Graptoloid cladistics, taxonomy and phylogeny

Abstract

A cladistic analysis aimed at understanding the phylogenetic relationships of basal (Early to Mid Ordovician) planktic graptolites resulted in a well-resolved majority rule consensus tree that displays a series of major monophyletic groups. This tree was used to produce a classification system for the planktic Graptoloida, one of the major groups of the Graptolithina. Resolution within the individual taxonomic units is low, however, as only few species of the individual groups were included in the analysis. Tree statistics such as the retention index show that the proximal end characters used in the analysis are more reliable indicators of synapomorphy, whereas distal rhabdosome characters show significantly more homoplasy on the consensus tree. This result supports previous qualitative assessments suggesting that the initial part of the colony appears to have been relatively conservative during the evolution of the graptolites, whereas the distal features are more easily modified. Thus, the proximal end structure is especially relevant for revealing shared common ancestry through the presence of distinct homologies. Based on the structure of the consensus tree, we recognize a series of nested holophyletic clades, primarily total clades (names of which are identified by the prefix Pan-) and crown clades. The proposed names conserve the traditional names applied to common graptolite higher taxa as much as possible and yet precisely describe the branching history of the clade. The Graptoloida is the clade that descended from the first graptolithine to possess a nematophorous sicula and planktic habit and comprises a stem lineage of "anisograptids" (e.g., Rhabdinopora flabelliformis and Anisograptus matanensis), plus the crown clade Supercohort Eugraptoloida. The Eugraptoloida includes all the remaining graptoloids, which form the sister taxa, Cohort Pan-Reclinata nov. and Cohort Pan-Sinograpta. Many familiar genera formerly included among the Dichograptidea form a stem lineage to the crown clade Subcohort Reclinata nov. The Reclinata includes the sister clades, Superorder Pan-Tetragrapta nov. and Superorder Pan-Bireclinata nov. The Pan-Bireclinata comprises stem group bireclinatids (including Isograptus) and the crown clade Order Bireclinata nov., which in turn consists of the sister total clades Suborder Pan-Glossograpta nov. and Suborder Pan-Axonophora nov. The Pan-Axonophora are represented in this analysis only by stem lineage species (Arienigraptus gracilis to Exigraptus uniformis), but the group contains the roots of the diverse and well-known diplograptids and monograptids, which dominate subsequent Middle Ordovician to Early Devonian graptolite faunas.•

The classification of the Pterobranchia (Cephalodiscida and Graptolithina)

“…A system with a higher number of specified ranks may better represent the detailed evolutionary history, but may overburden our taxonomic system with names and being less practical. The extreme pectinate form of the results of the recent cladistic analyses of graptolite taxonomy (Mitchell et al 2007) already led to an increasing number of proposed taxonomic ranks within the Graptolithina, most fully expressed in Maletz et al (2009). The ranked and named taxa in Maletz et al (2009), however, show genuine taxonomic relationships and strictly describe monophyletic clades in a stacked succession.…”

“…The extreme pectinate form of the results of the recent cladistic analyses of graptolite taxonomy (Mitchell et al 2007) already led to an increasing number of proposed taxonomic ranks within the Graptolithina, most fully expressed in Maletz et al (2009). The ranked and named taxa in Maletz et al (2009), however, show genuine taxonomic relationships and strictly describe monophyletic clades in a stacked succession. These taxa show details of the evolutionary relationships not explicitly stated in the taxonomic system.…”

“…Examples of names that today are unfamiliar include the Monophyontes, Mono-Amphiphyontes and Amphiphyontes of Tullberg (1883) (Table 1) and the differentiation of the Axonophora and Axonolipa of Frech (1897). However, Frech's Axonophora is used in modern graptolite taxonomy, re-introduced by Maletz et al (2009). Some of the early taxonomic concepts show some remarkable insight and a deep understanding of the differentiation of the major groups of graptolites.…”

“…Hennig 1950Hennig , 1965PhyloCode), but so many previously established taxa have been shown to be paraphyletic or polyphyletic and to make our taxonomic approach a nightmare. Mitchell et al (2007) and Maletz et al (2009) largely avoided the problem of paraphyly in graptolites by naming only monophyletic groups and extracting paraphyletic taxa as unnamed stem groups. Whilst there is no doubt that polyphyletic taxa should be avoided at all costs, the question is whether we should abandon all paraphyletic taxa.…”

“…They reflect general phylogenetic relationships, but do not represent them in detail. Pushing a (named) ranked system upon this analysis could lead to the erection of numerous named and ranked taxa (see Maletz et al 2009) as potentially every node can be named. Thus, a cladistic system works best without explicit ranking, but every cladistic diagram includes an implicit ranking through the fixed succession of nodes.…”