New Vetulicolian from Australia – its Bearing on chordate Affinities of an Enigmatic Cambrian Group

Australia: The Land Where Time Began

New Vetulicolian from Australia – its Bearing on chordate Affinities of an Enigmatic Cambrian Group

Among fossil groups from the Cambrian Vetulicolians are of the most problematic and controversial, and have been regarded by various scientists as arthropods, chordates, kinorhynchs, or their own phylum. In this paper Garcίa-Bellido et al. say there is mounting evidence that suggests Vetulicolians are deuterostomes, though there are still unresolved issues with their affinities with crown group phyla.

Nesonektris aldridgei gen. et ap. nov., preserves an axial, rod-like structure in the posterior body region resembling a notochord in its morphology and taphonomy, and shows notable similarity to early stages of decay of the notochord of extant cephalochordates and vertebrates.

Garcίa-Bellido et al. conclude that phylogenetic analyses resolve a monophyletic Vetulicolia as a sister group to tunicates (Urochordata) within crown Chordata, which holds if this holds even if they are scored as unknown for all notochord characters. It is suggested by the hypothesis that vetulicolians, that were free-swimming, are the nearest relatives of tunicates, that a life-cycle involving perpetual free-living was the primitive condition of tunicates. Among the characters of the common ancestor of Vetulicolia + Tunicate was a body with distinct anterior and posterior body regions – the former being a non-fusiform filter feeder, and the latter originally being segmented with an terminal mouth, pharyngeal bars absent, the notochord restricted to the posterior region of the body, and the gut extending to the end of the tail.

Conclusions

Between the vetulicolians and tunicates the phylogenetic affinity has significant implications for the morphology of the ancestral Urochordata (tunicates), which has been a continuing conundrum, according to Garcίa-Bellido et al., in chordate phylogeny (Nielsen, C., 2012, Animal Evolution – Interrelationships of the Living Phyla, 3^rd edition, Oxford: Oxford University Press). It is suggested by the Olfactores concept that successive living sister taxa to tunicates are vertebrates and cephalochordates (Delsuc, Brinkmann, Chourrout, Philippe, 2006; Delsuc, Tsagkogeorga, Lartillot & Philippe, 2008), therefore indicating the tunicates were derived from a amphioxus-like animal with a notochord, neural crest cells (or homologous cell populations), neural tube, striated heart muscle and segmented lateral muscles, though with no atrium, that was free-swimming and segmented (Nelson, 2012; Swalla & Smith, 2008; Delsuc, Brinkmann, Chourrout & Philippe, 2006; Ruppert, 2005). The dorsal segmentation on the tail in Shankouclava from the Cambrian (Chen et al., 2003) points to ascidian-like early tunicates that were actively moving. The common ancestor to the vetulicolian + tunicate clade is inferred to have been a pelagic animal with a thick cuticle for protection, a large pharynx for filter feeding in the anterior body region, and a posterior body region that was segmented for propulsion, that contained a notochord and an anus. Garcίa-Bellido et al. suggest that if the free-swimming appendicularians are considered to be basal to all other tunicates (Swalla & Smith, 2008), then it is implied by the most parsimonious scenario that the common ancestor of tunicates then all of these traits were also retained by the common ancestor of tunicates; in many tunicates such as ascidians the benthic sessile adult morphology optimises as a specialisation that has been derived (Swalla & Smith, 2008; Delsuc, Brinkmann, Chourrout, Philippe, 2006).

It is suggested that in the Cambrian the vetulicolians could have filled the ecological niche of medium-large suspension feeders, which supports the proposal that chordates arose as a clade of suspension feeders (as in the case of the early vertebrates such as Haikouichthys, and also for yunnanozoans and modern amphioxus, tunicates and larval lampreys), and so not as predators. They would have relied on their size and tough cuticle for protection as they would have been blind and slow moving. There have been problems with the interpretation of the morphology of vetulicolians (Gee, 2001; Aldridge et al., 2007), though evidence has been mounting, notably the presence of gill pharyngeal slits Ou et al., 2012), that indicates they belong to the deuterostome total group (Smith, 2012). New support for placing the vetulicolians more crownwards has been provided by evidence of a possible notochord in vetulicolians, a consequence of which was a substantial increase in the diversity, disparity and abundance of chordates in the Cambrian (Conway Morris & Caron, 2012; Shu, 1999; Swalla & Smith, 2008; Chen et al., 2003; Hou, 2002).

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