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Not to be confused with Graptolitha, a genus of moths.

Graptolites Temporal range: Survive to the present via the living genus Rhabdopleura. No other genera known past the Middle Permian (Roadian).
A collection of graptolites. Clockwise from top left: Rhabdopleura compacta, an extant rhabdopleurid, Didymograptus murchisoni, an Ordovician graptoloid, Oktavites spiralis, a Silurian graptoloid, and Dictyonema sp., a Silurian dendroid.
Scientific classification
Kingdom:	Animalia
Phylum:	Hemichordata
Class:	Pterobranchia
Subclass:	Graptolithina Bronn, 1849
Subgroups
Family Rhabdopleuridae Family †Cysticamaridae Family †Wimanicrustidae Family †Cyclograptidae Family †Dithecodendridae Order †Dendroidea Order †Graptoloidea

History of research

1727–1850

In this period, relatively very little was written about graptolites.

The first known notice and description of a possible graptolite fossil was published in 1727 by Magnus von Bromell in his Lithographiæ Suecanæ, in which he described a "swinestone" rock with an imprint of a "branched moss" that he compared to dendrite crystals. According to Elles and Wood (1902), this fossil was probably the species Dictyonema flabelliforme (now Rhabdinopora flabelliformis). In the same work, von Bromell also described leaf-like impressions in a black fissile rock, which later authors such as Göran Wahlenberg interpreted as a shale containing graptolites.

The name "graptolite" is derived from Graptolithus, a term used by Carl Linnaeus introduced in the first edition of his major work Systema Naturae, published in 1735. Linnaeus used the term for 'pictures resembling a fossil', as he regarded dendrite-like markings and the like as not true fossils but inorganic markings resembling fossils. In his 1751 Skånska Resa, Linnaeus figured and described markings on a slab as a "fossil or graptolite, of a strange kind", which can today be recognised as graptolites. Linnaeus' graptolite specimens were later identified as the graptolite species Climacograptus scalaris and Monograptus triangulatus (now Normalograptus scalaris and Demirastrites triangulatus, respectively), though the current location of the material is unknown. In 1768, in the twelth edition of Systema Naturae, Linnaeus again used Graptolithus for inorganic markings, dividing it into species and adding the statement "a fossil, properly speaking, is not a graptolite". Of the species included under Graptolithus, G. sagittarius was later recognised as a fossil of a plant from the Carboniferous period, either Lepidodendron or Sigillaria, while G. scalaris was recognised as a graptolite, being the same form as that described in Skånska Resa in 1751.

Carl Linnaeus's original drawing of "Graptolithus"

[Walch, 1771: figures two species as minute cephalopods like Orthoceratites]

[Wahlenberg, 1821: recognised that G. scalaris is a true fossil, but interpreted graptolites as Orthoceratites]

[Brongniart, 1828: described two species as plants]

[Priodon proposed by Nilsson as replacement for Graptolithus, but it was preoccupied; Lomatoceras was proposed as a replacement]

[various other authors described species]

1850–1865

[Barrande, 1850: Graptolites de Bohème]

[M'Coy, 1850: recognised Graptolites for uniserial forms, Diplograpsis (now Diplograptus) for biserial forms]

[Hall, 1847, 1865]

1866–1880

[British workers]

1880–1918

[Scandinavian workers]

21st century

A phylogenetic analysis in 2012 by Jörg Maletz and other paleontologists and biologists revealed that Rhabdopleura is an extant graptolite.

Description

All graptolites are colonial organisms composed of numerous individual animals called zooids, which are all connected by a stalk-like structure called the stolon. The zooids secrete an organic housing composed of the interconnected living tubes (called thecae) of the individual zooids. Historically, this housing was called the coenecium for living pterobranchs and the rhabdosome for fossil graptolites, but in recent literature, the term tubarium, originally suggested by E. R. Lankester in 1884 for the housing constructions of the extant genus Rhabdopleura, is most widely used.

