Graptolite

Graptolites
Temporal range: Cambrian (Fortunian)–Present
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 Edit this classification
Kingdom: Animalia
Phylum: Hemichordata
Class: Pterobranchia
Subclass: Graptolithina
Bronn, 1849
Subgroups

Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian (Miaolingian, Wuliuan) through the Lower Carboniferous (Mississippian). A possible early graptolite, Chaunograptus, is known from the Middle Cambrian. Recent analyses have favored the idea that the living pterobranch Rhabdopleura represents an extant graptolite which diverged from the rest of the group in the Cambrian.

Fossil graptolites and Rhabdopleura share a colony structure of interconnected zooids housed in organic tubes (theca) which have a basic structure of stacked half-rings (fuselli). Most extinct graptolites belong to two major orders: the bush-like sessile Dendroidea and the planktonic, free-floating Graptoloidea. These orders most likely evolved from encrusting pterobranchs similar to Rhabdopleura. Due to their widespread abundance, planktonic lifestyle, and well-traced evolutionary trends, graptoloids in particular are useful index fossils for the Ordovician and Silurian periods.

The name graptolite comes from the Greek graptos meaning "written", and lithos meaning "rock", as many graptolite fossils resemble hieroglyphs written on the rock. Linnaeus originally regarded them as 'pictures resembling fossils' rather than true fossils, though later workers supposed them to be related to the hydrozoans; now they are widely recognized as hemichordates.

Etymology

Both the word "graptolite" and the name for the subclass Graptolithina are derived from the genus name Graptolithus Linnaeus, 1735, which in turn originates from the Greek words grapho "write" and lithos "stone." As a result, "graptolite" roughly translates to "writing in the rocks", which later became the title of a popular book on the fossils.

History of research

Early research

The first description of a possible graptolite fossil was published in Magnus von Bromell's 1727 work Lithographiæ Suecanæ, where he noted a "swinestone" rock with an imprint of a "branched moss" (likely a specimen of Rhabdinopora flabelliformis) and "leafy impressions" in a "black fissile rock" that probably represented graptolites in shale. However, von Bromell did not figure his specimens.

Carl Linnaeus's original drawing of Graptolithus in his 1751 work Skånska resa

The second person to describe graptolites was the "great systematist" Carl Linnaeus, who conferred the name Graptolithus to dendrites and other markings that were widely accepted to not be fossils in the first edition of his Systema Naturae. Indeed, in the twelfth edition of the Systema Naturae, Linnaeus wrote "a fossil, properly speaking, is not a graptolite." However, in his 1751 work Skånska resa, Linnaeus figured and described specimens, under the description of a "Fossil or Graptolite, of a strange kind," of what are now considered graptolites. Today, these specimens are considered to represent the graptoloids Normalograptus scalaris and Demirastrites triangulatus, both of which are common from the locality where he collected the slab. Linnaeus divided Graptolithus into many species in the twelfth edition of the Systema Naturae, including one species (Graptolithus scalaris, now Normalograptus scalaris) that was identical to the form described in the Skånska resa.

The first authors to recognize that graptolites actually represented fossils were Johann E.I. Walch, who in 1771 described two species (now Lituigraptus convolutus and Monograptus priodon) as miniature cephalopods, and Göran Wahlenberg, who in 1821 recognized that Graptolithus scalaris Linnaeus represented the fossils of an animal. While Wahlenberg agreed that most species of Graptolithus were not fossils, he chose to use the name to represent the true fossils allied to G. scalaris, and set G. scalaris as the type species of Graptolithus. Like Walch, Wahlenberg believed that Graptolithus were the fossils of tiny, unmineralized cephalopods related to Orthoceratites. Wahlenberg's work drew attention to graptolite fossils, and future paleontologists would use the names "Graptolithus" and "graptolite" exclusively to refer to graptolites in Wahlenberg's sense. Over the next several decades, numerous authors would describe species of Graptolithus and would provide interpretations of the graptolites as cephalopods, hydrozoans, corals, plants, and even foraminiferans.

The Graptolithina was formally named in 1849 by Heinrich G. Bronn as a group within the Anthozoa. Frederick M'Coy erected the family Graptolitidae in 1850 as the first family in the Graptolithina and, in the same paper, divided graptolites into two genera: Graptolithus for species with a single row of thecae (uniserial) and Diplograpsis (now Diplograptus) for species with two rows of thecae (biserial), taking the first step in graptolite taxonomy. The first members of the Dendroidea were discovered in the 1850s, with Prout (1851)'s description of Graptolithus hallianus (now Dendrograptus hallianus) and Hall (1851)'s description of the genus Dictyonema.

In an 1852 monograph, Hanns B. Geinitz created the name Monograpsus (now Monograptus) for uniserial graptolites and suggested discarding the genus Graptolithus as the word 'graptolite' was being used for all graptolites. This was supported by Charles Lapworth, who in an 1876 paper elected to use Monograptus over Graptolithus for uniserial graptolites, and by R. R. Gurley in an 1896 paper on North American graptolites, and the name Graptolithus subsequently fell into disuse. In 1954, the ICZN finally suppressed the genus Graptolithus Linnaeus, 1768 and the name Graptolithus scalaris Linnaeus, 1768.

Advances in taxonomy, morphology, and stratigraphy

The monographs of Joachim Barrande (1850) on Bohemian graptolites and James Hall (1847, 1865) on North American graptolites laid the foundation for the research of graptolites. Barrande, in his 1850 monograph Graptolites de Bohême, described the structure of graptolites in detail and argued against a cephalopod affinity for the Graptolithina, suggesting that they instead represented corals related to the Pennatulacea. Barrande also hypothesized on the mode of life of graptolites, suggesting that uniserial forms were planktonic while biserial forms had their proximal end buried in the seabed. Hall's 1847 monograph Graptolites of the Inferior Strata of the New York System mainly described new species, but his 1865 monograph The Graptolites of the Quebec Group revised the classification of the Graptolithina and described the structure of graptolites in further detail. Hall (1865) suggested that both biserial and uniserial graptolites were planktonic, while dendroid graptolites were rooted onto the seabed.

Charles Lapworth ushered in the study of graptolites for biostratigraphic purposes with his 1878 publication The Moffat Series, in which he established a series of Early Silurian biozones based on graptolites and used them to successfully interpret the geological structure of the Southern Uplands. Lapworth further recognized that graptolites were "unsurpassed" worldwide for early Palaeozoic biostratigraphy, which was confirmed by the North American graptolite biozones established by Rudolf Ruedemann's 1904 paper on the graptolites of New York. Graptolite faunas and biostratigraphy were described in detail in Britain by Gertrude L. Elles and Ethel M.R. Wood's monograph (1904-18) on British graptoloids and Oliver M.B. Bulman's monograph (1927-67) on British dendroids, and in North America by Rudolf Ruedemann's 1947 monograph The Graptolites of North America.

