| Articulata | |
|---|---|
 | |
| Extinct and modernarthropods | |
 | |
| Glycera sp. (Annelida) | |
| Scientific classification | |
| Kingdom: | |
| (unranked): | |
| Superphylum: | Articulata H. Bruce Boudreaux, 1979[1] |
| Subordinate taxa | |
TheArticulata is a proposed higher taxon of animals with segmented bodies, consisting ofAnnelida andPanarthropoda. This theory states that these groups are descended from a common segmented ancestor. The Articulata hypothesis is an alternative to the hypothesis thatecdysis (the shedding of outer cuticle) is a primitive characteristic – this would place Panarthropoda in the groupEcdysozoa.
The Articulata hypothesis originates from thephylogenetic analyses ofGeorges Cuvier in his 1817 published workLe Règne animal, distribué après son organization. In this work, Cuvier theorized that allorganisms exist as a functional whole, meaning that all of the physiological structures of an organism are important for survival. By studying these physiological structures, Cuvier was able to group the knownanimal kingdom according to structural similarities resulting from what he referred to as special "ground-plans", which are analogous to blueprints. Each of these ground-plans, he further argued, evolved separately from the others and structural similarities were due to common function and not to common ancestry. From these ground-plans, Cuvier separated the known animal kingdom into four branches orembranchements:Vertebrata, Articulata,Mollusca andRadiata. From this phylogenetic grouping, the Articulata hypothesis was born.[2]
The Articulata hypothesis, simply stated, is the phylogenetic grouping of the phylum Annelida (which includespolychaetes,oligochaetes, andleeches) together with the phylum Arthropoda (arachnids,insects andcrustaceans) into the commontaxon Articulata. Cuvier grouped these diverse phyla together according to the common structural feature: the segmented body plan. This hypothesis further implies that all segmented organisms have a common ancestral origin.[3]
Since its original formulation in 1817, there have been significant challenges and modifications to the Articulata hypothesis as new theories have been accepted (Charles Darwin'stheory of evolution) and new technologies have become available (confocal microscopy,DNA sequencing,genomics). Additionally, the discovery ofOnychophora as its own phylum was incorporated into the theory by H. Bruce Bordreaux[4]
Darwin's theory of evolution had a large, yet often understated impact of the Articulata hypothesis. Cuvier's original Articulata hypothesis was based on his assumption that current species no longerevolved because to evolve would cause loss of integral structures necessary for the survival of thespecies. While the general acceptance of the theory of evolution weakened Cuvier's general theory of the unique ground-plans as the origin of moderntaxa, it strengthened the Articulata hypothesis by organizing annelids and arthropods into aclade descended from a common segmented ancestor.[5]
While each advance in modern molecular biology has shaken thephylogenetic tree ofBilateria, advances inmolecular biology techniques led to further data supporting the Articulata hypothesis but also led to the development of conflicting theories. Advances inconfocal microscopy technology led to the discovery ofembryoniccleavage patterns, which differs between the annelids and arthropods.[6] Annelids show spiral cleavage, meaning that each embryonic cleavage occurs at progressive 90-degree angles with respect to the animal–vegetal axis. Arthropods, on the other hand, display a heterogeneous mix of embryonic cleavage patterns including spiral-like cleavage and radial cleavage patterns. This led researchers to two theories: The first was that the arthropods lineage must have lost the ability to spiral cleave since differentiating from the last common ancestor between annelids and arthropods. The second is that this showed similarities between annelids andmollusks who also spirally cleaves but lacks that the segmented body plan. This was not the only interpretation of this data but other hypotheses were seen to have less data or merit. Other studies such as those looking a neural patterns within the Articulataclade showed mixed patterns and thus mixed results.[5][7]
The advancements in DNA sequencing techniques and the development of phylogenetic analysis algorithms led to the splitting of the Articulata clade. Original phylogenetic studies on the sequences of 18S and 28Sribosomal DNA sequence led to the suggestions that the annelids and arthropods had evolutionarily diverged much earlier than was previously thought but such limited genetic studies led to limited and often mixed results. As more genes were added to the studies, it became apparent that arthropods were genetically closer tonematodes and other molting organisms whereas the annelids were closer evolutionary to mollusks. ThisEcdysozoa hypothesis is generally accepted today as the best supported evolutionary hypothesis for annelids and arthropods.[8][9][10] However, a 2025 paper still suggests Articulata is monophyletic, alongsideAtelocerata, despite the vast majority of papers favouring Mandibulata and Ecdysozoa instead. While Ecdysozoais recovered within the paper, cycloneuralians are instead found to be simplified lobopodians. Alongside this, Articulata is hypothesised to have descended fromspintherid-like crawling annelids.[11]