Colony shape

The morphologies of graptolite tubaria are highly variable due to a variety of factors, including genetic controls on the construction of the tubarium as well as environmental conditions. Variation due to environmental factors is strongest in benthic graptolites. Graptoloid colonies show less ecological variation and have more symmetrical colonies, which may assist in maintaining a stable position in the water column.

Zooids and stolon

Understanding of graptolite zooids is mostly limited to the zooids of the living genus Rhabdopleura.

Fossil zooids

Fossil stolons

Thecae

The thecae are the individual living tubes of the zooids, and each theca is occupied by a single zooid.

Ecology

Evolutionary history

Fossil record

The earliest known graptolites are fragments of the rhabdopleurid Sokoloviina costata found as small carbonaceous fossils from the earliest Cambrian (Fortunian) of Ukraine. Graptolites from Cambrian Series 2 (the upper Lower Cambrian) are rare, with only three species known: Malongitubus kuangshanensis and Yunotubus gemmatus from the Chinese Cambrian Stage 3 Chengjiang and Xiaoshiba biotas, respectively, and the rhabdopleurid Sphenoecium annularoides (originally Dalyia annularoides), from the Cambrian Stage 4 of Pennsylvania. Graptolite faunas become much more diverse during the Miaolingian epoch (the Middle Cambrian), though still dominated by rhabdopleurids. Sphenoecium, which contains most Cambrian species previously referred to Rhabdopleura, is a common element of Miaolingian graptolite faunas, being spread over multiple continents and containing multiple species. The earliest erect graptolites, the dithecodendrids, originated at the base of the Miaolingian. The dithecodendrids are important for understanding the evolution of later erect forms, but they are generally poorly preserved and their phylogenetic position is uncertain.

Various Middle Cambrian species of graptolites. (a-c): Archaeolafoea skeatsi. (c) is a detail of the fusellum. (d): Sphenoecium wheelerensis. (e): Tarnagraptus robustus.

The earliest dendroids can be found in the upper Miaolingian, represented by Acanthograptus, Callograptus, Dendrograptus, Desmograptus, and Dictyonema; however, dendroids only begin to dominate in the succeeding Furongian (the Late Cambrian) epoch. The Furongian has a poorly studied fossil record, causing the interval to be termed the Furongian Gap. This gap was recently filled with the discovery of the Guole biota from the middle Furongian of China, which contains a diverse graptolite fauna dominated by the Dendrograptidae, Callograptidae, and Dithecodendridae. The Guole biota also preserves the earliest fossils of true Rhabdopleura, two poorly preserved specimens resembling Rhabdopleura compacta encrusting tubes of the cnidarian Sphenothallus.

Three families of encrusting graptolites, the Cyclograptidae, Cysticamaridae, and Wimanicrustidae, are known mostly from isolated material in Ordovician glacial erratics. All three families' earliest occurences are from the Tremadocian (earliest Ordovician). Cysticamaridae is only known until the Sandbian (early Late Ordovician), but Cyclograptidae and Wimanicrustidae range to the Silurian.

The earliest graptoloid and only Cambrian graptoloid is possibly Rhabdinopora wutingshanense from the Furongian of China. However, graptoloids truly began to diversify in the Tremadocian with the Great Ordovician Plankton Revolution. Tremadocian faunas primarily consisted of multiramous anisograptids like Rhabdinopora in the early Tremadocian and Adelograptus in the mid- to late- Tremadocian. The Dichograptina and Sinograptina, two groups that first evolved in the late Tremadocian by losing bithecae, began to diversify in the earliest Floian and rapidly replaced the Anisograptidae. The Isograptidae, a simplified group with only two stipes, diverged from dichograptine ancestors at the start of the Dapingian (early Middle Ordovician).

