Bireclinata

Structure of Pseudisograptus manubriatus, from Lykova & Sennikov (2021).

Belongs within: Reclinata.
Contains: Axonophora.

In the arms of Pseudisograptus
Published 25 April 2021

From the Ordovician to the early Devonian, the graptoloids were a major component of the oceanic fauna. These colonial floaters are among the characteristic fossils of the early Palaeozoic and have received a lot of attention due to their use in biostratigraphy. The evolution of graptoloids has been presented as a process of increasing simplification, of progressive reductions in colonial complexity and density. Like all illustrations of evolutionary trends, this is an overly simplistic representation of how things actually occurred but it’s not entirely incorrect. The history of graptoloids was indeed marked by a number of significant transitions were particular growth forms overran their predecessors. One genus that may have played a significant role in the lead-up to one of these turnovers was Pseudisograptus.

Pseudisograptus manubriatus koi, entire fossil and close-up diagram of initial thecae, from Cooper & Ni (1986).

The graptoloid genus Pseudisograptus has been collected from rocks in Australia, North America and eastern Asia dating to the latter part of the Floian stage of the early Ordovician, a bit over 470 million years ago (Cooper & Ni 1986). It is characterised by colonies growing in two branches (stipes) with the stipes spreading outwards and upwards like a pair of wings (indeed, one species of this genus luxuriates in the name of Pseudisograptus angel). In large specimens, the stipes reach about two centimetres in length and about three millimetres wide (from inner margin to the outer apex of the individual thecae). Pseudisograptus species are very similar to, and until 1972 where classified with, species of the related genus Isograptus. They differ, however, in the arrangement and growth of the earliest thecae in the colony. Whereas Isograptus stipes grow outwards immediately from the oldest theca, Pseudisograptus have the first few thecae on each stipes elongated and growing downwards before the stipes makes a later sharp turn upwards. As a result, between the two ‘wings’ of the stipes there is a more or less distinct triangle (referred to as the manubrium) formed from the bases of the early thecae. At the top of the manubrium is an upright thread, the nema. In a number of graptoloid fossils, an inflated structure has been identified at the top of the nema that probably functioned as a float. I don’t know if such a structure has ever been identified in a Pseudisograptus fossil but I imagine it would quite easily be lost in the course of preservation.

Basal section of Cardiograptus amplus, from Fortey et al. (2005). Not a true biserial graptoloid but illustrative of the way biserial forms may have evolved from biramous ancestors.

Pseudisograptus‘ disappearance from the fossil record coincides with one of the aforementioned turnovers in graptoloid diversity, the appearance of the biserial graptoloids. These forms, which had two rows of thecae arising from a single central line, rapidly replaced most of the earlier branched forms. The rapid appearance of the biserial graptoloids has made their origins difficult to work out but current thinking is that they arose from a form similar to Pseudisograptus, in which the upward growth of the stipes became steep enough that they met in the middle along the nema. One interesting detail is that two lineages appear to have achieved biseriality at about the same time from closely related but separate ancestors. In the glossograptids, the conjoined stipes met each other side-by-side; in the diplograptids, they met back to back. Pseudisograptus has, at different times, been implicated in the ancestry of both of these groups. Cooper & Ni (1986) regarded Pseudisograptus as paraphyletic and including the direct ancestors of the glossograptids. In contrast, more recent studies by Fortey et al. (2005) and Maletz et al. (2009) have placed Pseudisograptus closer to the diplograptids. These studies have been more agnostic as to whether Pseudisograptus was a direct ancestor or a close relative. If the former is the case then, while the exact Pseudisograptus morphotype would disappear at the end of the Floian, their genetic lineage would continue strong for nearly ninety million more years.

