Royal Tyrrell Museum Lecture Series: On the Origin of Chordates in the Cambrian Explosion

Nesonektris aldridgei Shows Cambrian Vetulicolians Were Vertebrate Cousins

A new vetulicolian from Australia and its bearing on the chordate affinities of an enigmatic Cambrian group

Authors:

García-Bellido et al

Abstract:

Background

Vetulicolians are one of the most problematic and controversial Cambrian fossil groups, having been considered as arthropods, chordates, kinorhynchs, or their own phylum. Mounting evidence suggests that vetulicolians are deuterostomes, but affinities to crown-group phyla are unresolved.
Results

A new vetulicolian from the Emu Bay Shale Konservat-Lagerstätte, South Australia, Nesonektris aldridgei gen. et sp. nov., preserves an axial, rod-like structure in the posterior body region that resembles a notochord in its morphology and taphonomy, with notable similarity to early decay stages of the notochord of extant cephalochordates and vertebrates. Some of its features are also consistent with other structures, such as a gut or a coelomic cavity.
Conclusions

Phylogenetic analyses resolve a monophyletic Vetulicolia as sister-group to tunicates (Urochordata) within crown Chordata, and this holds even if they are scored as unknown for all notochord characters. The hypothesis that the free-swimming vetulicolians are the nearest relatives of tunicates suggests that a perpetual free-living life cycle was primitive for tunicates. Characters of the common ancestor of Vetulicolia + Tunicata include distinct anterior and posterior body regions – the former being non-fusiform and used for filter feeding and the latter originally segmented – plus a terminal mouth, absence of pharyngeal bars, the notochord restricted to the posterior body region, and the gut extending to the end of the tail.

Conodont Teeth, Vertebrate Skeleton Are Examples of Parallel Evolution

A comparison between the growth of the ‘teeth’ of the paraconodont

For decades, it was thought that our skeleton and all its characteristic bony tissues originated in the predators, known as 'conodonts'. However new research, led by the University of Bristol and published today in Nature, shows that they were evolutionary copy-cats who evolved tooth-like structures and tissues independently of other vertebrates. The origin of our skeleton is to be found in the armour of our mud-slurping ancestors who evolved bony armour to protect themselves from such predators.

Palaeontologists from Bristol, Peking University and the US Geological Survey collaborated with physicists from Switzerland to study the tooth-like skeleton of conodonts using high energy X-rays at the Swiss Light Source at the Paul Scherrer Institut in Switzerland. They showed that the tooth-like structures found in the mouths of conodonts evolved within their own evolutionary lineage, rather than in an ancestor shared with other vertebrates.

Lead author, Duncan Murdock of the University of Bristol said: "We were able to visualise every tissue, cell and growth line within the bony teeth, allowing us to study their development. We compared the tooth-like skeleton of conodonts to that of their 'paraconodont' ancestors and to teeth in living vertebrates, demonstrating that the tooth-like structure of conodonts was assembled through evolutionary time independently of other vertebrates."

Co-author, Professor Philip Donoghue of the University of Bristol's School of Earth Sciences said: "This removes a key piece of evidence from the hypothesis that teeth evolved before the skeletal armour, and suggests that the common ancestors of conodonts and other vertebrates likely lacked a mineralized skeleton. Rather, it seems that teeth evolved from the armour of our meek filter-feeding ancestors."

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