Reconstruction of "Diplograptus pristis" (now Orthograptus quadrimucronatus) in Ruedemann (1895).

Rudolf Ruedemann, in his 1895 paper Development and the mode of growth of Diplograptus M’Coy, reconstructed synrhabdosomes (aggregations of radially arranged colonies) of "Diplograptus pristis" (now Orthograptus quadrimucronatus) as being joined by a central disc and possessing a gas-filled float called a pneumatophore and four sac-like structures called gonangia. This reconstruction has become widespread and has been copied by papers, textbooks, museum displays, and on the Internet. However, it is unsupported, as the central disc, pneumatophore, and gonangia are not present in the specimens from which he based his reconstruction and have not been found in any other fossils. Ruedemann initially used his interpretation of the pneumatophore and gonangia to argue for a hydrozoan affinity for the Graptolithina, but later decided that graptolites were primitive bryozoans. His opinion was based on the bilateral symmetry (in contrast to the radial symmetry of hydroid thecae) and mode of budding of the thecae, as well as the presence of circular marks interpreted as muscle scars. Similar marks were interpreted as muscle scars in the Monograptidae, but instead represent the thickened bases of the interthecal septum.

Isolated specimens and affinities

The first studies of chemically isolated graptolites were performed by Karl Wilhelm Gümbel, who in 1878 described isolated specimens of Monograptus priodon. However, Gerhard Holm's 1895 paper on isolated specimens of Didymograptus, Tetragraptus, and Phyllograptus was far more influential and is regarded as a "milestone in graptolite research", as it provided important information on the structure of graptolites. Carl Wiman's 1895 thesis Ueber die Graptolithen provided similarly important results through the study of isolated fossils and serial sections of both graptoloid and dendroid graptolites.

In 1931, a diverse Tremadocian fauna of graptolites was discovered by the Polish paleontologist Roman S.J. Kozłowski, who was able to isolate the specimens from the surrounding matrix using hydrofluoric acid. Kozłowski presented a preliminary report on his observations in 1938 but, due to the onset of World War II, was only able to publish his full monograph in 1949. Kozłowski's fauna included a diverse array of benthic graptolites, including both the erect Dendroidea and three new orders of encrusting graptolites: the Camaroidea (now Cysticamaridae), the Tuboidea (now Cyclograptidae), and the Stolonoidea (now thought to be stolons of Rhabdopleuridae). In addition, the exceptional preservation of the material allowed Kozłowski to discover the stolon system and the fusellar construction of the tubarium, and based on these new observations, Kozłowski determined that the Graptolithina were hemichordates closely related to pterobranchs.

One difference that Kozłowski noted between extant pterobranchs and graptolites was the presence of a layered cortex on the exterior of the tubarium. Kozłowski suggested that the cortex was secreted by an epithelial layer on the outside of the theca (the extrathecal tissue model), which was supported by workers such as Nancy H. Kirk as well as by Oliver Bulman (though with reservations) in his 1970 edition of the Treatise on Invertebrate Paleontology. However, the independent discovery of cortical bandages by Andres (1977) and Crowther & Rickards (1977) proved that cortex was not a simple layered structure but was instead 'plastered' onto the surface of the theca by a pterobranch-like zooid, thus making the extrathecal tissue model unnecessary.

Peter N. Dilly's 1993 description of Cephalodiscus graptolitoides as a living graptolite sparked excitement, with some authors accepting C. graptolitoides as a living graptolite and others preferring to keep it within the Pterobranchia. The prevailing view at the time was that Pterobranchia was a monophyletic taxon and the sister taxon to the Graptolithina. However, a cladistic analysis published in 2012 by Charles E. Mitchell and others showed that Pterobranchia included the graptolites and that Rhabdopleura, rather than Cephalodiscus, was a living graptolite.

Description

Key parts of the graptolite tubarium. (1) Isograptus mobergi showing two reclined stipes and overlapping thecae connected by the common canal. (2) Rhabdopleura normani showing zooids, thecal tubes, and the stolon. (3) Heisograptus macropoma showing a theca constructed out of fuselli.

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. Graptolite colonies develop from the sicula, which is the housing for the first zooid of the colony and the only zooid produced through sexual reproduction. Parts of the tubarium, stipes, or thecae are frequently described as proximal or distal: proximal refers to parts that are formed early and are closer to the prosicula (the initial portion of the sicula and the first secreted structure of the tubarium), while distal refers to parts that are formed late and are farther from the prosicula.

Colony shape

The graptolite tubarium is composed of branches called stipes, which in turn are composed of interconnected thecal tubes budding from either a stem or a common canal. However, graptolite colonies can form diverse sheet-like forms in encrusting colonies and bushy, fan-like, or conical forms in erect benthic and planktonic taxa. Encrusting graptolites form two-dimensional colonies over surfaces and can exhibit high levels of variation, depending on the shape of the substrate. Encrusting graptolite colonies may be thigmophyllic or 'touch-loving' (as in extant Rhabdopleura compacta and the Cysticamaridae), where the tubaria form dense masses of thecal tubes in close contact with each other, or may form runner-type colonies (as in extant Rhabdopleura normani and the Wimanicrustidae), where the tubaria loosely cover large areas and the individual thecal tubes are widely spaced and thus never touch.

Erect benthic taxa are multiramous (frequently branching) and can form three-dimensional, bushy colonies as in Dendrograptus or sheet-like colonies that can be curled to form a fan-like or conical shape. In the benthic Dendrograptidae and the planktonic graptoloids, the stipes are composed of a single row of serially arranged thecae with all their apertures oriented in one direction (the ventral side of the stipe), while in the Callograptidae the stipes can be more complicated with thecae oriented in multiple directions. Conical taxa typically had their stipes oriented so the thecal apertures were oriented towards the middle of the tubarium, while fan-like taxa had stipes oriented in the same direction and bushy taxa had variably oriented stipes.

Diagram illustrating the orientation of graptolite stipes to the nema and the terminology.

The earliest planktic taxa were conical or disk-shaped and multiramous, with their morphologies being modifications of the original conical shape of Rhabdinopora. Later forms became pauciramous (with 4 or fewer stipes) and took on a wide variety of shapes based on the inclination of the stipes with respect to the sicula. Scandent taxa, where the stipes grew straight up to enclose the sicula, can be uniserial (one-stiped) like the Monograptidae, biserial (two-stiped) like the other Axonophora and the Glossograptidae, or quadriserial (four-stiped) like some members of the Phyllograptidae.