Various Ordovician graptoloids. From left to right: the anisograptid Rhabdinopora flabelliformis, the dichograptine Didymograptus murchisoni, the isograptid Arienigraptus zhejiangensis, and the primitive axonophoran Levisograptus primus.

By the Late Ordovician, most dominant groups of Early and Middle Ordovician graptoloids start to decline: the Dichograptina and the Isograptidae go extinct in the Darriwilian (Middle Ordovician) while the Sinograptina go extinct in the Sandbian. The Glossograptidae, an unusual scandent clade derived from the Isograptidae, persist until the Katian (Late Ordovician) as minor but important elements in Late Ordovician faunas. However, the axonophorans, a group of scandent graptoloids that evolved in the middle Dapingian from ancestors close to the isograptid Pseudisograptus, rapidly diversified in the later Ordovician to become the dominant component of Late Ordovician faunas. Due to this radiation, graptoloids reach their peak in diversity during the Nemagraptus gracilis biozone at the start of the Late Ordovician. Late Ordovician graptoloid faunas exhibit high provincialism: low-paleolatitude, tropical waters have diverse faunas dominated by the Diplograptina, including varied diplograptids, climacograptids, and dicranograptids. Meanwhile, the graptoloid faunas at higher-paleolatitude waters close to the South Pole have less diverse faunas, dominated by species of Archiclimacograptus in the Sandbian that are replaced by members of the Normalograptidae in the Katian.

Various Hirnantian graptolites. Graptolites from the late Hirnantian Metabolograptus persculptus zone. (a, c): Korenograptus elegantulus. (b): Normalograptus sp. (d) Neodiplograptus lanceolatus (e): ?Metabolograptus parvulus (f): Normalograptus anjiensis (g): Avitograptus avitus. Scale bars, 1 mm

The beginning of the Hirnantian saw the onset of the Late Ordovician glaciation, which caused a global Late Ordovician extinction event that wiped out between 49% and 60% of genera and almost 85% of species. Graptoloids were particularly hard-hit by the extinction; species diversity decreased from a pre-extinction high of 61 species in the latest Katian to 28 species in the Hirnantian, while only eight genera survived past the Ordovician. The Diplograptina went extinct, being replaced by neograptines, primarily represented by a few normalograptids and neodiplograptids, invading from south polar latitudes. The Retiolitoidea became abundant in the early Hirnantian and diversified throughout the mid-late Hirnantian, giving rise to more derived neodiplograptids and the earliest retiolitids. The Monograptoidea diversified later than the Retiolitoidea, with the earliest dimorphograptids originating in the late Hirnantian and the first true monograptids originating in the earliest Silurian.

Ordovician-Silurian dendroids

Silurian-Devonian graptoloids

Devonian-Carboniferous dendroids

Later occurences of Rhabdopleura

External relationships

Ambulacraria	Echinodermata Hemichordata Enteropneusta Harrimaniidae Spengelidae Ptychoderidae Torquaratoridae Pterobranchia Cephalodiscus Graptolithina benthic forms (paraphyletic) Rhabdopleura †Graptoloidea

Relationships of Hemichordata to each other and to echinoderms. Following Treatise on Invertebrate Paleontology, Part V, Second Revision.

Interrelationships

Pterobranchia	Cephalodiscus Rhabdopleura Cysticamara Bithecocamara Wimanicrustidae Ellesicrusta Wimanicrusta Cyclograptidae Tubidendrum Reticulograptus Epigraptus Dendrotubus Cyclograptus Dithecodendridae Sotograptus Archaeolafoea Mastigograptidae Mastigograptus Dendroidea Callograptidae Acanthograptus Koremagraptus Dendrograptidae Dendrograptus "Dictyonema" Graptoloidea Rhabdinopora Anisograptus Calyxdendrum	Cysticamaridae

Phylogeny of the Graptolithina based on Mitchell et al. (2013)'s analysis of an updated version of the Durman (1992) matrix. Rhabdopleura and Cephalodiscus were used as dual outgroups.