New research on early tetrapod feeding habits

A study of the jaws of one of the earliest known limbed vertebrates shows the species still fed underwater, not on land.

Scientists from the University of Lincoln, UK, University of Zurich, Switzerland, University of Cambridge and University of Bristol, developed an innovative new method to infer the feeding mechanism of Acanthostega – one of the earliest and most primitive tetrapods.

Tetrapods – the four-legged limbed vertebrates – evolved from fish and include today’s amphibians, reptiles, birds and mammals.

Acanthostega is regarded as one of the best known early tetrapods, and has played a key role in debates about tetrapod origins since spectacular new specimens were discovered in Greenland in 1987. Dating back to some 360 million years ago (end of the Devonian period); it has often been seen as a near-perfect fish-tetrapod intermediate.

The UK and Swiss researchers employed advanced statistical methods from a range of disciplines to explore the anatomical, functional and ecological changes associated with the emergence of tetrapods.

They examined the movement and structure of the lower jaws of Acanthostega and several other early tetrapods and tetrapod-like fish. Their observations suggest the Acanthostega jaw was more geared towards feeding under water, indicating that this tetrapod retained a primarily aquatic lifestyle.

Dr Marcello Ruta, from the University of Lincoln’s School of Life Sciences, said: “The origin of tetrapod from fish is an iconic example of a major evolutionary transition. The fossils of Acanthostega continue to play an unsurpassed role in our understanding of the fish-tetrapod transition.

“Acanthostega retained many primitive and fish-like features while also displaying unquestionable tetrapod features such as fingers and toes. Its broad snout appears to be consistent with aquatic feeding habits (suction feeding), but its complex cranial joints appear to be similar to those of terrestrial vertebrates and would suggest direct biting on land environments as a means of prey capture. This paradox prompted our study.”

The team examined patterns of jaw shape variation to assess whether the Acanthostega jaw is overall more similar to the jaws of fish or those of tetrapods. They then used advanced engineering methods to simulate biting action.
Dr Ruta said: “The lower jaw of Acanthostega can be shown to be anatomically and functionally similar to the jaws of some early fish and contemporary fish-tetrapod intermediates.

“Its upturned anterior extremity and rearward-facing anterior fangs appear to be adaptations for a snapping action. All these observations imply that this type of jaw was capable of fast closure for efficient capture of fast prey, in support of a predominantly if not exclusively aquatic feeding action.”

Their results are published on Wednesday, 26th February in Proceedings of the Royal Society B.

Neenan JM, Ruta M, Clack JA, Rayfield EJ. 2014 Feeding biomechanics in Acanthostega and across the fish–tetrapod transition. Proc. R. Soc. B 20132689.
http://dx.doi.org/10.1098/rspb.2013.2689

Land animals kept fish-like jaws for millions of years

Research has confirmed how early land vertebrates, which evolved from fish, developed weight-bearing limbs and other adaptations long before their feeding systems adjusted to a vegetation-based diet.

Now, for the first time, fossil jaw measurements have demonstrated this gap in evolutionary development.

Scientists from the University of Lincoln (UK), the University of Massachusetts, Amherst, and the University of Oxford (UK), examined the lower jaws of 89 fossils of early tetrapods (four-footed animals) and their fish-like predecessors.

The fossils ranged in age from about 300 to 400 million years old and the team were interested in how the mechanical properties of the jaws of these animals differed through time.

They used 10 biomechanical metrics to describe jaw differences. One of these, called mechanical advantage, measured how much force an animal can transfer to its bite.

Dr Marcello Ruta, from the School of Life Sciences, University of Lincoln, said: Our study is the first of its kind to address changes in biomechanical properties of the lower jaw across the transition from fish to land vertebrates using a diverse range of extinct species. This work paves the way to in-depth analyses of the rates of evolutionary transformation in other anatomical structures during this major episode in vertebrate history. It also lays the foundations for integrative research that explores themes as diverse as the origin of the first terrestrial food webs, the impact of acquisition of new structures on the diversification of major animal groups, and patterns and processes of functional change.”

So it turns out that just moving into a new environment is not always enough to trigger functional adaptations.

The team discovered that the mechanical properties of tetrapod jaws did not show significant changes in patterns of terrestrial feeding until some 40 to 80 million years after the four-legged creatures initially came out of the water. Until then, tetrapod jaws were still very fish-like, even though their owners had weight-bearing limbs and the ability to walk on land.

In the paper, which has been published in an early online edition of the journal Integrative and Comparative Biology, the authors say the results may be explained by an earlier hypothesis: a shift from gilled to lung breathing in later four-footed animals was necessary before they could adapt their jaw structure to eating plants.

This finding suggests tetrapods may have shown a limited variety of feeding strategies in the early phases of their evolution on land.

Lead author Dr Phil Anderson, from the University of Massachusetts, said: “The basic result was that it took a while for these animals to adapt their jaws for a land-based diet. They stayed essentially fish-like for a long time.”

Dr Matt Friedman, lecturer in palaeobiology at the University of Oxford, said: “The thing that is really interesting is that the diversity of jaw function didn’t really take off until around the origin of amniotes – creatures that lay hard-shelled eggs on land rather than being tied to water for reproduction like fishes and amphibians. It is in amniotes and their closest relatives that we see the first evidence for dedicated herbivory – until that point tetrapods had basically been carnivores. So this means it took at least 50 million years of evolution after the origin of features like limbs, fingers and toes before tetrapods achieved dietary diversity that began to resemble what we see today.”

The statistical methods developed in this work could be used in future studies of more subtle biomechanical patterns in fossil animals that may not be initially clear.