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

Diagram of a graptolite zooid based on the zooids of extant Rhabdopleura.

Understanding of graptolite zooids is mostly limited to the zooids of the living genus Rhabdopleura. Like in other hemichordates, the zooids of Rhabdopleura are divided into three portions: an anterior proboscis (also called the cephalic shield or the protosome), a collar (mesosome) bearing a single pair of arms, and a trunk (metasome). The cephalic shield is a flattened secretory organ that is used to construct the tubarium. The cephalic shield also contains a single coelomic cavity in the cephalic shield (the protocoel) that houses the stomochord and an anterior excretory complex (also called the heart-glomerulus complex) that is composed of the heart and a kidney-like bundle of blood vessels called a glomerulus. The mouth is immediately behind the cephalic shield and opens into the pharynx.

The collar of Rhabdopleura contains a pair of coelomic cavities (the mesocoels) that completely surround the pharynx. The mesocoels extend into a single set of paired arms (as opposed to the multiple pairs of arms in Cephalodiscus), with each arm bearing two rows of tentacles (which, in turn, possess three rows of hair-like cilia) formed from extensions of the arm coelom. The mesocoels also extend into a pair of ventral projections called oral lamellae, which bear a row of cilia that transports food from the arms to the mouth. The trunk of Rhabdopleura also contains paired coelomic cavities (the metacoels) surrounding the U-shaped gut and the single asymmetrically disposed gonad. Unlike all other hemichordates, Rhabdopleura completely lacks gill pores, but does possess a single pair of grooves that remain as traces of the lost gill pores. The muscular zooidal stalk originates from the posterior end of the trunk and connects the zooid to the stolon.

Traces of graptolite zooids are extremely rare in the fossil record, and usually do not show any details of anatomy. Pyrite blobs in the early Ordovician graptoloid Psigraptus and in early Silurian graptolites may represent the remains of decayed zooids, while remains of the zooidal stalks of the monograptids Rastrites geinitzii and Neolagarograptus sp. have been reported from the Silurian of Latvia. The best-preserved remains of zooids come from a specimen of the Cambrian rhabdopleurid Sphenoecium obuti, which preserves one arm and the cephalic shield of the zooid.

Stolon system

Stolon system in (1) Rhabdopleura normani, showing monopodial growth, and (2) Adelograptus tenellus, showing sympodial growth

The zooids of graptolites are connected by the stolon system, which is composed of the stolon proper and the zooidal stalk. The stolon itself is a connecting stalk that is divided into the pectocaulus or black stolon, which is coated in a black, sclerotized stolon sheath, and the gymnocaulus or naked stolon, which lacks a stolon sheath. In Rhabdopleura, the gymnocaulus is the stalk of the permanent terminal zooid and buds into new zooids before secreting the stolon sheath and becoming the pectocaulus, while in fossil taxa the gymnocaulus is unpreserved but likely also develops into the pectocaulus. The sclerotized pectocaulus is present in the erect benthic Dendroidea and most encrusting taxa, while in the planktonic graptoloids the entire stolon is unsclerotized and is considered the gymnocaulus. In taxa with a pectocaulus, the stolon system is rigid and often easily preservable in the fossil record, but in the graptoloids the stolon is not preserved.

Diaphragm complex of Rhabdopleura compacta

In Rhabdopleura and more derived graptolites such as the Wimanicrustidae, Cyclograptidae, and the Dendroidea, the pectocaulus of each zooid ends in a structure called a diaphragm complex, which may aid in the development of new zooids and serve as a protective case for developing zooids and for contracted adult zooids. The diaphragm complex is composed of the stolon diaphragm, a ring-like dividing wall on the inside of the stolon sheath, and an expanded, conical portion of the stolon sheath that forms the base of the developing theca. The stolon diaphragm has a central opening that the zooidal stalk passes through to connect to the stolon.

Graptolite colonies grow through the creation of new zooids, which occurs via asexual budding from the stolon. Stipes are lengthened along a growing axis, which may take the form of a stem or of a thecate stipe. The most primitive mode of growth for graptolites is monopodial growth, which is found in the living genus Rhabdopleura and is also known from the dithecodendrid Tarnagraptus. Monopodial growth is characterized by the presence of a permanent terminal zooid, which has a distinct bifid cephalic shield in Rhabdopleura, and by new zooids budding from the side of the growing axis and behind the permanent terminal zooid. In graptolites with monopodial growth, the permanent terminal zooid creates a stem, which can be encrusting (as in Rhabdopleura) or erect (as in Tarnagraptus), and the openings for new thecae are formed by resorption of part of the stem. In Rhabdopleura, the stems are divided by walls called transverse septa, which create compartments for the growth of new zooids.

Sympodial growth, which is characterized by new zooids (the daughter zooids) successively budding from the previous zooid (the mother zooid) at the tip of the growing axis, first evolved in the Furongian and is present in the erect benthic Dendroidea and their descendants, the planktonic Graptoloidea. In taxa with sympodial growth, there is no permanent terminal zooid; rather, each zooid acts as a temporary terminal zooid before creating additional zooids. Taxa with sympodial growth have thecae connected by a continuous cavity called a common canal which runs through the immature portions of all of the thecae in the colony and provides evidence of a truly colonial lifestyle.

Thecae

The thecae (sing. theca) are the individual living tubes of the zooids, and each theca is constructed and occupied by a single zooid. The initial parts of each theca are connected to the previous theca, forming branches called stipes. The individual thecae of each stipe are repetitive, clonal units that generally are either identical or grade between different thecal morphologies. The openings of the thecae are called thecal apertures and can be simple and round or can be modified, commonly with a tongue-like process on the dorsal or ventral sides of the thecal aperture called a rutellum.

Each zooid constructs its theca out of slightly overlapping, spindle-shaped growth increments called fuselli (sing. fusellus) that are secreted by the cephalic shield. The fuselli can be full rings or half rings, and the boundary between the wedge-like ends of fuselli forms an angled oblique suture. Fusellar full rings have a single oblique suture marking the start and end of secretion of the fusellum and can be irregularly oriented. Meanwhile, fusellar half rings are secreted in pairs to form one full ring, creating a dorsal and ventral suture on opposite sides of the theca. In taxa with fusellar half rings, the dorsal and ventral sutures are regularly placed to form dorsal and ventral zigzag sutures.

