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

Why Earth’s greatest mass extinction was the making of modern mammals

Featured image credited to Roger Smith, Iziko Museums of South Africa Social History / Natural History / Art Collections.

The ancient closest relatives of mammals – the cynodont therapsids – not only survived the greatest mass extinction of all time, 252 million years ago, but thrived in the aftermath, according to new research published today (28th August).

The first mammals arose in the Triassic period, more than 225 million years ago. These early fur balls include small shrew-like animals such as Morganucodon from England, Megazostrodon from South Africa and Bienotherium from China.

They had differentiated teeth (incisors, canines, molars) and large brains and were probably warm-blooded and covered in fur – all characteristics that stand them apart from their reptile ancestors, and which contribute to their huge success today.

However, new research suggests that this array of unique features arose gradually over a long span of time, and that the first mammals may have arisen as a result of the end-Permian mass extinction – which wiped out 90 per cent of marine organisms and 70 per cent of terrestrial species.

The research was conducted by the University of Lincoln, UK, the National Museum in Bloemfontein, South Africa, and the University of Bristol, UK, and has been published today in Proceedings of the Royal Society B.

Lead author Dr Marcello Ruta, evolutionary palaeobiologist from the University of Lincoln’s School of Life Sciences, said: “Mass extinctions are seen as entirely negative. However, in this case, cynodont therapsids, which included a very small number of species before the extinction, really took off afterwards and were able to adapt to fill many different niches in the Triassic – from carnivores to herbivores.”

Co-author Dr Jennifer Botha-Brink of the National Museum in Bloemfontein, South Africa, said: “During the Triassic, the cynodonts split into two groups, the cynognathians and the probainognathians. The first were mainly plant-eaters, the second mainly flesh-eaters and the two groups seemed to rise and fall at random – first one expanding, and then the other.  In the end, the probainognathians became the most diverse and most varied in adaptations, and they gave rise to the first mammals some 25 million years after the mass extinction.”

Co-author Professor Michael Benton, of the University of Bristol, UK, added: “We saw that when a major group, such as cynodonts, diversifies, it is the body shape or range of adaptations that expands first. The diversity, or number of species, rises after all the morphologies available to the group have been tried out.”

The researchers concluded that cynodont diversity rose steadily during the recovery of life following the mass extinction, with their range of form rising rapidly at first before hitting a plateau. This suggests there is no particular difference in morphological diversity between the very first mammals and their immediate cynodont predecessors.

 

Ancient mammal relatives cast light on recovery after mass extinction

A study of how the ancient relatives of modern mammals recovered after mass extinction raises fresh questions about the capacity for life to recover from cataclysmic events.

A growing number of species in the modern world face extinction due to global climate change, habitat destruction and over-exploitation.

The fossil record has been used by scientists in a bid to understand how mass extinctions came about, and how species and ecosystems recovered in the aftermath.

Research suggests that the survivors of mass extinctions are often presented with new ecological opportunities. The loss of many species in their communities allows them to evolve new lifestyles and new anatomical features as they fill empty niches.

However, it turns out that not all survivors respond in the same way, and some may not be able to fully exploit the new opportunities arising after a mass extinction.

Dr Marcello Ruta (University of Lincoln, UK), Dr Kenneth Angielczyk (Field Museum of Natural History, Chicago), Professor Jörg Fröbisch (Museum für Naturkunde, Berlin) and Professor Michael Benton (University of Bristol) examined how a group of ancient relatives of mammals called anomodonts responded in the aftermath of the largest mass extinction in Earth’s history.

Their findings, published this week in Proceedings of the Royal Society B and titled “Decoupling of morphological disparity and taxic diversity during the adaptive radiation of anomodont therapsids”, show that anomodonts remained anatomically conservative even as the number of species recovered.

The mass extinction at the end of the Permian Period – about 252 million years ago – had profound effects on organisms on land and in the sea, with as many as 90 per cent of marine organisms and 70 per cent of terrestrial species wiped out.

Lead author Dr Marcello Ruta, of the University of Lincoln, UK, said: “Groups of organisms that survive such a mass extinction are said to have passed through an evolutionary bottleneck similar to the genetic bottleneck that may occur in a population if many of its members die off.  A genetic bottleneck sometimes allows the population to move to a new evolutionary trajectory, but other times it constrains the future evolution of the population. Near the end of the Permian, a large number of anomodont species existed that displayed a wide range of body sizes and ecological adaptations, including terrestrial plant eaters, amphibious hippo-like species, specialized burrowers and even tree-dwelling forms.”

The variety of anatomical features found in anomodonts declined steadily over their history. Even in the aftermath of the mass extinction, when there should have been a lot of empty ecological space, anomodonts did not evolve any fundamentally new features. This suggests that the evolutionary bottleneck they passed through during the extinction constrained their evolution during the recovery.

This is the first study of its kind to address simultaneously changes in species number and anatomical diversity in anomodonts, and to quantify their response to the most catastrophic extinction on record.

Anomodonts are abundant, diverse, and well-studied, which makes them ideal models for evolutionary analyses.

Dr Ruta added: “The results underscore that recoveries from mass extinctions can be unpredictable, a finding that has important implications for the species extinctions being caused by human activity in the world today. We cannot just assume that life will return to the way it was before the disturbances.”

Funding was provided by the Natural Environment Research Council, the Deutsche Forschungsgemeinschaft, the Alexander von Humbolt Foundation and the German Federal Ministry for Education and Research.