Dichograptacea from the Early to Middle Ordovician has been one of my main interests for many years. The Early to Middle Ordovician faunas of Scandinavia (Sweden, Norway) and eastern North America (Quebec, western Newfoundland) are quite diverse and sometimes excellently preserved. Thus, a detailed phylogenetic analysis is possible and ongoing cladistic interpretations have started to yield promising results.

 
Figure 2. The proximal end development in pendent didymograptids from the Arenig in reverse view. A. Xiphograptus mantuanus (based on Riva). B. Didymograptellus bifidus. C. Didymograptus spinulosus. The prosicula is shown in black (Maletz, unpublished).

Figure 3. Relief specimens of dichograptid graptolites often show their proximal development in the reverse view. The left specimen is Isograptus victoriae lunatus from the Tøyen Shale, Oslo region, Norway. The right specimen is Expansograptus latus, taken from a latex cast of a specimen from Diabasbrottet, Hunneberg, Sweden. The proximal development in both specimens is identical, but the arrangement of proximal thecae and stipe orientations differ considerably (Maletz, unpublished).

Paleogeography

In many cases, the paleogeographic distribution of graptolites is useful for biogeographic interpretations and plate tectonic reconstructions. In general, Graptolites show a differentiation from cold water faunas (Atlantic Faunal Province) to warm water faunas (Pacific Faunal Province) in the Ordovician. Also, shallow water and deep water faunal elements can be differentiated and used to outline continental shelves. Research in Scandinavia (Atlantic Faunal Province) and eastern North America (Pacific Faunal Province) helps to understand the faunal differentiation as well as individual ecological requirements of faunal elements. The investigation also includes studies on the faunal diversities and changes through time.

Figure 4. The paleogeographic distribution of the Pacific Faunal Province element Didymograptellus bifidus in the mid Arenig of North America (Laurentia). Arrows show where lithological successions are thrusted onto the Laurentian platform during later orogenies. The Laurentian biofacies refers to the shallow water shelf faunas, while the oceanic biofacies indicates deep water graptolite faunas (Maletz, unpublished).

Biostratigraphy

Graptolites are the most important faunal elements for biostratigraphic purposes in the early Palaeozoic. They are used to define Global stratotype sections (GSSP’s) for a number of chronostratigraphic time intervals from the early Ordovician to the Devonian.

The isograptids in the mid-Arenig to mid-Darriwilian especially, show promise for both a phylogenetic analysis and a high potential for biostratigraphic purposes as their proximal structures and rhabdosome shapes changes quickly through time. Still, they are easily identified and differentiated when well preserved.

Figure 5. Isograptid biostratigraphy in the Chewtonian to Yapeenian (early Middle Ordovician) of the Cow Head Group of  western Newfoundland, one of the standard – and most complete - successions for Middle Ordovician isograptids and pseudisograptids (Maletz, unpublished).

Evolution of the Diplograptina

The early Middle Ordovician isograptids evolved into the most important Ordovician faunal elements: the Diplograptaceans, during the late Arenig time interval. These biserial graptolites show extremely complex proximal developments that can be figured out only through investigation of relief specimens and series of isolated growth stages, a work that is mainly done by SEM photography today. However, transmitted light investigation can still provide important data used to help untangle the complex interactions of the thecal tubes with the use of their fusellar structure.

Figure 6. Juvenile specimens of biserial graptolites show important characters to interpret their proximal development. A: Dicranograptus sp., Viola, Limestone. B, C, D. Archiclimacograptus sp., Table Head Group, western Newfoundland. SEM photos (Maletz, unpublished).

Figure 7. Isolated and bleached graptolites from shales and limestones showing fusellar structure. A-C. Archiclimacograptus sp., Table Head Group, flattened specimens, growth series. D. Expansograptus abditus, relief, Cow Head Group. E. Nicholsonograptus fasciculatus, flattened, no fusellar structure visible, Table Head Group. F. Isograptus victoriae lunatus, flattened, Cow Head Group. All specimens from western Newfoundland. Transmitted light photographs (Maletz, unpublished).

Silurian Monograptids

It is often difficult to identify Silurian monograptids, as their elaborate thecal apertures are flattened on shale surfaces and details are not visible any more. Thus, isolated specimens are highly sought after in order to help understand the structure, but are known from few regions. In the Llandovery of Dalarna, central Sweden, limestone lenses can be collected from the Kallholn Shale, which bares a diverse fauna of Llandovery biserial and uniserial (monograptid) graptolites.

Figure 8. Llandovery graptolites from Dalarna, Sweden. A. Normalograptus scalaris, juvenile in obverse view. B, G. thecal aperture of Streptograptus dalecarlicus, proximal (G) and distal (B) thecal apertures. C. Oktavites contortus, thecal apertures. D, F. Lituigraptus convolutus, thecal aperture and rhabdosome fragment. E. Metaclimacograptus sp., reverse view.

Retiolitid rhabdosome architecture

In the Silurian, one group of biserial graptolites, the retiolitids, looses the typical periderm seen in most graptolites. These species are preserved only as a meshwork of lists that can be shown to be formed originally on a membrane surface. Unfortunately, this membrane is rarely preserved in the fossil record. The reason for the list development is unknown. It can make identification of material extremely difficult and, as with many specimens, it is severely distorted by post-mortem preservational effects. The taxonomy and biostratigraphy of the retiolitids remains difficult, but exciting work.

Figure 9. SEM photos of Silurian biserial graptolites. A. Plectograptus mailentus. B. Spinograptus spinosus. C. Spinograptus munchi. D. Petalolithus sp. A-C are retiolitids from glacial erratic boulders, collected in northern Germany, specimens isolated by Hermann Jaeger, Berlin, Museum für Naturkunde. D is an isolated specimen from the Kallholn Shale in the Llandovery of Dalarna, Sweden (Maletz, unpublished).