The thecae in encrusting graptolites like Rhabdopleura are composed of an creeping initial portion constructed out of fusellar half rings with a dorsal zigzag suture and an erect portion constructed out of fusellar full rings. The creeping tubes usually have a flattened and featureless ventral wall that contacts the substrate, while the erect tubes are isolated and typically have a circular cross section. In Rhabdopleura and some other rhabdopleurids, the erect tubes possess distinct protruding fusellar collars at the border between fuselli. Encrusting taxa often have thecae that branch irregularly from the creeping main stipe.

Morphology of simple (1) and hooked (2), serially arranged thecae showing protheca (in orange) and metatheca (in yellow)

In the Dendroidea and Graptoloidea, the stipes are fully erect branches composed of serially arranged thecae, with each theca budding from the previous one. The thecal apertures are all oriented towards the ventral side of the stipe, while the side of the stipe opposite to the thecal apertures is termed the dorsal side. The thecae in a stipe often overlap, forming a shared dorsal wall called an interthecal septum which can be constructed by either the zooid of the first theca or by both zooids.

The thecae of dendroids and graptoloids can be divided into two portions: the initial portion of the theca, called the protheca and the distal portion of the theca, the metatheca. The protheca is the portion of the theca that is constructed before the theca is fully differentiated and forms the wall of the common canal. Meanwhile, the metatheca is constructed after the theca is fully differentiated. In species with an interthecal septum, the start of the metatheca is marked by the beginning of the interthecal septum; however, in species without an interthecal septum, the start of the metatheca is instead marked by the point where the opening for the daughter theca is separated from the opening for the common canal.

Thecal types

Most graptolites have two main types of thecae: the autothecae, which are larger and are present in all graptolites, and the bithecae, which are present in most benthic graptolites (though notRhabdopleura) but are absent in most of the planktonic graptoloids. Several theories were proposed to explain the purpose of bithecae, including that bithecae represented the thecal tubes of diminutive male zooids (and that their loss in Graptoloidea was associated with the evolution of hermaphroditic zooids) or, alternatively, that they represented the thecal tubes of specialized cleaning individuals. However, the purpose of bithecae is still unknown. The stolotheca was originally coined to represent the thecal tube surrounding the black stolon and was thought to give rise to both autothecae and bithecae. However, in Dendroidea and Graptoloidea, the stolotheca does not have a distinct zooid but instead represents the immature portion of the autotheca.

Morphology of graptolite stipes showing a bithecate stipe (1), a non-bithecate stipe with plaited thecal overlap (2), and a non-bithecate stipe with dorsal thecal origins (3). Bithecae in dark orange, autothecae in alternating light orange and white.

Both new autothecae and bithecae are created through budding from the stolon. In diad budding, which is present in most encrusting graptolites (like Rhabdopleura) and is the most primitive form of budding in graptolites, each budding event causes the stolon to bifurcate (split into two branches), creating two thecae at each division of the stolon. Diad budding is often irregular (for example, in Reticulograptus, one diad budding event can create any pair of autotheca, bitheca, and stolotheca other than two autothecae). However, the encrusting Wimanicrustidae and the erect branching Dendroidea evolved triad budding, where each budding event causes the stolon to trifurcate (split into three branches), creating three thecae at each division of the stolon. Triad budding follows a highly regular pattern called the Wiman rule, where each budding event creates one new autotheca (a 'stolotheca') on one side of the previous autotheca and a bitheca on the other side, and the origin of bithecae alternates between the left and right side of the stipe. The lateral origin of thecae in taxa with triad budding creates plaited thecal overlap, where alternating autothecae overlap each other laterally. Most graptoloids both lack bithecae and have autothecae originating from the dorsal edge of the stipe without plaited thecal overlap; however, some early graptoloids lack bithecae while still having lateral origins of thecae and plaited thecal overlap.

Several genera in the Cyclograptidae possess unique types of thecae that likely represent modified autothecae. Conothecae, which are known from Reticulograptus, Discograptus, and Epigraptus, are conical thecae that lack fusellar structures. Conothecae are modified autothecae and may ensure that autothecae are regularly spaced despite the irregular branching of the stolon. Microthecae, which are solely known from Tubidendrum, are modified autothecae with a narrow apertural "neck" and thecal apertures facing in the opposite direction from those of normal autothecae. Umbellate thecae, only known from Galeograptus, are modified autothecae close to the encrusting portion of the tubarium with an umbrella-shaped autothecal apparatus. The "umbrellas" of these thecae fuse with similar structures from adjacent stipes and stipes on the opposite side of the tubarium, completely covering the space between stipes.

Branching

Structure of the dicalycal theca in forms with triad budding and bithecae (1, 2) and without bithecae (3).

In Rhabdopleura and other graptolites with monopodial growth, branching occurs through the creation of a new permanent terminal zooid, which creates a new growing axis. However, in graptolites with sympodial growth, branching occurs through the creation of a dicalycal theca, which creates two new autothecae rather than one, in a process called dichotomous branching. In taxa with triad budding and bithecae, dichotomous branching occurs when the bitheca is replaced at the dicalycal theca with a second new autotheca. The process is similar in taxa which have lost bithecae, but the daughter thecae are offset rather than paired. In dichotomous branching, the two daughter stipes typically diverge at an identical angle from the mother stipe, though in some genera (e.g. Schizograptus and Trichograptus) one daughter stipe follows the direction of the mother stipe. Each dicalycal theca can only produce one additional dicalycal theca at most, and in most derived graptoloids a dicalycal theca only gives rise to normal (monocalycal) thecae which only produce one new autotheca.

Some graptoloids add branches through the production of cladia, which originate from the thecal apertures of the mother theca rather than from the dorsal side of the stipe. Cladia are secondarily produced by mature zooids away from the growing end of the main stipe, rather than at the tip of the stipe as in dichotomous branching. Cladia are most well-known in monograptids like Cyrtograptus, but are also known from some pterograptids (e.g. Pterograptus) and dicranograptids (e.g. Nemagraptus).

Sicula

All graptolite colonies originate from a sicula (pl. siculae), which is the housing secreted by the founding zooid of the colony. The siculae of all graptolites are composed of two portions: the prosicula, which is the initial (proximal) portion of the sicula and represents the earliest growth stage of graptolites that can be preserved in the fossil record, and the metasicula, which is the distal portion (the latest-formed portion that is farthest from the point of origin) of the sicula and is constructed out of fusellar increments.

