Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite

Abstract

A phylogenetic analysis of morphological data from modern pterobranch hemichordates (Cephalodiscus, Rhabdopleura) and representatives of each of the major graptolite orders reveals that Rhabdopleura nests among the benthic, encrusting graptolite taxa as it shares all of the synapomorphies that unite the graptolites. Therefore, rhabdopleurids can be regarded as extant members of the Subclass Graptolithina (Class Pterobranchia). Combined with the results of previous molecular phylogenetic studies of extant deuterostomes, these results also suggest that the Graptolithina is a sister taxon to the Subclass Cephalodiscida. The Graptolithina, as an important component of Early–Middle Palaeozoic biotas, provide data critical to our understanding of early deuterostome phylogeny. This result allows one to infer the zooid morphology, mechanics of colony growth and palaeobiology of fossil graptolites in direct relation to the living members of the clade. The Subdivision Graptoloida (nom. transl.), which are all planktic graptolites, is well supported in this analysis. In addition, we recognize the clade Eugraptolithina (nov.). This clade comprises the Graptoloida and all of the other common and well‐known graptolites of the distinctive Palaeozoic fauna. Most of the graptolites traditionally regarded as tuboids and dendroids appear to be paraphyletic groups within the Eugraptolithina; however, Epigraptus is probably not a member of this clade. The Eugraptolithina appear to be derived from an encrusting, Rhabdopleura‐like species, but the available information is insufficient to resolve the phylogeny of basal graptolites. The phylogenetic position of Mastigograptus and the status of the Dithecoidea and Mastigograptida also remain unresolved. □ Biodiversity, Cambrian, Hemichordata, Deuterostomia, Ordovician.

A reexamination of Yuknessia from the Cambrian of British Columbia and Utah

“…It differs sufficiently from other fossil and living pterobranchs to be maintained as a separate genus, but interpretation of broader-scale relationships within the Pterobranchia must await treatment of this and similar Cambrian taxa within a comprehensive phylogenetic analysis similar to that recently conducted by Mitchell et al (2013), the outcome of which indicates that living rhabdopleurids nest within the graptolite clade.…”

“…Rhabdopleura also has a stolon system that appears to be lacking in Yuknessia. The absence of this key feature in Yuknessia, however, is regarded to reflect the lack of a sclerotized sheath for the stolon system, as has been documented for other early colonial pterobranchs (reviewed in Mitchell et al, 2013), rather than the actual absence of a stolon.…”

“…Tuboid graptolites can have zooidal tubes with a form similar to the erect tubes of Yuknessia. In this case, however, the zooidal tubes are characterized by distinct thecal dimorphism (autothecae and bithecae) (Maletz et al, 2005;Mitchell et al, 2013), and no evidence of such a condition is present for Yuknessia.…”

Modern Antarctic acorn worms form tubes

“…Fossil evidence [13][14][15] reveals that graptolites, and even rhabopleurids and cephalodiscids (modern pterobranch lineages), were present in the Middle Cambrian, making S. tenuis contemporary with established pterobranch (including graptolite) lineages. As the split between enteropneust and pterobranch lineages would have been before the Middle Cambrian, the tube of S. tenuis was not a precursor to the pterobranch coenecium.…”

The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

“…Erection of additional genera originally was not deemed necessary and most were introduced only much later, leading to the more than 600 genera now available. Today, more than 275 years after Linnaeus, we understand the graptolites to be a group of pterobranch hemichordates with numerous extinct and a few extant taxa (Mitchell et al 2010(Mitchell et al , 2013. We have learned a lot about their evolutionary patterns over more than 500 million years from the Early to Middle Cambrian until today (Maletz 2014).…”