Systematics of Bireclinata
<==Bireclinata [Amphiphyontes]MCM09
|--Pan-Axonophora [Arienigraptidae, Arienigraptinae, Pseudisograptidae]MCM09
| |--Arienigraptus Yu & Fang 1981M10, M14
| | |--A. geniculatusM14
| | |--A. gracilisMCM09
| | `--A. zhejiangensis (Yu & Fang 1981)M10
| `--+--AxonophoraMCM09
| `--Pseudisograptus Beavis 1972MCM09, C73 [Pseudisograptinae]
| | i. s.: P. geniculatusFC86
| | P. jiangxiensis (Yu & Fang 1981)R93
| |--P. angelFC86
| `--+--P. manubriatusFC86
| `--+--P. dumosus (Harris 1933)FC86, R93
| |--P. gracilisFC86
| `--P. hastatus (Harris 1933)FC86, R93
`--Glossograptidae [Corynoididae, Corynoidinae, Glossograptinae, Kalpinograptidae, Pan-Glossograpta]MCM09
| i. s.: Lonchograptus Tullberg 1880B70
| `--*L. ovatus Tullberg 1880B70
| Nanograptus Hadding 1915B70
| |--*N. lapworthi Hadding 1915B70
| `--N. phylloidesB70
| Kalpinograptus Jiao 1977M14
| Apoglossograptus Finney 1978M14
| Corynites Kozłowski 1956 non Berk. & Curtis 1853 (ICBN)M14
| `--*C. wyszogradensis Kozłowski 1956B70
| Corynoides Nicholson 1867 non Gray 1821 (ICBN)M14 [incl. Corynograptus Hopkinson & Lapworth 1875B70]
| |--*C. calicularis Nicholson 1867B70
| |--C. curtus Lapworth 1876R93
| `--C. gracilisFC86
| Mimograptus Lapworth in Elles & Wood 1908M14
| Rogercooperia Sherwin & Rickards 2000M14
| Sinoretiograptus Mu et al. 1974M14
| Skiagraptus Harris 1933M14
| `--*S. gnomonicus (Harris & Keble 1916) [=Diplograptus gnomonicus]B70
| Tonograptus Williams 1992M14
|--Bergstroemograptus Finney & Chen 1984MCM09, M14
| `--B. crawfordiMCM09
`--+--Paraglossograptus Mu in Hsü 1959MCM09, M14
| |--*P. latus Hsu 1959B70
| |--P. etheridgeiJ79
| `--P. tentaculatus (Hall 1865)M10
|--Cryptograptus Lapworth 1880MCM09, B70 [Cryptograptidae]
| |--*C. tricornis (Carruthers 1859) [=Diplograpsus tricornis]B70
| |--C. antennarius (Hall 1865)R93
| |--C. hopkinsoni (Nicholson 1869)R93
| |--C. insectiformis Ruedemann 1908R93
| `--C. schaeferi Lapworth 1880M10 [=C. tricornis schaeferiB70]
`--Glossograptus Emmons 1855MCM09, B70 [=GlossograpsusB70]
|--*G. ciliatus Emmons 1855B70
|--G. acanthus Elles & Wood 1901–1918R93
|--G. hincksiB70
|--G. holmiB70
|--G. quadrimucronatusK48
`--G. sinicusB70

*Type species of generic name indicated

References

[B70] Bulman, O. M. B. 1970. Graptolithina with sections on Enteropneusta and Pterobranchia. In: Teichert, C. (ed.) Treatise on Invertebrate Paleontology pt V 2nd ed. pp. V1–V149. The Geological Society of America, Inc.: Boulder (Colorado), and the University of Kansas: Lawrence (Kansas).

[C73] Cooper, R. A. 1973. Taxonomy and evolution of Isograptus Moberg in Australasia. Palaeontology 16: 45–115.

Cooper, R. A., & Ni Y. 1986. Taxonomy, phylogeny, and variability of Pseudisograptus Beavis. Palaeontology 29 (2): 313–363.

[FC86] Fortey, R. A., & R. A. Cooper. 1986. A phylogenetic classification of the graptoloids. Palaeontology 29: 631–654.

Fortey, R. A., Y. Zhang & C. Mellish. 2005. The relationships of biserial graptolites. Palaeontology 48 (6): 1241–1272.

[J79] Jaanusson, V. 1979. Ordovician. In: Robison, R. A., & C. Teichert (eds) Treatise on Invertebrate Paleontology pt A. Introduction. Fossilisation (Taphonomy), Biogeography and Biostratigraphy pp. A136–A166. The Geological Society of America, Inc.: Boulder (Colorado), and The University of Kansas: Lawrence (Kansas).

[K48] Kozłowski, R. 1948. Les graptolithes et quelques nouveaux groupes d’animaux du Tremadoc de la Pologne. Palaeontologica Polonica 3: i–xii, 1–235.

[M10] Maletz, J. 2010. Xiphograptus and the evolution of virgella-bearing graptoloids. Palaeontology 53 (2): 415–439.

[M14] Maletz, J. 2014. The classification of the Pterobranchia (Cephalodiscida and Graptolithina). Bulletin of Geosciences 89 (3): 477–540.

[MCM09] Maletz, J., J. Carlucci & C. E. Mitchell. 2009. Graptoloid cladistics, taxonomy and phylogeny. Bulletin of Geosciences 84 (1): 7–19.

[R93] Rickards, R. B. 1993. Graptolithina. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 537–542. Chapman & Hall: London.

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