Siculae of the Graptolithina, showing prosicular dome (dark orange), prosicula with helical line (light orange), metasicula (white), and the first theca (3, 5-6, white). Siculae of the encrusting forms Rhabdopleura spp. (1, 3), Epigraptus bidens (2) and Kozlowskitubus erraticus (4), of the erect benthic Dendrograptus communis (5), and of the planktonic Graptoloidea (6).

The prosicula in the extant genus Rhabdopleura, as well as other encrusting genera like Epigraptus, takes the form of a dome with a flattened dorsal wall that attaches the colony to the substrate. The wall of the dome is featureless and is secreted by glands in the ventral surface of the developing larva. The opening to the metasicula in Rhabdopleura and Epigraptus is a hole formed in the wall of the dome by the developing zooid, creating a sharp angle at the transition between the prosicula and metasicula.

In contrast, the prosicula of the erect benthic Dendroidea and the planktonic Graptoloidea lacks a dome and is tubular (in the Dendroidea) or conical (in the Graptoloidea) throughout. The prosicula in these taxa is secreted out of a single, continuous band of fusellar material, creating a spiraling suture called a helical line that extends along the entire length of the prosicula. The opening for the metasicula in these taxa is a primary opening, meaning that it was originally present rather than being a hole opened in a closed wall, and as a result, there is a smooth transition between the prosicula and metasicula. The prosicula of the cyclograptid Kozlowskitubus is unique, as it has an initial portion like the dome of rhabdopleurids but also has both a primary opening for the metasicula and an erect, tubular distal portion with a helical line like dendroids and graptoloids.

The metasicula is a distally open tube that is composed of fusellar increments and can typically be easily distinguished from the prosicula. The fuselli of the metasicula are usually regular half rings and have dorsal and ventral zigzag sutures. However, the metasicular fuselli in the cyclograptid Kozlowskitubus are irregular throughout and lack zigzag sutures, while the metasicular fuselli in some dendroids and graptoloids are irregular without zigzag sutures only in the proximal (earliest-formed) portion of the metasicula and are regular half rings with zigzag sutures in the distal portion of the metasicula. The ventral apertural margin of the metasicula (and less commonly the dorsal margin) is frequently adorned with a tongue-like rutellum, which can be transformed into a spine called the virgella in some graptoloids.

The siculae of benthic graptolites are termed discophorous as they are attached to the substrate by a basal disc formed from cortical material (material that is laid down on the tubarium wall after the fuselli are secreted). In contrast, the siculae of the planktonic graptoloids are nematophorous, as they are conical and have a thread-like structure called the nema that extends from the apex of the prosicula.

Proximal development

The sicula and the first few thecae of a graptolite colony form the proximal end of the colony, and the development of the zooids and thecae (the astogeny) in the proximal end is termed proximal development. The founding zooid of a graptolite colony is the sicular zooid, which is the only zooid in the colony to be produced through sexual reproduction. The zooidal stalk of the sicular zooid forms the prostolon, which is a stalk that attaches to the proximal portion of the sicula and gives rise to the stolon system. The first thecal zooid buds from the prostolon and emerges through a hole in the sicula. In most graptolites, this hole is a resorption foramen pierced in the wall of the sicula; however, the hole is a primary foramen (meaning that it was created during the growth of the sicula rather than being a hole opened in the wall of the sicula) in derived members of the Monograptidae. The colony then grows through budding from the stolon.

The shape of the tubarium is determined through branching, which occurs due to the creation of either a permanent terminal zooid (as in Rhabdopleura) or a dicalycal theca (as in Dendroidea and Graptoloidea). In dendroids and graptoloids, initial branching occurs in highly regular sequences called proximal development patterns, which are often diagnostic for genera or species. In the benthic dendroid "Dictyonema" cavernosum (and presumably most dendroids), the first dicalycal theca is the third-formed autotheca or a later theca. However, the earliest graptoloids evolved isograptid-type development, where the earliest dicalycal theca is the second-formed autotheca (also called theca 12 or th12). A few early graptoloids (most predominantly species of the genus Didymograptus) possess artus-type development, where the dicalycal theca is the first-formed autotheca (th11), and the axonophorans evolved several proximal development types all characterized by a delayed dicalycal theca and complex proximal end structure.

Wall structure

Ultrastructure of the tubarium wall. (1) Three stacked fuselli, showing sheet fabric (sf) and fusellar fabric (ff). (2) Tubarium wall structure, showing fuselli (ff), ectocortex (ect) in the form of cortical bandages, and endocortex (en). Black dots and lines represent fibrils.

The wall of the graptolite tubarium is composed of a fusellar layer (also called fusellum), which is constructed out of stacked growth increments called fuselli (sing. fusellus), and cortical layers (also called cortex) that are laid on top of the fusellum. The wall material was historically called periderm, but this term is inappropriate as it suggests that the tubarium was secreted by an epidermis (the outermost layer of cells in the skin) like the exoskeletons of bryozoans when it was in fact constructed out of secretions from glands in the cephalic shield.

Each fusellus is arch-shaped in cross section and is composed of a central core of fusellar fabric, which is a three-dimensional meshwork of loosely packed and randomly oriented fibrils, that is bound by one layer of sheet fabric below the fusellar fabric and another layer of sheet fabric on top of the fusellar fabric. Both layers of sheet fabric contain fibrils that are parallel to each other and to the top edge of the fusellus. All three layers of the fusellus are secreted in one step.

Cortex is composed of secondary deposits, which are secretions that are laid on top of the thecal wall after the fusellum is constructed. Cortical deposits can be present both on the outer surface of the tubarium, in which case they are called ectocortex, and on the inner surface of the thecae, in which case they are called endocortex. These deposits are often secreted in the form of cortical bandages, which are elongate, parallel-sided structures that are either evenly distributed or, in some taxa, are concentrated to form rods of cortical tissue called lists. Each cortical bandage is composed of a body of cortical fabric, which is composed of tightly-packed fibrils oriented in parallel, that is bound by lower and upper layers of sheet fabric identical to the sheet fabric of the fusellum. While sheet fabric is present in most cortical bandages on the outside of the tubarium, those in the endocortex often lack sheet fabric. Cortical bandages are typically oriented parallel to the thecal axis and often radiate outwards from thecal apertures.

There are three main types of cortex: eucortex, paracortex, and pseudocortex, that are defined by the frequency and orientation of fibrils in the intersheet material between the layers of sheet fabric:

  • Eucortex is formed by cortical bandages and is composed of layers of sheet fabric and intersheet material of cortical fabric, which contains straight, parallel fibrils.
  • Paracortex is formed by the overlapping "limbs" of succeeding fuselli and is composed of layers of sheet fabric and intersheet material without cortical fabric, instead containing a meshwork of fibrils.
  • Pseudocortex is composed of layers of sheet fabric with scarce intersheet material lacking fibrils. The way it is formed is not fully understood, but some instances are likely formed in a similar manner to paracortex.

Some gerontic (aged) specimens of graptolites display cortical overgrowth, which occurs when the cortex is thickened as a result of excessive secretion of cortical bandages late in the development of the tubarium. Cortical overgrowth may seal the thecal apertures of graptolites and may obscure important details, which can make identifying specimens more difficult. Cortical overgrowth may have helped reinforce aged colonies and, in planktonic forms, to modify or maintain the center of gravity.

Ecology

Graptolites were a major component of the early Paleozoic ecosystems, especially for the zooplankton because the most abundant and diverse species were planktonic. Graptolites were most likely suspension feeders and strained the water for food such as plankton.

Inferring by analogy with modern pterobranchs, they were able to migrate vertically through the water column for feeding efficiency and to avoid predators. With ecological models and studies of the facies, it was observed that, at least for Ordovician species, some groups of species are largely confined to the epipelagic and mesopelagic zone, from inshore to open ocean. Living rhabdopleura have been found in deep waters in several regions of Europe and America but the distribution might be biased by sampling efforts; colonies are usually found as epibionts of shells.

Their locomotion was relative to the water mass in which they lived but the exact mechanisms (such as turbulence, buoyancy, active swimming, and so forth) are not clear yet. One proposal, put forward by Melchin and DeMont (1995), suggested that graptolite movement was analogous to modern free-swimming animals with heavy housing structures. In particular, they compared graptolites to "sea butterflies" (Thecostomata), small swimming pteropod snails. Under this suggestion, graptolites moved through rowing or swimming via an undulatory movement of paired muscular appendages developed from the cephalic shield or feeding tentacles. In some species, the thecal aperture was probably so restricted that the appendages hypothesis is not feasible. On the other hand, buoyancy is not supported by any extra thecal tissue or gas build-up control mechanism, and active swimming requires a lot of energetic waste, which would rather be used for the tubarium construction.

There are still many questions regarding graptolite locomotion but all these mechanisms are possible alternatives depending on the species and its habitat. For benthic species, that lived attached to the sediment or any other organism, this was not a problem; the zooids were able to move but restricted within the tubarium. Although this zooid movement is possible in both planktic and benthic species, it is limited by the stolon but is particularly useful for feeding. Using their arms and tentacles, which are close to the mouth, they filter the water to catch any particles of food.

Life cycle

The study of the developmental biology of Graptholitina has been possible by the discovery of the species R. compacta and R. normani in shallow waters; it is assumed that graptolite fossils had a similar development as their extant representatives. The life cycle comprises two events, the ontogeny and the astogeny, where the main difference is whether the development is happening in the individual organism or in the modular growth of the colony.

The life cycle begins with a planktonic planula-like larva produced by sexual reproduction, which later becomes the sicular zooid who starts a colony. In Rhabdopleura, the colonies bear male and female zooids but fertilized eggs are incubated in the female tubarium, and stay there until they become larvae able to swim (after 4–7 days) to settle away to start a new colony. Each larva surrounds itself in a protective cocoon where the metamorphosis to the zooid takes place (7–10 days) and attaches with the posterior part of the body, where the stalk will eventually develop.

The development is indirect and lecithotrophic, and the larvae are ciliated and pigmented, with a deep depression on the ventral side. Astogeny happens when the colony grows through asexual reproduction from the tip of a permanent terminal zooid, behind which the new zooids are budded from the stalk, a type of budding called monopodial. It is possible that in graptolite fossils the terminal zooid was not permanent because the new zooids formed from the tip of latest one, in other words, sympodial budding. These new organisms break a hole in the tubarium wall and start secreting their own tube.

Graptolites in evolutionary development

In recent years, living graptolites have been used as a hemichordate model for Evo-Devo studies, as have their sister group, the acorn worms. For example, graptolites are used to study asymmetry in hemichordates, especially because their gonads tend to be located randomly on one side. In Rhabdopleura normani, the testicle is located asymmetrically, and possibly other structures such as the oral lamella and the gonopore. The significance of these discoveries is to understand the early vertebrate left-right asymmetry due to chordates being a sister group of hemichordates, and therefore, the asymmetry might be a feature that developed early in deuterostomes. Since the location of the structures is not strictly established, also in some enteropneusts, it is likely that asymmetrical states in hemichordates are not under a strong developmental or evolutionary constraint. The origin of this asymmetry, at least for the gonads, is possibly influenced by the direction of the basal coiling in the tubarium, by some intrinsic biological mechanisms in pterobranchs, or solely by environmental factors.

Hedgehog (hh), a highly conserved gene implicated in neural developmental patterning, was analyzed in Hemichordates, taking Rhabdopleura as a pterobranch representative. It was found that hedgehog gene in pterobranchs is expressed in a different pattern compared to other hemichordates as the enteropneust Saccoglossus kowalevskii. An important conserved glycine–cysteine–phenylalanine (GCF) motif at the site of autocatalytic cleavage in hh genes, is altered in R. compacta by an insertion of the amino acid threonine (T) in the N-terminal, and in S. kowalesvskii there is a replacement of serine (S) for glycine (G). This mutation decreases the efficiency of the autoproteolytic cleavage and therefore, the signalling function of the protein. It is not clear how this unique mechanism occurred in evolution and the effects it has in the group, but, if it has persisted over millions of years, it implies a functional and genetic advantage.

Geological relevance

Preservation

Pendeograptus fruticosus from the Bendigonian Australian Stage (Lower Ordovician; 477–474 mya) near Bendigo, Victoria, Australia. There are two overlapping, three-stiped rhabdosomes.

Graptolites are common fossils and have a worldwide distribution. They are most commonly found in shales and mudrocks where sea-bed fossils are rare, this type of rock having formed from sediment deposited in relatively deep water that had poor bottom circulation, was deficient in oxygen, and had no scavengers. The dead planktic graptolites, having sunk to the sea floor, would eventually become entombed in the sediment and were thus well preserved.

These colonial animals are also found in limestones and cherts, but generally these rocks were deposited in conditions which were more favorable for bottom-dwelling life, including scavengers, and undoubtedly most graptolite remains deposited here were generally eaten by other animals.

Cyrtograptus canadensis from the middle Silurian of the Cape Phillips Formation, Nunavut.

Fossils are often found flattened along the bedding plane of the rocks in which they occur, though may be found in three dimensions when they are infilled by iron pyrite or some other minerals. They vary in shape, but are most commonly dendritic or branching (such as Dictyonema), sawblade-like, or "tuning fork"-shaped (such as Didymograptus murchisoni). Their remains may be mistaken for fossil plants by the casual observer, as it has been the case for the first graptolite descriptions.

Graptolites are normally preserved as a black carbon film on the rock's surface or as light grey clay films in tectonically distorted rocks. The fossil can also appear stretched or distorted. This is due to the strata that the graptolite is within, being folded and compacted. They may be sometimes difficult to see, but by slanting the specimen to the light they reveal themselves as a shiny marking. Pyritized graptolite fossils are also found.

A well-known locality for graptolite fossils in Britain is Abereiddy Bay, Dyfed, Wales, where they occur in rocks from the Ordovician Period. Sites in the Southern Uplands of Scotland, the Lake District and Welsh Borders also yield rich and well-preserved graptolite faunas. A famous graptolite location in Scotland is Dob's Linn with species from the boundary Ordovician-Silurian. Since the group had a wide distribution, fossils are also abundant in several parts of the United States, Canada, Australia, Germany and China, among others.

Evolutionary history

Fossil record

Temporal ranges of graptolite taxa following the Treatise on Invertebrate Paleontology:

GraptoloideaDendroideaDithecodendridaeCyclograptidaeWimanicrustidaeCysticamaridaeRhabdopleuridaeCambrianSpecial'colon'MyLanguage/OrdovicianSpecial'colon'MyLanguage/SilurianSpecial'colon'MyLanguage/DevonianSpecial'colon'MyLanguage/CarboniferousSpecial'colon'MyLanguage/PermianSpecial'colon'MyLanguage/TriassicSpecial'colon'MyLanguage/JurassicSpecial'colon'MyLanguage/CretaceousSpecial'colon'MyLanguage/PaleogeneSpecial'colon'MyLanguage/NeogeneSpecial'colon'MyLanguage/QuaternarySpecial'colon'MyLanguage/PaleozoicSpecial'colon'MyLanguage/MesozoicSpecial'colon'MyLanguage/CenozoicSpecial'colon'MyLanguage/Phanerozoic

Early evolution

The earliest known graptolites are fragments of the rhabdopleurid Sokoloviina costata found as small carbonaceous fossils from the earliest Cambrian (Fortunian) of Ukraine. Three species of graptolite are known from the Cambrian Series 2 (the upper Lower Cambrian): 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 growing on tubes of the cnidarian Sphenothallus.

The Dendroidea resided primarily in stable, inshore environments and changed relatively little over their evolutionary history, with most genera from the Cambrian persisting through the succeeding Ordovician and Silurian periods. The Dendroidea can be differentiated into two families: the Dendrograptidae, characterized by widening thecae with a ventral apertural lip, and the Callograptidae, characterized by tube-like, overlapping thecae with isolated apertures. In both families, dendroid evolution follows a trend from bushy, irregular colonies like Dendrograptus and Acanthograptus, through more ordered fan-shaped forms with parallel stipes assigned to Callograptus, to fan-shaped or conical forms with dissepiments, thecal bridges, or anastomosis assigned to Dictyonema and Desmograptus. While most dendroids were benthic, sessile organisms, several Ordovician dendrograptids of the genus Calyxdendrum evolved a planktonic lifestyle independently from Rhabdinopora and the graptoloids.

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

Planktonic forms

Various Ordovician (top row) and Silurian (bottom row) graptoloids. Top, from left to right: the dichograptid Dichograptus octobrachiatus and the primitive axonophoran Levisograptus primus. Bottom, from left to right: the retiolitid Paraplectograptus hermanni and the monograptid Rastrites guizhouensis.

The late Cambrian and early Ordovician saw a radiation in phytoplankton, which coincided with the diversification of the planktonic Graptoloidea in the Early and Middle Ordovician. The earliest graptoloids were members of the conical, multiramous genus Rhabdinopora, which evolved from benthic dendrograptids in the late Cambrian. The Ordovician graptolite radiation occurred in three successive faunas: the anisograptid fauna, comprising multiramous, bithecate members of the Anisograptidae, that dominated the Tremadocian; the dichograptid fauna, comprising multiramous or pauciramous members of the Dichograptina, Sinograptina, and Glossograptina, that originated in the middle Tremadocian and dominated the Middle Ordovician before declining and going extinct by the Katian; and the diplograptid fauna, comprising the scandent axonophorans (particularly the Diplograptina) that evolved in the Darriwilian and radiated to become the dominant group of graptoloids by the Katian. The end-Ordovician glaciation caused a period of faunal turnover (the Late Ordovician mass extinction) that stretched from the very end of the Katian to the end of the latest Ordovician Hirnantian stage. During this period, the Diplograptina, which had dominated tropical faunas and lived in dysoxic (low-oxygen), mesopelagic waters, slowly went extinct and were replaced by members of the Neograptina that had invaded from polar oceans and inhabited the oxygenated epipelagic zone.

The Neograptina were unharmed by the Hirnantian glaciation and diversified even during the LOME. The succeeding Llandovery epoch saw an explosion in morphological disparity fueled by the evolution of the uniserial Monograptidae in the early Rhuddanian and the evolution of the mesh-like Retiolitinae at the start of the succeeding Aeronian stage. Late Llandovery and early Wenlock faunas were composed of a diverse variety of monograptids and retiolitines but were decimated by the mid-Wenlock lundgreni event (or the Great Crisis), which exterminated nearly all monograptids (with only unspecialized members of the Pristiograptus dubius group surviving the event) but left the retiolitines relatively unharmed. However, the retiolitines went extinct in the succeeding leintwardensis event during the middle Ludlow, along with several lineages of monograptids. The surviving graptoloids were hit by additional extinctions at the end of the Ludlow (the kozlowskii event) and at the Silurian-Devonian boundary (the transgrediens event).

The transgrediens event in particular wiped out the long-lived Pristiograptus dubius lineage and nearly all other Silurian genera, with only the multi-branched Linograptus making it into the Devonian. Devonian faunas are characterized by the reappearance of the Lazarus taxon Uncinatograptus, possibly originating from the early Přídolí Uncinatograptus pridoliensis. Linograptus and its descendant Abiesgraptus went extinct before the end of the Lochkovian, leaving only Uncinatograptus and some related genera in the Pragian. The last known graptoloid was the monograptid Uncinatograptus yukonensis, which finally went extinct in the latest Pragian or earliest Emsian.

Later benthic graptolites

Dendroid graptolites survived past the Early Devonian, but are poorly known from post-Early Devonian strata. Devonian faunas are dominated by members of the Callograptidae, such as species of Callograptus, Dictyonema, Desmograptus, and Palaeodictyota. Dendrograptids are rare from the Devonian, with only the genera Ophigraptus, Aspidograptus, and possibly Dendrograptus being known from Devonian strata, and only Callograptidae have been found from the Carboniferous. The Dendroidea went extinct by the Serpukhovian, but the reasons for their extinction are unknown.

The last known graptolite remains not assigned to the genus Rhabdopleura are Middle Permian stolons of the rhabdopleurid genus Diplohydra. Rhabdopleura itself is known from the Cambrian (Rhabdopleura sp. from the Guole Biota), but the earliest named species that can be confidently assigned to the genus is Rhabdopleura hollandi from the Silurian. Fossil species of Rhabdopleura are also known from the Carboniferous (Rhabdopleura graysoni and Rhabdopleura delmeri), the Jurassic (Rhabdopleura kozlowskii), the Cretaceous (Rhabdopleura vistulae), and the Eocene (Rhabdopleura eocenica). Rhabdopleura is also known from 12 extant species from all major oceans and at a wide range of latitudes and water depths.

External relationships

Graptolites are members of the class Pterobranchia, and can be united with other pterobranchs through the presence of asexually budding zooids and a tubarium constructed out of fusellar increments. Pterobranchia is a monophyletic group within the phylum Hemichordata, which also includes the free-living, worm-shaped enteropneusts (also called acorn worms). Hemichordates are deuterostomes and are most closely related to the Echinodermata, with the two phyla forming the clade Ambulacraria.

The following cladogram shows the relationships between graptolites and other ambulacrarians:

Ambulacraria

Interrelationships

Graptolites are a monophyletic group that includes all taxa more closely related to Rhabdopleura than Cephalodiscus. A clade composed of Rhabdopleura and the 'traditional' graptolites but excluding Cephalodiscus is well-supported, as Mitchell et al. (2013)'s cladistic analysis of the Graptolithina found resampling support for a clade composed of Rhabdopleura and other graptolites to be between 67% and 100% depending on the character coding schemes used.

Mitchell et al. (2013) found multiple synapomorphies that can be used to define the Graptolithina:

  • Truly colonial organization with zooids budding from a stolon system
  • The presence of regular fusellar sutures on the creeping tubes
  • The presence of fusellar full rings or regularly arranged half rings on the erect tubes
  • The presence of a sclerotized prosicula (though this is unknown in Cysticamara).

Mitchell et al. (2013) also named the clade Eugraptolithina, based on the presence of a helical line on the prosicula, that includes the family Cyclograptidae and the orders Dendroidea and Graptoloidea. While the Eugraptolithina may represent a natural clade, the sicula is poorly known from most benthic graptolites, which limits the usefulness of the clade.

Phylogeny of the Graptolithina based on Mitchell et al. (2013)'s main analysis.

Phylogeny of the Graptolithina based on Mitchell et al. (2013)'s analysis of an updated version of the Durman (1992) matrix.


Taxonomy

Representatives of the main groups of graptolites
Rhabdopleuridae
Cysticamaridae
Wimanicrustidae
Cyclograptidae
Dithecodendridae
Dendroidea
Graptoloidea

The Graptolithina include the two main orders Dendroidea and Graptoloidea, as well as several small families of uncertain phylogenetic placement.

Taxonomy of the Graptolithina following the Treatise on Invertebrate Paleontology, Part V, Second Revision:

Subclass Graptolithina Bronn, 1849


See also

References

  1. 1 2 Maletz, Jörg; Bates, Denis E. B.; Beli, Elena; Brussa, Edsel D.; Cameron, Christopher B.; Cooper, Roger A.; Gonzalez, Paul; Kozłowska, Anna; Lenz, Alfred C.; Loydell, David K; Rigby, Sue; Riva, John F.; Steiner, Michael; Toro, Blanca A.; VandenBerg, Alfons H. M.; Zhang, Yuandong; Zalasiewicz, Jan (2023-01-01). "Part V, Hemichordata, 2nd Revision, Complete Volume: Enteropneusta, Pterobranchia (Graptolithina)". Treatise on Invertebrate Paleontology: i–548. doi:10.17161/tip.vi.25184. ISSN 2153-621X. Retrieved 2026-04-13.
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    2. 1 2 3 4 5 p. 181.
    3. 1 2 p. 182.
    4. ↑ pp. 239, 244.
    5. 1 2 3 p. 183.
    6. ↑ p. 184.
    7. 1 2 p. 185.
    8. ↑ pp. 205–231.
    9. ↑ p. 29.
    10. 1 2 3 p. 40.
    11. 1 2 3 p. 170.
    12. ↑ p. 30.
    13. ↑ pp. 31–33.
    14. 1 2 p. 32.
    15. 1 2 3 4 5 p. 33.
    16. 1 2 p. 66.
    17. 1 2 p. 31.
    18. 1 2 p. 19.
    19. ↑ pp. 56–57.
    20. 1 2 p. 22.
    21. ↑ pp. 22–23.
    22. 1 2 p. 45.
    23. 1 2 p. 48.
    24. ↑ p. 178.
    25. ↑ pp. 47–48.
    26. ↑ pp. 177–178.
    27. 1 2 3 4 5 6 7 8 9 p. 34.
    28. ↑ p. 53.
    29. ↑ pp. 177, 179.
    30. 1 2 3 4 5 6 7 p. 49.
    31. ↑ pp. 49, 53.
    32. 1 2 3 4 5 6 p. 54.
    33. ↑ pp. 170–171.
    34. 1 2 p. 228.
    35. ↑ pp. 54–53.
    36. ↑ pp. 54–55.
    37. 1 2 pp. 55.
    38. ↑ pp. 63.
    39. 1 2 p. 64.
    40. ↑ p. 65.
    41. ↑ pp. 40–45.
    42. 1 2 3 4 5 6 7 p. 41.
    43. 1 2 3 p. 43.
    44. 1 2 3 4 5 p. 59.
    45. ↑ pp. 45–47.
    46. 1 2 3 p. 37.
    47. 1 2 pp. 35–36.
    48. 1 2 p. 78.
    49. 1 2 p. 210.
    50. ↑ pp. 233–236.
    51. ↑ pp. 237–241.
    52. 1 2 p. 236.
    53. 1 2 pp. 215–231.
    54. ↑ p. 329.
    55. ↑ p. 452.
    56. ↑ p. 233.
    57. ↑ p. 214.
    58. ↑ p. 3.
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